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Aquarium.Net March 97

Ron Shimel writes on crabs and the aquarium. Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

So... Now You've Found Out You've Gotten The Crabs...

By Ronald L. Shimek

A Caribbean arrow crab, Stenorhynchus seticornis.

and sometimes they even get them by choice. But what are "crabs?"

My old invertebrate zoology professor used to say, "The crab habitus was a conservative and common form in crustaceans." What he meant by this was that a lot of different and not terribly closely-related groups all had some species that were called, or looked like, crabs. So, what is a crab? Generally, a crab is a relatively large crustacean with a more-or-less squat body form and no visible long abdomen. As far as the everyday person is concerned these are sufficient characters to define a "crab." Additionally, crabs generally are thought to be walkers or runners, but consider that many swim very well; for example, the common blue crab of the east coast, Callinectes sapidus , is a very active swimmer.

For a good idea of some crustacean and crab diversity follow this link:

For the hair-splitting invertebrate watcher, or the discerning aquarist, however, there is a lot more to crabs than this. The diversity of forms hiding under the name of "crab" IS rather high. This diversity of form means an equivalent diversity of life style. There is an old ecological dictum that states that no two species with exactly similar habits can coexist, and it follows that if there is a diversity of form, there is an equivalent diversity of natural history and biological interactions.

Certainly, however, there are some basic commonalities that all crabs share. All true crabs are crustaceans and members, therefore, of the phylum Arthropoda which is characterized jointed legs and the presence of an external skeleton or shell. The crustaceans are primarily marine or aquatic arthropods differing from the other major arthropodan groups, the uniramians (insects, centipedes, and millipedes) and chelicerates (scorpions, spiders and mites) in a number of significant ways. Individuals of one group of chelicerates go by the name of "crabs." These are the "horseshoe crabs" and they are not crustaceans and will be discussed below. Horseshoe crabs are truly fascinating and wonderful animals, but are more like large sea-going stingless scorpions than true crabs.

All arthropods share a hardened exterior surface. Commonly called the "shell," the shell is really a complex structure consisting of a multi-layered, secreted cuticle and the underlying epidermis. This integument is a dynamic structure continually being altered and modified. The crustacean integument is often hardened with calcium and other mineral salts

Mithrax spinosissimus , a large Caribbean reef crab. This individual was about 50 cm across the leg span.

(some predatory planktonic crustaceans have opal-tipped jaws - "All the better to crush you with, grandma.") Insects and most chelicerates do not have calcium in their integumental layers.

Other differences between insects and crustaceas are found as well. While the insects or spiders have limbs without branches, the basic crustacean appendage has two branches coming from the single basal limb segment. There is a lower (or inner) branch which is often used for walking in the crabs, and an outer (or upper) branch which, in the crabs, is generally bent upward under the carapace or armor covering the top of the animals. This upper branch is generally used as a gill.

All crustaceans have jaws, but unlike our jaws which bite from top to bottom, the crustacean jaws bite from side to side, and are really modified swimming appendages. Chelicerates, such as the horseshoe crabs or spiders, do not have jaws and must eat either very soft food or liquids.

The whole of crustacean - and hence, crab - biology must revolve around the outer integument or shell. This structure is not like the shell of a mollusk. Molluscan shells are basically calcareous deposits laid down over proteinaceous framework. This makes a very strong rigid material, not unlike re-enforced concrete. However, once laid down, the mollusk is done with it, unless it needs to be repaired due to injury. The other fundamental difference between the two types of shell is that the mollusk shell only covers the outer top or dorsal surface of the animal while the crustacean integument covers the whole outer surface of the animal.

The reader might reflect that the lining of the gut is actually an outer surface of any animal, and in crustaceans, the outside integumental covering extends over this surface as well as the visible outer surface. This difference between the two shells has a significant result for the animals involved. Mollusks can grow continuously through their lives by constantly secreting more shell material on top. Crustaceans are surrounded by their skeletal integument and can only get bigger by shedding the integument, enlarging themselves and then rebuilding a new integument. This latter process is called molting or ecdysis, and is difficult and fraught with problems; consider, for example, in addition to shedding the outer covering, the lining of the gut has to be pulled out of the mouth and anus. The physical and chemical complexities at molt result in a finite molting mortality at each molt. The percentage of animals that die at each molt can be calculated. In normal natural situations, somewhere between 5 and 15 percent of the animals will not successfully complete any given molt and die. This molting mortality can have a profound effect on the animal's biology.

