A Spineless Column by Ronald L. Shimek, Ph.D.



Crabs are a group of animals both familiar and enigmatic to many hobbyists. Virtually everybody knows what a crab looks like, but at the same time, that definition of a crab seems to fall apart under scrutiny, as all sorts of widely different animals are described as crabs. This ambiguity of description is not surprising. One of my former professors, the late Dr. Paul Illg, a noted authority on crustaceans, once said, "The crab habitus (or body form) is commonly found among many groups of crustaceans, and it can be very difficult to distinguish them."

There are horseshoe crabs, Alaskan king crabs, hermit crabs, and mole crabs, none of which are found within the group of animals that a biologist will refer to as crabs or brachyurans. Sometimes called, "the true crabs," to distinguish them from all the "pretenders," these animals are amongst the most highly evolved or derived forms of crustaceans. By that, I mean that they are linked to the ancestral crustacean by a long evolutionary history, and are very different from that ancestor. In this regard they are similar to humans, birds, and other highly derived vertebrates, all of which are very dissimilar to the wormlike animals that were our distant progenitors.

Although they share many characteristics with other crustaceans, the crabs really are a group of amazing animals, all with similar body architectures. As in the old saying, "The clothes make the man," the crab shell and form make the crab, and it is worth some time and effort to learn a bit about this morphology.

Shells, Shapes, and Skeletons

Everybody seems to know that the arthropods, those animals with an exoskeleton and jointed legs, are the most diverse of all animals. Because of this, most folks tend to think that there are somewhere on the order of a gazillion species of marine arthropods. This really isn't the case. Most arthropods are insects, and the number of species of insects is truly awesome, for example there are over 600,000 scientifically described species of beetles. Even more awesome is that there is probably at least twice that number remaining to be discovered and described. And beetles are only one type of insect. To put that number in perspective, there are probably around 10,000 species of birds, and maybe 4,000 species of mammals. The total number of insect species is almost beyond imagining. However, the rest of the arthropodan groups are nowhere near as diverse as are the insects.

The levels of diversity are much lower among marine arthropods. Most marine arthropods are crustaceans, only a few species of insects are actually found living wholly in marine environments. The total number of marine crustaceans is estimated to be on the order of 30,000 to 40,000 species. This is a lot of species to be sure, but it is not beyond the diversity found in other groups. Several other animal groups, such as the fishes, mollusks, nematodes, and annelid worms, may contain as many or more species than the crustaceans.

Crustaceans are typically small animals; most of them are animals such as copepods, cladocerans (water fleas) or mysids. These animals may reach astounding population sizes in the mid-oceanic regions, but they are typically miniscule and seldom seen by the casual observer. It takes both specialized collecting gear, coupled with microscopic examination of the collected materials, to really observe and appreciate these animals. The crustaceans that people typically see are the larger ones, such as the crabs, shrimps and hermit crabs.

Interestingly enough, virtually all of the larger crustaceans belong to the same major taxonomic group. This group, called the "Class Malacostraca," contains the crabs, shrimps, krill, amphipods, mysids, isopods, and a number of smaller groups, most of which contain only small animals. Almost all large crustaceans are crabs and shrimps.

The basic body form of all of the larger marine crustaceans is ultimately similar to, or derived from, a shape that looks like a shrimp.  This is a bilaterally symmetrical animal, composed of segments (or delineated body regions), and all appendages and structures are found in pairs on both sides of the animal.  This shrimp-like form has a series of characteristics.

  • The head and thorax are covered by a common “shell,” or covering termed the “carapace.”

  • The front end of the carapace has “the rostrum,” a spike-like projection from the front end.

  • The eyes are on stalks and there are two sets of sensory appendages termed antennae.

  • Each of the first pair of antennae has two branches.

  • Each of the second pair of antennae has one long branch and a smaller scale-like branch at the base.

  • The primitive forms have eight thoracic segments.

  • Each thoracic segment bears an appendage that may be used for walking.
  • Each thoracic appendage may bear a gill found up under and protected by the carapace.

  • The abdomen projects beyond the carapace and has six segments.

