Open Access is an initiative that aims to make scientific research freely available to all. To date our community has made over 100 million downloads. It’s based on principles of collaboration, unobstructed discovery, and, most importantly, scientific progression. As PhD students, we found it difficult to access the research we needed, so we decided to create a new Open Access publisher that levels the playing field for scientists across the world. How? By making research easy to access, and puts the academic needs of the researchers before the business interests of publishers.
We are a community of more than 103,000 authors and editors from 3,291 institutions spanning 160 countries, including Nobel Prize winners and some of the world’s most-cited researchers. Publishing on IntechOpen allows authors to earn citations and find new collaborators, meaning more people see your work not only from your own field of study, but from other related fields too.
To purchase hard copies of this book, please contact the representative in India:
CBS Publishers & Distributors Pvt. Ltd.
www.cbspd.com
|
customercare@cbspd.com
In the Mediterranean Sea, in addition to the two historically known species belonging to the Squalus genus (Squalus blainville and Squalus acanthias), a third species, Squalus megalops, has been reported. This last specie is a subject of debate between authors. S. acanthias is quite distinct from the other species of the genus Squalus, while S. blainville and S. megalops are very similar morphologically. This similarity has resulted in considerable confusion over their taxonomy. The lack of a well-preserved holotype for S. blainville, misidentifications in databases and in the literature, description, and figure of Risso (1827) not conforming to any known species of Squalus are impediments to the proper taxonomic identification and the potential revision of the genus. This chapter aims to clarify the state of the species of the genus Squalus in the Mediterranean Sea, taking into account all the studies carried out on this subject.
National Institute of Marine Science and Technology (Sfax Center), Sfax, Tunisia
Sami Karaa
National Institute of Marine Science and Technology (Sfax Center), Sfax, Tunisia
Othman Jarboui
National Institute of Marine Science and Technology (Sfax Center), Sfax, Tunisia
*Address all correspondence to: sondes.marouani@instm.rnrt.tn
1. Introduction
The Mediterranean Sea is a semi-enclosed sea covering less than 1% of the surface of the global oceans, even though it constitutes a general richness hotspot of total species on a global scale [1, 2]. This richness of cartilaginous and bony fishes is likely the result of the recolonization of the Mediterranean basin after the Messinian crisis. As demonstrated for the great white shark [3], pulses of species immigrations occurred during the glacial and interglacial periods of the quaternary [4].
It is a heterogeneous biogeographic area that shows a high level of biological diversity. This sea constitutes a complex marine ecosystem within which elasmobranchs play a basic role in controlling trophic relationships [5]. This is related to multiple factors from its geological history to its peculiar oceanographic and ecological features [6].
The Mediterranean Sea is considered a Chondrichthyes-rich basin. Recently, a total of 88 chondrichthyan species were listed, representing 30 families and 48 genera in the area [6]. This list includes 48 species of sharks, belonging to 18 families and 27 genera, 38 species of batoids, belonging to 11 families and 19 genera, and two chimeras belonging to two different genera.
Despite its richness, it encloses the highest proportion of threatened species in the world [7], in the Mediterranean Sea, where at least 53% of the species are classified by the IUCN as vulnerable, endangered, and critically endangered [8, 9]. Quite a large proportion of species (13%) are still classified as data deficient [8].
There are information gaps with respect to species richness and abundance of elasmobranchs in the Mediterranean Sea. These gaps often make it hard for international organizations to assess the conservation status of populations. Either the knowledge of the abundance and richness of this group, which has played a crucial ecological part in Mediterranean trophic webs, is also significant to any future strategic plan for the conservation of marine biodiversity in the region [10, 11].
The gaps are due to many reasons, the most important of which is the misidentification of species in databases and in the literature. Generally, elasmobranchs have suffered major taxonomic constraints that have led to misidentification issues related to by-catch and fisheries, which were usually solved by grouping data at higher taxonomic levels, such as genus or family [12, 13], or to morphological and biological similarities among some species, which have led to considerable confusion over their taxonomy such the case of the Squalus genus species [14].
Dogfish are scientifically classified as the Squalidae family, categorized under the squaliform order, which encompasses seven families in total, including Squalidae. The latter, more commonly known as dog sharks or spiny dogfish, have two dorsal fins different in shape with long spines without grooves and anal fin, with a cylindrical body and a small mouth. Their jaws are furnished with powerful cutting teeth and sometimes present only on the lower jaw; the upper jaw plays, in this case, only the role of holding the prey. The species of this family are generally small sharks, which frequently generally more or less accentuated bottoms except for the spiny dog S. acanthias, which does not descend below 150 meters.
Some species are highly valued and important as a major fish resource for food and liver oil. Some species are threatened due to overfishing and because of their biological characteristics, namely a long lifespan, late maturity, and low fecundity, as is the case with all elasmobranchs.
The Squalidae family itself contains two separate genera: Cirrhigaleus and Squalus, numbering together 37 species, and has the most species in the group, including Squaliolus laticaudus, one of the smallest known sharks with a size of 15 cm.
The genus Squalus Linnaeus, 1758 is distributed worldwide [15]. Until 2013, 25 species were known: 14 species documented as valid and 11 species added latterly from the western Indo-Pacific ocean [16, 17, 18]. But, this number has recently increased due to the resurrection of Squalus acutipinnis (Regan 1908) from South Africa and the description of four new species (S. albicaudatus, S. bahiensis, S. lobularis, and S. quasimodo) from the southwest Atlantic [19, 20, 21].
It was stated that the species diversity within the group is still poorly characterized [22]. For instance, 20 species have been described or resurrected in the last decade in the Indo-Pacific region [23, 24, 25] and the south Atlantic [19, 20].
In fact, cryptic speciation among elasmobranchs is very common [26, 27] and the number of new descriptions, redescriptions, and resurrections of species is growing with the increasing application of molecular tools and integrated taxonomic methodologies. Thus, the number of valid species in the genus was doubled and a significant amount of “hidden” diversity in the group has been revealed. Consequently, about 50% of the Squalus species are considered data deficient consistent with the International Union for the Conservation of Nature (IUCN) red list of threatened species [22].
The Squalus species inhabit the waters of the continental shelf and upper slope, between 300 and 700 m of depth [28, 29, 30], as well as some seamounts and the waters around oceanic islands [22, 31]. They have been divided into four assemblages based on their morphology: the “acanthias” group, the “blainville” group, the “megalops cubensis” group, and the “asper-barbifer” group [32, 33, 34]. However, members of the “asper-barbifer” group are assigned to the genus Cirrhigaleus [17]. Nevertheless, in recent years, different genetic studies have attempted to identify Squalus species using mitochondrial COI and NADH2 genes [22, 35, 36, 37]. Generally speaking, three well-defined groups within the genus have been described: group I, including Squalus suckleyi and S. acanthias; group II, including S. blainville/S. megalops/Squalus raoulensis/Squalus brevirostris; and a third group, “the Squalus mitsukurii complex” including Squalus edmundsi, Squalus japonicus, Squalus grahami, Squalus clarkae, and S. mitsukurii [22, 38, 39, 40].
In the Mediterranean, in addition to the two historically known species the longnose spurdog S. blainville (Risso, 1827) and the spiny dogfish S. acanthias (Linnaeus, 1758), a third species, the shortnose spurdog S. megalops (Macleay, 1881), has been reported [40, 41, 42, 43].
In the following, we try to clarify the state of the species of the genus Squalus in the Mediterranean Sea taking into account all the studies carried out on this subject.
2. Status of squalus genus in the Mediterranean Sea
2.1 Ecobiology
As many elasmobranchs, Squalidae are K-selected species with slow growth rates, low fecundity, and late sexual maturation; those species tend to aggregate by sex and size [31, 44, 45]. These features make such a taxon greatly vulnerable to overfishing. Thus, increased understanding of their ecobiology is crucial to develop an assessment for conservation strategies and developing effective fisheries management.
