Eco-Morphology of Some Decapod Crustaceans in a Tropical Coastal Marine Waters

Abstract

The relationship among the morphology, population of crustaceans and water quality of the coastal marine waters of Ondo State, Nigeria, was assessed in order to accentuate the sustainability of biodiversity in the coasts. Standard methods were employed to identify and examine the effect of the environment on the crustaceans. The DO (7.58 mg/l), temperature (29.53°C), pH (6.69), turbidity (44.03NTU), salinity (16.48‰), hardness (85.88 mg/l), biochemical oxygen demand (21.22 mg/l) and conductivity (41.55 μS cm−1). The population structure of decapod crustaceans follows the order Nematopalaemon hastatus > Farfantepenaeus notialis > Holthuispenaeopsis atlantica > Macrobrachium macrobrachion > Sanquerus validus > Ocypode africana > Callinectes marginatus. The sampled organisms (F. notialis, M. macrobrachion, N. hastatus and Holthuispenaeopsis atlantica) had mean total length (cm) (9.41 ± 1.62, 7.14 ± 0.77, 6.69 ± 0.81 and 11.78 ± 0.60) and body weight (g) of (3.21 ± 1.63, 2.37 ± 0.79, 1.34 ± 0.56 and 6.72 ± 0.47 g), respectively. C. marginatus, Ocypode africana and Sanquerus validus had a mean carapace length (cm) of 5.50 ± 0.71, 4.83 ± 1.27 and 8.31 ± 3.50, respectively, and mean body weight (g) of 4.69 ± 0.95, 3.41 ± 4.72 and 66.21 ± 50.45, respectively. PCA revealed strong correlation among BOD, DO and the morphological parameters of each species. Also, Single Factor and Comprehensive Pollution Indices revealed a slightly and moderately polluted aquatic ecosystem, respectively. Thus, adequate control of all activities in the ecosystem for healthy growth and survival of aquatic species is essential.

1. Introduction

Crustaceans are among the most diverse, numerous, and widely distributed decapods. They are found all across the tropics and are key commercial fisheries for a country’s economy. The majority of the species are only found in estuaries, and many of them require brackish or coastal water for larval development [1].

The relationships between the forms and functions of organisms, as well as the ecological qualities connected with the utilization of acquired resources, are studied in eco-morphological studies [2, 3]. The research answers fundamental concerns about organism niches, shared resources, and community organization, as well as basic techniques to developing a “fit” between organisms and their environments. Climate change, habitat destruction, and other forms of pollution are all contributing to the loss of marine biodiversity around the world. Because to changing environmental variables, fish have faced a greater vulnerability threat. Ecomorphological research has been increasingly important in recent years, particularly as the environment’s impact on organism growth and survival has increased. Among vertebrates, aquatic species have the most morphological variety and exhibit a wide and strong link between form and function [4]. Thus, they can serve as a vital tool in understanding and establishing relationships between ecology and morphology.

Coastal waters are essential to life and concentrate more than half of the world’s population of aquatic life. Although the areas have long been considered an unlimited resource, the health and biodiversity of the resource of the zones are being threatened with some fish stocks collapsing due to uneven environmental factors, changing chemistry, over-exploitation, pollution etc [5]. These have a profound impact on reproduction and survival of various species, ecological adaptation and community and productivity of the ecosystem [6].

The coastal marine waters of Nigeria have been adduced to be areas of high biodiversity of aquatic species with the composition, distribution, abundance and growth widely linked to numerous ecological factors. Several authors [7, 8, 9, 10, 11, 12] have worked on decapods in the coastal marine waters of Ondo State, Nigeria and reported the overall wellbeing of the species as well as the level of pollution in the environment. The authors strongly suggested the need to examine the eco-morphology of decapods in the coastal environment in order to determine the effecsof the environment on the growth and well-being of the crustaceans. Recommendations from the various authors and dearth of information on ecomorphology of widely distributed decapods in coastal marine waters of Ondo State, Nigeria necessitated this study.

Since the organization of any community (atmospheric, aquatic, terrestrial) can best be understood through the relationship of organisms and how they acquire, use and share resources with each other, it is therefore essential to assess and study the eco-morphology of decapods in the coastal marine waters of Ondo State, Nigeria which is known for a diverse assemblage of fish species to understand the conditions of the ecosystem.

