Elpidia minutissima

Elpidia minutissima is a deep-sea swimming sea cucumber belonging to the family Elpidiidae. It feeds on detritus and lives in the northern Pacific Ocean. The species shows physical variation between populations. Specimens from the Aleutian Trench averaged 13 mm in length and had three pairs of dorsal podia.

In contrast, specimens from Station M in the northeastern Pacific, at a depth of about 4,000 meters, had four pairs of podia: two on the dorsal surface and two on the ventral surface, with the rearmost ventral pair being shorter than the others. When population density was low at Station M, the average length matched that of the Aleutian Trench specimens, but during periods of high abundance, average length increased to 25 mm. It is possible these two groups are not the same species. Ecologically, E. minutissima consumes detritus from the seabed, which originates as "marine snow"—a mix of mucus, feces, and organic debris sinking from surface waters.

This species is one of several echinoderms that experience dramatic swings in population density. Over a sixteen-year study, both E. minutissima and another deep-sea holothurian, Peniagone vitrea, declined in density by one to two orders of magnitude. Researchers suggest that broadcast spawning and planktotrophic larvae increase the likelihood of such large fluctuations. These traits create a positive feedback loop: once populations drop, recovery is very slow because individuals are farther apart, making fertilization less likely and reducing the number of available larvae for recruitment.