Opisthobranch of the Week Data
There is a close resemblance between the above featured sea cucumber and several species of phyllidiid nudibranchs, notably Phyllidia coelestis and P. varicosa. The conventional thinking on this relationship is that over a long period of evolutionary history, juveniles of Pearsonothuria graeffei have come to resemble the highly distasteful phyllidiid nudibranchs; in so doing they gain a advantage by not being preyed upon by certain predatory fishes. This relationship is sometimes referred to as a model and a mimic system. Highly toxic secretions from the phyllidiid deter fish from eating it and fish learn to avoid the Phyllidia-appearing cucumber and the Pearsonothuria graeffei juveniles are at least partially protected from predation. In this case, any animal that looks like Phyllidia will gain some protection from predation by tricking predatory fish into thinking they are a distasteful Phyllidia species. As the cucumber grows in size it undergoes an adult color change to a brown and white mottled color. I've added an additional page showing several images of the same above cucumber juvenile, an adult, and, in addition to the above link, an example of Phyllidia varicosa.
The sea cucumber Pearsonothuria graeffei is considered to be a relatively common holothuroidean in Okinawan waters. The adults are frequently seen in reef environments although the juveniles are only occasionally seen. The above featured animal is the only collected individual although I've seen perhaps two or three other comparable-sized animals here. There are several images of juvenile Pearsonothuria graeffei (as Bohadschia graeffei) on the Sea Slug Forum and Gosliner, et al (1996) has photos of several intermediate stages of the cucumber.
There seems to be a fair amount of confusion in both the popular and scientific literature as to the generic placement of the animal. Several researchers have rectified the confusion (Samyn, 2000, Kerr, et al, 2000). The following is taken from Samyn (2000, see also 2003) which explains the current taxonomic thinking of this animal:
The reef-dwelling species Pearsonothuria graeffei (Semper, 1868) was originally described as Holothuria graeffei Semper, 1868. Examination of spicule morphology showed that this species is unrelated to the species in the genus Holothuria. Hence, it was transferred to the genus Bohadschia since the rosettes of the body wall bear some resemblance to the rosettes found in the genus Bohadschia. The taxonomic status of Bohadschia graeffei (Semper, 1868) was critically examined later by Levin et al. (1984), who found that the nature of the chemical characters of this species needed a new genus name: Pearsonothuria Levin, Kalinin & Stonik, 1984. Indeed, by assigning the genus name of Pearsonothuria, the anomalous structure of the typical 'racket-shaped' spicules, and the translucent and weakly developed calcareous ring, now get a more appropriate systematic position. However, the name Bohadschia graeffei still appears in numerous papers that deal with conservation of holothurians.