2. Materials and methods

2.1 Study area

The study was carried out in Ayetoro (Station I), Idiogba (Station II), Asumogha (Station III) and Bijimi (Station IV) of Ilaje Local Government Area of Ondo State from September to December 2011. The study area falls within Latitude 5° 50′N–6° 09′N and Longitude 4° 45′E – 5° 05′E of the Greenwich Meridian. The stations were selected based on extensive shrimp fishing in the communities.

2.2 Sample collection and identification

Shrimps were collected monthly with the assistance of artisanal fishermen and were immediately preserved in ice chest before being transferred to the Fisheries and Aquaculture Laboratory of the Federal University of Technology, Akure where identification was done according to FAO Species Identification Sheets, (Volume VI) [13].

2.3 Morphometric measurement

Measurements were taken for total length (tip of rostrum to edge of telson), carapace length (posterior margin of carapace to edge of telson), rostral length (tip of rostrum to the posterior end of the orbit), and body length (posterior margin of the orbit to the edge of telson) to the nearest 0.01 cm using graduated measuring board while weight was determined with top loading digital Sartorius weighing balance (Model 1100) to the nearest 0.01 g.

2.4 Water sampling and analysis

Water samples from each station were collected monthly at sub-surface level with 250 ml sampling bottles and transported in ice chest to the Fisheries and Aquaculture Laboratory of the Federal University of Technology, Akure for analysis. Samples handling and preservation were done following the standard method [14]. The temperature, turbidity and conductivity of the water were done in-situ with a standard mercury-in-glass thermometer, turbidity meter and Knick Portamess conductivity Meter (Model 913) respectively while pH, Salinity and Dissolved Oxygen (DO) were determined using the Hanna multi-parameter kit (Model H19828).

2.5 Data analysis

Data obtained from physical and chemical measurements were subjected to multivariate analysis of variance (MANOVA) using the Statistical Package for Social Sciences (SPSS), Version 20.0 and was tested at P < 0.05 for significance. The mean values were compared with the water quality criteria of World Health Organization (WHO) and Nigerian Federal Environmental Protection Agency (FEPA). Morphology and water parameter data were related with Principal Components Analysis (PCA) while the water pollution status was determined using both single-factor pollution index (SFPI) andcomprehensive pollution index (CPI).

The single-factor pollution indexis defined as [15]:

$$P_i=\frac{C_i}{S_i}$$E1

Where: P_iis the pollution index of pollutant i,

C_iis the measured concentration of the pollution indicator (mg/l).

S_iis the National water quality standard permissible limit for the pollution indicator in surface water.

The water quality factor P_iis classified into five grades, as listed in Table 1 [16].

Pi	Pollution grades
Less than 0.4	Non-pollution
0.4–1.0	Slight pollution
1.0–2.0	Medium polluted
2.0–5.0	Heavy polluted
More than 5.0	Serious polluted

Table 1.

Standard grades for single-factor pollution index (PI).

The comprehensive pollution index (CPI)is defined as follows [17]:

$$\mathrm{CPI}=\frac{1}{n}\sum _{i=1}^n\frac{C_i}{S_i}$$E2

Where: CPI = the comprehensive pollution index,

C_i = the measured concentration of the pollution indicator (mg/l),

S_i = National water quality standard permissible limit for the pollution indicator in surface water.

n = the number of chosen parameters.

CPI is classified into the water quality levels listed in Table 2 according to [17].

The comprehensive pollution index (CPI)	Level	Explain the water quality grades
Less than 0.2	I	Cleanness
0.21–0.4	II	Sub-cleanness
0.41–1.0	III	Slight pollution
1.01–2.0	IV	Moderate pollution
More than 2.01	V	Severe pollution

Table 2.

Standard surface water quality categories based on CPI.

3. Results

3.1 Population structure of sampled organisms

The population structure of the sampled organisms is presented in Table 3. The table revealed a total of 225 Farfantepenaeus notialis, 27 Macrobrachium macrobrachion, 1999 Nematopalaemon hastatus, 134 Holthuispenaeopsis atlantica, 2 Callinectes marginatus, 3 Ocypode africanaand 18 Sanquerus validus. The most predominant species was N. hastatus. M. macrobrachionwas not recorded in Station III, while C. marginatusand Ocypode africanawere only recorded in Station III.

