Featured Invertebrate Data
Pseudoceros imitatus is considered to be relatively rare in the waters of Okinawa's main island. I've photographed only two animals but have seen perhaps a half-dozen others, but unfortunately none of these were collected. There's a remarkable association between the featured polyclad flatworm, Pseudoceros imitatus and a nudibranch, Phyllidiella pustulosa. This relationship was described (Newman, Cannon, & Brunckhorst) in 1994. Bill Rudman, in a Sea Slug Forum discussion on mimicry (23 July, 2004) concerning these two 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 Pseudoceros imitatus is relying on its similarity to Phyllidiella pustulosa for protection."
The following original description of Pseudoceros imitatus is from Newman, Cannon & Brunckhorst, 1994.
Colour and pattern. Dorsal surface black with large irregularly shaped creamyellow spots, each raised medially, some fused to form pustules of varying shapes and sizes. Margin composed of smaller spots which merge together. Tips of the anterior marginal tentacles are black. Colour pattern of the Australian specimen varies slightly; spots are more numerous and fused to form elongate longitudinal groups of pustules, and black pigmentation is reduced forming irregular lines between the pustules. Preserved colour of the holotype is black, the chromatophores embedded in the parenchyma, and irregular beige spots. Ventral surface grey-black fading to cream centrally, with cream margin.
Morphology. Body elongate and oval, soft, relatively small (holotype 13 mm long x 7 mm wide preserved, paratype 8 mm long x 5 mm wide preserved, Madang specimen 18 mm long x 10 mm wide preserved). Margin with few shallow ruffles, anteriorly forming two pointed folded pseudotentacles. Cerebral eyespot composed of about 60 eyes (in largest specimen) in horseshoe shaped cluster. Marginal eyes obscured due to the black pigment, few eyes laterally on the pseudotentacles. Pharynx anterior, highly ruffled with numerous bifurcating folds and mouth central. Male pore mid-body, near and posterior to the pharynx, female pore directly posterior to male pore. Sucker mid-body, 2.4 mm posterior to female pore.
There are quite a few sites with images of Pseudoceros imitatus, several of these have excellent photographs, as seen at the following links:
Philippine Sea Slugs.com
South-west Indian Ocean Flatworm site
I've added a second page w/ a series of images of both Pseudoceros imitatus and Phyllidiella pustulosa.
In addition, I've added a link to the current and past featured invertebrates via a list and thumbnails.