Limulus or king crab is a marine animal and found along the north western Atlantic Coast and the Gulf of Mexico. It lives on soft bottoms in shallow waters. During breeding season, both male and female come on land (called nuptial visit) and dig holes in sand at the upper limit of high tides to lay eggs. Limulus is a scavenger and feeds on molluscs, worms and other organisms including bottom-dwelling algae.
Limulus is not a true crab. It is known as a living fossil, because it has remained unchanged since the Triassic period nearly 200 million years ago. It is believed that Limulus, together with trilobites, originated from some common arthropod ancestor but remained universally isolated.
Limulus is not a true crab. It is known as living fossil, because it has remained unchanged since the Triassic period nearly 200 million years ago. It is believed that Limulus, together with trilobites, originated from some common arthropod ancestor but remained universally isolated.
Limulus (L. polyphemus) reaches a length of 60 cm and is dark brown in colour. Its body is divisible into an anterior prosoma and a posterior opisthosoma. The prosoma is convex above, covered by a broad, horse-shoe shaped carapace bearing a pair of median simple eyes and a pair of compound eyes. Limulus can detect movement through eyes. On its ventral side, prosoma bears six pairs of appendages surrounding the mouth. The first pair of chelicerae are small, trisegmented and chelate. The next five pairs are walking legs which serve both the function of locomotion and food procurement. The first four pairs of walking legs are chelate. The coxa of each leg contains spines, called gnathobases. The coxae of these legs bear a short process, the flabellum. Flabellum is used for cleaning the gills. A pair of small, oval and plate-like appendages, called the chilaria, are also present a little behind the mouth. Chilaria are directed vertically downwards and represent the degenerated seventh pair.
The opisthosoma is hexagonal and movably articulated with prosoma. It consists of six segmented mesosoma, unsegmented metasoma and a long telson or caudal spine. Mesosoma bears six pairs of appendages. The first pair is fused and forms the genital operculum and bears two genital pores. Remaining five pairs of appendages are flap-like and membranous. The undersurface of each flap forms leaf-like folds called gills or branchial lamellae, for respiration.
A long triangular caudal spine or telson articulates to the posterior end of abdomen. Telson is highly mobile and is used for correcting the body when it is accidentally turned over.
In Limulus, excretion takes place by coxal glands or brick red glands. Sexes are separate and sexual dimorphism is distinct. Fertilization is external and cleavage of its centrolecithal egss is complete or holoblastic. The larva emerges from the egg as a free swimming trilobite larva. Average life span of Limulus is 19 years and it attains its sexual maturity within 9-12 years. Limulus has an economic importance too; it is fed to chickens and pigs and sometimes consumed by human beings.