In the Mediterranean Sea, many investigations on the life history traits of Squalidae were conducted toward filling the information gap and to develop practical conservation strategies for those species in the area. As shown in Tables 1–4, a survey of the available published literature was carried out through a bibliographic study.
Studies on Squalus blainville, S. megalops, and Squalus acanthias diet carried out in the Mediterranean Sea (−; +; ++; +++: Increasing gradient of prey importance according to the calculated IRI%/ Chond.: Chondrichthyes; Teleo.: Teleosteans, crus.: Crustaceans; moll.: Mollusks; Ann.: Annelids; Echino.: Echinoderms; Oth.: Others).
Studies on squalus blanville, S. megalops, and Squalus acanthias mass-lenght relationship, carried out in the Mediterranean Sea (N: Number of individuals; M: Male; F: Female; L: Lenght; W: Weight; GT: Growth type; I: Isometry; (+): Positive allometry; (−): Negative allometry).
Studies on Squalus blainville, S. megalops, and Squalus acanthias age, carried out in the Mediterranean Sea (VBGM Von Bertallonffy; L∞: The asymptotic length at which growth is zero; K is the growth rate; T0: Constant value).
Until now, the historical traits of the Mediterranean population of S. megalops remained poorly studied, which may be attributed to its taxonomic problem in the area.
Generally speaking, Squalidae species are aplacental viviparous, with a long gestation, estimated up to 2 years for S. acanthias [81]. Their uterine fecundity was estimated to be from 1 to 12 embryos per litter. Females reach maturity at up to 70 cm, 56.41 cm, and 88 cm total length (TL), whereas males matured at up to 55.0 cm, 44.39 cm, and 70.0 cm TL for S. blainville, S. megalops, and S. acanthias, respectively (Table 1). The estimated size at maturity differed between males and females confirming the marked sexual size dimorphism of those species.
Concerning the food habits of those species, they are active predators, which feed on similar preys but with different importance according to the index of relevant importance of each prey item consumed (IRI%) calculated (Table 2). Species are reported to feed mainly on bony fishes, cephalopods, and crustaceans.
Sexual size and mass dimorphism were observed, with females attaining larger TL and greater mass than males (Table 3). This pattern is common among viviparous sharks since for females, due to their more energetically demanding reproductive mode, there is a strong selection pressure for larger body size [82].
Squalidae are long-lived animals, with females attaining greater age than males, as it is typical of most elasmobranchs. Using the von Bertallonfy model, the investigations were conducted to confirm this pattern of differential growth between males and females (Table 4). The maximum ages observed for males were 22 years, 26 years, and 23 years and for females were 28 years, 29 years, and 36 years of S. blainville, S. megalops, and S. acanthias, respectively.
2.2 Geographic distribution
S. acanthias and S. blainville are mostly found in the northern part of the Mediterranean Sea and the Adriatic, including the Black Sea [30, 83, 84, 85]. Some authors have reported the spiny dogfish (S. acanthias) to be one of the most frequent shark species captured in the Mediterranean [30, 86, 87]. Whereas, its congener S. blainville is stated to be one of the most important species of the demersal assemblages in the eastern Ionian Sea [88], as well as throughout the basin, principally in its central-western part (eastern Corsica and southern Sicily) and the eastern Ionian and Aegean seas [87]. The species was found to be more abundant on the slope than on the shelf [89].
The reason for this replacement between those sharks could be related to taxonomic problems afflicting the Squalus genus in different areas of the Mediterranean [38, 84]. Indeed, recent studies [53] highlight that S. acanthias showed a limited geographic distribution in the past, suggesting an inaccurate classification of these two species [90].
To clarify the real presence of the Squalus species in the Mediterranean Sea, numerous scientific studies, such as MEDITS trawl survey, have been conducted. According to those studies, S. acanthias and S. blainville are mostly found in the northern part of the Mediterranean Sea, Adriatic, and Black Sea [25, 84, 85, 91].
Concerning S. megalops, it is recorded from the northern coasts of the Canary Islands, Morocco, and southern Spain (Malaga), but it is present mainly along the African coasts of Tunisia [41].
2.3 Catches
Fundamental problems in accurately identifying and classifying species have hampered the collection of robust biological and ecological data. This fact, together with erroneous reports of reported catches (usually lower than actual catches; [30, 92]) makes stock estimates for these fish difficult to assess. This situation is of particular concern in elasmobranchs taken as targets or as bycatch in several fisheries around the globe, as their conservative life history strategies make them extremely vulnerable to overexploitation [93].
Worldwide, Indonesia and Spain remain the top three shark catchers in the world [94]. In the Mediterranean Sea, countries, contributing more to the elasmobranch landings during the last years, are Tunisia and Libya, which contributed more than 70% of production. Tunisian landings did not show any distinguished variations from 1980 to 2015. Those from Libya appear for the first time in FAO statistics and seem to be significant. Turkey and Italy register a dramatic decrease in catch after being known to be the major elasmobranch-fishing countries within the Mediterranean, between 1980 and 2008. It should be noted that the Mediterranean landings of carcharhiniformes, the most represented group among the elasmobranchs and the most commercially fished, recorded a notable decrease [92]. The most commonly caught species are skates (Rajidae) and catsharks (Scyliorhinus spp. and Galeus spp.) [95, 96]. Different species of pelagic sharks, as well as eagle rays (Myliobatidae) and stingrays (Dasyatidae), are bycatch of pelagic and demersal fisheries [97, 98].
Squalidae, it represents one of the most commercially targeted families among elasmobranchs [24]. Capture production for Squalidae in the Mediterranean and the Black Sea during the last decades is illustrated in Figure 1 [99].
Figure 1.
Capture production for Squalidae in the Mediterranean and Black Sea during the last decades.
Indeed, several species belonging to this family are landed by up to 50 countries in direct fisheries or as bycatch [24]. The genus Squalus is highly represented in bycatch and several studies have focused on the mitigation of the fishery impact on this group [100, 101, 102].
In the Mediterranean Sea, S. blainville constitutes an important landing from bottom trawlers, longlines, and gill nets [103, 104]. Moreover, the presence of S. megalops and S. blainville was reported on African coasts of Tunisia in bycatch of bottom trawl and longline fisheries [41, 92].
Although longlines are considered selective, they bring several nontarget species, including S. acanthias [92]. In fact, a significant decline in the bycatch of the former species is perceived according to the fishermen’s perception on the evolution of shark populations in the northern Catalan coast (north-western Mediterranean Sea) [105]. The low presence of the piked dogfish (Annex III of the Barcelona Convention) in the subregion’s bycatch composition could confirm the recorded decrease in biomass of this species.
The bycatch of elasmobranchs is an issue of global concern, particularly in high-seas pelagic longline fisheries, where 25% of the catch is nontarget sharks and rays [106]. Thus, in order to keep the populations of these fishes in balance, incisive management programs are required to guarantee stability for the populations. Among these management measures, the elimination or at least the reduction of the bycatch cannot be missing.
2.4 Taxonomic status
Dependable data on species richness are crucial for any biodiversity study and conservation policies, even though it is every so often difficult to discriminate a species based on extremely similar morphological characters [107]. Therein, reliable species identification is the principal step for the application of conservation policies and maintainable exploitation of natural resources [108], all the more so considering the currently elevated biodiversity crisis induced by human activities [109].
Overall, sharks belonging to the Squalus genus exhibit conserved body morphology, making identification based entirely on morphological characters complicated, leading to misidentifications [110]. This complexity is amplified even similarly via the high overlap of morphological characters among species, as identification is often based on limited and insufficiently consistent characters, like the number of vertebrae and morphometric data [10, 16, 20, 38, 111, 112]. In fact, morphological and biological similarities among squalids have led to considerable confusion over their taxonomy [14]. Some taxonomic and nomenclatural problems affect the group of species in question. Excluding S. acanthias, easily recognizable thanks to its specific pattern, characterized by the presence of white spots on the back or narrowly round to acutely angular rear tips and inner margins of the pectoral fins, which permits an easier identification and discrimination from the other two species [84], and the correct identification of the other two species, which do show a very similar morphology, requires the observations of the dermal denticles, meristic features, or even genetic analysis.
