(from the Greek words arthros, jointed, and podes feet)
Note: This article is the sixth in a series about marine animals you and your diving pals are likely to encounter when underwater. The pieces are presented in order according to the taxonomic system used by scientists when they describe and classify plants and animals. An overview of the articles and the taxonomic system is provided in the February 2001 issue.
While many humans consider our species the supreme beings on Earth, it is clear that the most successful animals are the arthropods. Collectively, they inhabit the land, air and sea, and make up more than 75 percent of all known fossil and living animal species. To date, more than 1 million species of arthropods have been described, but some experts maintain that an accurate count of living species exceeds 10 million.
The phylum Arthropoda not only contains all of the world’s lobster, shrimps, crabs, barnacles, isopods, copepods and amphipods, but also all of the world’s many insects. The distribution of animals described in the phylum Arthropoda ranges from the deep sea to the peaks of tall terrestrial mountains. Representatives of this group range in size from the Japanese spider crab with its 12-foot “arm spans” to microscopic-sized insects and zooplankton. And they provide culinary treats that range from chocolate-covered insects to lobster Newburg and crawfish jambalaya. The arthropods are a varied group, indeed.
With so many members, it should not come as too big of a surprise that the phylum Arthropoda is divided into three subphylums which contain seven classes. Fortunately for divers, the vast majority of the species we encounter are described in a single class, Crustacea, a group that contains 35,000 different animals. All lobsters, shrimps, crabs, barnacles, isopods, copepods and amphipods are crustaceans. Many of these species are extremely abundant and some are quite a draw for those of us who seek game or dive with cameras.
The Basic Body Plan of Crustaceans
Crustaceans are equipped with jointed appendages and a hard external skeleton (exoskeleton) which we often call the shell. The shell is usually jointed. Most soft body parts are contained within the shell. The bodies of crustaceans display bilateral symmetry, meaning that the body is organized fore and aft. Crustaceans have a definite brain in the head at the front of the body. The eyes of many species are at the end of elongated stalks attached to the head. Many species possess pincer-like claws that can be used for defense and for gathering or capturing food. Lastly, crustaceans are the only arthropods that possess two pair of antennae.
In most species, the hardened exoskeleton is impregnated with calcium carbonate, adding rigidity and strength. In addition, this protective shield greatly assists in the animal’s dexterity and locomotion. It provides muscles with key points of attachment, a significant development in the course of evolution.
While some crustaceans spend their lives drifting in the water column as plankton, the species most familiar to divers are the lobsters, shrimps and crabs encountered along the sea floor. Many of these species possess well-developed senses of touch, taste and vision. Antennae and stiff body hairs assist greatly in the sense of touch.
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 eyes.
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.
For many years barnacles were mistakenly classified as members of the phylum Mollusca (mollusks) instead of Arthropoda. Before that, some “biologists” believed barnacles to be closely related to birds, hence, the common name of the species known as the gooseneck barnacle. However, in 1830 a British biologist recognized the many similarities that barnacles share with other arthropods. Barnacles develop from an egg into a larval stage in a manner similar to lobsters, shrimps and crabs. Barnacles also possess the jointed appendages found in other crustaceans. But barnacles only display some of these characteristics as larvae. Once they become adults, barnacles remain hidden within a shelled fortress of their own making.
Barnacles live in habitats that range from the intertidal zones to the skin of whales. As adults, many barnacles depend on currents to bring them food which is captured with the barnacle’s feather-like feet which are known as cirripedia. The feet are repeatedly drawn through the water in a mesmerizing, sweeping motion to catch food particles. The food is then drawn into the shell where it can be ingested.
Being permanently attached to the reef as an adult also presents problems where reproduction is involved. It is only logical to ask, “how does one gain access to a potential mate?” Barnacles solve this problem via the fact that males have the longest penis-to-body-size ratio known in the animal kingdom, making mates readily available.
Isopods, Copepods and Amphipods
You are not likely to ever see the animals known as isopods, copepods and amphipods described in any scientific literature as the “pods.” However, it is common for divers to encounter some member of the group and because they are not sure exactly what they saw, they will simply say they saw one of those “pod” animals. Often the pod in question was attached to a fish or some other creature, but that is not always the case.
While it is true that very few divers are drawn to our sport by the “pods,” these are not insignificant creatures. Copepods, for example, fill an important slot in many food chains being a major component in the plankton supply. Some copepods and isopods live their lives attached to the bodies of larger animals including bony fishes, sharks and rays, dolphins, whales and turtles.
