Follow a Clam Through a Lobster: Internal Anatomy

A lobster locates its prey, a clam for instance, with its excellent sense of smell. Then it breaks open the shell with its large claws. It uses its mouth parts and first two sets of walking legs to manipulate the food into its mouth. A lobster is a messy eater. It creates a cloud of debris as it tries to coordinate all these claws to move food into its mouth.

Lobster: Side View

As a clam slides down the lobster's gullet, three stomachs begin the work of digestion. The first stomach contains a gastric mill, a set of grinding teeth, that pulverize the clam. Then the clam, now mush, passes into the digestive gland (which contains that yummy green stuff, tomalley). Here in the digestive gland the food is absorbed into the body. Finally, wastes pass through the intestine and out the anus at the tip of the lobster's tail.

Armored Arthropod: (External anatomy)

The first thing you notice about Homarus americanus, the "Maine" or "American" lobster, is its two strong claws: a big-toothed crusher claw for pulverizing shells and a finer-edged ripper claw resembling a steak knife, for tearing soft flesh. The lobster uses these claws, as well as smaller appendages around its mouth (mandibles and maxillipeds), for gripping and shredding its food. Besides its formidable front claws, the lobster also has eight walking legs, giving it ten legs altogether, which is why people who classify things call it a decapod.

Lobster: Bottom View

The lobster usually crawls forward on its walking legs, but if it needs to make a quick exit, it contracts its tail forcefully and scoots backwards. When you first pick up a lobster, it frequently exhibits that flight response. Lobstermen call young lobsters, who do this a lot, "snappers." Under stress, a lobster may also "throw" a claw or a walking leg, but it will eventualy regenerate a new, fleshy, "limb bud." At the next molt, the lobster deposits a skeleton on the new limb.

The lobster wears its skeleton on the outside (an exoskeleton), like a suit of armor. Because of this crusty covering, the lobster is included in the category of crustacean, along with crabs, shrimp, and barnacles. Crustaceans belong to a larger group known as Arthropods, which includes spiders, insects, and horseshoe crabs.

Sense-sational facts: chemoreceptors and other lobster senses

A lobster paces the ocean bottom in a shadowy world where vision is not all that important. Each eye, set on a movable stalk, has up to 10,000 facets that operate like many tiny eyes. The lobster probably doesn't see images, but its eyes can detect motion in dim light. In bright light, a lobster is probably blind.

Because of its poor vision, the lobster learns about its environment primarily through touch, taste, and smell. In fact, you could say the lobster's whole body is a sense organ. The long pair of antennae and tiny hairs that cover the entire body, including the walking legs, are sensitive to touch. Short bristles called "hedgehog hairs" line the insides of the pincers on the walking legs. These are akin to our taste buds, so a lobster can be said to taste with its feet. If it likes what it picks up, it passes the food along to its mouth where sensory bristles on its mouth parts also detect taste.

The shorter set of antennae, antennules, perceive distant odors or chemical signals carried by the seawater. These "chemoreceptors" help a lobster find food, choose a mate, and decide whether to fight or flee. Delicate hairs on the antennules have more than 400 types of receptors, sensitive enough to distinguish between a horse mussel and a blue mussel, for instance. If the antennules of a lobster are removed, it can only find food if it literally bumps into it.

Dr. Richard A. Wahle of Bigelow Laboratory for Ocean Sciences has found in his research that lobsters respond to the "chemical presence" of fish predators such as sculpin. In lab experiments where water was siphoned from a separate tank containing a sculpin, very small lobsters would spend more time in shelter. Larger lobsters would emerge from their shelters in an aggressive claw display.

Dr. Jelle Atema of the Marine Biological Laboratory in Woods Hole, Massachusetts has been studying the senses of lobsters for over twenty years. He is constantly amazed at how much information their chemical receptors can glean from the sea water. According to Dr. Atema, "They may enable the animal to detect the species, sex, and even the mood of another animal."

What color is a lobster?

There are always stories about people "from away" who, accustomed to seeing their lobsters cooked, are horrified by the color of a living lobster. They are sure it must be moldy. A live lobster is greenish-black on top and orange below, with accents of blue on the joints of its claws. That is because a lobster's shell is composed of three pigments: red, blue, and yellow.

When one or more of these pigments are missing at birth, a lobster may be red, blue, albino (white), or calico (dark with yellow spots). Blue lobsters occur once in every 3-4 million lobsters. Red lobsters (live ones) occur once in every 10 million. Except for albinos, all the color variations of lobsters turn red when they are cooked.

How big can lobsters grow?

Unlike a human, a lobster continues to grow throughout its life, although it molts less frequently as it grows older. Accounts from colonial times reported lobsters that were five to six feet long. The record for the largest documented lobster goes to a lobster taken off Nova Scotia in 1977. It weighed 44 lbs., 6 oz. and was between three and four feet long. It may have been 100 years old. The record for the largest lobster caught in Casco Bay (in the Gulf of Maine) is 36 lbs.

So many variables affect when a lobster will molt and grow that deducing the age of any lobster is little more than an educated guess.