Compared with Squalus acanthias and S. asper, a close similarity between S. blainville and S. megalops was pointed out [113, 114]. Moreover, despite that the relationships between the snout tip and nostril distance and the distance from the nostril to the preoral clefs were proposed as the best features for discriminating between species of the genus Squalus, and it is proved that they were of little use [115].
The taxonomic status of S. blainville is problematical as there are no extant types and the original description and figures do not correspond to any known species of Squalus [42, 116]. Consequently, in a review of the Australian species of Squalus, Squalus griffini and Squalus fernandinus (Molina, 1782) were incorrectly synonymized with S. blainville [32]. However, in a review of Japanese Squalus, S. blainville was defined as a species with high dorsal fins and long dorsal-fin spines [116]. The same review revealed that Squalus, referring to S. fernandinus and S. blainville by some authors, had short dorsal-fin spines and were more similar to S. mitsukurii from Japan, and suggested that nominal S. blainville from New Zealand could be identical to S. mitsukuri [112, 117]. It was also noted that dogfish resembling S. mitsukurii occurred in Australia and New Zealand [31]. Outside its main distributional area (Mediterranean Sea and eastern Atlantic), S. blainville has also been recorded erroneously in Australia and New Zealand [112]. It was thought to be widespread in the Atlantic, Indian and Pacific Oceans [34, 117] as well as in Japan [116]. The distribution of S. blainville was restricted to the Mediterranean Sea and eastern Atlantic, and its records in the Pacific records were questioned [17]. The confusion is due largely to the poor original description and the lack of type material.
Taxonomic research on the genus Squalus conducted in the Tunisian waters revealed that S. blainville in this zone is not characterized by its high first dorsal fin and spine as was well thought-out by some authors, but rather it is a short-spine species [31, 116]. Comparing their data for S. blainville with the measurements of the same species in different regions, they noted that data generally agree despite there being some differences in the morphometrics between populations, and that Tunisian S. blainville specimens examined and the specimens studied by some authors [41, 42, 43, 114, 115, 118] have similar vertebral counts.
Recently, according to the studies conducted in other areas of the Mediterranean Sea and based on morphological and genetic (COI sequences) analyses, only one spurdog species, S. blainville, occurs in the Ionian, Libyan, Aegean Seas, and the Sardinian waters [38, 119].
These findings spotlighted the stretch of sea between Tunisia, southern Sicily, Malta, and Libya, known as the Strait of Sicily, as a more interesting area for spurdog species. The presence of S. blainville in the Maltese waters was assessed through the use of the DNA barcoding approach [84]. In the same region, some authors [37] collected and analyzed individuals belonging to the nominal S. blainville and genetically clustering within clade B [22], while three individuals were classified as Squalus sp. by the authors as clustering in the genetic clade C [22].
Regarding S. megalops, it is described for the first time in the Mediterranean in 1984 [43]; its occurrence is still debated and several scientific studies contributed to clarifying the real presence of this species in the area [21, 22, 37, 39, 41, 42, 84, 85, 119].
It was suggested that the southern Australian S. megalops might be endemic to Australia [91]. However, the tale can be even extra complex. Morphological research has proven that more than a single form of this species exists in Australian seas [28].
Recently, few research within the context of integrative taxonomy had been successfully accomplished in the genus Squalus aiming at the integration of new molecular taxonomy techniques to more classical morphological analyses with the purpose to make clear taxonomic ambiguities among some of the species [41, 84].
To highlight the taxonomic uncertainties in relation to the occurrence of Squalus species in the central Mediterranean Sea, a study including other morphometric characters and a molecular study was carried out to confirm that S. megalops occurs as a valid species in the Mediterranean Sea [41]. In fact, two species of spurdog of the genus Squalus occur in the Gulf of Gabès (southern Tunisia and central Mediterranean): S. blainvillei and S. megalops cubensis group. Morphometric and meristic data as well as genetic analyses (DNA inter-simple sequence repeat markers and molecular barcoding methods) support the assignation of this short-snout spurdog to S. megalops.
The Tunisian S. megalops species are consistent for characters typifying the “megalops-cubensis” group and fit the description of S. megalops from Australian waters [23], as well as the eastern Atlantic-Mediterranean [42] and Mediterranean waters [43]. Specimens described from other areas clearly agree with the Tunisian samples of S. megalops for most of the morphometric characters and had similar vertebral counts to those studied by other authors (Indo-Pacific, [118]; South Africa, [115]; Mediterranean coasts of Spain, [34]; east Atlantic, [42]; south western Australia, Queensland [23]).
Some authors stated that the number of chondrocranial lateral processes is the most important character to discriminate between S. blainville and S. megalops [34, 42], but they can be discriminated also based on other morphological features, such as teeth and dermic denticles morphology. These findings have been confirmed in the Gulf of Gabès through morphometric, meristic, and genetic analyses, suggesting that S. megalops could be even more common than S. blainville in these waters (Figures 2 and 3) [41]. In addition to the differences cited between those species, the study of their traits of the life history in the area revealed that they differ also in those terms [50, 51, 63, 66, 77].
Figure 2.
Squalus blainville (adult female 96 cm TL) off Tunisian waters (a: Lateral view, b: unicuspid flank denticles, c: teeth with a single cusp deeply notched and outward end strongly oblique d: bent claws and massive spurs, e: two cartilaginous processes in the basal plate, f: sharpen palatoquadrate).
Figure 3.
Squalus megalops (adult female 76 cm TL) off Tunisian waters (a: lateral view, b: unicuspid flank denticles, c: teeth with a single cusp deeply notched and outward end directed strongly laterally, d: bent claws and massive spurs, e: two cartilaginous processes in the basal plate, f: sharpen palatoquadrate).
Differently from the former study [41], a study on the intraspecific morphological variability in S. blainville did not identify diagnostic features (e.g., dermal denticles), which could effectively discriminate between S. blainville and S. megalops. The authors asserted that species identification based only on morphological characteristics can easily lead to taxonomic misidentifications, especially when multiple anatomical characters (e.g., skull and teeth morphology) are used [84].
To aid in clarifying the taxonomic status of Squalus species in the eastern Atlantic and Mediterranean, some authors assessed species diversity at the molecular level and evaluated the consistency in species identification in the region [22]. They confirmed unreliable species identification in the eastern Atlantic and Mediterranean Squalus and reinforced the need to revise the status of S. megalops and S. mitsukurii as they may include several distinct species distributed around the world. Nonetheless, the results provided by those authors suggest that a different species from the “true Australian” S. megalops, which remains unidentified, can occur in the eastern Atlantic and Mediterranean waters [22].
In any case, specimens of S. megalops for which the identification is considered feasible were rarely reported in the catches [38, 51]. Most of the catches of these species are recorded from the northern coasts of Canary Islands, Morocco, and southern Spain (Malaga). However, the real presence of S. megalops is still unclear not only for the Mediterranean Sea but also for the neighboring Atlantic area [18, 22] and some evidences confirm the inconsistency of the species identification keys to distinguish between the Atlantic and Mediterranean Squalus, concerning S. blainville and S. megalops [22].
Studies conducted in the Sardinian waters showed that morphological and genetic analysis revealed the presence only of S. blainville in the region, despite the observation of chondrocranial lateral processes initially allowing the investigated specimens to be subdivided into two groups. Indeed, the comparison of chondrocranial and body morphology of the specimens examined indicated that none of the considered measurements could differentiate the two squalid groups [38].
