Crustaceans (make up a very large group of the Arthropods which include the crabs, lobsters, crayfish, shrimp, krill, barnacles brine shrimp, copepods, ostracods and mantis shrimp.

As divers, we think of lobsters in a variety of ways. No doubt, some of us begin to salivate when we think of “bugs,” the name hunters commonly use to refer to lobsters. Many photographers consider lobsters wonderful subject matter, and almost any diver enjoys seeing them on a reef.

During the day, it is common for many lobsters to seek cover under ledges and overhangs and in cracks, crevices and caves. Often, when they have taken cover, you can spot lobsters by seeing their long antennae sticking out into the open. At night, many species are more active and they readily leave their hiding places to forage for food. Their food generally consists of dead organic matter and a variety of snail, worms, mollusks, other crustaceans and other animals that they can crush with their powerful mandibles.

Despite their stiff bodies and robot-like appearance, lobsters can move rapidly over short distances by swimming backwards when they so desire. Rapidly tucking their tail provides backward thrust, and when swimming, as opposed to walking across the sea floor, that is the direction they tend to go.

A variety of species of lobsters are known as spiny lobsters. These species lack claws but their exoskeletons are often covered with numerous sharp spines at the base of their antennae and on their shells. These barbs provide some defense against natural predators and divers. In addition, their heavily armored antennae can inflict a tearing wound when they thrash them about in a whip-like fashion.

Spiny lobsters are social animals and often gather in groups of a dozen or more. In some instances they are believed to warn other lobsters of danger with loud rasping sounds made by rubbing the base of their antennae against the ridges below their eye

Divers sometimes observe an interesting phenomenon known as a “lobster march.” Hundreds, sometimes thousands, of spiny lobsters form long columns to migrate en masse, often after a storm. Why they march remains a mystery.

Whether spiny lobsters are really lobsters or crayfish is a point of debate among some specialists. Unlike their counterparts in New England, the various species of spiny lobsters found in the Caribbean, California and many other parts of the world lack the large pinching claw found in Maine lobster, although females possess a small pincer on the last pair of walking legs. As strange as it sounds, many people refer to spiny lobsters as crayfish because they lack claws, but the real fly in the ointment here is that freshwater crayfish possess claws. Go figure.

Also known as the American lobster, the Maine lobster (Homarus americanus) occurs from the Canadian Maritimes southward to North Carolina, but it tends to be most abundant in Maine waters. This well-known crustacean possesses two strong claws: a big-toothed crusher claw for pulverizing shells and a finer-edged serrated claw used for tearing soft flesh. Far more solitary than spiny lobsters, the American lobster is one of about 30 clawed species worldwide.

A variety of species are known as slipper lobsters. Some claim that these lobsters look like a flattened marine armadillo. Whether you have that much imagination is a personal matter, but the bodies of slipper lobsters are more flattened from top to bottom and more heavily armored than their spiny cousins. Upon first sighting, it is sometimes hard to tell the front from the rear in slipper lobsters because of their broad, flat body and very short antennae. Although they are edible, slipper lobsters are not as heavily pursued by commercial fishermen as spiny lobsters and Maine lobster.

Exactly where the best places to find lobsters of legal size is an issue that is hotly debated by hunters. At different times legal-sized individuals can be found in rocky reef and coral reef communities in depths ranging from the tide pools to well below sport diving limits.

Worldwide there are more than 2,000 species of shrimps. Shrimps play important roles in almost every known marine niche from the tidal zones to the deep sea and from the tropics to the poles. With so many different species occurring in such a range of diverse habitats, it only makes sense that shrimps are a very diverse group of animals. Most species are scavengers, but others play vital roles as cleaners and many species are important sources of food for many marine creatures, especially a variety of predatory fishes.

Shrimps have elongated bodies typically divided into two major parts. One section, the cephalothorax, is composed of the head and thorax, which are fused. The other major section is the abdomen, which is segmented. The abdomen and tails of shrimps are proportionally longer than those of crabs and lobsters. Shrimps use their tails and abdomens to create thrust when they swim. By quickly flexing muscles in their abdomen and tail, shrimps can propel themselves backwards at surprisingly fast speeds over short distances. This sudden burst of speed provides them with a valuable means of avoiding predators. The underside of the tail of shrimps is equipped with several wide, well-developed appendages known as pleopods, structures which enable many species to swim slowly forward while maintaining control of their attitude.

Most shrimps are superb swimmers, but as divers we spot the majority of species as they rest or walk about the reef. During the day, many species tend to seek cover, hiding under ledges, in sponges, in, on and under sea anemones and in the darker cracks and crevices in reefs. However, at night many shrimps emerge to look for food.

