Sea Cucumber Crab

Lissocarcinus orbicularis grows to about 4 cm (1.6 in), and has a smooth, sub-circular carapace with a curved frontal margin that lacks defined teeth or a medial notch. L. orbicularis can be distinguished from L. holothuricola due to its generally smoother carapace. The carapace is broader than it is long, ranging from 0.3 to 1.4 cm in length, and has a highly medial convex dorsal surface.

The type locality for this species is Fiji, and it is widely distributed in shallow waters of the Indo-West Pacific, including Tanzania, Kenya, Somalia, Seychelles, Madagascar, Sri Lanka, Japan, Taiwan, Australia, Hawaii, and the eastern coast of South Africa. Nigam and his associates at the Zoological Survey of India also recorded Lissocarcinus orbicularis in the Maldives, Marshall Islands, Palau, Moorea Island, Society Islands, Philippines, Gulf of Tadjoura in Djibouti, Moorea Island, Society Islands, Mayotte Island, Comoros Islands, Caroline Islands, New Caledonia, Oman, Papua New Guinea, Vanuatu, Europa Island, Iles Eparses, and most recently India, in addition to the previously listed regions. It is also found in the Red Sea.

L. orbicularis has an obligatory relationship with sea cucumbers of the genera Thelenota, Bohadschia and Holothuria, feeding on the surface tissue of these tropical echinoderms. Although these crabs feed off the upper integumentary layer, they do not cause their hosts any serious damage, classifying them as having a commensal relationship. While their hosts provide a significant component of nutrition, this species is also known to feed on a diverse array of food including sea grasses, and they also absorb and digest organic matter. Gastric mills, a moveable grinding apparatus in the pharynx, are used to eat the organic matter that their hosts intake or that is found on their exterior. A large proportion of these gastric mills have been discovered to be empty, suggesting that they do not continuously nor regularly feed.

Only three of nine species of the genus Lissocarcinus are known to be obligate symbionts of holothurians. The inter-specific interaction between Harlequin crabs and their sea cucumber hosts is a defining characteristic of this species. L. orbicularis specifically has a commensal relationship with certain sea cucumbers and has no known negative effects on their hosts. It can be found along the outer body integument within the gut cavity, or near the oral tentacles and anus of sea cucumber species Holothuria whitmaei, Actinopyga obesa, and most commonly Holothuria atra. Sea cucumbers are known to produce "cytotoxic secondary metabolites" known as saponins, to repel predators. Interestingly, these emissions also act as kairomones, which are chemical signals that attract symbiotic animals, including the Harlequin crab.

Unlike many marine invertebrates, the Harlequin crab partakes in a monogamous mating system and can be found inhabiting sea cucumbers in heterosexual pairs or along with their progeny.

This species also displays weak sexual dimorphism in which female crabs are significantly larger than their male counterparts. The larger size of females allows for greater resource allocation and energy utilization for producing offspring. Females have been observed to reproduce throughout the year, rearing roughly 500 eggs at a time. For most of the year, females also dominate in population numbers. These crabs undergo internal fertilization, with many females brooding their eggs until they are prepared to hatch as free-swimming, planktonic larvae.