Copepods burrow into the skin of their host while isopods hold on by clinging. While some species of copepods and isopods are parasitic, their presence tends to be more of an irritant than life-threatening. Many species primarily acquire nutrition by eating away at the skin and body of their host; many feed by removing food from the water that flows past them. While they are certainly not limited to these species, copepods can often be seen in the mouths and on the dorsal fin of mako sharks in the waters off southern California, while isopods are often observed attached to the head of soldierfishes and squirrelfishes in tropical seas.
Some species of isopods can be seen in many areas with rocky beaches before you ever enter the water. Isopods bear some resemblance to the terrestrial crustaceans known as pill bugs. They can be seen scurrying around rocks and other structures as they hide in cracks and crevices along shorelines in the tidal zones where they make their livings by scavenging on a variety of algae and organic debris.
The name amphipod refers to myriad species of crustaceans known as sand fleas, beach hoppers and water fleas. While they are relatively small creatures, they, too, often occur in numbers too high to count, and collectively they serve as a valuable food source for many larger creatures. Amphipods are common from the shoreline to the deep sea. Beach dwelling amphipods tend to bury themselves in the sand by day and emerge as the sun gets low on the horizon. Their bites are irritating and can itch for several days.
Growth Is A Dangerous Time
Having a hard shell has its downside. Growth is difficult and it can be especially dangerous. In order to grow, crustaceans must discard their existing shell in an act known as molting and then create a newer, larger shell. During this time, their soft body parts are exposed to predators. Creating a newly hardened shell requires time. The length of time varies, but it is usually a matter of a few hours before the shell begins to harden enough to provide significant protection, and it only takes a little bad luck in the blink of an eye to end up as someone else’s dinner.
Molting is controlled by hormones that are produced by glands in the elongated eye stalks. Before shedding their existing skeleton but as part of the molting process, crustaceans undergo a period in which they feed heavily and store fat reserves. Once sufficient fat has been stored, the animal begins to form the foundation of a new shell underneath the existing shell. Hormonal changes cause the old shell to fracture and split in strategic places. Once the old shell has sufficiently deteriorated, the animal climbs out of the remains and the new shell begins to expand and harden. In some instances the old shell is simply abandoned, but in others the molting animal will eat some of the old shell in order to reacquire calcium that will help the new shell harden.
Once the old shell has been shed, the animal is quick to seek cover in a hiding place where it can wait safely as the new shell takes shape and hardens. However, in many crustaceans it is only during this soft-shelled period that females can be mated.
During the period while the shell is soft, important hormonal processes prevent the loss of body fluids and encourage body tissues to swell and growth to occur. In time, the newly created shell hardens over the enlarged body. As the hardening occurs, another hormonally triggered event causes excess body fluids to be expelled and the soft body of the animal inside shrinks, leaving ample room for body growth before a new shell is needed again.
Larval crustaceans often molt as many as a dozen times in one year. Once they have settled out of the water column and become members of reef communities, the frequency of molts tends to lessen, but the animals tend to molt several times annually with the number of events continuing to slow as they age.
The Sex Lives of Crustaceans
In most crustaceans the sexes are separate. However, there are some rather fascinating exceptions. For example, some shrimps are hatched as males, but after they have undergone one mating cycle they are transformed into females.
The intricate courtship and mating rituals of many crustaceans are typically seasonal affairs. While many less advanced invertebrates simply cast their gametes into the water column and allow fate to takes its course, crustaceans tend to provide some degree of parental care, an act that is considered to be more typical of more highly advanced animals. As a rule, it is the female that provides the care as she carries and guards the eggs attached to her body until they hatch. Females usually produce hundreds of eggs in a single brood.
Carried by currents, the larvae range far and wide, in many cases for the better part of a year, and they are often caught in sampling nets hundreds of miles from the nearest reef where their parents might have mated or where they might settle.
Research has demonstrated that it can take years for many species to reach sexual maturity, as many as seven years in the case of the California spiny lobster.
Many species of larval crustaceans bear little resemblance to their adult forms. The larvae are often attracted to lights as night divers know all too well, but they are usually so active and so small that it is difficult, if not impossible, to get a good look at a single animal. On occasion, larvae are so numerous that they reduce visibility and severely inhibit underwater photography and filmmaking. While this can be frustrating at the moment, it illustrates just how numerous crustaceans are and provides some insight into how important they are in many marine food chains as they are preyed upon by myriad fishes and other open ocean animals including the great, or filter feeding, whales.