Lobster Facts, Photos, and Biology: Interesting Invertebrates
To many people, a lobster is simply a source of tasty meat that's fun to eat at social events. Living lobsters can be fascinating animals to observe and study, however. They deal with the problems of living very differently from humans. They have gills instead of lungs, different sense organs, and a nervous system that has ganglia—nerve centers—but not a true brain. Nevertheless, lobsters are very successful creatures and are found in all oceans of the world.
My interest in the animals began when I was at a high school event shortly after I arrived in Canada from the UK. A container of live lobsters was present to feed the crowd. Another student told me that the lobsters were going to be put into boiling water to cook, something that I had never heard of before. I was horrified and refused to eat any lobster meat. Boiling the animals alive seemed incredibly cruel to me. I did eat other types of meat, however. It didn’t occur to me that this too could be seen as a form of cruelty to animals, depending on how the animals are treated.
Genus (for the American lobster) Homarus
Species (for the American lobster) americanus
External Anatomy of a Lobster
A lobster's body is divided into two sections. The first is known as the cephalothorax. This contains the head and the thorax and is made of fused segments. The eyes, antennae, and mouth parts are attached to the head, or cephalon, and the legs are attached to the thorax. The second body section is the abdomen, which consists of a visibly segmented region and a wider tail at the end. The underside of the abdomen has multiple pairs of swimmerets (or pleopods) attached to it, which help the lobster to move.
Lobsters are crustaceans (members of the class Crustacea) and belong to an order of animals called the Decapoda. The order name refers to the ten legs, which are arranged in five pairs. The first pair of legs of many lobsters are greatly enlarged to form claws. One claw is larger than the other and is known as the crusher claw while the smaller claw is called the pincer claw. These claws are used for manipulating objects but not for walking. Interestingly, some lobsters have the crusher claw on their right side while others have it on the left side, so lobsters have a form of handedness. The remaining four pairs of legs are the walking legs.
Lobsters usually move by walking along the ocean floor. When they're threatened they can swim backwards very rapidly, curling the underside of their abdomen and tail up toward the underside of the cephalothorax and then uncurling it again. Some reports claim that lobsters can move at up to five meters a second using this procedure.
Spiny lobsters belong to the family Palinuridae in the order Decopoda. They look quite similar to true lobsters but have some important differences.
The Exoskeleton and Molting
The lobster's skeleton is on the surface of its body instead of on the inside and is known as an exoskeleton or shell. As the animal grows, it periodically sheds its exoskeleton in a process called molting. This is necessary because the exoskeleton is too hard to allow the lobster's body to expand.
The animal that emerges from the old shell is in a very delicate state. It's covered by a new exoskeleton that formed under the old one. The new covering is soft and needs time to harden. The softness of the new shell allows the lobster's body to increase in size but also makes the animal vulnerable to predators. In the wild, molting generally happens in a secluded place, such as a burrow.
Not all animals with "lobster" in their name are true łobsters. True łobsters belong to the family Nephropidae. Spiny lobsters, squat łobsters, and slipper lobsters don't belong to this family. The molting process of the spiny lobster that's shown in the video above is quite similar to that of a true lobster, however.
The American Lobster
The American lobster (Homarus americanus) lives on the east coast of North America in Canada and the United States. In the descriptions given below, the word "lobster" refers to this animal. European lobsters (Homarus gammarus) are close relatives of the American animal.
Most American lobster shells are olive green or red brown in color. The shell may have orange highlights, however, and may sometimes have blue markings around the joints. Very rarely, the animal is completely blue. Red lobsters are even rarer. Yellow lobsters are extremely rare. Albino lobsters—those with no pigment—also exist. The video below shows blue, yellow, and white animals. The next one shows a calico lobster.
According to the Lobster Institute at the University of Maine, an estimated one in two million American łobsters is blue, one in ten million is red, and one in thirty million is yellow. The frequency of calico (orange and brown) lobsters is also one in thirty million.
Sight and Vibrations
The eyes of lobsters are located at the ends of little stalks and are movable. Lobsters have compound eyes which give them a 180 degree view of the world. It’s thought that the eyes are sensitive to light intensity and movement but don’t produce a very clear image.
Lobster eyes are interesting because they contain mirrors instead of lenses. Each of our eyes—and the eyes of most other animals—contains a lens that refracts (bends) light rays so that they hit the retina, the light-sensitive layer at the back of the eyeball. The lobster eye is made of many tube-like segments, each containing reflective surfaces that act as mirrors instead of a lens. The mirrors reflect the light rays onto the retina.
"Sound" is created by vibrations at the specific frequencies that an ear can detect. When the ear is stimulated it sends a signal to the brain, which creates the sensation of sound. Lobsters almost certainly can't sense sounds in the way that we do, since they don't have any sense organ that behaves like our ears. They do detect vibrations, however. They also produce low frequency vibrations themselves by contracting and relaxing special muscles at the base of their antennae. This causes the carapace (the exoskeleton over the cephalothorax) to vibrate. The function of the vibrations is uncertain, but they may play a role in defence.
Sense of Smell, Taste, and Touch
Lobsters have an excellent sense of smell. They use the first, shorter pair of antennae to detect scents. These short antennae, which each consist of two branches, are actually known as antennules. The many tiny hairs on the antennules pick up a wide range of odors.
The mouth parts and the legs of a lobster have taste receptors. The second, longer pair of antennae are sensitive to touch. The exoskeleton has fine hairs that also sense touch.
In 2016, a 23-pound lobster was caught in the province of New Brunswick in Canada. It was estimated to be a hundred years old. There are claims that far heavier lobsters than this have been caught, including a 44-pound, three-and-a-half-foot long animal that is reported to have been caught off the coast of Eastern Canada in 1977.