To give you some idea of the actual numbers involved, I will use an example of the Alaskan King Crab, Paralithodes camtschatica . After hatching, individuals in this species molt about 60 to 65 times to reach sexual maturity. This means if the molting mortality is 5 percent, there need to be about 27 eggs produced to allow one animal to reach sexual maturity; if the molting mortality is 15 percent, then 34,000 eggs are needed; for 20 percent, 1,600,000 eggs are necessary. And this is mortality exclusive of all other types of mortality... Obviously there is a significant reproductive advantage to any king crab that has some behavior or physiological attribute that results in even slightly lower mortality.

When crabs grow by molting, they pass through a process whereby they change tissue mass between the molts, and then at the molts they change size to match the growth in mass. Crabs that are starved often still molt, but they can shrink in size. The process of molting is often used to repair injuries as well. After an injury, such as the loss of limb, the area scabs over (crustacean blood clots REALLY well) and the underlying tissues rebuild an appendage primordium. During the next molt, a rudimentary appendage is formed at the site where one was lost. Remodeling will continue through at least a couple of more molts. Depending on the seriousness of the injury and the size of the animal it can take up to seven or eight molts to completely repair the structure.

All crabs pass through a series of larval stages, as do all crustaceans. However, crabs often have larvae that are significantly different from the other crustaceans that an aquarist might be familiar with. Brine shrimp, Artemia salina , are primitive crustaceans, and hatch out as small swimming larvae with only three pairs of tiny appendages. This larval type, called a "nauplius," is found throughout the crustacea. Many crustaceans have an initial larvae that is almost indistinguishable from the nauplius of the brine shrimp. As these larvae feed and grow, they must molt, and with each molt they must shed their outer integument as well as the lining of their guts. Probably because of mortality during a long planktonic larval period, crabs have undergone evolution from the basic larval series, reducing the number of larval stages.

The early crab larvae are now all passed within the egg shell, and the crabs generally hatch out as a relatively large larvae called a zoea. Zoea larvae look like small shrimp, and some of the crabs they may be as long as a centimeter or so. The zoea can spend from a few weeks to more than a year in the plankton, feeding, growing and molting. With each molt, they add appendages or change their shapes. Eventually, crab zoea change into a larva called a megalops.

Megalops larvae look like small crabs, but their abdomen sticks out behind themselves like that of a shrimp. These larvae swim down to the bottom and look for the "right" place. When they find a habitat that suits them, they undergo a molt from the final larval stage to that of the first juvenile crab stage. Juveniles often look much like the adults, but lack the developed sexual organs and characteristics of the adult animals. From here on out growth is primarily dependent upon food and the vagarities of survival. Depending on the size of the adult crab, sexual maturity can be reached in less than a year, or take as long as decade.

The above discussion relates to just about all crabs. Now it is time to discuss some the differences between the various critters called crabs.

Horseshoes - The crabs that aren't.. .

Along the eastern coasts of North America and Asia are a few species in a group called the horseshoe crabs. These are animals with a head-thorax region that is horseshoe-shaped in outline, hence the common name. Behind this area is rather rectangular abdomen, and a projecting tail spine. These are rather flattened animals with a smooth streamlined front end. Two large compound eyes are found on ridges on the head-thorax area, and smaller single eyes are found on the dorsal midline anteriorly. All the appendages are tucked under the animal and not generally visible from above.

