Urchin covering by ocean depth: List of the 15 urchin genera and their use of covering behavior, organized by the depth ranges in which they are found.
Abstract
Covering behavior in sea urchins is an important aspect of many species’ ecology and has a variety of perceived benefits including food source, mechanical defense, shielding from sunlight, and predator protection. The goal of this study was to determine whether an urchin genus’s main benefit from this form of crypsis is correlated with either phylogenetic relationships or environmental factors (ocean depth and climate). To evaluate this hypothesis, a literature review was conducted on 15 urchin genera that use the covering reaction. The function of this behavior for the aforementioned genera was both mapped onto a phylogeny and evaluated, based on the climate and depth of the genera’s habitats to determine whether the patterns exist. The results suggest that phylogenetic relationships provide a more functional predictive tool for determining the purpose of covering in an urchin genus than its environment. This conclusion is useful for understanding the biology of sea urchins as well as how the covering reaction relates to the many other cryptic behaviors used by animal species.
Keywords
- crypsis
- ecology
- protective behavior
- phylogenetics
1. Introduction
The crucial roles that sea urchins play in aquaculture and coral reef ecosystems warrant the study of their ecology and behavior [1, 2]. Many species exhibit a special form of crypsis called covering behavior [3–5], also known as “dressing” or “heaping” [6] (Figure 1), which involves securing algae, rocks, and other items in their environment to their tests using a combination of spines and tube feet [7]. While the mechanics of this reaction are well understood, the reasons for it remain nebulous.
Two conflicting hypotheses exist with respect to this form of crypsis: the “reflexive” hypothesis, which proposes that the covering reaction is automatic, and the “functional” hypothesis, which suggests that heaping is used for specific purposes. The reflexive hypothesis has been supported by several studies. One such study examined the covering behavior of
In contrast, the functional hypothesis theorizes that covering behavior is a conscious decision by sea urchins. Many species have been shown to choose when to cover and to what extent, based on a variety of environmental stimuli [4, 5, 7, 11–13]. One study noted the differences between both species and individuals in the amount of covering even in similar environmental conditions, which suggest that this crypsis is a decision, as otherwise all urchins would cover themselves to an equal extent [14]. Additional research has found that urchins choose objects of particular sizes with which to cover themselves [15], a result that would not be expected were the action reflexive. While both hypotheses have convincing evidence, the functional hypothesis has been assumed for the current study based on the variety of species-specific reactions to environmental stimuli, in addition to the fact that not all urchin species use heaping behavior.
Covering can provide sea urchins with protection from predators [16], mechanical damage (wave surge or floating debris) [11, 17], overexposure to light [7], or can act as a food source [18]. However, no prior studies have considered a relationship between sea urchin biology and the reaction. Such a relationship would provide both a framework to understand why such a variety of applications for covering have been observed and a tool for predicting what function crypsis serves in an unstudied species of urchin.
One source for such a framework might be environmental considerations. The reason for crypsis and the extent to which it is seen differs widely between species [14]. However, the behavior is also affected by size and mobility [19] as well as the energetic cost of covering [20]. It is possible, then, that the differences in the covering reaction between species are due to variations in animal size and mobility as well as energetic costs, rather than genetics. In this case, the reason for covering might be deduced not by a species’ evolutionary relationships, but rather by the climate and ocean depth at which it is found.
It has been shown, though, that differences in the heaping reaction persist even for species such as
The purpose of this study was to review all existing studies of cryptic sea urchins to determine whether covering behavior can be predicted from either (1) the depth and climate within which an urchin resides (the species’ environment), or (2) the species’ relationship to other cryptic sea urchins.
2. Methods
Studies of 15 urchin genera were researched to determine the reason for their cryptic behavior, chosen based on a previous paper that researched which species of sea urchins use covering [5]. The genera considered were
Four main reasons for covering were considered: predator defense, protection from mechanical damage (wave surge/floating debris), use as a food source, and protection from bright light (sunlight/UV). When it was possible, this list was narrowed down to one main use for crypsis, though in some cases, two uses had equal evidence and were both noted as main covering reasons. For some genera (
2.1. Phylogenetic methods
To determine whether evolutionary relationships provide a solid framework for urchin’s covering motivation, the use of heaping by eleven of the 15 urchin genera was mapped onto an existing phylogeny [5] (Figure 2). The genera included in the phylogeny were
2.2. Environmental condition methods
To consider whether the environmental conditions are the best indicator of covering purposes, an additional four urchin genera for which the reaction has been noted (
3. Results
3.1. Covering as sunlight/UV protection
Protection from sunlight was the most common reason for using the behavior in the urchin genera considered, with six of the total fifteen genera covering primarily for this reason.