  • The first five abdominal segments have flap-like swimming appendages found underneath it.

  • The last abdominal segment bears a pair of lateral flaps and is typically flattened.  Together with the lateral flaps, it is often called the tail fan.

This set of characteristics, sometimes called the “caridoid facies,” really describes a shrimp.

And it doesn't look much like a crab.

Figure 1. A hypothetical animal showing the series of characteristics known as the "caridoid facies." The carapce is an extension of the back exoskeleton down over the sides of the animal enclosing the head, thorax, and the bases of the thoracic appendages. In crabs and shrimps, only the fourth through eighth pairs of thoracic legs are used for walking or prey manipulation. Thoracic leg four is the leg that forms the big claw of crabs. Modified from Meglitsch, 1972.

Arthropods with this group of characteristics are those within the crustacean Class Malacostraca, but this class is diverse containing many subgroups. The largest of those subgroups is one known as the Eucarida, or "true shrimps," which contains the shrimps, the crabs and a few other groups. There have apparently been several adaptive radiations from ancestral forms in the eucarids, and there are a lot of different types of them. Some, such as the many various and diverse array of shrimps and prawns, are specialized as swimming animals. Others such as the crabs, or brachyurans, are specialized for walking.

Crabs have numerous modifications of the basic caridoid facies described above.

  • Crabs are flattened from top to bottom. In contrast, shrimps are compressed from side to side.

  • Crabs may lack the rostrum, or anterior projection.

  • Three of the thoracic segments are fused into the head, and their pairs of appendages are called maxillipeds and are modified to handle and process food.

  • The first pair of remaining thoracic appendages ends in large pinching claws.

  • The last four pairs of thoracic appendages are walking legs.

  • The first abdominal segment is small.

  • The abdominal appendages are small and modified, and not used for swimming.
  • The abdomen is curved under the thorax where it fits in a groove on the animal's underside.

In fact, a crab may be thought of as a shrimp, flattened from top to bottom, with its abdomen tucked under its body and with a pair of large claws on the first functional thoracic appendages. Interestingly enough, the evolutionary change from a shrimp-like ancestor to a present day crab is reflected in the larval history of the crabs. Crabs generally hatch from the egg as larvae that look like a small shrimps. These larvae grow and metamorphose into larvae called megalopa (the singular of this word is "megalops"). A megalops larva looks like a small crab with a shrimp's tail; and it is able to swim. The final metamorphosis from the larval stage results in the megalops folding its tail under itself, and settling to the bottom as a small crab.

click here for full size picture
Figure 2. Dorsal, or top, view of a crab, Erimacrus eisenbeckii, showing the major visible body
parts. The four pairs of walking legs are numbered, the claws are in front of these legs and are
modified walking legs. A few of the body structures are named. Every potential bump, groove, or
body segment has one or more names. This makes discussions of crab morphology very precise,
and very filled with jargon. It also makes it impossible for most non-specialists to identify most
crabs. Click for larger image.

click here for full size picture
Figure 3. Ventral, or bottom, view of a crab, Erimacrus eisenbeckii, showing the major visible
body parts. The abdomen is visible tucked in a groove under the body. This animal was a male,
the abdomen of the female would be broader and more rounded. Click for larger image.

Internal Anatomy

The exterior of the "basic crab" is really a pretty labile and changeable surface. There are many trends and developments throughout the group reflecting various evolutionary pressures. As a result, crabs come in a wide variety of shapes and sizes, but they are derived from the same beginning body form. Internally, crabs are complex and very similar from animal to animal.

click here for full size picture
Figure 4. A side view of the internal anatomy of a typical crab, drawn as if the animal were cut open just to the side of the midline. The location of the claws and walking legs are given. The heart and major blood vessels are shown in blue, the gut is in green, and the nervous system is in red. Specific structures are labeled. Modified from McLaughlin, 1980. Click for larger image.

