Friend or Foe Part 8 Friend or Foe The Crustaceans Part 2 – True Crabs DIGITAL IMAGES BY THE AUTHOR TRISTAN LOUGHER B.Sc. graduated from Manchester University in 1992 with a degree in Zoology. He has worked at CheshireWaterlife n the last article of this series we looked Most crab species have a similar life for five years. at the Arthropods- segmented , "joint- cycle consisting of a planktonic larval Ifooted" animals that include insects, development. Figure one shows how the crustaceans and their kin. We centred on the body form of the crab changes from the assemblage within the crustaceans that is first zooeal stage up to the adult form. Prior some which can be harmful depending on know as the Order Decapoda and which to the zooeal stages the life cycle resembles circumstance, and some which are arguably includes some of the most familiar that of the shrimp depicted in the previous beneficial. If this article does not cover a crustaceans – shrimps, crabs and lobsters article in this series. We generally only ever particular species that you have and almost 25% of all known crustaceans. experience the adult stage although this will encountered then it is up to you as the This article will focus on a further sub- undergo further moulting as it grows to responsible aquarist to observe the division of the Decapoda called the maximum size. specimen to determine whether it should be Brachyura-a group containing the most removed or not. I am perfectly familiar crustaceans of all – the true crabs happy to house unwanted crabs as which is represented by over 4500 most will do well in aquarium described species. situations and sumps can be decent True crabs can be distinguished from places to house any rogues that you other decapod crustaceans , including may wish to keep away from the porcelain crabs and squat lobsters, by the aquarium proper. fact that their tails are not used in True crabs are widely locomotion instead the tail is curved encountered by marine aquarists underneath the body. Sexes are separate in because many species are extremely these animals and the tail can be used as a hardy. It is not uncommon to open a means to ascertain the sex of an individual: box of live rock and find several Female crabs incubate their eggs beneath live species present. In the days of the tail. You may have noticed this on some Florida live rock I can remember of the crabs in your aquarium as the egg opening one box and twenty plus mass is often brightly coloured- boxer crabs individuals of one species went (Lybia spp.) often have bright orange egg Life cycle stages of a typical true crab. There are four scuttling across the floor! If this masses present when imported. Sexes can zooeal stages in this situation ( each separate larval hardiness is shared by most species be differentiated as the tail of the female is stage is called a zooea ), a "big eyed" or Megalops of true crab (and is suspect it is) much broader and more rounded than the stage which resembles a tailed adult and finally the then it is no real surprise that we can tail of a male in any given species. adult stage has the tail tucked beneath the body encounter so many species in a reef characteristic of all brachyurans. situation. I will cover the removal of problem species later in the article I have long since stopped being unless a specimen requires individual surprised by comments made by attention. non-aquarists as they inspect the contents of the aquaria here at Sub-Phylum : Crustacea Cheshire WaterLife. Brittle stars Class: Malacostraca have been mistaken for octopi, all Superorder: Eucarida corals are plants and so on. But Order: Decapoda almost everybody from the age of Infraorde: Brachyura two, and sometimes much younger, Family: Majidae can identify a crab as a crab. If all Genu : Mithrax/ Mithraculus? crabs were benign this would make their location and treatment in I am basing the name of this crab species aquarium situations extremely easy on comparisons with the emerald or gorilla but there are several species Underside of a teddybear crab. The broad tail shows crab Mithrax sculptus. Emerald crabs are regularly encountered by marine found throughout the Caribbean region in that this is a female. aquarists that are definitely harmful, MARINE WORLD – DECEMBER/JANUARY 2003 • 19 Friend or Foe crabs where I find them as I have never observed them doing any damage to the aquarium inhabitants. If you have different experiences with them then remove them but until this happens I would recommend giving them the benefit of the doubt. Many other representatives from the family Majidae are imported either by design or accident. They include the well known and popular pruner of corals the Decorator crab plus a wide Mithrax sp. Note the rounded nodules on the variety of other species including carapace but particularly those spoon shaped pincer sponge crabs. Many species can Sponge crab. The living sponge tips. arrive by accident on specimen conceals the crab well by breaking pieces of coral. Their adaptations for up its outline. shallow water where they feeds on algae camouflaging themselves may make some which is scraped from rocks using the blunt members of the Majidae difficult to spot Xanthid crabs ended, almost ladle-shaped, claws. It is this but their removal is preferred as they can be last feature, shared by the unidentified, yet quite destructive as they prune corals and Xenia crab extremely common, crab in the photo. other invertebrates to enhance their Whether all individuals bearing these disguise. pincers are one species or represent a group of closely related species requires further investigation but the characteristics of all individuals I have encountered are the same. They have bristly or hairy legs to some extent – colour of the body varies from cream coloured through light brown to dark brown to black. Some appear uniformly coloured whereas others are mottled. They all without exception have The white colouration of the Xenia these blunt ended pincers which look as if crab can make it difficult to spot but they have been scooped out. This shape the observant aquarist should be able to locate one if present. appears to be ideal for the removal of algal films from the surface of rocks but I also seen them employed in breaking off small chunks of coralline algae or even some of Xanthid crabs superficially resemble the tough wiry forms of macroalgae swimming crabs (see later) but note typically found on Caribbean live rock. that the hind legs are of the clinging Emerald crabs are cited in the type in Xanthids not paddle shaped. literature as being capable of removing Over recent years there is one species of crab which has become Valonia spp algae but this seems to depend Camposcia retusa -Decorator crab with decoration. much more commonly encountered very much on the individual concerned and A fascinating species that causes damage to corals how much other food is available to the by pruning them. A good natural propagator? than in the past. This is due to the crab. This species grows to around 3-4 increased amount of pulsing Xenia inches across the carapace which in the spp. imported for the aquarium context of most reef aquaria is a large beast. trade. The white pearlescent Xenia I have not had experience of such beasts crab is often found ensconced in but reports state that they are unproblematic even surprisingly small colonies of at this size. Certainly small Mithrax species Xenia soft coral. The nature of the seem to behave impeccably. beast is difficult to ascertain. Early It must be stated here that there are encounters suggested that the crab very few species of crab which will turn was a harmless commensal species down a free protein meal. Even if a crab perhaps living off the mucus of the spends much of its life eating algae, the coral. Due to the increased volume arrival of a large lump or protein in the of encounters with this animal a form of e.g. a fish – dead or dying is likely different story seems to be to be a windfall rarely rejected. The fish is emerging. That the pulse coral is unlikely to have fallen prey to a Mithrax bothered by the presence of the crab crab because they do not seem to be is certain. I have witnessed small aggressive, but several specimens have colonies shrink in size due to its been blamed for reaping the harvest that a presence. It could be that larger A closely related species – this one covers itself dead fish represents. I always leave these colonies are not bothered unduly by with algae to remain concealed. Possibly Huenia sp. its behaviour as the disturbance will 20 • MARINE WORLD – DECEMBER/JANUARY 2003 Friend or Foe be shared by most of the colony rather than will shape a piece of sponge or other and a more rounded carapace than a small area as in the case of smaller encrusting animals such as tunicates or soft Pilumnus spp. crabs .These depend on their pieces. However, there are reliable reports corals and then hold it over the body with coral hosts for their survival. As they move of the crab consuming the Xenia itself the small hind legs – an identification about corals such as Acropora the hairs often after the colony has become damaged feature of this family. They are unlikely to collect mucus which is harvested by the in some way. For this reason I must be harmful in a reef aquarium other than crab for food.