Species	Station I	Station II	Station III	Station IV	Total
Farfantepenaeus notialis	51	73	53	48	225
Macrobrachium macrobrachion	13	4	0	10	27
Nematopalaemon hastatus	526	601	416	456	1999
Holthuispenaeopsis atlantica	24	29	20	61	134
Callinectes marginatus	0	0	2	0	2
Ocypode africana	0	0	3	0	3
Sanquerus validus	5	2	6	5	18

Table 3.

Population structure of sampled organisms.

3.2 Morphological parameters of sampled organisms

The morphological features of the sampled organisms are presented in Tables 4 and 5. The morphometric measurements of sampled shrimps varied across the sampled species. F. notialis, M. macrobrachion, N. hastatusand Holthuispenaeopsis atlanticahad mean total length (cm) of 9.41 ± 1.62, 7.14 ± 0.77, 6.69 ± 0.81 and 11.78 ± 0.60 respectively and mean rostral length (cm) of 2.72 ± 0.47, 2.48 ± 0.25, 2.31 ± 0.35 and 2.78 ± 0.17 respectively. The mean carapace length (cm) of F. notialis, M. macrobrachion, N. hastatusand H. atlanticaobtained in the aquatic ecosystem was 2.70 ± 0.44, 2.36 ± 0.25, 2.20 ± 0.40 and 2.80 ± 0.22 cm respectively while the corresponding mean body length (cm) was 5.63 ± 0.92, 4.15 ± 0.53, 4.49 ± 0.54 and 6.30 ± 0.51respectively. The body weight (g) of F. notialis, M. macrobrachion, N. hastatusand H. atlanticaobtained in the aquatic ecosystem was 3.21 ± 1.63, 2.37 ± 0.79, 1.34 ± 0.56 and 6.72 ± 0.47 respectively (Table 4). The table further shows that N. hastatushad varying ranges of morphological features across the stations.

Parameter	Species	Station I	Station II	Station III	Station IV	Mean
Total length (cm)	Farfantepenaeusnotialis	9.12 ± 1.74	9.34 ± 1.68	9.67 ± 1.44	9.56 ± 1.57	9.41 ± 1.62
	Macrobrachium macrobrachion	7.40 ± 0.80	6.70 ± 0.39	—	6.98 ± 0.77	7.14 ± 0.77
	Nematopalaemon hastatus	6.70 ± 0.45	7.00 ± 0.13	6.09 ± 1.15	6.85 ± 0.97	6.69 ± 0.81
	Holthuispenaeopsisatlantica	11.51 ± 1.87	11.94 ± 1.77	12.80 ± 1.70	11.04 ± 2.53	11.78 ± 0.60
Rostral length (cm)	Farfantepenaeus notialis	2.63 ± 0.37	2.75 ± 0.49	2.78 ± 0.50	2.69 ± 0.48	2.72 ± 0.47
	Macrobrachium macrobrachion	2.46 ± 0.17	2.50 ± 0.08	—	2.49 ± 0.36	2.48 ± 0.25
	Nematopalaemon hastatus	2.50 ± 0.20	2.53 ± 0.09	1.98 ± 0.36	2.19 ± 0.38	2.31 ± 0.35
	Holthuispenaeopsisatlantica	3.18 ± 0.40	3.38 ± 0.48	3.35 ± 0.50	3.10 ± 0.67	2.78 ± 0.17
Carapace length (cm)	Farfantepenaeus notialis	2.67 ± 0.39	2.66 ± 0.46	2.83 ± 0.47	2.68 ± 0.41	2.70 ± 0.44
	M. macrobrachion	2.38 ± 0.22	2.50 ± 0.00	—	2.29 ± 0.32	2.36 ± 0.25
	N. hastatus	2.38 ± 0.26	2.51 ± 0.03	1.81 ± 0.42	2.05 ± 0.40	2.20 ± 0.40
	Holthuispenaeopsis atlantica	3.18 ± 0.42	3.22 ± 0.59	3.35 ± 0.57	3.04 ± 0.65	2.80 ± 0.22
Body Length (cm)	F. notialis	5.32 ± 1.06	5.51 ± 0.86	5.88 ± 0.89	5.85 ± 0.77	5.63 ± 0.92
	M. macrobrachion	4.20 ± 0.48	4.60 ± 0.39	—	3.90 ± 0.54	4.15 ± 0.53
	N. hastatus	4.60 ± 0.40	4.86 ± 0.25	3.98 ± 0.62	4.45 ± 0.51	4.49 ± 0.54
	Holthuispenaeopsis atlantica	6.78 ± 0.89	7.03 ± 1.15	7.26 ± 0.92	6.68 ± 1.21	6.30 ± 0.51
Weight (g)	F. notialis	3.39 ± 2.14	2.88 ± 1.33	3.41 ± 1.58	3.29 ± 1.46	3.21 ± 1.63
	M. macrobrachion	2.67 ± 0.77	2.47 ± 0.29	—	1.94 ± 0.80	2.37 ± 0.79
	N. hastatus	1.11 ± 0.37	1.34 ± 0.53	1.51 ± 0.71	1.43 ± 0.60	1.34 ± 0.56
	Holthuispenaeopsis atlantica	6.24 ± 4.14	7.22 ± 3.33	8.34 ± 4.45	6.24 ± 3.68	6.72 ± 0.47

