Crab trouble

Horseshoe crabs, survivors from the Paleozoic Era, now face the threat of extinction

Published: Monday 15 March 1999

 Living in the past: ancient h (Credit: amit shanker / cse) though called a "crab", the horseshoe crab is not really one in essence. It derives its name from shape of its shell, which closely resembles a horse's hoof. While most crabs have two pairs of antennae, a pair of mandibles or jaws and five pairs of legs, the horseshoe crab can only boast of small pincers in the front for maneuvering its food. It lacks the antennae and the mandibles. The most arresting feature of the species is its nine eyes and consequently, the horseshoe crab has one of the largest optic nerves in the animal kingdom.

Once widespread and flourishing in the Paleozoic Era, just four species of Horseshoe crabs survive today. These belong to three genera: Limulus, Carcinoscorpius and Tachypleus. And all three belong to the family Limulidae , also known as "Living Fossil". Limulus and Carcinoscorpius has one species each -- Limulus polyphemus ( L polyphemus ) and Carcinoscorpius rotundicauda ( C rotundicauda ). Tachypleus has two species: Tachypleus tridentatus ( T tridentatus ) and Tachypleus gigas ) ( T gigas ). T gigas and C rotundicauda are found in coastal Orissa and West Bengal. These crabs are benthic or bottom-dwelling anthropoids that use both estuarine and continental shelf habitats. They inhabit shallow area with soft or sandy bottoms, often areas of seagrass and legal growth where even the most vicious storms cannot harm these ancient creatures.

Commercially, the crabs mean big business. One of its most important contribution is Limulus Amoebocyte Lysate ( lal ), a reagent processed from its blood. lal has helped save many lives from bacterial diseases. The lal test is widely used for screening medical equipment for bacterial contamination and processing lal is a booming business in many countries such as the us .

However, like all creatures, horseshoe crab populations can become endangered if their environment is not suitability conserved. Though the Orissa coastline -- home to many of these crabs -- is relatively free from marine pollution, developmental activities along the state's Balramgarhi and Avana regions could bring in undesirable changes to the as-yet-pristine crab habitat.

Recent reports say the T tridentatus species of the crabs are disappearing along Japan's coastal belt. Habitat destruction and reproductive failure have contributed to considerable population decline here. To rescue the dwindling horseshoe crab population, Japan's authorities declared the crabs as national treasure. Exploitation of horseshoe crabs for biomedical research has also resulted in a decline in horseshoe crab populations in other parts of the world. It is indeed ironic that these creatures, around since the Paleozoic Era, are suffering population losses at the hands of humans today. An evaluation of the threats these crabs are facing has thus become crucial.

Erosion of nesting beaches can ruin the suitable nesting habitat of horseshoes crabs, while accumulation of beach sediment can kill the eggs in the nest. The most serious threat nesting female crabs face is from increased human presence on the beach. Beach disturbance can cause the horseshoe crabs to move to different beaches for spawning activity. Mechanised trawling, especially during the high tides, can also prevent the crabs from arriving on the shore to breed. Horseshoe crabs often get entangled in gill nets, hooks, traps, longlines, ropes, buoys and other marine and fishing equipment.

Beach armoring can also endanger the crabs. Construction of seawalls, structures, jetties, barriers and barbed wires threaten the horseshoe crab nesting habitat. As these break apart with time, they spread debris that impedes access to suitable nesting sites.

Beach nourishment, pumping, trucking, or depositing sand on the beach to replace what has been lost through erosion, cyclones, floods and storms, are known crab enemies. Sand brought in might differ from the native beach sediment and can affect the nesting selection site, incubation temperature, moisture content of the nest and the hatching success of horseshoe crabs. Nourishment projects involve heavy machinery, pipelines, increased human activity and artificial lighting.

Batch nourishment can also result in severe compaction or concretion of the beach. Another curious enemy of horseshoe crabs is the mosquito. For a horseshoe crab, mosquito bites can prove fatal. Stagnant water along the coast line, dead animals and fish create a breeding ground for mosquitoes. Further, studies have reported that horseshoe crab eggs and larvae are often eaten by fishes and birds . Crows, for instance, are reported to predate frequently on horseshoe crab's eggs.

Like their Japanese counterparts, horseshoe crabs along the Indian coast need to be viewed as a national treasure as well if they are to be preserved. The perspective plan should be scientifically-sound while appropriate management strategies must be evolved for sustainable utilisation of the crab population. A Coastal Zone Management Action Plan needs to be formulated for demarcating the horseshoe crab area. Documented effort describing the distribution and nesting habitat is desirable. The habitat of horseshoe crabs along the Orissa and West Bengal Coast needs to be carefully developed by using the guidelines of Marine Protected Areas ( mpa s). These mpa s act as management tools for the protection of marine habitats and species.

There is a need for identification and selection of certain beaches to be declared as protected areas and measures for proper protection should be executed. Protected zones should be set up at Balramgarhi and Avana on the lines of the rookery set up at Gahiramata beach which is a marine sanctuary to protect the turtles which are an endangered species. Preservation of areas for scientific research also need to be addressed. In order to achieve this it is essential to develop centers of excellence in marine ecology, mariculture, biotechnology and taxonomy for conserving the horseshoe crab population.

Stringent legislation and regulations pertaining to protection of horseshoe crabs should be enacted and enforced these should involve scientists, non-governmental organisations ( ngo s), governmental agencies and entrepreneurs in the formulation of research project design, its implementation and funding.

Further, horseshoe crab conservation efforts are strongly dependent on mobilisation of public awareness. Hence the masses need to be made aware of the problems these ancient wonders are facing today.

Subscribe to Weekly Newsletter :

Comments are moderated and will be published only after the site moderator’s approval. Please use a genuine email ID and provide your name. Selected comments may also be used in the ‘Letters’ section of the Down To Earth print edition.