Lobster Biology


Lobster Classification

Kingdom: Animilia
Phylum: Arthropoda
Class: Crustacea
Order: Decapoda
Family: Nephropidae
Genus: Homarus
Species: Americanus

Since the 1700’s, scientists have attempted to group all living things into familial groups to produce a hierarchy referred to as nomenclature. The Atlantic Lobster that we harvest off the East coast of Canada and parts of the US are scientifically referred to as Homarus americanus as distinguished by its binomial name for its genus and species. Homarus americanus is known by many names including: Atlantic Lobster, Canadian Lobster, Northern Lobster, Maine Lobster, American Lobster and Canadian Reds.

In zoological classification, the hierarchy of animals are divided into two major groups: Vertebrata, which refers to animals possessing a backbone and Invertebrata, which are those without (lobster are invertebrates). Lobsters are placed into the phylum Arthrapoda that encompasses the largest group of invertebrates and includes: shrimps, crabs insects, spiders and various other creatures with jointed appendages and outer skeletons (exoskeleton). The class Crustacea refers to animals that have flexible outer shells and are distinguished from hard-shell animals such as oysters clams and mussels.

Lobsters, crabs and shrimp belong to the order Decapoda (Greek deca, ten) and as a name suggests it constitutes decapod’s that have five pairs of legs. Decapod’s with long tails such as a lobster are further subdivided into a suborder called Macrura Reptantia and is distinguished from the crab which has a short tail tucked under their body. The two commercially important families of lobster in the North America are broken into two distinct infraorders. The Homarus americanus belongs to Astacida while the spiny lobster (or rock lobster) belongs to Palinuridea. Atlantic Lobster are distinguishable from other lobster types because they have claws on the first three pairs of legs with the first set bearing the largest.

Lobster Anatomy

If an Atlantic Lobster was cut into two equal parts from head to tail one would find that both halves would be the mirror image of the other (with the exception of the front claws). This characteristic is common to all arthropods and is referred to as being bilaterally symmetrical. The body plan of an Atlantic Lobster is essentially comprised of two parts termed the cephalathorax and the abdomen. The cephalathorax, a shield like covering, is comprised of the cephalon (head) and the thorax (midsection), which are fused together. What is often referred to as the tail is in fact called the abdomen. From a marketing perspective, however “lobster abdomen” lacks the sex appeal we often associate with the much sought-after “lobster tail.” The cephalathorax actually consists of 14 fused segments (somites) with each bearing a pair of appendages. These appendages include: eyes, antennules, antennae, mouthparts and thoracic walking legs.

Lobsters have compound eyes. These are located at the end of movable stalks (rostrum) on the first segment. Lobsters lack the ability to discern much beyond detecting basic images and varying levels of light intensity. They predominantly rely on their eyes for the detection of movement.

The antennules and antennae act as the sensory organs of the lobster and are found on the second and third segments respectively. The antennules are highly sensitive chemical receptors that can detect minute amounts of amino acids (found in animal tissue). The antennae are tactile receptors and perform touching, movement and even hearing functions.

The mouth of the Atlantic Lobster is highly complex and is found on the six segments following the antenna (4-9). The mouthparts perform a variety of functions from gripping and passing food to crushing it for digestion. Along with the legs, the mouthparts contain the taste organs for the lobster.

In the last five sections of the cephalathorax the legs of the lobster are found. The first set of legs bare the large claws for which the Atlantic Lobster is renowned. The front claws of a lobster differ in shape and size enabling the lobster to cut and crush its food. The larger more powerful claw is referred to as the crusher claw, while the other end is called the pincer claw. The next two pair of legs contain smaller pincers at the end that are used for gripping food and passing it to the mouth. The last two sets of legs are adapted for walking grooming and have pointed tips (dactyl).

The abdomen consists of six segments and unlike those in the cephalathorax, these are not fused. Each segment has a pair of swimmerets (pleopods) with the exception of the last, which contains the tail fan. The broad tailfin is actually comprised of five parts, which include a central telson and modified pleopods on each side (uropods). In the lobster male the first two sets of pleopods are adapted for sperm transfer. In the female the last four swimmerets, which bear longer hairs then the male (flagella) and are used as the site of egg attachment.

Lobster Physiology

The Atlantic Lobster possesses all the major physiological processes common to most animals and include: the digestive system, the circulatory system, the reproductive system, the nervous system, the excretory system, the musculature system and the respiratory system.

The lobster does not have a centralize brain as is common in many animals. Instead, they have a bilaterally symmetrical nervous system, which consists of one ganglia (group of nerves) per body segment. These ganglia are joined together by a ventral nerve cord and perform different body functions. It is surprising that if the connections between the ganglia are severed, the lobster can still carry on seemingly normal behavior.

Reproduction holds particular interest to the lobster fishery, since a clear understanding is necessary in order to incorporate sustainable management practices. Atlantic Lobster generally mate shortly after the female sheds her old exoskeleton (within 20 – 40 minutes) normally in the summer months. Males generally mature at a shorter length (40 to 45 mm carapace length) compared to the female (90 mm). Small males cannot usually mate successfully with larger females. While males are capable of producing spermatozoa at 45 mm (carapace length), they are not capable of mating until they are at least as large as the smallest mature females.

The female lobsters’ ovaries are located in cephalathorax with paired oviducts emerging below the heart and running down to the base of the third pair of thoracic legs. This is where they meet the seminal receptacle where the male deposits the spermatophores. The male cradles the female off the ocean bottom while he mounts her and inserts his modified first pair of pleopods into the seminal receptacle of the female. After depositing the spermatophores, the male covers the females’ receptacle with a gelatinous material. It may take the female up to 15 months after mating before she produces the eggs at which point they are fertilized and attached to her swimmerets. A female carrying eggs is said to be “in berry” and the eggs may remain there for 10 to 11 months before the larvae hatch.

Lobster Moulting

Since the hard exterior shell of a lobster is inelastic, moulting is essential for lobster growth. In the first year of its life a lobster will moult about 10 times. As they get older lobster tend to moult less frequently, as few as once every few years for large lobsters. It takes approximately six years for lobster to reach a weight of one pound.

To prepare for moulting the lobster produces a new exoskeleton underneath the old one. Minerals and blood from the old exoskeleton are removed causing the shell to shrink. When a lobster is a few days away from shedding, the exoskeleton is very soft and appears bluish color. The lobster then absorbs large amounts of water, which causes in the exoskeleton to swell and push apart the old carapace. The lobster then roles over on its side and bends into a V shape, and over a period of several hours will remove its extremities from the old shell. The absorbed minerals are passed back into the blood and lymphatic fluid and are then redeposited into the new soft shell. It generally takes several months before the new shell is fully hardened.

Lobster Habitat

Homarus americanus is found in the Northwest Atlantic from Labrador to North Carolina. It is found on sub-tidal bottoms from the intertidal to upwards of 480 meters in depth. Most lobsters are commonly found in four to fifty meters of water in rocky areas where they can hide in crevices for protection. Lobster larvae assume a planktonic existence for the first 5 to 6 weeks of their life and live in the water column near the surface. It is here that the young lobster are most susceptible to predation and it is estimated that as little as 1/10th of one percent survive this stage. At the fifth stage, lobster sink to the bottom where they will spend the rest of their life. Although movement is achieved by walking (using tail for quick escapes) the Atlantic Lobster has been known to migrate as far as 15 kilometers.


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