Abstract
We compared the covering behavior of four sea urchin species, Tripneustes gratilla, Pseudoboletia maculata, Toxopneustes pileolus, and Salmacis sphaeroides found in the waters of Malapascua Island, Cebu Province and Bolinao, Panagsinan Province, Philippines. Specifically, we measured the amount and type of covering material on each sea urchin, and in several cases, the recovery of debris material after stripping the animal of its cover. We found that Tripneustes gratilla and Salmacis sphaeroides have a higher affinity for plant material, especially seagrass, compared to Pseudoboletia maculata and Toxopneustes pileolus, which prefer to cover themselves with coral rubble and other calcified material. Only in Toxopneustes pileolus did we find a significant corresponding depth-dependent decrease in total cover area, confirming previous work that covering behavior serves as a protection mechanism against UV radiation. We found no dependence of particle size on either species or size of sea urchin, but we observed that larger sea urchins generally carried more and heavier debris. We observed a transport mechanism of debris onto the echinoid body surface utilizing a combination of tube feet and spines. We compare our results to previous studies, comment on the phylogeny of sea urchin covering behavior, and discuss the interpretation of this behavior as animal tool use.
Highlights
Several species of sea urchins cover their exposed body surfaces, in a special form of crypsis, with debris collected from their environment [1,2,3,4,5]
The sea urchins actively manipulate these debris fragments with their tube feet and combine these with their spines to transport them onto their upward facing body surface. Suggested reasons for this behavior are UV protection, olfactory camouflage
The aim of this study was to compare the covering behavior of four species of sea urchin, Tripneustes gratilla, Pseudoboletia maculata, Salmacis sphaeroides, and Toxopneustes pileolus, which are species found near reefs at shallow depths in the tropical Indo-Pacific, and the Philippines
Summary
Several species of sea urchins cover their exposed body surfaces, in a special form of crypsis, with debris collected from their environment [1,2,3,4,5]. The sea urchins actively manipulate these debris fragments with their tube feet and combine these with their spines to transport them onto their upward facing body surface Suggested reasons for this behavior are UV protection (demonstrated for Tripneustes gratilla [5,6]), olfactory camouflage (suggested for covering species living in light-deficient deep-sea environments [7,8], and use as a ballast for weighing down of the animal by increasing the relative density of the sea urchin in the face of currents or surge (shown for Strongylocentrotus droebachiensis [2] and for Paracentrotus lividus [9]) as well as a direct anti-predatory function [1,10]. Since the covering material often acts as an effective platform for algal growth and is often moved from and across the main body of the animal toward the mouth, situated on the aboral surface, covering has been suggested as a food storage behavior [11,12]
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