They noted that considering the brief half-life and fast replacement rate of the dermal denticles [120]. In fact, the different development stages of denticles observed in the analyzed skin portion could explain this particular aspect [120]. Moreover, dermal denticles, teeth, and dorsal fin spines were reported as common diagnostic morphological structures, which could vary in shape with the ontogenetic development [121, 122]. Consequently, the morphology of the dermal denticles should be further investigated before it can be properly used as a suitable classification tool, as also suggested particularly for the genus Squalus [84]. The same authors stated that considering the finding of sporadic divergent sequences [22, 38, 41, 119] different from S. blainville and S. acanthias but also S. megalops from Australia, the occurrence of a third species in the Mediterranean (apart from S. acanthias and S. blainville) cannot be ruled out.
Similar efforts were undertaken recently for those species [39]; it was noted that S. megalops does not occur in the eastern Atlantic and Mediterranean waters and that individuals composing clade C [22] should be considered a new species that needs formal description and proper taxonomic assessment [22, 123]. Besides, the species described as short-snout spurdog by other authors was considered a rare species and an occasional visitor with high morphological similarity to the S. megalops and S. blainville but is genetically distinct from both [39].
According to some authors [32], molecular data alone do not replace traditional taxonomy in the delimitation of species [124]. This integrative approach has been used over the years and has proven to be quite effective in elasmobranchs [124, 125] and in other groups of organisms [125, 126]. However, because of the difficulty of morphologically defining Squalus species, many sequences available in genetic databases indicate misidentifications or identifications only at the genus or family levels, making them not very useful for molecular identification purposes.
On their part, some authors stated that despite S. acanthias is the type species of the genus and is one of the most easily distinguished species of Squalus some sequences of Mediterranean specimens originally recognized as S. acanthias clustered in clade B. This misidentification is surprising but reveals the confused state of Squalus taxonomy in the region [22].
These findings further support current inconsistencies in species identification within the genus Squalus and the need for an accurate redescription of Squalus species, especially in the Mediterranean Sea, to stabilize the systematic and facilitate specimens’ identification.
The Mediterranean Sea represents some of the most intensively studied regions of the world’s oceans; however, this wealth of information does not translate into a good understanding of the species diversity and raises additional concerns regarding accurate identification of elasmobranchs. This concerns, among others, the genus Squalus, which the taxonomic confusion on it, is intrinsically related to difficulties in morphologically separating congeners and to incessant applications of synonyms due to the lack of appropriate taxonomic scrutiny with disregard for detailed morphological assessments that are essential for understanding possible variations and defining species.
In conclusion, since the first comprehensive revision on the genus in Africa [115] and after over 40 years gap, it is clear thus that an integrative approach includes both morphological and genetic tools with rigorous participation of taxon experts in the systematics of elasmobranch fishes still need to be strengthened to reduce this “taxonomic obstacle” and to faster actions in conservation and management of target species that were formerly unknown. Therefore, the establishment of a coordinated international effort to implement a comprehensive and integrated taxonomic assessment of this genus which represents an irreplaceable component of the biodiversity of the area is welcomed.
The authors gratefully acknowledge the Regional Activity Center for Specially Protected Areas (SPA/RAC) for its contribution in funding the cost of editing this chapter.
References
1.Tortonese T. Osservazioni comparative intorno alla ittiofauna del Mediterraneo e dell’Atlantico occidentale (Florida e isole Bahamas). Natura. 1989;1:1-20
2.Derrick DH, Cheok J, Dulvy NK. Spatially congruent sites of importance for global shark and ray biodiversity. PLoS One. 2020;15(7):e0235559. DOI: 10.1371/journal.pone.0235559
3.Leone A, Puncher GN, Ferretti F, Sperone E, Tripepi S, Micarelli P, et al. Pliocene colonization of the Mediterranean by great White shark inferred from fossil records, historical jaws, phylogeographic and divergence time analyses. Journal of Biogeography. 2020;47(5):1119-1129. DOI: 10.1111/jbi.13794
4.Bianchi CN, Morri C, Chiantore M, Montefalcone M, Parravicini V, Rovere A. Mediterranean Sea biodiversity between the legacy from the past and a future of change. In: Stambler N, editor. Life in the Mediterranean Sea: A Look at Habitat Changes. Vol. 1. New York: Nova Science Publishers, Inc; 2012. p. 55. ISBN: 978-1-61209-644-5
5.Katsanevakis S, Coll M, Piroddi C, Steenbeek J. Ben Rais Lasram F, Zenetos a, Cardoso AC: Invading the Mediterranean Sea: Biodiversity patterns shaped by human activities. Frontiers in Marine Science. 2014;1:32. DOI: 10.3389/FMARS.2014.00032
6.Serena F, Abella AJ, Bargnesi F, Barone M, Colloca F, Ferretti F, et al. Species diversity, taxonomy and distribution of Chondrichthyes in the Mediterranean and Black Sea. The European Zoological Journal. 2020;87:497-536. DOI: 10.1080/24750263.2020.1805518
7.Dulvy NK, Allen DJ, Ralph GM, Walls RHL. The Conservation Status of Sharks, Rays and Chimaeras in the Mediterranean Sea [Brochure]. Malaga, Spain: IUCN; 2016. p. 14
8.Dulvy NK, Fowler SL, Musick JA, Cavanagh RD, Kyne PM, Harrison LR, et al. White WT: Extinction risk and conservation of the world’s sharks and rays. eLife. 2014;3:e00590. DOI: 10.7554/eLife.00590
9.Otero M, Serena F, Gerovasileiou V, Barone M, Bo M, Arcos JM, et al. Identification Guide of Vulnerable Species Incidentally Caught in Mediterranean Fisheries. Malaga, Spain: IUCN; 2019. p. 204
10.Menesini E. Ittiodontoliti Miocenici di Terra d’Otranto (Puglia). Palaeontographia Italica. 1968;65:1-61
11.Ramírez-Amaro S, Ordines F, Puerto MÁ, García C, Ramon C, Terrasa B, et al. New morphological and molecular evidence confirm the presence of the Norwegian skate Dipturus nidarosiensis (storm, 1881) in the Mediterranean Sea and extend its distribution to the western basin. Mediterranean Marine Science. 2017;18(2):251-259. DOI: 10.12681/MMS.1950
12.Zeeberg J, Corten A, Graaf E. Bycatch and release of pelagic megafauna in industrial trawler fisheries off Northwest Africa. Fisheries Research. 2006;78(2):186-195. DOI: 10.1016/j.fishres.2006.01.012