Above the walking legs on each side of a lobster’s body is a space under the exoskeleton called the branchial chamber (or sometimes the gill chamber). The lobster’s gills are located in these chambers. If the shell on the side of a lobster is removed, the gills can be seen. The branchial chamber is lined with a thin layer of shell on its inner wall.
The internal anatomy of a crayfish is very similar to that of a lobster. In the crayfish anatomy video below, the gills are shown, although the narrator never refers to them. The gills are the segmented clumps of tissue on each side of the crayfish above the walking legs. They look like clumps because they are dried out and stuck together. When the gills are placed in water they separate, revealing a feathery structure.
There are twenty gills in each branchial chamber of a lobster. Each gill consists of a central rod with projections extending all around it. Most of the projections are long and look like filaments. The gill is often said to resemble a bottlebrush. The base of each gill is attached to the wall of the branchial chamber or to the legs. Therefore when a leg is removed from a lobster a gill may be removed as well.
Sea water reaches the gills through the opening at the bottom of the branchial chamber. As the water moves upwards and forward over the gills, the gills extract oxygen from the water. The oxygen is then transported by the blood to the lobster's cells. The blood picks up carbon dioxide waste from the cells and transports it to the gills, where the carbon dioxide is released into the water that is flowing over the gills. The water in a branchial chamber moves out through an opening at the front of the chamber.
The current that moves the water over the gills is created by a structure called the gill bailer, or scaphognathite. This is a flap attached to a mouth part and is almost constantly beating. Sometimes the gill bailer changes the direction of its beat for a short time in order to reverse the direction of water flow and sweep sea water over the gills, removing any debris that has become trapped on them.
Lobsters may be able to live out of water for one or two days if they stay cool and their gills stay moist.
Lobsters aren't scavengers, as was once thought. They prefer to catch live prey, such as fish, crabs, clams, snails, and starfish. The mouth parts of the lobster begin the breakup of the prey. The small pieces of food then pass into the esophagus.
The esophagus sends the food into the first stomach, which is called the cardiac stomach. This contains teeth-like structures that form the gastric mill. The mill breaks the food into smaller particles. The second stomach is the pyloric stomach. This filters the materials that enter it according to particle size.
The tiny food particles from the pyloric stomach pass into the intestine and are absorbed through its lining. Indigestible material is excreted as fecal pellets through the anus.
The digestive gland of the lobster plays a role somewhat similar to that of our liver and pancreas and secretes digestive enzymes. The gland is sometimes known as tomalley. It's a soft and green material that some people consider to be very tasty. It may collect toxins, however.
Circulatory and Excretory Systems
Lobsters have an "open" circulatory system. Their heart pumps blood (technically called hemolymph) into arteries, but the arteries lead to blood cavities called sinuses instead of to other blood vessels. The blood travels through sinuses and channels back to the heart. The animals have colorless blood, which turns slightly blue when exposed to oxygen. Their respiratory pigment is called hemocyanin.
Like our cells, those of a lobster produce waste substances that must be removed from the body. The excretory glands are called the green glands and are located at the base of the antennae. The glands release the waste substances into the surrounding water. They shouldn't be confused with the green digestive gland, or tomalley, which is located beside the digestive tract.
Lobsters can regenerate their claws, their walking legs, and their antennae. They may drop a claw spontaneously, which is useful if they want to escape from a predator.
A lobster's nervous system is based on ganglia and nerves. These are made from neurons, or nerve cells. A neuron consists of a cell body, which contains most of the cell's organelles, and a fiber called an axon extending from the cell body. A ganglion is a group of cell bodies from several neurons. A nerve is a group of axons bundled together.
A lobster has a large pair of ganglia in its head near its eyes, which is sometimes referred to as a brain. These ganglia don't have the complex structure of a true brain, however. A double nerve cord extends from the "brain" to the lower part of the lobster’s body and then travels towards the rear of the animal. The nerve cord has a pair of ganglia in almost every segment of the lobster and gives off nerves that go to the various parts of the body.
Do Lobsters Feel Pain?
Do lobsters and their relatives feel pain? Researchers can't answer for certain. There are scientists on both sides of the debate. Some claim that lobsters and other invertebrates do feel pain and stress; others say that it's unlikely that they feel pain due to their relatively simple nervous system.
To me it seems unlikely that lobsters and other invertebrates have evolved without being able to perceive some kind of pain sensation. Feeling pain is a protective mechanism to prevent damage to an organism's body. The brain of lobsters doesn't have a cerebral cortex, the part of our brain that perceives pain. This doesn't rule out the possibility that lobsters are perceiving pain by a different mechanism than we use. In any case, since no scientist can guarantee that lobsters are unable to feel pain, the onus should be on us to kill them humanely if we want to eat them.
In the American lobster, the female releases a pheromone to attract a male. The male's first pair of swimmerets are rigid and grooved. They are used to insert sperm into the female's sperm receptacle.
The female retains her unfertilized eggs in her body for many months. Eventually she releases her eggs, which are fertilized by the sperm from her receptacle and then stick to her swimmerets. Here they stay until they hatch.
The youngsters that leave the swimmerets are tiny larvae. They molt as they grow and go through several developmental stages. Eventually (if they survive predation), they develop a typical lobster form.
There are probably many more facts about the lives of lobsters to be discovered. They are interesting animals and have some impressive characteristics. It's a shame that many people think of them only as food.
- Lobster biology is described at the website of The Lobster Conservatory.
- The NOAA fisheries website gives information about the American lobster.
- The biology of the European lobster is described at the National Lobster Hatchery
- Research suggests that crustaceans feel pain, as described in this nature.com article.
- Facts about a giant 23-pound lobster caught in Canadacan be found at the CTV News website.
Questions & Answers
© 2012 Linda Crampton