The diet of the Harlequin crab Lissocarcinus orbicularis, an obligate symbiont of sea cucumbers (holothuroids) belonging to the genera Thelenota, Bohadschia and Holothuria

The present paper characterizes, for the first time,
the diet of the Harlequin crab Lissocarcinus orbicularis, an
obligate symbiotic crab that associates with sea cucumbers
(holothuroids) belonging to the genera Thelenota, Bohadschia
and Holothuria. These tropical holothuroids host a rich symbiotic
community in the Indo-West Pacific Ocean of which the
Harlequin crab is the best known. The diet of L. orbicularis
was characterized by analyzing the microscopic, molecular
and isotopic signatures obtained from its gastric content. The
presence of sea cucumber ossicles in the gastric mills of the
crabs suggests that symbionts eat the superficial integument of
their host and this was supported by the fact that Holothuroid
DNA was detected in the stomach of L. orbicularis after
DGGE and sequencing of the 18S rDNA gene. The stable
isotopic δ13C and δ15N values of crab tissues were compared
with diverse potential food sources including three holothuroids,
three algae, one sea grass as well as the organic matter contained
in the water column, in the sediment, and the second most
abundant symbiont, the polychaete Gastrolepidia clavigera.
The low δ15N values of crabs suggests that the crabs do not
exclusively feed on sea cucumber tissue but assimilate diverse
food sources such as sea grasses and organic matter contained in
sediment that have similar δ13C values. There were no differences
between the feeding of males and females but there was a
positive correlation between the carapace length and the stable
isotopic values indicating a shift of the food source as crabs
grow larger.