Opisthobranch of the Week Data
[a data range is usually given for Okinawa-collected specimens of the featured animals, but
in this instance, due to its common occurrence, a range has not been supplied]
Phyllidiella pustulosa is considered to be one of the most common nudibranchs found within the tropical Indo-Pacific, and it is most assuredly the most frequently observed of the Okinawan opisthobranchs. One of the earliest references to Japanese opisthobranchs (Stimpson, 1855) cites P. pustulosa (as Phyllidia pustulosa) being found in the Japanese seas; the earliest work on Okinawan opisthobranchs (Baba, 1936), reports that the animal had been collected in June of 1934 from Okinawa.
Several years ago I recall finding several very attractive green-pigmented individuals which were photographed in situ as well as at home under "photo tank" conditions. At the time, I thought perhaps I had an undescribed species of Phyllidiella; when the photos were returned several weeks later, I discovered that the animals were apparently no longer green and that perhaps I'd made a mistake in recording my field and photo notes, and I chalked it up as being an error. The next time I found one of the green forms I took special note of the color prior to photography, however, upon receiving the developed film, again they were without the green pigmentation. Brunckhorst (1993) points out that some individual animals appear to the human eye to be green underwater, but mentions incontrovertible evidence that these animals are members of the same species, based on observations of green-pigmented individuals mating with the more usual pink-pigmented animals.
I've included a photo of a remarkable association found here, and elsewhere, between Phyllidiella pustulosa and a polyclad flatworm, Pseudoceros imitatus. This relationship was described (Newman, Cannon, & Brunckhorst) in 1994. Bill Rudman, in a Sea Slug Forum discussion (a 25 June, 1998 response to a photo submitted by Wayne Ellis) concerning the above animals, believes the association is one involving mimicry; presumably, the flatworm is "copying" very closely the color and even the notal pustules of the nudibranch. In doing so, the flatworm derives a fair amount of protection from predation, possibly similar to the relationship between some butterflies, notably the palatable African Swallowtail (Papilio dardanus), and that of several distasteful species belonging to several dissimilar genera (a "model" and "mimic" system). In the cited Sea Slug Forum discussion above, Rudman points out that "both flatworms and chromodorids produce distasteful chemicals, but it seems most probable that Chromodoris geometrica and Pseudoceros imitatus are relying on their similarity to Phyllidiella pustulosa for protection."
As noted above, range data hasn't been supplied with the featured animal, primarily due to its very common status on Okinawa. I can't recall, not seeing these animals on any dive, including the majority of the variable habitats investigated to date, and during all times of the year.