13.Coelho R, Erzini K. Identification of deep water lantern sharks (Chondrichthyes: Etmopteridae) using morphometric data and multivariate analysis. Journal of the Marine Biological Association of the United Kingdom. 2008;88(1):199-204. DOI: 10.1017/S0025315408000271
14.Myakov NA, Kondyurin VV. Dogfishes, squalus (Squalidae) of the Atlantic Ocean and comparative notes on the species of this genus from other regions. Journal of Ichthyology. 1986;26(6):1-18
15.Ebert DA, Stehmann M. Sharks, Batoids, and Chimaeras of the North Atlantic. Vol. No. 7. Rome: FAO Species Catalogue of fishery Purpose; 2013. p. 523
16.Ebert DA, Fowler S, Compagno LJV. Sharks of the World: A Fully Illustrated Guide. London: Wild Nature Press; 2013. p. 528
17.Compagno LJV, Dando M, Fowler S. A Field Guide to the Sharks of the World. London: Harper-Collins; 2005. p. 496
18.Last PR, White WT, Pogonosky JJ. Descriptions of the New Dogfishes of the Genus Squalus (Squaloidea: Squalidae). Hobart: CSIRO Marine and Atmospheric Research; 2007. p. 130. DOI: 10.4225/08/58615c42b11ce
19.Viana ST, de Carvalho MR. Redescription of Squalus acutipinnis Regan, 1908, a valid species of spiny dogfish from southern Africa (Chondrichthyes: Squaliformes: Squalidae). Copeia. 2016;104:539-553. DOI: 10.1643/CI-14-217
20.Viana ST, de Carvalho MR, Gomes UL. Taxonomy and morphology of species of the genus squalus Linnaeus, 1758 from the southwestern Atlantic Ocean (Chondrichthyes: Squaliformes: Squalidae). Zootaxa. 2016;4133:1-89. DOI: 10.11646/ZOOTAXA.4133.1.1
21.Bellodi A, Porcu C, Cannas R, Cau A, Marongiu MF, Mulas A, et al. Life-history traits of the longnosed skate Dipturus oxyrinchus. Journal of Fish Biology. 2017;90(3):867-888. DOI: 10.1111/JFB.13205
22.Veríssimo A, Zaera-Perez D, Leslie R, Iglesias SP, Seter B, Grigoriou P, et al. Molecular diversity and distribution of eastern Atlantic and Mediterranean dogfishes squalus highlight taxonomic issue in the genus. Zoologica scripta. 2017;46(4):414-428. DOI: 10.1111/zsc.12224
23.Last PR, Edmunds M, Yearsley GK: Squalus crassispinus sp. nov., a new spurdog of the 'megalops-cubensis' group' from the eastern Indian Ocean. In Last PR, White WT, Pogonoski JJ editors. Descriptions of new dogfishes of the genus Squalus (Squaloidea:Squalidae). CSIRO Marine and Atmospheric Research Paper No 14, World Register of Marine Species. 2007; pp. 11-22
24.Ebert DA, White WT, Goldman KJ, Compagno LJ, Daly-Engel TS, Ward RD. Resurrection and redescription of Squalus suckleyi (Girard, 1854) from the North Pacific, with comments on the Squalus acanthias subgroup (Squaliformes: Squalidae). Zootaxa. 2010;2612(1):22-40. DOI: 10.11646/zootaxa.2612.1.2
25.White WT, Iglesias SP. Squalus formosus, a new species of spurdog shark (Squaliformes: Squalidae), from the western North Pacific Ocean. Journal of Fish Biology. 2011;79(4):954-968. DOI: 10.1111/j.1095-8649.2011.03068.x
26.Borsa P, Shen KN, Arlyza IS, Hoareau TB. Multiple cryptic species in the blue-spotted maskray (Myliobatoidei: Dasyatidae: Neotrygon spp.): An update. Comptes rendus biologies. 2016;339(9-10):417-426. DOI: 10.1016/j. crvi.2016.07.004
27.Borsa P, Arlyza IS, Hoareau TB, Shen KN. Diagnostic description and geographic distribution of four new cryptic species of the blue-spotted maskray species complex (Myliobatoidei: Dasyatidae; Neotrygon spp.) based on DNA sequences. Journal of Oceanology and Limnology. 2018;36(3):827-841. DOI: 10.1007/s00343-018-7056-2
28.Whitehead PJP, Bauchot ML, Hureau JC, Nielsen J, Tortonese E. Fishes of the North-Eastern Atlantic and the Mediterranean (FNAM). Paris: Unesco; 1984. p. 1473 ISBN 92-3-002308-6
29.Cannizzaro L, Rizzo P, Levi D, Gancitano S. Age determination and growth of Squalus blainvillei (Risso, 1826). Fisheries Research. 1995;23(1-2):113-125. DOI: 10.1016/0165-7836(94)00333-R
30.Serena F, Papacostantinou C, Relini G, Gil De Sola L, Bertrand J. Distribution and abundance of spiny dogfish in the Mediterranean Sea based on the Mediterranean international trawl surveys program. Biology and management of dogfish sharks. In: Gallucci VF, McFarlane GA, Bargmann GC, editors. American Fisheries Society. Maryland: Bethesda; 2009. pp. 139-149. DOI: 10.47886/9781934874073.ch13
31.Compagno LJV. Squaliformes-Dogfish sharks. In: Fischer W, Nauen C, editors. FAO Species Catalogue, Volume 4. Sharks of the World: An Annotated and Illustrated Catalogue of Shark Species Known to Date. Part I. Hexanchiformes to Lamniformes. Vol. 125. Rome: FAO Fisheries Synopsis; 1984. pp. 111-113
32.Garrick JAF. Studies on New Zealand Elasmobranchii. Part XII. The species of squalus from New Zealand and Australia; and a general account and key to the New Zealand Squaloidea. The Transactions and Proceedings of the Royal Society of New Zealand. 1960;88:519-557
33.Garrick JAF, Paul LJ. Heptranchias dakini Whitley, 1931, a synonym of H. perlo (Bonnaterre, 1788), the sharpnosed sevengill or perlon shark, with notes on sexual dimorphism in this species. Zoological Publications from Victoria University of Wellington. 1971;54:1-14
34.Bigelow HB. Schroeder WC 1957: A study of the sharks of the suborder Squaloidea. Bulletin of the Museum of Comparative Zoology. 1957;117:1-150
35.Daly-Engel TS, Koch A, Anderson JM, Cotton CF, Rubbs RD. Description of a new deep-water dogfish shark from Hawaii, with comments on the Squalus mitsukurii species complex in the West Pacific. Zookeys. 2018;798:135-157. DOI: 10.3897/zookeys.798.28375
36.Ziadi-Künzli F, Soliman T, Imai H, Sakurai M, Maeda K, Tachihara K. Re-evaluation of Deep-Sea dogfishes (genus squalus) in Japan using phylogenetic inference. Deep-Sea Research Part I: Oceanographic Research Papers. 2020;160:103261. DOI: 10.1016/j.dsr.2020.103261
37.Vella A, Vella N, Schembri S. A molecular approach towards taxonomic identification of elasmobranch species from Maltese fisheries landings. Marine Genomics. 2017;36:17-23. DOI: 10.1016/j.margen.2017.08.008
38.Bellodi A, Porcu C, Cau A, Marongiu MF, Melis R, Mulas A, et al. Investigation on the genus squalus in the Sardinian waters (Central-Western Mediterranean) with implications on its management. Mediterranean Marine Science. 2018;19:256-272. DOI: 10.12681/mms.15426
39.Ferrari A, Di Crescenzo S, Cariani A, Crobe V, Benvenuto A, Piattoni F, et al. Puzzling over Spurdogs: Molecular taxonomy assessment of the squalus species in the strait of Sicily. The European Zoological Journal. 2021;88:181-190. DOI: 10.1080/24750263.2020.1849436
40.Ward RD, Holmes BH, Zemlak TS, Smith PJ. DNA barcoding discriminates Spurdogs of the genus squalus. In: Last PR, White WT, Pogonoski JJ, editors. Descriptions of New Dogfishes of the Genus Squalus (Squaloidea: Squalidae). Hobart, Australia: CSIRO Marine and Atmospheric Research; 2007. pp. 117-130. Available from: http://hdl.handle.net/102.100.100/124477?index=1
