The blue-blooded sea creature on Odisha’s coastline

Horseshoe crabs have been around for millions of years, but their habitats are increasingly being degraded. While scientists have now begun tagging them to track their movements and study them better on the beaches they come to nest on, The Hindu finds that they are also in demand for the medical industry

Strolling along Odisha’s Balasore coast, Bharat Jena, a fisherman in his 70s, talks proudly of encountering countless marine fishes, prawns, and crabs over his half a century of fishing. But there is one creature that never excites him: a hard-shelled, 12-legged crab. When he does find it caught in his net, Jena disentangles it and tosses it back into the water. To him and other marine fisherfolk along the Odisha coast, the horseshoe crab is worthless, because it is not eaten. Jena doesn’t encounter them that much any more though; their numbers have gone down over the years.

For ecologists and marine biologists though, the horseshoe crab is of particular interest. Found only on select coasts around the world, it has survived 445 million years, as fossils show, without undergoing any morphological change. It belongs to a class called Merostomata, living fossils, or those organisms that haven’t changed in millennia. Today, its existence is threatened by anthropogenic changes in the ocean.

The dwindling population of horseshoe crabs and their shrinking spawning grounds have raised concerns among scientists. In August this year, for the first time in over a century, scientists of the Zoological Survey of India (ZSI) in collaboration with the Odisha Forest Department began tagging horseshoe crabs — 12 to begin with — to understand their population dynamics and habitat use, a critical step towards their conservation and management.

Early findings suggest that one tagged horseshoe crab travelled nearly 40 km, while another moved 2.5 km. Unfortunately, two crabs were found dead, having been caught in trawlers’ fishing nets. Each tag contains a serial number and a phone number, encouraging fishermen and marine enthusiasts to report sightings. This helps scientists track the crabs’ movements and better understand their mortality.

“If research shows a significant decline in horseshoe crab populations, the government will be advised to either restock the population through artificial breeding or protect spawning grounds,” says Basudev Tripathy, a senior scientist at ZSI, who is leading the tagging study.

On Odisha’s coastline

Nanigopal Mandal, 55, is mending his fishing net in Inchidipai village, 18 km from Chandipur, home to India’s missile testing centre. Struggling to recall the local name for the horseshoe crab, he explains how fishermen call it ‘Ramlakhan’ after the inseparable mythological brothers Ram and Lakshman, inspired by the sight of male crabs clasping onto females as they head to shore together. In Odisha, some refer to the crab as ‘Ramlekhani’; in Bengal it is called ‘Lakhan’.

The ZSI first reported the species in 1909. Although the beaches of Balasore were once teeming with horseshoe crabs, their numbers have drastically declined since then. Despite their ability to survive millennia, no major scientific research in India has been conducted on them. The International Union for Conservation of Nature (IUCN) also slots horseshoe crabs in the ‘data deficient’ category, highlighting the lack of comprehensive information about the species. However, the Wildlife Institute of India (WII) conducted the first detailed survey of horseshoe crabs along the east coast of India in 2005. This survey resulted in the listing of the crab in the Schedule IV of the Wildlife (Protection) Act, 1972.

The crab’s blue blood is used to produce a rapid diagnostic reagent, limulus amebocyte lysate, essential for testing the toxicity of injectable drugs. One litre of horseshoe crab blood can fetch a few lakh rupees due to its biomedical applications in America. A report in Fortune magazine says 30% of the blood is drained before the crab is put back into its habitat. In the process, up to 30% of crab lives can be lost.

In the late 1980s, marine biologist Anil Chatterji, then working with the National Institute of Oceanography (NIO) in Goa, studied the distribution of horseshoe crabs along Odisha’s Balasore coast, particularly at Balaramgadi beach. “In 1987-88, I visited the Odisha and West Bengal coasts in search of horseshoe crabs. A pharmaceutical lab was interested in producing a diagnostic reagent from the crab, and it had asked one of the Council of Scientific and Industrial Research laboratories to investigate its population in India,” Chatterji recalls.

The task to locate horseshoe crab locations was assigned to the NIO. “We started our survey in the Sundarbans, but it was a very disappointing experience — no crabs were found. We then moved on to Digha (West Bengal) and further south of Odisha to Balasore, Puri, and Paradip, but the initial survey ended in failure, as no one spotted a single horseshoe crab,” he says.

On the advice of fisherfolk in these coastal areas, the scientists finally visited Balasore’s Balaramgadi beach on a full moon night. They waited. “Then the sun rose, and around 6 a.m., the pristine beach was covered with horseshoe crabs that had come to breed,” says Chatterji, who still remembers the sight with wonder.

