|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!