Hebivory
As mentioned, Menaethius monoceros is a generalist mesograzer (Cruz-Rivera & Paul 2006), that consumes a large variety of algae (Capper et al. 2006; Cruz-Rivera & Paul 2006) and seaweed.

Prey
Being part of the food chain, Menaethius monoceros also serves as food for organisms on the higher trophic level, such as coral reef fishes. Leray et al. (2012) found body remains of M. monoceros in the stomach of a coral reef fish, Paracirrhites arcatus.

Symbiont
Menaethius monoceros is also called decorator crab because of its decorating behaviour. It usually puts a large variety of materials on its back for camouflage, such as sea weed (Falciai 2002), algae, sponges and hydroids (Henderson 1893; Sankarankutty 1962). On the other hand, Korn and Rybakov (2001) also found that there were barnacles, Sacculina pilosella, attached to the carapace of M. monoceros sample. Commensalism might developed between M. monoceros and the attached organisms. While the attached organisms provide camouflage protection for M. monoceros, movement of M. monoceros may allow the algae and seaweed to absorb more light (fig. 1, top tight) and the food debris from M. monoceros itself may feed the sponges, hydroids and barnacles.

In addition, there were isopods found in some Menaethius monoceros samples, being an internal parasite. These species includes Tylokeponbiturus and Portunicepon tiariniae (An 2009). Isopod, Tylocepon bonnieri, was also found attached to the branchial chamber of M. monoceros (Pillai 1966).