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Abstract. Members of the genus Echinometru (Echinoideu: Echinometridue) inhabit hard substrata in shallow waters where they live in self‐excavated dwellings. Boring by Echinometru spp. causes a secondary structure of the surface, thus forming additional microhabitats. In this study the effects of boring activities of Echinometru lucunter (L.) along the Caribbean coast of Colombia and of Echinometra vunbrunri A. Agassu in the Colombian Pacific is examined. Several species inhabit the boreholes occupied by these urchins. The associates live underneath the echinoid on the bottom of the borehole, where they find shelter from exposure and predators. The co‐inhabitants of E. lucunter include the porcellanid Clusroroechus vunderhorsri (Schmitt), the recently described brit‐tlestar Ophiorhrix synoecim (Schoppe), and the clingfish Acyrrus rubiginosus (Poey). The species co‐occumng with E. vunbrunti include the porcellanid crab Clusroroechus gorgonensis Werdingamp; Haig and the clingfish Arcos decoris Briggs. With the exception of A. decoris, all of these species are obligatorily associated with the Echinomerra host.

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Johngarthia malpilensis (Faxon, 1893) is the least studied of the eight American species of Gecarcinidae. This land crab is considered endemic to Malpelo, an oceanic island of the Colombian Pacific. Several aspects of its ecology were investigated between 2003 and 2006. We estimated its population density, distribution, daily activity, reproduction, interactions, and diet by marking and monitoring 909 individuals. During our visits we recorded crabs of sizes from 5 to 82 mm carapace width. Johngarthia malpilensis shelters mainly in fissures and hollows between rocks. It is distributed all over the main island except in very steep sectors. An average density of 0.41 adults m À2 and 0.55 juveniles m À2 produced an estimated total population of 833,000. Johngarthia malpilensis showed high mobility, with crabs covering distances over 450 m in a few days on highly irregular surfaces. Activity was higher from dusk till dawn and lowest around noon. Release of larvae took place during the high tides associated with the new moon, at least during the rainy season. It is omnivorous and opportunistic, consuming practically every available resource. The crab is occasionally preyed upon by an endemic lizard and migratory birds. Its general ecology is very similar to that of J. planatus, a closely related species. As a voracious omnivore J. malpilensis is one of the most important components of Malpelo's food web.

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