Table 4.

Morphological parameters of sampled shrimps.

Parameter	Species	Station I	Station II	Station III	Station IV	Mean
Carapace Length (cm)	Callinectes marginatus	—	—	5.50 ± 0.71	—	5.50 ± 0.71
	Ocypode africana	—	—	4.83 ± 1.27	—	4.83 ± 1.27
	Sanquerus validus	7.96 ± 3.46	8.05 ± 2.33	8.97 ± 4.23	7.96 ± 3.92	8.31 ± 3.50
Weight (g)	Callinectes marginatus	—	—	4.69 ± 0.95	—	4.69 ± 0.95
	Ocypode africana	—	—	3.41 ± 4.72	—	3.41 ± 4.72
	Sanquerus validus	53.17 ± 45.66	31.65 ± 3.07	88.48 ± 58.89	66.37 ± 53.28	66.21 ± 50.45

Table 5.

Morphological parameters of sampled crabs.

The morphometric measurements of sampled crabs which also varied across the sampled species are presented in Table 5. C. marginatus, O. africanaand S. validushad a mean carapace length (cm) of 5.50 ± 0.71, 4.83 ± 1.27 and 8.31 ± 3.50 respectively, and mean body weight (g) of 4.69 ± 0.95, 3.41 ± 4.72 and 66.21 ± 50.45 respectively (Table 5).

3.3 Water quality parameter of the coastal marine waters of Ondo State Nigeria

The water quality parameters obtained in the coastal marine waters of Ondo state is presented in Table 6.

Station	Station I	Station II	Station III	Station IV	Mean	FEPA [18], FEM [19]
DO (mg/l)	7.66 ± 0.22^a	7.54 ± 0.14^a	7.60 ± 0.20^a	7.53 ± 0.15^a	7.58 ± 0.18	5.0–9.0
Temperature (°C)	29.50 ± 0.90^a	29.50 ± 0.52^a	29.71 ± 0.45^a	29.42 ± 0.51^a	29.53 ± 0.61	28.0–30.0
pH	6.68 ± 0.11^a	6.71 ± 0.13^a	6.66 ± 0.05^a	6.69 ± 0.14^a	6.69 ± 0.11	6.5–8.5
Turbidity (NTU)	44.87 ± 4.97^b	43.94 ± 2.86^ab	45.36 ± 2.18^b	41.95 ± 4.42^a	44.03.89	≤50
Salinity (ppt)	16.55 ± 5.16^a	16.65 ± 5.36^a	16.35 ± 4.95^a	16.36 ± 5.11^a	16.48 ± 4.98	1.0–15
Hardness (mg/l)	87.16 ± 3.47^a	84.75 ± 1.84^a	84.57 ± 1.80^a	87.03 ± 5.13^a	85.88 ± 3.47	50–100
BOD (mg/l)	22.10 ± 4.96^a	22.50 ± 3.90^a	20.74 ± 6.45^a	19.55 ± 2.62^a	21.22 ± 4.70	3.0–6.0
Conductivity (μS cm^−1)	41.84 ± 18.95^b	41.10 ± 18.81^a	41.60 ± 12.97^b	41.67 ± 17.75^b	41.55 ± 16.97	<35,000

Table 6.

Water quality parameter of the coastal marine waters of Ondo state Nigeria.