41.Marouani S, Chaâba R, Kadri H, Saidi B, Bouain A, Maltagliati F, et al. Taxonomic research on Squalus megalops (Macleay, 1881) and Squalus blainvillei (Risso, 1827) (Chondrichthyes: Squalidae) in Tunisian waters (Central Mediterranean Sea). Scientia Marina. 2012;76(1):97-109. DOI: 10.3989/scimar. 03440.22B
42.Muñoz-Chápuli R, Ramos F. Morphological comparison of Squalus blainvillei and S. megalops in the eastern Atlantic, with notes on the genus. Japanese Journal of Ichthyology. 1989;36(1):6-21. DOI: 10.1007/BF02905668
43.Muñoz-Chápuli R, Ramos F, Garcia-Garrido CM. Squalus megalops (McLeay, 1882) en el Mediterraneo. Notas sobre su diagnosis, sistematica y distribucion. Butlleti de la Societat Catalana d'Ictiologia i Herpetologia. 1984;9:16-21
44.King JR, McFarlane GA. Marine fish life history strategies: Applications to fishery management. Fisheries Management and Ecology. 2003;10:249-264. DOI: 10.1046/j.1365-2400.2003.00359.x
45.Dell’Apa A, Bangley CW, Rulifson RA. Who let the dogfish out? A review of management and socio-economic aspects of spiny dogfish fisheries. Reviews in Fish Biology and Fisheries. 2015;25:273-295. DOI: 10.1007/s11160-014-9379-1
46.Quignard JP. Recherche sur la biologie de Squalus blainvillei, (Risso, 1826). Travaux du Laboratoire de la Biologie Halieutique Université de Rennes. 1971;5:125-141
47.Capapé C. Observations sur la sexualité, la reproduction et la fécondité de 8 sélaciens pleurotrêmes vivipares placentaires des côtes tunisiennes. Archives de l’institut Pasteur de Tunis. 1974;51:329-344
48.Capapé C, Quignard JP. Recherche sur la biologie de Squalus blainvillei, (Risso, 1826) (Pisces, Squalidae) des côtes tunisiennes: relations taille-poids du corps, du foie et des gonades. Coefficients de condition. Rapports hépato et gonosomatique. Croissance embryonnaire. Archives de l’institut Pasteur Tunis. 1980;57(4):385-408
49.Sion L, D'Onghia G, Tursi A, Mytilineou C. First data on distribution and biology of Squalus blainvillei (Risso, 1826) from the eastern Mediterranean Sea. The Journal of Northwest Atlantic Fishery Science. 2003;31:213-219. DOI: 10.2960/J.v31.a16
50.Marouani S, Kadri H, Bradaï MN, Saïdi B, Bouaïn A. Size at maturity of Squalus blainvillei and Squalus megalops in the Gulf of Gabès (Tunisia). Rapports des réunions Commission internationale pour l'exploration scientifique de la Mer Méditerranée. 2013;40:466. DOI: 10.1029/2005PA001169
51.Marouani S. Systématique et éco-biologie de deux requins Squalus blainvillei (Risso, 1827) et Squalus megalops (Macleay, 1881) (Chondrichtyens, Squalidae) du golfe de Gabès (Méditerranée centrale). Tunisia: Faculté des sciences de Sfax: Université de Sfax; 2013
52.Kousteni V, Megalofonou P. Reproductive biology and embryonic development of Squalus blainvillei in the eastern Mediterranean Sea. Scientia Marina. 2011;75(2):237-249. DOI: 10.3989/scimar.2011.75n2237
53.Anastasopoulou A, Mytilineou C, Makantasi P, Smith CJ, Kavadas S, Konstantia EL, et al. Life history aspects of two species of the squalus genus in the eastern Ionian Sea. Journal of the Marine Biological Association of the United Kingdom. 2018;98(4):937-948. DOI: 10.1017/S0025315416001818
54.Capapé C, Reynaud C. Maturity, reproductive cycle and fecundity of the spiny dogfish Squalus acanthias (Chondrichthyes: Squalidae) off the Languedocian coast (southern France, northern Mediterranean). Journal of the Marine Biological Association of the United Kingdom. 2011;91(8):1627-1635. DOI: 10.1017/S0025315411000270
55.Chatzispyrou A, Megalofonou P. Sexual maturity, fecundity and embryonic development of the spiny dogfish, Squalus acanthias, in the eastern mediterranean sea. Journal of the Marine Biological Association of the United Kingdom. 2005;85(5):1155-1161. DOI: 10.1017/S0025315405012233
56.Yigin CC, Ismen A. Reproductive biology of spiny dogfish Squalus acanthias, in the North Aegean Sea. Turkish Journal of Fisheries and Aquatic Science. 2013;13:169-177. DOI: 10.4194/1303-2712-v13_1_20
57.Bargione G, Donato F, La Mesa M, et al. Life-history traits of the spiny dogfish Squalus acanthias in the Adriatic Sea. Scientific Reports. 2019;9:14317. DOI: 10.1038/s41598-019-50883-w
58.Kousteni V, Karachle PK, Megalofonou P. Diet and trophic level of the longnose spurdog Squalus Blainville (Risso, 1826) in the deep waters of the Aegean Sea. Deep Sea Research Part I: Oceanographic Research Papers. 2017;124:93-102. DOI: 10.1016/j.dsr.2017.04.008
59.Bonnici L, Bonello JJ, Schembri PJ. Diet and trophic level of the longnose spurdog, Squalus Blainville (Risso, 1826) in the 25-nautical mile fisheries management zone around the Maltese islands. Regional Studies in Marine Science. 2018;19:33-42. DOI: 10.1016/j.rsma.2018.03.001
60.Bengil EGT. Biology and ecology of squalus Blainville (Risso, 1827) from the eastern Mediterranean. Thalassas. 2022;124:93-102. DOI: 10.1007/s41208-022-00482-w
61.Barria C, Coll M, Navarro J. Unravelling the ecological role and trophic relationships of uncommon and threatened elasmobranchs in the western Mediterranean Sea (supplement). Marine Ecology Progress Series. 2015;539:225-240
62.Güzin G, Demirel N. Evaluation of the comprehensive feeding strategy and trophic role of overexploited mesopredator species in the sea of Marmara (northeastern Mediterranean). Estuarine, Coastal and Shelf Science. 2021;259:107448. DOI: 10.1016/j.ecss.2021.107448
63.Marouani S, Kadri H, Karaa S. MN Bradai: Feeding ecology of the piked spurdog Squalus megalops (Chondrichthyes: Squalidae) in the Gulf of Gabès (Central Mediterranean Sea). Marine and Freshwater Research. 2017;69(1):48-55. DOI: 10.1071/MF17018
64.Merella P, Quetglas A, Alemany F, Carbonell A. Length-weight relationship of fishes and cephalopods from the Balearic Islands (Western Mediterranean). Naga, The WorldFish Center. 1997;20(3/4):66-68
65.Pallaoro A, Jardas I, Šantič M. Weight-length relationships for 11 chondrichthyan species in the eastern Adriatic Sea. Cybium. 2005;29(1):93-96
66.Marouani S, Kadri H, Saidi B, Morize E, Bouain A, Bradai MN. Age, growth, longevity, natural mortality and maturity of the longnose spurdog Squalus blainvillei (Chondrichthyes: Squalidae) in the Gulf of Gabès (Central Mediterranean Sea). Cahiers de biologie marine. 2012;53:197-204. DOI: 10.21411/CBM.A.E6C632F8
67.Ismen A, Ozen O, Altinagac U, Ozekinci U, Ayaz A. Weight–length relationships of 63 fish species in Saros Bay, Turkey. Journal of Applied Ichthyology. 2007;23:707-708. DOI: 10.1111/j.1439-0426.2007.00872.x
68.Ismen A, Yigin C, Altinagac U, Ayaz A. Length–weight relationships for ten shark species from Saros Bay C (North Aegean Sea). Journal of Applied Ichthyology. 2009;25:109-112. DOI: 10.1111/j.1439-0426.2009.01263.x
69.Bok TD, Gokturk D, Kahraman AE, Alicli TZ, Acun T, Ates C. Length-weight relationships of 34 fish species from the sea of Marmara, Turkey. Journal of Animal and Veterinary Advances. 2011;10:3037-3042. DOI: 10.3923/javaa.2011.3037.3042