Probably the most familiar of the horseshoe crabs is Limulus polyphemus from the Atlantic coast of North America, but similar species are found from Asia. Small individuals of these tropical species are occasionally seen in the aquarium trade.Horseshoe crabs lack the big pincer-like claws found in most true crabs, and don't have any of the antennae or jaws that crustaceans have. Additionally, other peculiarities of their anatomy indicate that they are actually rather closely related to the arachnids such as spiders or scorpions. These animals are true relics of a bygone period. There are fossils about 250 million years old that are essentially identical to modern Limulus. During the ancient heyday of this group, the horseshoe crabs had some relatives called sea scorpions or Eurypterids. These were scorpion-like predatory arthropods, and some of them apparently reached lengths in excess of 3m, making them the largest arthropods that ever lived. Predation by eurypterids on early fishes has been proposed as the natural selective force resulting in the development of bony armor in primitive fishes. The skull bones of modern vertebrates are the modern descendents of such bony armor, so you may owe the bumps on your head to some ancient sea scorpions.

Horseshoe crabs, are not such dynamic predators. Rather they push their way through sediments collecting small bivalves and worms. These are crushed between the bases of their legs and passed forward to the mouth where they are eaten. A friend of mine, not enamored by soft muds, worms or clams, once said that horseshoe crabs have the perfect defense against mankind: "They can live in garbage and are economically unusable for anything." Howsoever, they do have an economic use. Their blood is blue and contains a copper-based pigment called hemocyanin. This chemical has some pharmacological uses and there is some processors that catch the crabs, drain off some of their blood, and harvest the chemical. The processors claim they are good vampires and don't kill their animals, but I would rather suspect that if the Limulus could talk, they would wish for their equivalent of garlic and crucifixes to keep the processors at bay.

For some illustrations of horseshoe crabs, living and fossil, follow this links and the subsequent links therein:

Crustaceans, the many kinds of crabs that are... .

The crab body form is especially common within one particular crustacean group. The crustacean Class Malacostraca has several different types of crabs found within it. Most of these are found in the Order Decapoda. As the name implies, these animals have ten legs arranged in five pairs. .

This North Pacific hermit crab, Orthopagurus minimus , inhabits the shells of scaphopods which are also known as "tusk shells." The name Orthopagurus means "straight Pagurus " and this hermit is not curved and cannot fit into snail shells.

This would be a good preliminary characteristic to use to discuss crabs, but unfortunately for the logical amongst us, seldom are all ten used for walking

In fact, in many cases all ten legs are simply not visible. In virtually all crabs, the first pair of legs is modified to form large and evident pinchers or claws. We, in the scientific fraternity, like to impress people by calling these claws by the term "chelae," a word derived from the Greek word "chele" meaning..."claws."

So if the first pair of appendages is used as claws, then most crabs have four pairs of walking legs. And FINALLY we have hit a character that we can use. All of the so-called "true crabs" have four pairs of walking legs and a pair of claws up front.

Another characteristic of the crabs that separates them from the rest of the decapod crustaceans, such as shrimps and lobsters, is that the true crabs have their abdomen tucked up underneath their body. Furthermore, the body is often flattened into a more-or-less discoidal or lenticular shape. The main part of the crab's body runs down the middle of this disk, and on either side are cavities covered by the exoskeleton that surround and protect the gills, which branch off the top of the first segment of each of the walking legs.

For more links to crabs and some illustrations of odd types follow these links:

Of course, to paraphrase, there is a hermit crab in the ointment.. ..

Hermit crabs are true decapods, but as befitting the confusing nature of these animals, they don't really look like crabs. A hermit crab is more like a fat shrimp with a hard front end and a soft back end.

The soft back end is protected by inserting it in a snail shell or some other such object. The hermit crabs follow the rule about the front pair of walking appendages being modified as claws, but they only have three additional pairs of walking legs. The most posterior set of walking legs,

A North Pacific hermit crab, Pagurus aleuticus . This individual abandoned his snail shell as I approached. Note the large curved abdomen.

just in front of the abdomen, are used to help hold the shells on and so are not used in walking, and they are not generally visible. So, all hermit crabs have three visible pairs of walking legs.

But everybody (if you're an crustacean) seems to aspire to having the discoidal crab shape, and several lineages related to, and possibly descendent from, hermit crabs have given up on snail shells and become secondarily much like the true crabs mentioned above. The porcelain crabs occasionally found in reef aquaria are in this category, and although they look like normal crabs, the observant aquarist will notice that they have only three pairs of walking legs. The reduced fourth pair of legs is present and visible, as the legs are carried dorsally over the point where the abdomen tucks under the body of the crab. The Alaskan king crabs is also related to hermit crabs and has only three pairs of walking legs.