The Table shows that the parameters excluding turbidity and conductivity had no significant differences across the stations. The mean DO concentration was 7.58 mg/l, while water temperature and pH had a mean concentration of 29.53°C and 6.69 respectively. The turbidity and salinity of the aquatic ecosystem had a mean concentration of 44.03NTU and 16.48‰ respectively. Hardness, biochemical oxygen demand (BOD) and conductivity of the water body recorded mean values of 85.88 mg/l, 21.22 mg/l and 41.55 μS cm⁻¹ respectively.

3.4 Water pollution assessment

The Single-Factor Pollution Index (PI) and Comprehensive Pollution Index (CPI) of the Coastal Marine Waters are presented in Table 7. The Table shows that the values obtained for DO (1.26), Conductivity (1.19) and pH (1.01) classified the study area to be moderately polluted as the mean values were within 1.0–2.0 (as earlier stated in Table 1), while Temperature (0.98), Turbidity (0.88), Salinity (0.47) and Hardness (0.86) classified the water as slightly polluted (with values within 0.40–1.0 as interpreted in Table 1). The mean BOD (4.24) indicated heavy pollution as the value was within 2.0–5.0 (as in Table 1). The mean CPI (1.36) showed moderate pollution as the value was within 1.0–2.0 of the standard surface water quality (Table 2).

Index	Parameter	Station I	Station II	Station III	Station IV	Mean
Single-Factor Pollution Index (SPI)	DO	1.28	1.26	1.27	1.25	1.26
	Temperature	0.98	0.98	0.99	0.98	0.98
	pH	1.01	1.02	1.01	1.01	1.01
	Turbidity	0.9	0.88	0.91	0.84	0.88
	Salinity	0.47	0.48	0.47	0.47	0.47
	Hardness	0.87	0.85	0.85	0.87	0.86
	BOD	4.42	4.5	4.15	3.91	4.24
	Conductivity	1.2	1.17	1.19	1.19	1.19
Comprehensive Pollution Index (CPI)		1.39	1.39	1.35	1.32	1.36

Table 7.

Single-factor pollution index (PI) and comprehensive pollution index (CPI) of the coastal marine waters of Ondo state Nigeria.

3.5 Relationship between the morphology of sampled organisms and water quality

The Principal Components Analysis of the morphological parameters of sampled organisms and water quality parameters is presented in Table 8 (Shrimps) and 9 (Crabs) as well as Figures 1–7. The results generally revealed that there was a strong correlation within the morphological parameters of each species of shrimps and crabs. Also, the important pollution indicator water quality parameters (Temperature, DO and BOD), all loaded positively (in the same principal component) with the morphological characteristic of each examined species (Tables 8 and 9). Figures 1–7 further buttressed that parameters (principal components) in the same circle showed a positive correlation with one another. Similarly, pH, salinity, conductivity and turbidity loaded positively in the same principal component but negatively with temperature, DO and BOD (Tables 8 and 9). This shows that the parameters contributed significantly to the survival, growth and abundance of each species of shrimps and crabs in the water body.

	Farfantepenaeus notialis		Macrobrachium macrobrachion		Nematopalaemon hastatus		Holthuispenaeopsis atlantica
	PC1	PC2	PC1	PC2	PC1	PC2	PC1	PC2
Total length(cm)	0.97			0.88	0.87		0.96
Rostral length (cm)	0.87			0.70	0.93		0.91
Carapace length (cm)	0.90			0.70	0.88		0.94
Body length (cm)	0.92		−0.21	0.78	0.92		0.96
Weight (g)	0.88		−0.13	0.80	0.35	−0.14	0.93
DO	0.27		−0.16	0.13			0.19	−0.10
Temperature	0.77		0.33	0.34	0.13		0.11	0.17
pH		0.69	0.71		0.31	0.60		0.68
Turbidity		0.57	0.55		0.21	0.78		0.76
Salinity		0.93	0.95	0.16	−0.13	0.94		0.95
Hardness		0.72	0.69	−0.18	−0.24	0.59	−0.14	0.68
BOD	0.14	−0.83	−0.86		0.37	−0.70	0.11	−0.80
Conductivity		0.91	0.90	0.14	−0.17	0.89		0.93

Table 8.

Relationships (principal components analysis) of the morphology of shrimps and water quality.

Figure 1.

Relationships (PCA) of the morphology ofFarfantepenaeus notialisand water quality.