The mouth opens on the bottom of the animal, behind a series of appendages referred to as mouthparts. All of these appendages are thought to be primarily manipulative and food-handling, although one of them seems also to function to pump water across the gills. There are large mashing jaws located on either side of the mouth and they smash food pretty well, but most of the chewing is done in the stomach. Because of crabs' hard exoskeleton, the mouth can't open very wide, so crabs are not biting carnivores like lions and tigers and bears, oh my… Instead, they are "tearing" predators or scavengers which use their large claws to tear off pieces of the food item. These pieces are passed to the maxillipeds covering the mouth and they rip it into progressively smaller pieces. Eventually, the food item is shoved between the jaws and into the small mouth. Food at this stage is a pulpy mass, but it is not ground really fine.

Figure 5. This is the grinding mill from the inside of a small shore crab's stomach, exposed by dissection. This is view from above, and the front of the animal is to the bottom of the image. The whole crab was about an inch across, and the largest visible teeth are each about 1/32nd of an inch high. Food that is eaten by the crab is ground into a very fine slurry by these teeth and filtered by the bristle-like fans behind the first row of teeth. The inside of the esophagus is the green horizontal structure visible between the tooth rows.

The food passes up a short esophagus into a large anterior stomach section containing an impressive array of teeth and stiff bristles. This is referred to as the "grinding mill" and this is where the crab really chews its food. Food then passes to the posterior part of the stomach where digestion really starts. As food leaves the stomach, it is separated into small masses and is surrounded by a thin membranous covering made of chitin. This membrane prevents the gut contents from actually contacting the gut walls and potentially abrading them. Digestive juices are released into the stomach and the midgut behind the stomach. Digestion occurs in this region and digested materials are moved into the digestive glands for absorption. There is a long dorsal pouch located off of this region of the gut, called the dorsal caecum. It may secrete some digestive enzymes as well. Food passes to the hindgut where the undigested food is compacted and the final digested materials are absorbed. There is another large pouch, of uncertain function, called the hindgut caecum, located nearby and opening into the hindgut. Small pelletized feces are released from the anus.

The foregut, including the stomach, and the hindgut, are lined with exoskeleton material called cuticle. This material, in fact, comprises the grinding mill. When the animal molts, the stomach lining, including the grinding mill must be pulled out of the mouth and a new version secreted internally. Likewise, the lining of the hindgut must be pulled out of the anus. These complications make molting a hazardous time, and occasionally, animals die from molting problems.

The circulatory system of such animals is large and complex. The heart is located on the top of the animal connected to, and suspended from, the dorsal body wall/exoskeleton. The heart is large, and basically triangular or rectangular in shape. Blood enters the heart from the body cavity surrounding the heart by means of four openings, called ostia, in the heart's upper surface. These ostia have valves to prevent backflow. The heart beats rapidly, forcing blood through a system of vessels to all parts of the body. These vessels are not built like arteries in mammals, and are generally lacking any muscles. Blood flows out of the vessels at their terminal ends, and flows back through the tissues bathing them with blood. Such a system lacks capillaries, and as the blood flow is not enclosed in vessels, is termed an open system. Although such a flow pattern may seem haphazard and inefficient, nothing can be further from the truth. Blood flow passes rapidly through the animal and efficiently distributes food and exchanges gases. The respiratory pigment present in crabs, is a copper-based pigment called hemocyanin that is blue when oxygenated and clear when deoxygenated. It is not as good a respiratory pigment as is hemoglobin, but it still allows the crabs to exist in many areas of low oxygen tension.

Figure 6. The viscera of the same small shore crab, Hemigrapsus nudus, seen in the previous figure. The front of the animal is to the top. The top of the carapace has been removed. The gills on the right side are visible. They are in the branchial chamber outside of the body. Note the blue color of the gills; this is due to the hemocyanin respiratory pigment found in crabs. The heart is dorsal in the crabs, and lies just below the carapace and is labeled. The ostia, through which blood enters the heart, are indicated. The ophthalmic artery transfers blood from the heart to the brain. The grinding mill (seen here unopened and intact) and the stomach muscles that move it are visible to the top of the image.