70.Güven O, Kebapçıoğlu T, Deval MC. Length–weight relationships of sharks in Antalya Bay, eastern Mediterranean. Journal of Applied Ichthyology. 2012;28:278-279. DOI: 10.1111/j.1439-0426.2011.01823.x
71.Eronat EGT, Özaydın O. Length-weight relationship of cartilaginous fish species from Central Aegean Sea (Izmir Bay and Sığacık Bay). Ege Journal of Fisheries and Aquatic Sciences. 2014;31(3):119-125. DOI: 10.12714/egejfas.2014.31.3.01
72.Kousteni V, Megalofonou P. Aging and life history traits of the longnose spiny dogfish in the Mediterranean Sea: New insights into conservation and management needs. Fisheries Research. 2015;168:6-19. DOI: 10.1016/j.fishres.2015.03.014
73.Lteif M, Mouawad R, Jemaa S, Khalaf G, Lenfant P, Verdoit-Jarraya M. The length-weight relationships of three sharks and five batoids in the Lebanese marine waters, eastern Mediterranean. Egyptian Journal of Aquatic Research. 2016;42:475-477
74.Öztekin A, Özekinci U, Daban İB. Lengthweight relationships of 26 fish species caught by longline from the Gallipoli peninsula, Turkey (northern Aegean Sea). Cahiers de Biologie Marine. 2016;57:335-342
75.Grace E, Bengil T. Biology and ecology of squalus Blainville (Risso, 1827) from the eastern Mediterranean. Thalassas: An International Journal of Marine Sciences. 2022. DOI: 10.1007/s41208-022-00482-w [In press]
76.Falsone F, Geraci ML, Scannella D, Gancitano V, Di Maio F, Sardo G, et al. Length-weight relationships of 52 species from the south of Sicily (Central Mediterranean Sea). Fishes. 2022;7(2):92. DOI: 10.3390/fishes7020092
77.Marouani S, Kadri H, Bradaï MN. First data on the age, growth and sexual maturity of the piked spurdog, Squalus megalops (Chondrichthyes: Squalidae), in the Gulf of Gabès (Central Mediterranean Sea). Marine and Freshwater Research. 2015;67(5):578-585. DOI: 10.1071/MF14356
78.Filiz H, Mater S. Preliminary study on length-weight relationships for seven elasmobranch species from North Aegean Sea. Turkey Journal of Fisheries & Aquatic Sciences. 2002;19(3-4):401-409
79.Filiz H. G bilge : Length–weight relationships of 24 fish species from the North Aegean Sea. Turkey. Journal of Applied Ichthyology. 2004;20(5):431-432. DOI: 10.1111/j.1439-0426.2004.00582.x
80.Gracan R, Zavodnik D, Krstinić P, Lazar B. Feeding ecology and trophic segregation of two sympatric mesopredatory sharks in the heavily exploited coastal ecosystem of the Adriatic Sea. Journal of Fish Biology. 2016;90(1):167-184. DOI: 10.1111/jfb.13158
81.Jones TS, Ugland KI. Reproduction of female spiny dogfish, Squalus acanthias, in the Oslofjord. Fishery Bulletin. 2001;99:685-690
82.Sims CA. Implications of rational inattention. Journal of Monetary Economics. 2003;50(3):665-690. DOI: 10.1016/S0304-3932(03)00029-1
83.Bat L, Erdem Y, Ustaoğlu S, Yardım Ö, Satılmıs HH. A study on the fishes of the Central Black Sea coast of Turkey. Journal of the Black Sea/Mediterranean Environment. 2005;11:281-296
84.Bonello J, Bonnici L, Ferrari A, Cariani A, Schembri P. Not all that clear cut: Intraspecific morphological variability in Squalus Blainville (Risso, 1827) and implications for identification of the species. Journal of the Marine Biological Association of the United Kingdom. 2016;96(8):1585-1596. DOI: 10.1017/S0025315415001915
85.Giarruso ME. Indagini morfologiche e genetiche sul genere Squalus nello stretto di Sicilia. Corso di laurea magistrale in biologia marina [thesis]. Italy: Tesi di laurea in struttura e connettività delle popolazioni marine; 2019. p. 31
86.Baino R, Serena F, Ragonese S, Rey J, Rinelli P. Catch composition and abundance of elasmobranchs based on the MEDITS program. Rapport de la Commision Internationale pour l’Exploration Scientifique de la Méditerranée. 2001;36:234
87.Damalas D, Vassilopoulou V. Chondrichthyan by-catch and discards in the demersal trawl fishery of the Central Aegean Sea (eastern Mediterranean). Fisheries Research. 2011;108(1):142-152. DOI: 10.1016/j.fishres.2010.12.012
88.Tsagarakis K, Mytilineou C, Haralabous J, Lorance P, Politou CY, Dokos J. Mesoscale spatio-temporal dynamics of demersal assemblages of the eastern Ionian Sea in relationship with natural and fisheries factors. Aquatic Living Resources. 2013;26:381-397. DOI: 10.1051/alr/2013067
89.Marongiu MF, Porcu C, Bellodi A, Cannas R, Cau A, Cuccu D, et al. Temporal dynamics of demersal chondrichthyan species in the central western Mediterranean Sea: The case study in Sardinia Island. Fisheries Research. 2017;193:81-94. DOI: 10.1016/j.fishres.2017.04.001
90.Follesa MC, Marongiu MF, Zupa W, Bellodi A, Cau A, Cannas R, et al. Spatial variability of Chondrichthyes in the northern Mediterranean. Scientia Marina. 2019;83(S1):81-100. DOI: ORG/10.3989/SCIMAR.04998.23A
91.Last PR. Stevens JD: Sharks and Rays of Australia. Melbourne, Australia: CSIRO; 1994. p. 513. DOI: 10.1017/S0025315400047858
92.Bradaï NM, Saidi B, Enajjar S. Overview on Mediterranean Shark’s fisheries: Impact on the biodiversity. In: Türkoglu M, Önal U, Ismen A, editors. Marine Ecology - Biotic and Abiotic Interactions. London: Intechopen; 2018. pp. 211-230. DOI: 10.5772/INTECHOPEN.74923
93.Musick JA, Burgess GH, Cailliet GM, Camhi M, Fordham SV. Management of sharks and their relatives(Elasmobranchii). Fisheries. 2000;25(3):913. DOI: 10.1577/15488446(2000)025<0009:MOSATR>2.0.CO;2
94.FAO: The State of World Fisheries and Aquaculture. Sustainability in Action. Vol. 2020. Rome: FAO; 2020. p. 244. DOI: 10.4060/ca9229en
95.Walker JMT, Joan MT, Wilkins AS, Dallaire JR, San dler HM, Hoover-Dempsey KV. Parental involvement: Model revision through scale development. The Elementary School Journal. 2005;106(2):85-104. DOI: 10.1086/499193
96.Cashion MS, Bailly N, Pauly D. Official catch data underrepresent shark and ray taxa caught in Mediterranean and Black Sea fisheries. Marine Policy. 2019;105(7437):1-9. DOI: 10.1016/j.marpol.2019.02.041
97.Damalas D, Megalofonou P. Occurrences of large sharks in the open waters of the southeastern Mediterranean Sea. Journal of Natural History. 2012;46:43-44. DOI: 10.1080/00222933.2012.716864
98.Echwikhi K, Saidi B, Bradai MN, Bouain A. Preliminary data on elasmobranch gillnet fishery in the Gulf of Gabès Tunisia. Journal of Applied Ichthyology. 2013;29:1080-1085. DOI: 10.1111/JAI.12022
99.GFCM (Mediterranean and Black Sea) Capture Production Quantity (1970 - 2020) https://www.fao.org/fishery/statisticsquery/en/gfcm_capture/gfcm_capture_quantity
100.Mandelman JW, Farrington MA. The physiological status and mortality associated with otter-trawl capture, transport, and captivity of an exploited elasmobranch, Squalus acanthias. ICES Journal of Marine Science. 2007;64(1):122-130. DOI: 10.1093/icesjms/fsl003
101.Stoner A, Kaimmer SM. Reducing elasmobranch bycatch: Laboratory investigation of rare earth metal and magnetic deterrents with spiny dogfish and Pacific halibut. Fisheries Research. 2008;92(2):162-168. DOI: 10.1016/j.fishres.2008.01.004