Highly mobile and rather active animals, shrimps rely heavily upon their highly evolved senses of sight and touch to help them maneuver around the potential hazards of the reef. Their eyes are positioned on movable stalks, a characteristic that helps them become aware of the world around them. In addition, their long, wispy antennae are used to feel their immediate surroundings. Shrimps are quick to withdraw and move away when they sense danger, and their elongated antennae provide them with a margin of safety in keeping vital and more vulnerable body parts away from threats. It is common for a predator to end up with a mouthful of antenna instead of shrimp.

Some shrimps play vital roles in the ecology of reef systems in tropical and temperate seas by providing cleaning services to a variety of fishes. The shrimps can often be seen crawling all over the bodies of their hosts, even entering the open mouths where they stay for several minutes as they help rid their hosts of unwanted ectoparasites, dead tissue, fungi and bacteria which the shrimps use as food. Often shrimps and cleaner fishes advertise their services and the location of their cleaning station via ritualistic displays. Cleaning stations are often associated with sponges, corals and anemones where the shrimps live.

A variety of species of shrimps are grouped and called popping, pistol or snapping shrimps. Found in a variety of habitats, they share the characteristic of being noisy as the various names suggest. These shrimps make snapping and popping sounds loud enough to startle a diver and keep boaters awake at night with sounds that travel through the hull of boats. Indeed, the so-called “silent world,” as the underwater realm was proclaimed to be years ago, is anything but.

It is believed that these shrimps use the sounds they produce to warn other shrimps to stay out of their territory, and perhaps to stun their prey which consists primarily of small fishes. These shrimps possess one larger pincer and one smaller pincer. To create the sound, the larger pincer is locked open and then the muscles that close the pincer start to contract. The locking mechanism prevents a gradual closure, and when the tension is suddenly released, the loud noise is produced as the parts of the claw collide.

Another group of shrimps known to scientists by the name stomatopods is called mantis shrimps. These shrimps are notorious for their large, lightening-fast, razor-sharp claws that are used for self-defense and capturing prey. Bearing some resemblance to the insect preying mantis because of the way the shrimps hold their claws, these often colorful shrimps can sometimes be seen peering out of the openings of self-made burrows in the sand or in holes at the top of a reef.

As cute as they are, these shrimps should not be taken lightly. There are plenty of anguished fishermen who by using their fingers can only count to nine for the rest of their lives after having tried to remove a mantis shrimp from a net. As adults, mantis shrimps range in size from less than 1 inch long to longer than 1 foot.

The design of the claws varies by species whose natural diets vary in turn. Those that prefer to feed on hard-shelled animals such as snails have strong, jackhammer-like claws that can quickly deliver a series of smashing blows. Mantis shrimps that primarily prey upon soft-bodied animals such as worms and small fishes have extremely sharp claws lined with small spines that help them grasp and rip their victims.

Crabs occur in a variety of forms. Some are small and cryptic. Others are large and stand out like a sore thumb. Some, like the widely admired arrow crabs, possess thin bodies and long, spindly legs. Others have bodies largely flat from top to bottom. 

A group of crabs known as decorator crabs cover themselves with debris, sponges, anemones, hydroids, barnacles and other organisms in order to make themselves less conspicuous. And hermit crabs literally carry around the vacated shells of snails on their back. Hermit crabs often trade shells when encountering other members of their species. Scientists separate crabs into two groups, the hermit crabs and the true crabs. Hermit crabs differ from true crabs in that only the anterior portion of the body in hermit crabs is protected by a hard exoskeleton. Not only is their abdomen unprotected, it is soft. Hermit crabs use their “borrowed” shells to cope with this vulnerability.

As soon as it molts for the first time, a juvenile hermit crab quickly begins to look for a vacant shell to call home. Once it finds a place to hang its hat, the crab will tote that shell until it becomes too small or until the opportunity to trade “up” in the marketplace presents itself.

Described in the order Decopoda (10-footed), all true crabs have 10 appendages with the forward two having been modified into pincers in most species. The other eight legs are used primarily for walking or in a few cases for walking and/or swimming.

The pincers and claws vary markedly from one species to the next. An examination of these appendages provides valuable insight into the natural history of the various species. Those that possess small, almost delicate-looking claws such as arrow crabs are likely to graze on a variety of algae to acquire nutrition. Crabs such as spider crabs and king crabs that have at least one large, powerful-looking claw that is heavily armored are far more likely to be carnivorous, feeding on a variety of snails, clams, fishes and other animals that must be crushed to be eaten. Many carnivorous species are also quick to scavenge when the opportunity presents itself. In turn, crabs are heavily preyed upon by octopuses and many fishes, including moray eels and bottom-dwelling sharks and rays.

Crabs display a remarkable variety of adaptations. For instance, the species known as porcelain crabs, which can be recognized by their flat, smooth bodies and single pair of antennae between their eyes, have the ability to detach their own claws during conflict to facilitate a wise retreat. But that is not all. The detached claws keep pinching while the crab makes its getaway.