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East coast rock lobster
(Panulirus homarus)
 Shovellar lobster
(Scyllarides elisabethae)
 
Lobsters are found in all oceans. They live on rocky, sandy, or muddy bottoms from the shoreline to beyond the edge of the continental shelf. They generally live singly in crevices or in burrows under rocks.

They are invertebrates, with a hard protective exoskeleton. Like most arthropods, lobsters must molt in order to grow, which leaves them vulnerable. During the molting process, several species change color. Lobsters have 10 walking legs; the front two adapted to claws.

As arthropods, lobsters have not developed the nervous system of cephalopod mollusks, nor do they have the advantages of good eyesight. They do, however, exhibit three remarkable evolutionary advances that have led to their great success. Their exoskeleton is a strong, lightweight, form-fitted external covering and support. They possess striated muscle: quick, strong, and lightweight, it enables rapid movement. Finally, articulated appendages allow their limbs to bend at specific points.

Lobsters are omnivores, and typically eat live prey such as fish, mollusks, other crustaceans, worms, and some plant life. They scavenge if necessary, and may resort to cannibalism in captivity; however, this has not been observed in the wild. Although lobster skin has been found in lobster stomachs, this is because lobsters eat their shed skin after molting.

Although clawed lobsters, like most other arthropods, are largely bilaterally symmetrical, they often possess unequal, specialized claws, like the king crab. The claw of a freshly caught lobster is full and fleshy, not atrophied. Lobster anatomy includes the cephalothorax which fuses the head and the thorax, both of which are covered by the chitinous carapace and the abdomen. The lobster's head consists of antennae, antennules, mandibles, the first and second maxillae, and the first, second, and third maxillipeds. Because lobsters live in a murky environment at the bottom of the ocean, they mostly use their antennae as sensors. The lobster eye has a reflective structure atop a convex retina. In contrast, most complex eyes use refractive ray concentrators (lenses) and a concave retina. The abdomen includes swimmerets and its tail is composed of uropods and the telson.

Lobsters, like snails and spiders, have blue blood due to the presence of haemocyanin, which contains copper. (In contrast, mammals and many other animals have red blood from iron-rich haemoglobin.) Lobsters possess a green organ, called tomalley by chefs, which serves as the hepatopancreas, functioning as both liver and pancreas.

In general, lobsters are 25–50 centimetres and move by slowly walking on the bottom of the sea floor. However, when they flee, they swim backwards quickly by curling and uncurling their abdomen. A speed of five meters per second has been recorded. This is known as the caridoid escape reaction.

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