Pseudoceros imitatus Newman, Cannon & Brunckhorst, 1994

Pseudoceros imitatus

Above: Pseudoceros imitatus gliding across the substrate out in the open in 12 metres of water at
Flinders Reef off Cape Moreton, Southern Queensland, Australia. (40 mm)

Pseudoceros imitatus Newman, Cannon & Brunckhorst, 1994

Family: Pseudocerotidae

Many marine flatworms display gaudy colours and patterns, and make no attempt at concealment as they swim out in the open with undulations of their thin body margins. This conspicuous behaviour serves to advertise the fact that they are highly toxic to potential predators such as fish. Experiments have proved that fish are wary of certain colour patterns. These flamboyant flatworms contrast with those that are cryptic in appearance and much more secretive in nature, living in crevices and slinking under rocks. There are also a few that directly imitate the appearance of toxic nudibranchs.

Pseudoceros imitatus is an excellent example of such a species. It has an uncanny resemblance, by colour, pattern and size, to the extremely toxic and common nudibranch, Phyllidiella pustulosa. To the casual observer the only clues are a flatter appearance and the more easily discernible movement of the flatworm compared to the nudibranch. There are no dorsal gills to simulate and the marginal pseudotentacles formed by upfoldings of the anterior margin do an excellent job of appearing like phyllidiid rhinophores. The dorsal surface of its body also has a lumpy texture like the sea slug it seeks to mimic.

This is not to suggest that this flatworm does not have its own store of toxic chemicals within its body. Although not yet proved this may in fact be the case because if the visual deterrence fails then by having distasteful or toxic flesh usually the worst damage is limited to a bite mark. Additionally not all potential predators have the faculty of sight. It is certainly a better outcome to be only just bitten rather than completely devoured however these flatworms are so thin it only takes a single vigorous shake to tear apart their thin delicate bodies.

Pseudoceros imitatus feeds upon colonial ascidians.

(Photos and text by David Mullins)


Phyllidiella pustulosa

Above: The toxic nudibranch Phyllidiella pustulosa at
the same site. The tubercles are most often pink but can
be white and even occasionally green. Also note the
dorsal anal opening on the midline posteriorly. (35 mm)

A damaged Pseudoceros imitatus

Above: A damaged Pseudoceros imitatus specimen
showing bite marks and a torn body, evidence of an
encounter with a predator that eventually had second
thoughts allowing the flatworm to escape.
Also at Flinders Reef. (40 mm)