Dear Damian,

Attached please find a publication that covers the answer to your question.

Examples of Eucidaris thouarsii as movile:

In contrast, populations of Eucidaris thouarsii around several of the Galápagos

islands were greater with adult densities between 10 and 50 ind./m2 and these

populations consisted of much larger individuals with maximum test diameters ranging from 4.3 to 6.2 cm (Glynn et al., 1979). Additionally, E. thouarsii in the Galápagos were neither sedentary nor cryptic during the day, but foraged over relatively large distances (1 to 3 m/day) during both day and night. The main prey of E. thouarsii on Galápagos reefs also differed from mainland sea urchins. In addition to feeding on crustose coralline algae, a common prey item for mainland populations, sea urchins in the Galápagos preyed heavily on live corals. The primary prey were Pocillopora damicornis, P. capitata, and P. elegans, although sea urchins also were observed to feed on Pavona clavus (Glynn et al., 1979). Feeding surveys indicated that on some reefs over 50% of the sea urchins were feeding on live coral with the remainder feeding on crustose coralline algae covering the dead coral skeletons. Most of the coral colonies were fed on from the tips of branches, as sea urchins usually did not have access to the interior portions of the colony.

This text was taken from page 11 of the attached file.

Examples of Eucidaris thouarsii as sedentary:

Eucidaris thouarsii

In most coral reef environments, cidaroid sea urchins are a minor component of the

predator and grazer guild and feed on a variety of encrusting animals and algae

(Lawrence, 1975; Birkeland, 1989a). In the Galápagos Islands, the cidaroid, Eucidaris thouarsii, has played a much more important role in the coral reef community. On the Eastern Pacific coral reefs of Panama and Ecuador, population densities of adult E. thouarsii ranged between 1 and 5 ind./m2, although abundances of juveniles occassionally exceeded 50 ind./m2. Adult sea urchins in these localities were relatively small with maximum test diameters between 2.7-3.5 cm (Glynn et al., 1979). Similar to cidaroids elsewhere, E. thouarsii in these coastal habitats are relatively sedentary, remaining cryptic during the day and foraging nocturnally. Adult populations of E. thouarsii in Panama and Ecuador appear to be kept in check by the abundance of echinoid predators, particularly fishes in the families Balistidae, Labridae, and Tetradontidae (Glynn et al., 1979; Glynn and Wellington, 1983).

This text was taken from page 10 in the attached file.

Hoping this will be helpful,

Rafik