Mangrove Horseshoe Crab

Appearance

The basic body plan of a horseshoe crab consists of three parts: the prosoma, the opisthosoma and the tail. The prosoma is the large, dome-shaped frontal part at the carapace. The smaller rear carapace with spines on the edge is the opisthosoma. The rear extension that looks like a spike is the telson, which is commonly described as the tail. Uniquely among the horseshoe crabs, the cross section of the tail is rounded. It is essentially triangular in the other species. The tail is used to turn itself right side up when overturned.

The mangrove horseshoe crab is the smallest of the four living species of horseshoe crabs. Like the other species, females grow larger than males. On average in Peninsular Malaysia, females are about 30.5–31.5 cm long, including a tail that is about 16.5–19 cm, and their carapace is about 16–17.5 cm wide. In comparison, the average for males is about 28–30.5 cm long, including a tail that is about 15–17.5 cm, and their carapace is about 14.5–15 cm wide. There are significant geographic variations in the size, but this does not follow a clear north–south or east–west pattern. Those from West Bengal in India average somewhat smaller than those from Peninsular Malaysia, with a carapace width of about 16 cm and 14 cm in females and males respectively. Elsewhere it averages even smaller, with the smallest reported from the Balikpapan and Belawan regions in Indonesia where the carapace width of females is about 13 cm and in males 11 cm. The largest females of the species may reach up to 40 cm in length, including the tail.

Uncommon for chelicerates, horseshoe crabs have two compound eyes. The main function of these compound eyes is to find a mate. In addition, they have two median eyes, two rudimentary lateral eyes, and an endoparietal eye on their carapace and two ventral eyes located on the underside by the mouth. Scientists believe the two ventral eyes aid in the orientation of the horseshoe crab when swimming. Each individual has six pairs of appendages. The first pair, the chelicerae, are relatively small and placed in front of the mouth. They are used to place food in it. The remaining five pairs of legs are placed on either side of the mouth and are used for walking/pushing. These are the pedipalps and the pusher legs. Most of the appendages have straight, scissor-like claws, but in males the first and second pair of walking legs have strongly hooked "scissors", which are used for grasping the female during mating. Located behind their legs are book gills. These gills are used for propulsion to swim and to exchange respiratory gases.

Naming

Despite their name, horseshoe crabs are more closely related to spiders and scorpions than to crabs. The mangrove horseshoe crab is the only species in the genus "Carcinoscorpius". There are four extant species of horseshoe crab. The biology, ecology and breeding patterns of "C. rotundicauda" and the two other Asian horseshoe crab species, "Tachypleus gigas" and "Tachypleus tridentatus", have not been as well documented as those of the North American species "Limulus polyphemus". All four extant species of horseshoe crabs are anatomically very similar, but "C. rotundicauda" is considerably smaller than the others and the only species where the cross section of the tail is rounded instead of essentially triangular.

Distribution

This species occurs only in Asia around the Indo-West Pacific region where the climate is tropical or subtropical. These horseshoe crabs can be found to exist throughout the Southeast Asia region in shallow waters with soft, sandy bottoms or extensive mud flats. The mangrove horseshoe crab is benthopelagic, spending most of its life close to or at the bottom of a body of their brackish, swampy water habitat, such as mangroves. This is the habitat for which it gains its common name: mangrove horseshoe crab.

Behavior

In the spring, horseshoe crabs migrate from the deeper water to the shallow, muddy areas. Nesting usually follows the cycle of the high tide of the full and new moons. During the mating period, the males will follow and cling to the backs of their potential mates using modified prosomal appendages for long periods of time before the egg-laying has occurred. Horseshoe crab species with low spawning densities and 1:1 sex ratios, such as the mangrove horseshoe crab, are found to be monogamous. In addition, the female does not choose her mate. Males find their female mates with the use of visual and chemoreceptive signals. Once a mate is found, the female digs a hole and lays the eggs while the male externally fertilizes them. Once the eggs are laid, the male and female head back to the ocean, and the eggs develop on their own. Their eggs are large, and after a couple weeks, the eggs hatch into miniature versions of the adults.

Mangrove horseshoe crabs in Singapore breed from August to April. Juveniles grow about 33% bigger each time they moult, and it takes the juveniles about five moults to grow from 2 centimetres to adult size.

Food

Mangrove horseshoe crabs are selective benthic feeders, feeding mainly on insect larvae, small fish, oligochaetes, small crabs and thin-shelled bivalves. Lacking jaws, it grinds up the food with bristles on its legs and places it in its mouth using its chelicerae. The ingested food then enters the cuticle-lined oesophagus and then the proventriculus. The proventriculus is made up of a 'crop' and a 'gizzard'. The crop can expand to fit the ingested food, while the gizzard grinds the food into a 'pulp'. Studies have found that mangrove horseshoe crabs have a strong preference for insect larvae over the other organisms on which it also feeds.

Evolution

Horseshoe crabs are commonly known by biologists around the world as a living fossil because they have remained practically unchanged in terms of shape and size for millions of years. Although their physiology has changed over the years, their typical three piece exoskeleton, consisting of a prosoma, opisthosoma, and telson, has remained since the mid-Paleozoic era. Fossils of horseshoe crabs that have been dated to over  million years ago look almost identical to those species that are still alive today. The long existence of this body plan suggests its success. It is estimated that the American species of horseshoe crab diverged from the three Indo-Pacific species approximately  million years ago.