The nervous system of these animals is decidedly different from that found in vertebrates such as fishes and aquarists. A large series of nerve cell aggregations called ganglia are present in front of the mouth and behind the eyes. This mass of ganglia is sometimes called "the brain," but that probably is not a good choice of words. This structure has some, but not all, of the functions, of the vertebrate brain. These ganglia are protected by skeletal ridges and walls. The main nerves run down the bottom of the animal to another large array of ganglia is located. This cephalothoracic gangalion is about as big as the so-called brain. These ganglia seem to control a lot of the body functions such as walking and mating. In a way, crabs seem to have two nervous structures that together function together to act as a "command and control" structure for the animal. They really don't have a single brain as we know it in vertebrates.

Sensory input comes from the large compound eyes, located at the front of the carapace, and from all the hairs and bristles covering the animal. These hairs and bristles are really complex sensory structures that can sense water movement and dissolved chemicals in the water. Crabs quite literally can taste with their whole body surface.

Figure 7. Lateral or side view of the top of a shore crab, at the front of the
carapace, showing the two antennae, and the compound eye from the right
side of the animal. These structures are sensory, but so are all the hairs
found all over the animal. Note the hairs around the base of the eyestalk.

Waste disposal removes the nitrogenous wastes from the animal through the action of an anterior gland that opens by a pore in the base of the antennae. This structure is referred to as the kidney or antennal gland. The fluid it releases is a concentrated urine.

Reproduction in crabs is a complicated process. Reproduction and spawning generally occur in conjunction with molting. The female molts first and then copulates. The males molt after copulation, and guard the female through her molt. Eggs are deposited and held between the abdomen and cephalothorax. The eggs hatch several weeks to several months after deposition and release a type of larva referred to as a zoea. Zoea live in the plankton for relatively long periods, weeks or months, molting several times. They finally molt into the megalops stage which is the stage found just prior to metamorphosis. This stage looks like a small swimming crab with a shrimp-like tail. The megalops chooses where to settle out of the plankton and lands there. Shortly thereafter, the megalops molts turning into a small crab.

Figure 8. A zoea or early larval crab collected from oceanic plankton.

Figure 9. A megalops photographed swimming in the plankton. This animal was about
one fourth of an inch long. Note the crab-like appearance of the body, including
the claws, but also notice the shrimp-like abdomen.

Growth in crabs is a complicated process. With the rigid exoskeleton found on the outside of the body, the only way the animal can grow is to shed this skeleton and grow a new one. However, you can probably imagine some of the problems with this, as the animal must avoid predators and still be able to move during this time. Molting is really a cyclic process that continues through the life of the animal. It may take 30 to 50 molts to go from a newly deposited egg to a sexually mature adult. If you consider the function of the crab's exoskeleton, you will realize that the musculature for all the body parts must attach to it. During molting, all the muscles have to detach from the molted skeleton and reattach to the newly secreted one underneath it. Additionally, the linings of both the fore- and hindgut, including the gastric grinding mill, are all exoskeleton and have to be shed along with outer layers. I have described molting elsewhere, and that is a place to go for details of the process, however, aquarists need to know that molting is normal, and it is hazardous for the crab. If you have a crab that is doing well, it may molt as frequently as every few weeks or as rarely as once a year, depending upon the species. During the molt, it will seek out a hiding place in which to perform the molt. Afterward, you may find what appears to be a crab's body, but if you remove this from your aquarium, you will find it is hinged along the front margin and opens from the rear. The crab literally backs out of the old skeleton and leaves it behind. It can either grow or shrink during a molt, generally by about a maximum of about ten percent either way. It takes about a day or two for the new skeleton to harden up and it will be ready to face the world again.

Identification of Crabs Likely To Be Found In Reef Aquaria

The identification of true crabs (excluding other crabs, such as the porcelain crab pictured on the cover of this issue of Reefkeeping) is relatively difficult. Crabs can and do change shape as they molt, and juveniles may look quite unlike adults. Additionally, distinguishing characteristics often are such small, but taxonomically significant, details as the distance between specific sets of spines on the carapace, or the relative proportional length of appendage segments. The moral of the above statement is that if you want a specific name for any given crab, the odds are that you will not be able to do it with any certainty.