Figure 2.

Relationships (PCA) of the morphology ofMacrobrachium macrobrachionand water quality.

Figure 3.

Relationships (PCA) of the morphology ofNematopalaemon hastatusand water quality.

Figure 4.

Relationships (PCA) of the morphology ofHolthuispenaeopsis atlanticaand water quality.

Figure 5.

Relationships (PCA) of the morphology ofOcypode africanaand water quality.

Figure 6.

Relationships (PCA) of the morphology ofSanquerus validusand water quality.

Figure 7.

Relationships (PCA) of the morphology ofCallinectes marginatusand water quality.

	Ocypode africana		Sanquerus validus		Callinectes marginatus
	PC1	PC2	PC1	PC2	PC1	PC2
Carapace length (cm)	1.00		−0.10	0.43	−0.21	0.98
Weight (g)	1.00		−0.06	0.41	0.16	0.99
DO	−0.15	0.99		−0.01		1.00
Temperature	0.91	−0.42		0.03	0.32	−0.95
pH	0.59	0.81	0.15	0.17	0.87	0.49
Turbidity	0.64	0.77	0.08	0.24	−1.00
Salinity	1.00		0.23	0.01	0.98	−0.20
Hardness	0.51	−0.86	0.21	−0.04	1.00
BOD	−0.89	0.46	−0.24	0.05	−0.98	0.18
Conductivity	0.99	0.12	0.25	−0.06	0.48	0.88

Table 9.

Relationships (principal components analysis) of the morphology of crab and water quality.

4. Discussion

The physical and chemical parameters of an aquatic ecosystem determine the quality of biodiversity in the environment, overall health and condition of the habitat. The parameters examined in the study area were within the tolerable range for coastal marine waters. The DO concentration was within the 4.5–8.5 mg/l recommended for the growth and survival of aquatic species within the ecosystem [18], while the slightly acidic water pH was similar to the observations of Deekae et al. [20] and Bolarinwa et al. [21]. The high DO concentration and slightly acidic pH value could result from photosynthesis by large amount of plants. Great uncontrolled plant growth, especially algal blooms increases the nutrient levels in the ecosystem, thus boosting the oxygen level of the ecosystem for a period of time. Water temperature recorded in the study was within the optimal limit for general metabolism, growth performance and enhancement of aquatic species [22].

The turbidity, salinity and conductivity concentrations were within the permissible range for coastal marine waters. There were variations in the turbidity and conductivity concentrations across the stations which could limit the distribution and abundance of organisms that requires stable concentration for reproduction, growth and survival. The hardness, biochemical oxygen demand (BOD) and electrical conductivity (EC) of the water body were extremely higher than the recommended limit for coastal marine waters [19], hence the ‘heavy pollution’ status of the BOD was not surprising. According to Yan et al. [15] these high concentrations could be adduced to the availability of a large amount of organic materials, a large quantity of urban runoffs, aggregation of both solid and domestic wastes and high concentration of dissolved ions in the ecosystem. These may also be responsible for the ‘moderate pollution’ status of the study area.

Moreover, the single factor pollution index revealed ‘slight pollution’ status for the study area in terms of all the parameters except for BOD that indicated ‘heavy pollution’ and in turn influenced the comprehensive pollution index to indicate a moderately polluted aquatic ecosystem. This shows that the pollution of the aquatic ecosystem spans from the physically observed human-mediated activities (such as domestic discharges, industrial effluents etc.) in the environment. The pollution level of the ecosystem reveals the environment to be relatively poor for the sampled species and other aquatic organisms in the water body. This was also in consonance with the findings of Bolarinwa et al. [21] on the same coastal waters.