For some links to hermit crabs and their kin, follow these:

So, now the aquarist has the crabs in a diverse way, with "true crabs," hermit crabs, king crabs, and porcelain crabs. Are there any commonalities of biology that we can use to our advantage in dealing with these animals? Well, of course, there are, but they have to be utilized in moderation.

Crab biology; crushing as a way of life...

Crabs can be basically considered as shrimps adapted to living on the bottom, with their front legs adapted for crushing. The claws of crabs are magnificent modifications of the small pinchers found on the tips of the front legs of shrimps. In the shrimps, these small pinchers are often used to grip the substrate or to pick up fine pieces of food. In the crabs, the delicate pinchers have been replaced by large, rugged structures used to crush or tear prey.

The claws on many crabs are not symmetrical. Generally, there is one claw that is more slender, often with very fine saw-like serrations on the inner surface. These claws appear to be primarily used to cut or tear food. The other claw is often much larger, more heavy-duty, and may have large bumps or tubercles on its inner surface. This claw is used for crushing prey. The crushing capabilities of some crabs are legendary, Coconut crabs can open coconuts, while Floridian stone crabs, Calappa flammea , and Puget Sound rock crabs, Cancer productus , can crush large snails. By the way, claws can also have a sexual function, mostly as secondary sexual devices which are used to signal to mates or rivals such as on fiddler crabs of the genus Uca , where the males have one large claw and one small one.

As many reef aquarists have discovered to their dismay, crabs are primarily predatory. Most crabs are meat eaters and most of those will also eat carrion, consequently they tend to make good scavengers while small. Even small herbivorous crabs, such the blue-legged hermit crabs sold as algal control animals will, and commonly do eat carrion. Occasionally, they will also attack other animals such as corals and remove food that they have eaten. However, this is often a small price to pay for their partial herbivory.

Crabs generally do not eat either large or rapidly moving prey, although there are exceptions (there are stories of the South Pacific Coconut crab, Birgis lato , chasing down, killing, and eating small dogs). Crabs must eat small pieces of food, and they use their claws to pull it into small pieces which are passed internally for further processing. Most crabs have a grinding mill in the gut made of hardened internal cuticle. They use this grinding mill to chew and mash the food that they have all ready eaten. In aquaria, they will tend to attack immobile or sleeping fish, sessile animals such as polyps that have recently fed (where they rip open the body to remove their gut contents), or tube-dwelling worms such as feather dusters. This predatory nature can be useful when the arrow crab eats a bristle worm, but rather less than useful when the same arrow crab eats a fire fish.

The porcelain or anemone crabs often found in aquaria are an exception to the general rule about the predatory nature of crabs. These animals use a pair of modified mouth parts to sweep the water and collect plankton which they eat. In reef tanks being kept as relatively sterile coral gardens, these particular animals generally do not do very well. Where the animals in the aquarium are feed regularly, these suspension-feeding crabs will do well.

Crabs are an interesting component of the marine environment, but are also found in fresh water. Occasionally, one sees fresh water crabs for sale in aquarium stores. These animals will do really well in a well-stocked community aquarium, and they will ensure that the aquarist will have to be continually stocking it as they remove slow moving or quiescent fishes, particularly at night.

In general, while small crabs and hermit crabs will do well in a reef aquarium, the predatory nature of most of the larger crabs precludes the long-term success of most crabs or crab-like crustaceans in such a space and refuge limited environment. The crabs are animals that, pure and simply put, are adapted to catching, cracking, crushing and crunching their prey. Eventually, many of the cute little crabs get will big and then they will wear out their welcome in a aquarium by grabbing and eating a favorite fish, or tearing apart a coral for its recent meal.

As always, if you have questions or comments, please feel free to contact me by email at

References and Suggested Readings.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

McLaughlin, P. A. 1980. The Comparative Morphology of Recent Crustacea. W. H. Freeman and Co. San Francisco. 177 pp.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

Schmitt, W. L. 1971. Crustaceans. University of Michigan Press. Ann Arbor. 204 pp.

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Last modified 2006-11-18 19:07