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
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 primitive forms have eight thoracic
- Each thoracic segment bears an appendage that may be used
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
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,
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
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
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.
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.
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.
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.
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
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 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
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
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
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
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
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
Figure 11. Majid,
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
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,
Figure 13. The Dromiid,
crabs are only occasionally encountered in aquaria as
hitchhikers. They are all rather easily distinguished
based on the criteria I have indicated.
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
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.