102.Tallack SML, Mandelman JW. Do rare-earth metals deter spiny dogfish? A feasibility study on the use of electropositive “mischmetal” to reduce the bycatch of Squalus acanthias by hook gear in the Gulf of Maine. ICES Journal of Marine Science. 2009;66(2):315-322. DOI: 10.1093/icesjms/fsn215
103.Séret B, Serena F. Distribution and abundance of Squalus acanthias and Squalus blainvillei in the Mediterranean Sea based on the Mediterranean international trawl survey program (MEDITS). In: The Mediterranean Chondrichthyan Fishes (Sharks, Rays, Skates and Chimaeras): Status and Priorities for Conservation. Tunis, Tunisia: United National Environment Programme (UNEP) Regional Activity Centres/Special Protected Areas (RAC/SP) Final Report; 2002
104.Marongiu MF, Porcu C, Bellodi A, Cannas R, Carbonara P, Cau A, et al. Abundance, distribution and reproduction of the data-deficient species (Squalus Blainville) around Sardinia Island (central western Mediterranean Sea) as a contribution to its conservation. Marine Freshwater Resources. 2020;72(1):118-130. DOI: 10.1071/MF19372
105.Nuez I, Gazo M. Cardona : A closer look at the bycatch of medium-sized and large sharks in the northern Catalan coast (north-western Mediterranean Sea): Evidence of an ongoing decline? Aquatic Conservation: Marine and Freshwater Ecosystems. 2021;31(9):2369-2380. DOI: 10.1002/aqc.3651
106.Mandelman JW, Cooper PW, Werner TB, Lagueux KM. Shark bycatch and depredation in the U.S. Atlantic pelagic longline fishery. Reviews in Fish Biology and Fisheries. 2008;18:427-442. DOI: 10.1007/s11160-008-9084-z
107.Last PR, White WT, Pogonoski JJ, Gledhill DC, Yearsley GK, Ward RD: Application of a rapid taxonomic approach to the genus squalus. In: Last PR, White WT, Pogonoski JJ editors. Descriptions of New Dogfishes of the Genus Squalus (Squaloidea: Squalidae). Hobart, Tas.: CSIRO Marine and Atmospheric Research; 2007. p. 1-10. DOI: http://hdl.handle.net/102.100.100/124361?index=1
108.Díaz-Jaimes P, Bayona-Vásquez NJ, Escatel-Luna E, Uribe-Alcocer M, Pecoraro C, Adams DH, et al. Population genetic divergence of Bonnethead sharks Sphyrna tiburo in the Western North Atlantic: Implications for conservation. Aquatic Conservation, Marine and Freshwater Ecosystems. 2021;31(1):83-98. DOI: 10.1002/aqc.3434
109.Ceballos G, Ehrlich PR, Barnosky AD, García A, Pringle RM, Palmer TM. Accelerated modern human–induced species losses: Entering the sixth mass extinction. Science Adverse. 2015;1(5):e1400253. DOI: 10.1126/sciadv.1400253
110.Geraci ML, Ragonese S, Norrito G, Scannella D, Falsone F, Vitale S: A tale on the demersal and bottom dwelling Chondrichthyes in the south of Sicily through 20 years of scientific survey. In: Filho LFDSR, de Luna Sales JR, editors. Chondrichthyes - Multidisciplinary Approach [Internet]. London: IntechOpen. 2017. p. 13-37. DOI: 10.5772/intechopen.69333
111.Pfleger MO, Grubbs RD, Cotton CF, Daly-Engel TS. Squalus clarkae Sp. Nov., a new dogfish shark from the Northwest Atlantic and Gulf of Mexico, with comments on the Squalus mitsukurii species complex. Zootaxa. 2018;4444:101-119. DOI: 10.11646/zootaxa.4444.2.1
112.Ariza AA, Adachi AMCL, Roque P, Hazin FHV, Vianna M, Rotundo MM, et al. DNA barcoding and species delimitation for dogfish sharks belonging to the squalus genus (Squaliformes: Squalidae). Diversity. 2022;14:544. DOI: 10.3390/d14070544
113.Ledoux JC. Affinités et origines du Squalus blainvillei de Méditerranée. Journees Ichthologique, C.I.E.S.M. 1970;3:65-69
114.Merrett NF. A new shark of the genus squalus (Squalidae, Squaloidea) from the equatorial western Indian Ocean, with notes on the Squalus blainvillei. Journal of Zoology. 1973;171:93-110. DOI: 10.1111/j.1469-7998.1973.tb07518.x
115.Bass AJ, D’Aubrey JD, Kistnasamy N. Sharks of the east coast of southern Africa. VI the families Oxynotidae, Squalidae, Dalatiidae, and Echinorhinidae, Investigation. Investigational Report Institute of Oceanographic Sciences. 1976;45:1-103
116.Chen C, Taniuchi T. Nose: Blainville’s dogfish, Squalus Blainville, from Japan, with notes on S. mitsukurii and S. japonicus. Japanese Journal of Ichthyology. 1979;26:26-42
117.Bigelow HB, Schroeder WC: Fishes of the western North Atlantic, part I: Lancelets, cyclostomes, sharks. In Tee–Van J, Breder CM, Hildebrand SF, Parr AE, and Schroeder WC, Editors. Fishes of the Western North Atlantic. Part 1. Sears Foundation for Marine Research, Yale University, New Haven; 1948. P 59-576) DOI: 10.1126/science.111.2874.95
118.Springer VG, Garrick JAF. A survey of vertebral numbers in sharks. Proceedings of the United States National Museum. 1964;116:73-96. DOI: 10.5479/si.00963801.116-3496.73
119.Kousteni V, Kasapidis P, Kotoulas G, Megalofonou P. Evidence of high genetic connectivity for the longnose spurdog Squalus Blainville in the Mediterranean Sea. Mediterranean Marine Science. 2016;17(2):371-383. DOI: 10.12 681/mms.1222
120.Kemp NE. Integumentary system and teeth. In: Hamlett WC, editor. Sharks, Skates and Rays: The Biology of Elasmobranch Fishes. Chapter 2. The John Hopkins University Press; 1999. pp. 43-68
121.White WT, Ebert DA, Naylor GJP, Ho HC, Clerkin P, Veríssimo A, et al. Revision of the genus Centrophorus (Squaliformes: Centrophoridae): Part 1— Redescription of Centrophorus granulosus (Bloch & Schneider), a senior synonym of C. acus Garman and C. niaukang Teng. Zootaxa. 2013;3752(1):35. DOI: 10.11646/ZOOTAXA.3752.1.5
122.Veríssimo A, Cotton CF, Buch RH, Guallart J, Burgess GH. Species diversity of the deep-water gulper sharks (Squaliformes: Centrophoridae: Centrophorus) in North Atlantic waters - current status and taxonomic issues. Zoological Journal of the Linnean Society. 2014;172(4):803-830. DOI: 10.1111/zoj.12194
123.Last PR, Stevens JD. Sharks and Rays of Australia. 2nd ed. Collingwood, Australia: CSIRO; 2009. p. 644
124.Schlick-Steiner BC, Steiner FM, Seifert B, Stauffer C, Christian E, Crozier RH. Integrative taxonomy: A multisource approach to exploring biodiversity. Annual Review of Entomology. 2010;55:421-438. DOI: 10.1146/annurev-ento-112408-085432
125.Gabbanelli V. Díaz de Astarloa JM, Gonzalez-Castro M, Vazquez DM, Mabragaña E: Almost a century of oblivion: Integrative taxonomy allows the resurrection of the longnose skate Zearaja brevicaudata (Marini, 1933) (Rajiformes; Rajidae). Comptes Rendus Biologies. 2018;341:454-470. DOI: 10.1016/j.crvi.2018.10.002
126.García-Melo JE, Oliveira C, Silva GJDC, Ochoa-Orrego LE, Pereira LHG, Maldonado-Ocampo JA. Species delimitation of Neotropical characins (Stevardiinae): Implications for taxonomy of complex groups. PLoS One. 2019;14:e0216786. DOI: 10.1371/journal.pone.0216786
Written By
Sondes Marouani, Sami Karaa and Othman Jarboui
Submitted: 30 August 2022Reviewed: 10 November 2022Published: 04 January 2023
Open access peer-reviewed chapter
