Friend or Foe

Part 8 Friend or Foe The Part 2 – True 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 species have a similar life for five years. at the - segmented , "joint- cycle consisting of a planktonic larval Ifooted" 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 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: have been mistaken for octopi, all Superorder: Eucarida 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,

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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 crabs. Many species can Sponge crab. The living sponge tips. arrive by accident on specimen conceals the crab well by breaking pieces of . 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 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

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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. This can result in some recommend removal of this undoubtedly pruning prize specimens of coral although minor damage to the coral but as there will beautiful creature. they follow the same rules as for all crabs – not be more than perhaps one or two protein meals are the best! individuals present in an aquarium their An interesting characteristic of many effects will be well spread out. Being such dromiid species is the fact they eggs hatch a specialised feeder its removal from its into young crabs rather than having the host will result in its starvation and death. planktonic stages typical of most . brachyurans. I am yet to experience a Menippidae -Red Eyed gravid female being imported but I could get overrun with them! Crabs and their relatives Eriphia spp. and relatives Pilumnidae Teddy Bear Crab Pilumnus spp.

Xenia shrimp. Apparently harmless. A villain of the peace – the red eyed crab Since writing the previous article in the series I have found a new species of shrimp that appears to live commensally with Xenia sp. soft coral. This shrimp The teddy bear crab – hated by some and never measures just under 11mm and appeared to even noticed by others. be completely harmless. It did not effect the Xenia in the short to medium term and seemed to forage amongst the polyps for The teddy bear crab is almost unmistakable food. as the entire body is covered in hair-like Other Xanthid crabs are imported projections. Some aquarists will never spot infrequently and most are potentially the entire animal perhaps only getting harmful. Remove them whenever possible. fleeting glimpses of a black pincered claw Eriphia sp. A different species from poking out from behind a rock when the the one above – but equally Family Dromiidae lights are out. This species can conceal destructive. itself in some apparently tiny holes that you would think it impossible There are a number of different species that for them to squeeze into. Offering are collectively called "red eye crabs" advice on teddy bear crabs is not obviously sharing the same characteristic straightforward- it really depends on of vivid red coloured eyes. Some are the personality of the individual in members of the family Menippidae but question. Certain specimens have others are more likely to be Xanthids . been known to cause great damage Individuals from either family are powerful in reef aquaria – consuming corals looking crabs often imported with live rock and attacking most animals present. and coral specimens. To the untrained eye Some have never been a problem they may be mistaken for the previously despite attaining a large size. The discussed Mithrax crabs but can be reason for this seems to be related to distinguished from the latter by having their diet prior to import. Removal pointed ends to the pincers, red eyes and must be recommended however, the often a lumpy looking- with the raised only problem is how? Trapping (see areas forming a symmetrical pattern later) can work well for certain resembling a "six pack" rather than Dromiid crab – unidentified species without individuals but the only sure way to rounded nodules. The eyes (which may be present. get them is to remove the rock in reddish in Mithrax spp. are more prominent which they hide and wait until they in Eriphia spp. and have a more stalked Dromiids are another brachyuran species vacate it. appearance. They are predators and will often referred to as sponge crabs. Typically Do not mistake the relatively harmless attack any animal fish or invertebrate that they have rather short yet powerful looking Cymo spp. hairy coral crabs for the teddy- they have a chance of subduing. They pincers and a rounded carapace. Dromiids bear crab. These have much shorted hairs should be removed without exception

