Picture details
Genus Thysanozon/Acanthozoon
Species sp.
Family Pseudocerotidae
Order Polycladida
Location Tulamben, Bali, Indonesia
Depth about 8 m
Date September 2002
© Copyright The Slug Site by Michael D. Miller
San Diego, CA, USA
Comments In the upper image the flatworm has the "pebble" wrapped or enfolded under its dorsal surface and resembles a bubble shell to some extent. Upon perturbing (moving), the flatworm retracts its folds and releases the white object and goes on its way. Mike reports that he has observed this flatworm and identical behavior in the Philippines several times before he was able to photograph it. The white pellet-like material was hard to the touch but its function is unclear. Several explanations are possible:

1) It is a relict of an calcareous invertebrate colony the worm was feeding on, per instance a bryozoan or coelenterate colony. The color looks like a bleached coral backbone after the predatory Acanthaster sp. has sucked out all polyps and living tissues.

2) It may be some kind of cocoon-like egg masses of this worm - although it seems to be quite a huge amount for this tiny worm. Maybe there are some glycoproteins that get highly hydrated after egg deposition and so the whole mass swells to manifold size and then hardens? That may serve as egg protection - same principle as our frogs in our garden ponds where eggs are protected by a thick gelatinous mass.

3) Could this be "Kauri-Mimicry"?

Any help and further opinions are highly appreciated!