There are four species of horseshoe crab: the mangrove (Carcinoscorpius rotundicauda), which inhabits the coastal waters of South and Southeast Asia; the Atlantic or American (Limulus polyphemus), found along the Atlantic coast of the United States and the southeastern Gulf of Mexico; the coastal (Tachypleus gigas), also native to South and Southeast Asia; and the tri-spine (Tachypleus tridentatus), found in Southeast and East Asia. India is fortunate to have two species: Carcinoscorpius rotundicauda and Tachypleus gigas, both found along the Odisha coast.

Procreating with nature

Scientists say tides play a crucial role in the arrival of horseshoe crabs on Odisha’s beaches. Chatterji says during high tide, the crabs come ashore, when the waves rise to 2.6 metres. Their numbers peak when the waves reach 3.1 metres. Full moon and new moon are the ideal occasions, with the waves rising high. Their choice of beach also depends on the sediment composition.

Horseshoe crabs choose sandy beaches where the grain size ranges between 63 and 120 microns, Chatterji says. This specific sand grain size range allows for optimal water retention, which occurs about six to seven inches below the surface. If the grains are larger, water retention is poor, making the site unsuitable for spawning.

Mature female crabs come to these beaches to lay eggs and release 400-500 eggs per clutch. Males are typically attached to the females during this process. Water is essential for the activation of the eggs, once they are produced; without it, the eggs remain dormant.

Nesting takes place along the margin of the high tide line, ensuring the eggs remain inundated as the tide rises and falls. Over a period of 40 to 42 days, the eggs incubate and the embryos develop. Temperature also plays a role in this process, and the Odisha coastal weather helps them breed through the year.

In Balaramgadi, the intertidal zone extends up to 7 km into the continental shelf, followed by a deeper zone. It is within this shelf area that hatching takes place. The horseshoe crab uses 10 legs to walk and two to feed.

A loss of habitat

The species is endangered by the gradual loss of its habitat. The spawning and nursery grounds along most parts of Odisha’s coast have been damaged. Tripathy identifies 18 sites where horseshoe crabs were once spotted. He says the crabs found the State’s sandy beaches and estuarine mudflats near mangrove forests favourable. “These crabs feed on decomposed insects and algae. However, the mangrove forests are thinning, and sandy beaches are undergoing transformation due to human activities. Shoreline fortifications like stone patching and the use of geotubes to protect the shore from erosion have worsened the situation,” he adds.

Being an intertidal species, horseshoe crabs do not naturally come ashore. Without their preferred habitats, they lay eggs irregularly and younger crabs become easy prey for natural predators like crows and dogs.

Scientists are frustrated that India is yet to establish a robust conservation plan for the crab. “This is an extremely important species. We need to develop a conservation strategy,” says ZSI director Dhriti Banerjee.

However, the Odisha government has conferred Sidharth Pati, a young researcher and conservationist, with the Biju Patnaik Award for Wildlife Conservation 2024. Pati, an IUCN committee member, runs the non-profit Association of Biodiversity Conservation and Research, which has rescued more than 2,000 horseshoe crabs trapped in fishing nets.

“Unregulated regular fishing activities, rough stone patching (stones placed to protect the beach from erosion), and discarding of fishing nets along the coast are major reasons behind horseshoe crab mortality. Crabs wash ashore in the tide and subsequently get trapped in the cracks of stone patching on the beach,” he says.

B.C. Choudhury, former scientist with WII and now associated with the Wildlife Trust of India, another non-profit, says all safe spawning grounds of horseshoe crabs along the Balasore coast have been documented, particularly where there is no fishing. “The Defence Research and Development Organisation has protected a fairly large stretch of the Balasore coast as a no-activity zone. A conservation reserve can be explored here to protect the crab’s spawning ground,” Choudhury suggests.

Odisha’s Principal Chief Conservator of Forests (Wildlife) Susanta Nanda says the areas inhabited by horseshoe crabs have already been declared Coastal Regulatory Zone-1 (A) under the Integrated Coastal Zone Management Project that aims to protect coastal ecosystems and coastal communities. “In those areas, activities are not permissible,” he says.

The fishermen say there are already turtle protection zones to facilitate natural breeding along the coast. Frequent cyclones and low pressure areas reduce their time at sea. “One more prohibition for horseshoe crabs will adversely impact our livelihood,” Mandal points out.

Published - October 11, 2024 03:04 am IST