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although placed in a small aquarium or As previously mentioned trapeze crabs necessary to locate where it is hiding but as sump they make interesting specimens. have well developed eyes located at the in all wars an hour ’s reconnaissance is front corner of the carapace. They can be worth a day in battle so try and establish Family Portunidae easily distinguished from swimming crabs where the crab is hiding before you get wet. Swimming Crabs which share the same trait by the hind legs. I know of several instances where every As we have seen the hind legs of swimming rock in an aquarium has been removed and crabs are paddle shaped whereas they are inspected an the target crab has not been pointed in trapeze crabs – an adaptation located! Remove the rock to another shared by the vast majority of brachyuran aquarium or take it to a sympathetic dealer crabs-for grasping. Of course, it is unlikely who may have the means to remove it and that you will find a trapeze crab out of the return the rock to you. Otherwise, get your context of a small polyp stony coral but this chisel out and chip around the rock until the does happen, particularly when a new coral crab falls out. Please note that I do not specimen is introduced and the trapeze crab condone the killing of any of the unwanted abandons ship. You should be able to animals found in reef aquaria. I appreciate reunite the crab with its home if this occurs. that in some case deaths are inevitable as specific dietary requirements of certain Catching Crabs(!) coral-eating animals will mean that they Unidentified swimming crab – note will die if removed. In this case it’s the Ahem, not as easy as it sounds this. It will the paddle-like hind legs and well coral or the predator so I have no problem developed eyes. never cease to amaze me just how small a with it. However, if an animal can be hole your average crab can run into when sustained elsewhere then it should be given alarmed. This is doubly true of teddy bear Anyone who has been rock-pooling along the chance. After all, it didn’t ask to be put crabs – they must need some grease and a the Western coast of the U.K. is likely to in your aquarium – it was minding its own shoehorn to get into some of the nooks and encounter the native member of the business on a reef before you got hold of it! crannies we find them in. The would-be Portunidae. Most British crab species will Seriously though, put unwanted crabs etc in crab hunter needs to use persuasion and either stay stock still when disturbed or try sumps or set up an aquarium for them. A guile to obtain his or her quarry. I find the to make a dash for freedom. The swimming simple one will usually be more than best technique is to use a tall glass tumbler crab rears up on its back legs with its adequate. Otherwise, see if your dealer will baited with a fish- based food such as menacing pincers outstretched and dares take it off your hands, you won’t get lancefish or silversides. Place the glass in you to get closer! Such feisty beasts are not anything for it but it will serve as an an upright position preferably between two creatures for the reef aquarium although educational tool for other aquarists. of rocks. It is very important that any they would make excellent specimens for This ends our look at the Brachyuran ornamental shrimp that you have are kept the specialist aquarium. Portunids can be decapods or crabs as we like to call them. occupied with meals of their own – easily identified by their paddle shaped In the next article I hope to tie up some something that will last them a while and hind legs used for locomotion in water. The loose ends from the hopefully cause them to have little or no eyes are well developed and, like Trapeze assemblage- Isopods, Amphipods and their interest in the baited trap. The problem here crabs (see later) they are located at the front relatives. is that we are working on the principle that corners of the carapace thus affording crabs either cannot swim (true for the vast excellent binocular vision. The specimen in majority) or that they cannot swim Bibliography and the photograph was only small – about the vertically ( usually true of Portunid crabs). size of a fifty pence piece but could still suggested further reading This does not count for shrimp. Their tails have presented a problem to small gobies • Barrett and Yonge. 1985. Guide to the are not curled under the body – they are and shrimp. The larger the specimen, the Seashore. Collins. lined with rows of specialised limbs which greater the range of potential prey items • Debelius 1999. Crustacea Guide of the propel the shrimp forward. This means that within an aquarium. World. Ikan Publishing your bait can be nabbed and scoffed by the • Debelius and Baensch 1994. Marine opportunistic shrimp before the target crab Family Trapezidae Atlas Vol I. Mergus Publishing. has even left its lair. Choosing a smooth Common Name: Trapeze crabs • Fossa & Neilson. 2000. The Modern sided glass will mean that in the majority of Coral Reef Aquarium. Vol 3. Birgit cases the crab will not be able to gain Most reef-keepers with small polyp stony Schmettkamp Verlag. enough purchase to climb out. This corals (SPS corals) such as Acropora spp. • J.G.Walls (Ed.). 1982. Encyclopedia of technique works very well for most crabs. I will be aware of these little crabs. Often Marine Invertebrates. T.F.H. should say that the best results with this found in pairs they reside within the Publications technique are achieved when your aquarium branching arms of the coral and were once • Ruppert and Barnes. 1994. is in total darkness. thought to be consuming the coral itself. Invertebrate Zoology Sixth Edition. Unfortunately, this removal method is Observations of aquarium specimens leads Saunders College Publishing not 100% effective. There is always a experts to believe that the trapeze crabs are • Sprung. 2001. Invertebrates – A quick hardcore of resistance to even the most actually beneficial to their hosts by reference guide. Ricordea publishing. subtle removal technique. It may be a removing unshed mucus, protecting the coincidence but one of the trickiest coral from predators and the removal of specimens to try and trap is the benign Acknowledgements detritus from its surface. An indication of teddy bear crab. This is the individual of Chris Higinbotham (TMC Manchester) how much beliefs in this hobby of ours can the species which doesn’t seem to be a pest for specimen acquisition. change is the fact that nowadays many in the aquarium but the aquarist would like Lisa Birchall – Illustrations aquarists will not purchase pieces of to remove just in case it becomes one. In Anyone who has ever brought me a crab Acropora if the crabs are not present! these situations a search of the aquarium is to identify.

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