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.

[7][3] Also designated true crabs, L. orbicularis and other Brachyurans, have all five pairs of legs fully visible and short tails typically hidden beneath the thorax.

[9] 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.

In many species the male may continue carrying the female after intercourse until her new integument has begun hardening to further ensure his sperm are properly established.

[11] Generally, they display a preference for larger holothuroids across the genera Thelenota, Bohadschia and Holothuria, likely due to their associated levels of toxicity and subsequent signaling.

[4] 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[3][7].

[11] 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.

[3] The type locality for this species is Fiji, and it is widely distributed in shallow waters of the Indo-West Pacific,[3] including Tanzania, Kenya, Somalia, Seychelles, Madagascar, Sri Lanka, Japan, Taiwan, Australia, Hawaii, and the eastern coast of South Africa.

[7] In India, L. orbicularis was also found to associate with echinoderms in a major coral reef community, including individuals of the Classes Crinoidea, Holothuroidea, and Echinoidea.

[5] The first collection of L. orbicularis, and a member of Portunidae in general, reported in Taiwan was in the 90s[4] andThelenota ananas was the first host of Harlequin crabs recorded near a Malaysian island in the north east of the Celebes Sea in 2012.