However, it is generally possible for an amateur to identify the true crabs to a major group and, as with most well-defined taxonomic groups of animals, knowing the group tells you something about the animal in it. To begin with, verify that the animal has one pair of large claws, and four pairs of large, evident, walking legs. Porcelain crabs, hermit crabs, and their kin, only have three pairs of walking legs and that characteristic would place them in a different group than the true crabs or Brachyurans. This month's ReefSlides shows some of the types of animals that may be called "crabs." About half of the images in the ReefSlides are of Porcelain crabs, which are not "true" crabs, and thus not discussed in this column. See if you can find the differences between porcelain crabs and "true" crabs."

The following series of illustrations gives the characteristics of the majority of true crabs likely to find their way into aquaria as hitchhikers or as purchases. The differences in the body shapes are indicated by the differences in the geometric figures.

Figure 10. Xanthid crabs are possibly the most commonly found hitchhiking crabs. The oval indicates the basic body shape, but it is only a guide. The edges of the oval will be often covered with short thick spines. The major characteristic of xanthids is the presence of claws that are large and black tipped. Xanthids are very destructive animals in any enclosed environment.

Additionally, some xanthids have also been shown to be quite toxic if eaten. So… Don't take your little crabs out and use them as a snack! Many of the small, and probably harmless, symbiotic crabs inhabiting branching corals are trapeziid crabs, a subgroup of xanthids.

Figure 11. Majid, or spider crabs, have a pentagonal or sometimes triangular carapace and typically have long spider-like legs.

Decorator crabs are majids. While these animals are not as destructive of aquarium life as the typical xanthids, they can devastate an aquarium. I once had a decorator crab that persisted in tearing apart soft corals to decorate his body. This living decoration is often transitory as it dies, and has to be released. The particular crab I had destroyed several colonies of soft corals over the course of a couple of weeks.

Figure 12. Grapsids, including the true Sally Lightfoot Crabs, are uncommon in aquaria except for the Percnon species misnamed in the aquarium trade as Sally Lightfoot crabs, but others do show up on live rock from time to time. They are characterized by a basically trapezoidal or squarish body, often with angled shoulders. They may be very fast and difficult to catch, and they are often quite destructive of mobile animal life in aquaria. They are quite capable of catching and eating fishes, for example.

Figure 13. The Dromiid, Calappid and Portunid crabs are only occasionally encountered in aquaria as hitchhikers. They are all rather easily distinguished based on the criteria I have indicated.

Aquarium Maintenance

All crabs may be rather easily kept in reef aquaria, but they are generally not "reef-aquarium" safe. They are generally hardy, opportunistic animals that adapt well to aquarium life. Their major drawback is that, with few exceptions, they are not specific predators on any one type of animal or alga. Instead they seem to be omnivorous, eating just about anything that strikes their fancy. Most of them have a predisposition to flesh, and they will attack snails, shrimps, worms and other mobile animals more-or-less indiscriminately. Additionally, some species are common predators on corals.

Coral crabs, such as the various species that are often found nestled among the branches of some corals are often considered to be commensal, causing no lasting damage to their host other than stealing an occasional meal. Nonetheless some of these crabs seem to capable of destroying and eating coral polyps, perhaps under conditions of starvation. With these little crabs, it is probably best to decide on a case-by-case basis whether or not you wish to keep them in your system.

Most hitchhiker crabs probably should be humanely disposed of, or relegated to a tank where they may be kept without damaging other desirable animals. Under such situations many crabs make delightful pets. Their behavior and their color patterns are truly unique and interesting to observe. They are natural reef animals, but they are not necessarily good animals for a reef aquarium.

If you have any questions about this article, please visit my author forum on Reef Central.

Suggested Readings and Cited References:

Bliss, D. E. (Ed.): 1982-1985. Biology of the Crustacea. 10 volumes. Academic Press, New York.

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.

Meglitsch, P. A. 1972. Invertebrate Zoology. Oxford University Press. London. 834 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.

Schram, F. R. 1986. Crustacea. Oxford University Press. New York. 700 pp.

Reefkeeping Magazine™ Reef Central, LLC-Copyright © 2008

Crabs - by Ronald L. Shimek, Ph.D. - Reefkeeping.com