For
Several other urchin genera also likely cover to shelter themselves from overexposure to sunlight.
3.2. Covering as mechanical protection
Of the urchin genera considered, two were found to cover primarily for mechanical protection (
3.3. Covering as a food source
Two urchin genera,
3.4. Covering as predator protection
For
3.5. Covering for unknown reasons
While both
3.6. Covering by evolutionary relationship
The various reasons for urchin heaping were mapped on to the existing phylogenetic tree (Figure 3). As previously noted,
In the
3.7. Covering by ocean depth
No relationship was found between the depth at which an urchin was found and its use for covering behavior (Table 1). For both 0–50 and 0–250 meter range of depths, UV protection was the most common reason for covering (Table 1). However, most urchins found in the range 0–100 meters had unknown reasons for heaping, and for the deeper ranging urchins (0–1000 and 0–1200 meters), light, mechanical damage, and predator protection were all reasons for covering (Table 1). These results suggest that there is no correlation between ocean depth of the urchin’s environment and the function of their cryptic behavior.
Depth (meters) | Genera | Climate(s) | Climate(s) |
---|---|---|---|
0–50 | Temperate | L | |
Temperate/polar | L | ||
Tropical | M | ||
0–100 | Polar | U | |
Temperate | U | ||
Temperate | L | ||
Temperate | F | ||
Tropical | L | ||
Tropical | U | ||
Tropical/temperate | U | ||
0–250 | Temperate | L | |
Temperate | M | ||
Tropical/temperate | L | ||
0–1000 | Temperate | P | |
Tropical/temperate | U | ||
0–1200 | Temperate | F | |
Temperate | L/M |
3.8. Covering by climate
Though urchins who cover to protect themselves from light tend to be found in warmer oceans, this rule does not apply for all genera (Figure 4a, b). Some urchins within tropical regions cover for mechanical reasons (Table 2; Figure 4b). Additionally, some urchins use the behavior to shield themselves from light are also found in temperate and polar waters (Table 2). Based on the studies reviewed, no clear trend was found between urchin crypsis and ocean climate.
Climate(s) | Genera | Depth (meters) | Covering Use |
---|---|---|---|
Tropical | 0–75 | L | |
0–100 | U | ||
2–50 | M | ||
Tropical/Temperate | 12–420 | U | |
0–250 | L | ||
0–60 | U | ||
Temperate | 0–1200 | F | |
5–65 | U | ||
0–180 | L | ||
0–1150 | L/M | ||
0–35 | L | ||
0–100 | L | ||
0–820 | P | ||
10–150 | M | ||
0–65 | F | ||
Temperate/Polar | 0–100 | Up | |
0–35 | L |
4. Discussion
Phylogenetic relationships are a better tool than environmental considerations for predicting why sea urchin species use cryptic behavior. Urchins that are found closer to the equator are more likely to cover to shield themselves from light, but
The covering reaction remains nebulous. For several genera, no studies have been performed to determine the function of the behavior, leaving gaps in the phylogeny as well as the climate and depth tables that might have impeded otherwise prominent patterns (Figure 3; Table 1; Table 2). It is also worth noting that for other genera that were assigned a most likely reason for covering, alternative hypotheses often were not tested.
It seems that heaping can perform several functions for a sea urchin, though within a genera there may be one primary reason for the behavior.
This study has important implications for the study of sea urchin heaping as a whole because of its novel suggestion that there might be a relationship between the various uses of covering and other aspects of urchin biology, be they genetic or environmental considerations. Understanding covering in sea urchins is particularly important because the ability is crucial to survival; one study has even shown that some sea urchins die if not provided with sufficient material with which to cover themselves [25].
Sea urchin ecology is important because of their role in coral reef systems, where they consume and control populations of algae that threaten corals [43], as well as in aquaculture, where they are part of the diet of humans in many countries [44]. Additionally, understanding the covering reaction itself, and how it may be determined by genetic and environmental factors, is important when considering it in relation to the multitude of cryptic behaviors that exist within the animal kingdom. Further studies of covering in urchin genera with currently unstudied covering behavior would be especially insightful for this study. Additional studies might also include ruling out other uses of the behavior for which one primary use has been considered, or studying the phylogenetic relationships of all urchin genera that cover.
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