Our of the animals purchased for their use as biological control in reef aquariums, Mithraculus sculptus is one that has caused a great deal of worry to aquarists. This species is strongly algivorous, and is often purchased for the purpose of algal control. Subsequently the crab gets blamed for damaging corals, and even the deaths of fishes are attributed to the crab. Much confusion exists as to the riskiness versus 'reef safety' of this controversial species. To aquarists this species is known as the emerald crab or simply 'the' mithrax crab. Historically there was much debate about the distinction, or otherwise, of Mithraculus from its relative Mithrax.
Mithraculus is extremely similar to Mithrax, so close overall that observations about the general diet of one genus, also apply to the other. M. sculpta is also known as the green clinging crab in some literature Whatever we call it, this is a crab of the Caribbean and adjacent eastern Atlantic, from Florida and the Bahamas south to Brazil. It grows to about 4 centimeters, or about 1 and a 1/2 inches long.
Although ecological studies tend to stress the herbivory of Mithrax senso lato, both field and laboratory studies of these crabs in the 1970s, have demonstrated their omnivorous feedings, on macroalgae, cnidarians, and detritus. In an experiment, the captive Mithrax eagerly consumed pieces of fish and clam, as well as such marine animals as polychaetes, gastropods, and echinoderms. Mithrax consume Aiptasia anemones, so they are able to consume rather large polyps. The extent of their predation on fleshy polyps and colonies is not known.
Very motile animals such as fish, shrimp, and crabs, were consumed only by scavenging. Mithrax are clumsy foragers, certainly not hunters, and they do not detect prey well, merely consuming animal protein opportunistically. M. sculpta and similar crabs really are primarily herbivorous, although they preferentially take animal derived foodstuffs where they are available. In other words, they have evolved to exploit animal protein if it is encountered whilst feeding on algae. Mithrax crabs are able to extract live snails from their shells, but appear unable to crack hard shelled prey open.
In shallow seagrass beds, and the margins of reef flats, M. sculpta uses the branching, coralline macroalga Neogoniolithon strictum as a host, and the alga benefits from the effectiveness of M. sculpta at controlling epibionts. M. sculpta is also a facultative symbionts of Porites stony corals. The residence of M. sculpta among P. porites, reduces intrusive algal cover on the corals by more than 85%.
Such a statistic is proof of the efficiency of these craps. By day, these crabs shelter in crevices and beneath ledges. Although they are nocturnal, their algal diet restricts them to areas of sunshine. They avoid areas that are shaded during the day, which would impair the growth of their algal food. The claws of M. sculpta are spatulate, an evolutionary tendency among crabs deviating towards herbivorous lifestyles.
In the absence of appropriate algal growth, emerald crabs will consume such foods as nori. They will obviously accept defrosted meaty foods, and some dry preparations. But their natural diet, which is mostly herbivorous, ought to be replicated. Good reason exists to presume M. sculpta is a risk to slow, motile animals like gastropods and starfishes in the aquarium, as well as to large fleshy polyps. But they do not hunt fish, shrimp, or other crabs.
Aquarists should note there is some overlap between the macroalgal genera eaten by emerald crabs, and those purchased deliberately, for ornamental reasons. These crabs are also nocturnal by nature, and many aquarists never see theirs by day, sometimes fearing their pet crab is dead, only to discover the same animal alive, some period later. The species requires places to reside in the rockwork, where it will avoid bright light.
What is reef safety, anyway? A reef is a living ecosystem, and even those organisms that do not consume other organisms directly, are competing for sunlight or space. The idea of 'reef safety' implies a simplification of how organisms in an environment really interact, and is semantically meaningless. It is like saying an animal is 'forest safe' or 'prairie safe'. No one talks about any other biome, the way 'reefers' do about coral reefs.