Exterior anatomy

Dorsal view and ventral view of lobster

Lobsters have three pairs of antennas: a large pair and two small pairs. They are sensory organs. The large antennas are used particularly for touching, and allow lobsters to find their way in their environment. The small antennas allow them to recognize various chemical signals in water. They are very sensitive to odours and help them locate food, among other things.

Lobster eyesEyes
Lobsters have two eyes located at the base of the antennas. They do not distinguish colors and do not see clear images. However, their eyes are very sensitive to light, and they are able to detect movement and shadows even though there is very little light. They find their way around mostly through touching and detecting odours.

Crusher claw (1) and cutter claw (2)

Crusher claw (1) and cutter claw (2)
Photos : Richard Larocque

Chelipeds (or claws)
The first pair of legs. They are very different from the other pairs because they are very large claws. There is a claw to crush and a claw to cut. The crusher claw (1) is bigger and has large "teeth" that enable the lobster to break prey shell or carapace. The cutter claw (2) is slightly smaller and has several little "teeth" that enable the lobster to tear or cut prey flesh. It also allows lobsters to quickly grab their prey.

Pereiopods (or walking legs)
The four other pairs of legs. They are used for locomotion. However, the two pairs located just after the chelae are equipped with small claws and are also used for eating.

The lobster's head and thorax are welded together and form the cephalothorax. It is the first part of the lobster's body.

The cephalothorax ends in a tip located between the two eyes, called the rostrum. This appendage is not a nose, as many people think. The rostrum simply protects the eyes when lobsters clash.

The abdomen corresponds to what is usually called the lobster tail. It is comprised of 6 segments and is articulated.

Telson and uropods of lobster

The telson is at the end of the abdomen. It is the segment in the middle of the spread tail.

Lobster tail

The segments on each side of the telson forming the spread tail.

The anus is under the abdomen, between the last segment of the abdomen and the telson.

Male and female

Swimmerets (or pleopods)
Under the abdomen, there are appendages resembling small fins. Those are the swimmerets. There are 5 pairs. The first (closest to the cephalothorax) is different from the four others and can be used to differentiate males from females. For males, the first swimmerets are large, hard and whitish, and are used for mating. They are called gonopods. For females, they are small and soft. The other pairs of swimmerets help lobsters move or make the water inside their shelters circulate. For females, they are also used to carry and ventilate eggs.

Facing and ventral view of lobster mouth

The mouth is just below the rostrum, under the eyes and between the antennas. It includes the maxillipeds and mandibles. Lobsters use the maxillipeds to bring food to the opening of the mouth and the mandibles act as teeth.

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Interior anatomy

Various parts of male and female

In adult lobsters, the brain is no larger than a pea. It is just behind the eyes.

The heart is on the back, behind the cephalothorax, just before the abdomen.

There are two of them. The first is located in the lobster's "head", just behind the eyes and brain. It is called the cardiac stomach. The second one is right after the first. It is called the pyloric stomach. It extends to the abdomen.

The intestine begins at the pyloric stomach and extends along the entire abdomen up to the anus. When a lobster tail is cut lengthwise, we can often see a small black tube: that is the intestine.

Only females have ovaries. There are two of them and that's where the eggs are produced before being fertilized (ovocytes). The ovaries extend along part of the abdomen and are located above the intestine. When cutting a cooked lobster tail lengthwise, we can sometimes see a large red line. Those are the ovaries with unfertilized eggs. They can be eaten. Sometimes, there's a black slimy substance. This is due to egg resorption. Those are eggs that were about to be laid when the female was caught. Instead of being laid, they were liquefied. The vitello-proteins contained in the eggs were re-circulated in the blood, giving it a black color. It is not very appetizing but the lobster is good nevertheless. It simply needs to be rinsed.

The testicles are directly below the heart and appear as two white lines.

Digestive gland
This organ takes up most of the cephalothorax and represents the liver, to some extent. This is the green substance we see in a cooked lobster when the cephalothorax is opened. Connoisseurs like it.

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Incredible but true

Lobsters smell with their "feet". In fact, lobsters have several receptors on their claws and their legs, which enable them to locate and recognize food in their immediate surroundings. Lobsters have other receivers on their antennas and mouth that enable them to detect odours (chemical signals).

Lobsters have two stomachs: the cardiac stomach and the pyloric stomach. In the first stomach (cardiac stomach), there are teeth to crush the food.

If a lobster loses a claw, a leg or an antenna, it restores it (it grows back) in the next moult.

When a lobster flees, it swims backwards quickly (up to a few metres/second) by swiftly curling and uncurling its abdomen.

Lobster blood is colorless. It is transparent. It becomes bluish when in contact with oxygen.

The crusher claw can be on the left or on the right. Lobsters can be either "left-handed" or "right-handed". Sometimes a lobster has two identical claws (generally 2 cutter claws).

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Different colours

Basic colours of lobster

The color of the carapace is made up of a basic pigment (red), which is associated to other pigments like blue and yellow. The mixture of the different pigments [red and yellow pigments are carotenoids, and blue pigment is a cyanin] gives lobsters their particular color. Generally, lobsters are brown (sometimes brown-green or brown-blue), with some more or less clear parts. Sometimes articulations can be blue, while the stomach area is orange. However, there are exceptions in nature. We hardly ever see an entirely blue (1), or entirely yellow lobster (2). It happens on the odd occasion that a lobster's carapace has two separate colors (3-4), or other cases it is an albino (white; 5) or leopard lobster (dark brown with a large amount of big yellow spots; 6).

Various colours met on lobster

Photos 1 and 5 : Marc Lanteigne
Photos 2, 3 and 4 : © Crewdog, Lobsterman's Page
Photo 6 : Gilles Savard

All lobsters become red when they are cooked, except albino lobsters. Heat changes the pigments associated to red and destroys their binding. The red pigment is thus released. Since it is the basic pigment of the carapace and that it is heat resistant, the carapace turns red with cooking. Albino lobsters remain white when they are cooked because they do not have colour pigment.

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