Tool use by four species of Indo-Pacific sea urchins

Abstract

We compared the covering behavior of four sea urchin species, Tripneustes gratilla, Pseudoboletia maculata, Toxopneutes 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 urchin, and, in several cases, the recovery of debris cover after stripping the animal of its cover.\n\nWe found that Tripneustes gratilla and Salmacis sphaeroides have a higher preference for plant material, especially sea-grass, compared to Pseudoboletia maculata and Toxopneutes pileolus, which prefer to cover themselves with coral rubble and other calcified material. Only for Toxopneutes pileolus did we find a decrease in cover with depth, confirming previous work that the covering behavior serves UV protection. We found no dependence of particle size on either species or urchin size, but we observed that larger urchins 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. The transport speed of individual debris items varied between species.\n\nWe compare our results to previous studies of urchin covering behavior, comment on the phylogeny of urchin covering behavior and discuss the interpretation of this behavior as animal tool use.