Big lobsters in the sea are crayfish.

The chelonian lobster genus has a distinct mid-dorsal groove on the dorsal surface of the cephalothoracic armor from the frontal angle to the posterior end, and low subdorsal ridges on both sides of the thorax. The posterior neck groove is distinct in the upper part. No ridges other than marginal ridges (no strong posterior ridges behind antennal spines).

Abdominal carapace smooth and without grooves, no ridges between dorsal and lateral armor. Abdominal segment 1 ventral carapace without mesoscopic spines. Caudal segment narrower posteriorly, with rounded convex terminal margin and a spine at each posterior lateral angle. 2nd jawed exopod with multi-segmented whip, 1st pair of steppe macropods varying from side to side, smooth surface, no grooves or ridges, only a few spines.

Chelal palpi and both fingers flattened, broadly spreading, especially firm, 2nd and 3rd pairs of stepped chelicerae tiny, 4th and 5th stepped chelicerae simple. Thoracic ventral armor narrow, with interstices between step 4 in females. The 1st abdominal limb becomes an alternator in males, divided into two segments but not movable. 2nd ventral limb in males thick and extended, with spines at the end. Caudal limb protarsus with dorsal lobe ending in a tooth, inner and outer limbs without longitudinal dorsum, outer limb with a transverse suture, and a small spine at the outer terminal corner of the basal segment.

Extended information:

The American crayfish is 20-60 centimeters long and weighs 0.5-4 kilograms, according to King's World Records, the largest American crayfish weighed 20.14 kilograms, with a full length of 1.06 meters from tail to tip of the cheliped, and was caught off Nova Scotia, Canada, in 1977, and is the world's heaviest marine crustacean to which it can grow. It is the heaviest marine crustacean in the world.

Body color is normally olive green or greenish-brown, orange, reddish-brown or black individuals are not difficult to see, but about one in 2 million is blue; yellow variant is even rarer, the chance of occurrence of about one in 30 million. The biggest difference with Asian lobsters is that the first step is a pair of large chelae, one side of the chelae is slightly larger than the other. The body surface is smooth and the antennae are thin.

Baidu Encyclopedia-Crayfish