There are more
than 15 species in several genera of shell-less mangrove slugs all
over Singapore mangroves, burrowing in the mud, on the forest floor,
under wood and even climbing trees!
Species identification is difficult, requiring an examination of the
texture and features on the dorsal and ventral surface, and even the
arrangement of internal organs! Hence they are often referred to collectively
as 'Onchs' - which means lump! (Onchidium is the Latinised
derivation of the Greek word for 'little lump'!).
the centre is the foot.
It is usually narrow in
has an orange margin
on the ventral surface
has a yellowish
orange foot (above)
and black specks
around the foot (below)
is a gregarious species
(length 1.5 cm)
(the smaller specks on
the sandback surface)
emerge from the
burrows to feed
the ellobiid snails, 'Onchs' are hermaphrodites,
with a complicated system of ducts, glands and sacs to ensure cross-
but not self-fertilisation.
Most species probably graze on detritus and surface algae.
most common species in our mangroves is Onchidium aberrans,
and many of the tree-climbers are species of Platyvindex.
Paraonchidium sp. is a small, burrowing species and may occur
in densities of up to 200 individuals per square metre on some patches
of muddy sand during low tides. Being air-breathers, they burrow to
form S-shaped tubes, forming an air pocket from which they breathe
during immersion. On sandy banks smoothened by the receeding tide,
they create a spectacular sight of churned sand as they emerge from
their burrows to graze above ground.