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.We 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.We 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.

Sea Urchins as an Inspiration for Robotic Designs

“…Microalgae is known to grow epiphytically on seagrass especially [53], and sea urchins have been recorded transporting the fragments down their bodies and towards the oral surface of their test. This behavior is ecologically very interesting, and has been deemed as tool use by some workers in the field [49]. From the point of view of a roboticist, the covering behavior shows that a collection of semi-independent sensory-motor elements can collaborate to enable a complex type of behavior, i.e., tool use, believed to be unique to humans until a few decades ago [54].…”

“…Spines and tube feet act together to select covering material and to transport it from the sea-bed up and onto the body of the animal (Figure 1). Different species of echinoids use different types of covering materials, including plant material, coral rubble, and calcium carbonate based shell fragments [48,49]. Some species drop these fragments at night and collect other fragments the next day.…”