The population structure of decapod crustaceans in the coastal marine waters follows the order N. hastatus>F. notialis> H. atlantica> M. macrobrachion > S. validus > O. africana > C. marginatus. It revealed that the decapods were highly predominant with Nematopalaemon hastatuswhich represented about three-quarter of the sampled crustaceans. This can be attributed to the breeding pattern of the shrimps as it breeds all year round with peaks in June and November [23, 24]. Also, the shrimp’s ability to tolerate and adapt to diverse ecological niche and environmental conditions could be responsible for its predominance. Farfantepenaeus notialisand Holthuispenaeopsis atlanticawere the second and third most dominant species in the ecosystem corroborating the assertions of Powell [24] and Olawusi-Peters and Ajibare [10] that the two species were of secondary importance to N. hastatusin Nigerian coastal waters. The low population structure of the other sampled organisms could be attributed to the moderately polluted water quality of the ecosystem as revealed by the results of the principal components analyses. Factors such as pH, salinity, temperature, dissolved oxygen (which were slightly polluted) and BOD (which was heavily polluted) have shown to influence the distribution and abundance of organisms. For example, Deekae et al. [20] observed a positive relationship between the population of shrimps and temperature, salinity, dissolved oxygen etc. in Luubara Creek, Nigeria and strongly recommended the effective control of all activities in the ecosystem.

The morphological features of the sampled organisms were within the morphological ranges recommended by FAO [13] and Powell [24]. Holthuispenaeopsis atlanticarecorded the highest morphological features when compared with the other species. The species had been conspicuously noticed and identified to be carnivorous when compared to other species, especially the penaeid shrimps. Powell, [24] further explained that the species feed on small crustaceans in the river mouth, thus accounting for its large size. Moreover, the total and body lengths were within the expected maximum size of 9–12 cm and 6–9 cm respectively for the species, thereby indicating the species to be in their adult stage. The most abundant species in the ecosystem (N. hastatus) showed diverse ranges of morphological features.

The average total length observed for N. hastatusin this study was within the range (4.87–9.71 cm) earlier stated by Ajibare et al. [25, 26] for white shrimps in the brackish waters of Ondo State, Nigeria. The shrimp’s ability to tolerate wide ecological niche and environmental conditions could have been the reason for this [23]. Similarly, the observed morphological characteristics of M. macrobrachionin this study agree with the findings of Jimoh et al. [27] who studied female M. macrobrachionin Badagry creek, Nigeria and observed mean weight of 5.65 g. However, it was lower than the observations of Oyekanmi [28] and Ajibare et al. [29] who reported a body weight of 66.14 g and 76.25 g for M. macrobrachionin Asejire reservoir respectively. Alphonse et al. [30] also reported 15.74 g as the average weight of M. macrobrachionin Mono-River coastal lagoon system in the Republic of Benin. These variations may be as a result of the differences in the sex, season, location and pollution status of the habitats [25, 26]. Also, Daniels et al. [31] opined that widely distributed fish species have high variation in morphology features. This is substantiated by the higher carapace length recorded in the study when compared to the findings of Enin et al. [32] who worked on the population dynamics of estuarine prawns off the southeast coast of Nigeria and obtained CL of (1.67–2.01 cm).

The results of principal components analysis showed that there was a strong correlation within the morphological parameters of each studied species of shrimps and crabs. Also, Temperature, dissolved oxygen and the biological oxygen demand (which are important pollution indicators) had positive correlation and loaded in the same principal component with morphological parameters of each examined species. This indicated that the growth, size, morphology and abundance of each species of the shrimps (F. notialis, M. macrobrachion, N. hastatusand H. atlantica) and crabs (C. marginatus, O. africanaand S. validus) might probably be affected by the temperature and the concentrations of DO and BOD since temperature reduces the DO (available to the biota), which in turn increases the BOD of the water [7, 22]. Also, pH, salinity, conductivity and turbidity (which had negative correlation with temperature, DO and BOD) contributed significantly to the survival, growth and abundance of each species of shrimps and crabs in the water body.

5. Conclusion

This study has used principal component analysis to examine the relationship between the tropical crustaceans and the environment and establish a baseline data on the eco-morphology of the coastal marine waters of Ondo state, Nigeria. The study revealed that the sizes of the decapod crustaceans were within the morphological range recommended by FAO and that all water quality parameters indicated slight pollution except BOD that indicated heavy pollution of the study area. However, the water quality (which was moderately polluted) can still sustain the biodiversity if the anthropogenic influence is regulated. The results of the principal components analysis (PCA) established that temperature, DO, and BOD had strong and positive correlation with the morphological parameters and therefore influenced the morphology/size of the crustaceans. Thus, improved management of the ecosystem is recommended in order to achieve healthy growth and survival of the aquatic species.

Funding disclosure

The authors of this research publication received no research funds/compensation from any organization. The research project and publication were sponsored by all the authors.

Competing interest statement

The authors have declared that no competing interest exists in the manuscript.