Bengal's horseshoe crabs in dire need of protection

Imagine a species that has been around for the past 450 million years predating dinosaurs, and their blood being a valuable component in the testing of medicines, vaccines and sterile medical equipment, slowly disappearing from the coasts of West Bengal over the last 5-7 years.

Horseshoe crabs, which are marine chelicerate arthropods living in shallow coastal waters along the Bengal coastline, once found in abundance, are now moving away from the said region due to anthropogenic activities, social and environmental factors. India finds two among the four different species of horseshoe crabs. They are the Tachypleus gigas and Carcinoscorpius rotundicauda.

Interestingly, horseshoe crabs are commonly referred to as living fossils as they have kept their morphology more or less unchanged for millions of years. As they are surviving for a long time, they are tightly woven into their environment. Their vanishing symbolises an impending threat to the ecological balance of the region.

According to the Zoological Survey of India (ZSI) report in 2014-15, Tachypleus gigas (TG) was found in sufficient numbers in some places of South 24-Parganas and stretches of East Midnapore. Around 100 sightings were reported along the coastline of Bakkhali, Canning, Namkhana and Sagar Islands in South 24-Parganas while in Digha, Shankarpur, Mandarmani along the East Midnapore coasts the number remained 200-300 during 2014-15. When a study funded by the Department of Biotechnology, Government of India, searched these areas in the recent past, it barely found one or two male horseshoe crabs in these areas.

While 236 TG were found in Digha in 2014-15 (out of which around 180 were part of mating pairs), Shankarpur accounted for a total of 161 TG (of which 120 were reported as mating pairs). In Sagar Island out of 67, around 32 were mating pairs while in Frazerganj out of 59, around 30 were mating pairs. In Prentice Island, there were 40 mating pairs out of a total of 81 reported in that same period. Around 30 mating pairs were found in Canning out of a total of 74. The survey is mainly carried out after the advent of monsoon till January/February. However, over the past two years, these sites reported no mating pairs and even the male TGs are rarely found along the Bengal coast.

"Over the past two years, we spotted two Tachypleus gigas males in Sagar Island, one at Shankarpur, none at Digha, three in Canning, and one in Bakkhali (Frazerganj). All were males. No mating pairs were found at all. This study was done in July to February over the two years, a time when you expect them to come to these shores to breed in abundance," said Dr Sumit Biswas, Associate Professor, Department of Biological Sciences, BITS, Pilani - K K Birla Goa Campus.

The situation is not that bleak when Carcinoscorpius rotundicauda (CG) is considered. While the previous data (2014-15) reported around 250 to 350 in these sites, the species is still being found, though not in the same abundance as the earlier reports.

Utpal Mallick, a research scholar working on the conservation of horseshoe crabs, said: "Scientific reports suggest two species are found in Bengal, Odisha and also in Bangladesh. In the last two years, our team visited Sagar Island, Digha and other coastal areas on three-four different occasions, but failed to find one in multiple erstwhile sites. To our surprise, we did not even find one female TG in the Sagar Island zone and no animals in Digha. Due to construction along the beaches, the texture of the sand changed."

Researchers believe that horseshoe crabs, more specifically the Tachypleus gigas might probably be finding better habitats along the Odisha coasts. Dr Biswas stated that as the number of horseshoe crabs is still abundant in the Odisha region, it is presumed that the species are moving away from the Bengal coast. The disappearance is an indicator of the ecological damage and the ecosystem along the Bengal coastline that is severely disturbed, leading to changes in the coastal morphology and the subsequent loss of habitat, Dr Biswas added.

The horseshoe crab's milky-blue blood contains a special clotting agent called Limulus Amebocyte Lysate (LAL), which detects a contaminant called endotoxin in pharmaceutical samples. Even a minute amount of endotoxin present in vaccines or injectable drugs can lead to dangerous consequences. The blue blood of the horseshoe crab provides this invaluable asset (LAL) used to test vaccines, drugs and medical devices to ensure they are not contaminated with dangerous bacterial toxins.

According to a story published by National Geographic in July 2020, horseshoe crab blood was key to the launch of the COVID-19 vaccines, albeit at a cost to the ecosystem. According to Prof Biswas, over the last two years, the rollout of the Covid vaccines was earmarked by one essential step — the test for detection of endotoxins in these vaccines. The blood of the horseshoe crab, rich with LAL, was the saviour. For decades, the pharmaceutical industry has been using the LAL test as a unique benchmark for endotoxin detection in any vaccine or medicine.

As per established evidence, the animal evolved together with trilobites during the Paleozoic Era (540-248 million years). Other primitive arthropods called trilobites disappeared at the end of the Paleozoic era. But the horseshoe crab survived several mass extinctions, including the Cretaceous-Tertiary (K-T) extinction event that obliterated dinosaurs approximately 65 million years ago.

Experts said the primary causes behind the declining number of horseshoe crabs can be attributed to the erosion of the coast and anthropogenic activities. Since female TGs prefer a sandy substrate for breeding, the increase in the silt content in these areas of West Bengal has led to an aversion to these sites. The natural habitat is destroyed due to unplanned embankments, the use of mechanised boats, and often Chinese fishing nets in which hundreds of these animals are trapped and left to die.

Debasis Shyamal, president of Dakhsinbanga Matsyajibi Forum and a member of West-Bengal-based National Platform for Small Scale Fish Workers, said: "We haven't seen horseshoe crabs in large numbers along the Bengal coastline for the past 4-5 years. Pollution and unscientific fishing like bottom trawling and destructive fishing gears are destroying the natural habitat. Even the marine fish catch has decreased by 60 per cent in the last 10-15 years. As a result, coastal ecology is also getting impacted. No awareness is conducted to inform the fishermen about the valuable species."

Prof Dipak Ranjan Mandal, a retired Vice-Chancellor of Sidho-Kanho-Birsha University in Purulia and an expert in the field of parasitology said the abundance of horseshoe crabs is drastically falling in this region. "Even there is no evidence that animals are mutually fighting. There may be some changes happening with climate change. If this trend continues, it is quite alarming. They withstood the evolutionary forces for a long time because of their immune system," said Prof Mandal.

According to an article published in 'The Pharmacologist', a publication by the American Society for Pharmacology and Experimental Therapeutics, Frederik Bang, a pathologist who routinely used marine organisms to gain insights into biological mechanisms of clinical significance and was later appointed Chairman of the Department of Pathology at Johns Hopkins School of Public Health in 1953, discovered special cells in horseshoe crab blood called amoebocytes. These cells attach to the bacteria forming a viscous gel that prevents it from invading the horseshoe crab's body. Bang realised that these cells could be used as a fast and efficient way to test pharmaceutical drugs for the presence of bacteria. He travelled extensively all the way to Calcutta and researched parasites and diarrhoea as well.

Biomedical companies persistently harvest blood from horseshoe crabs to produce Limulus Amebocyte Lysate (LAL). In the USA, Japan, and East Asia, the animals are captured in huge numbers for extracting LAL used by pharmaceutical companies. Even NASA is now testing the use of LAL in space to assist in the diagnosis of astronauts.

Over the years, several applications have been using the LAL of horseshoe crabs and it still remains a unique and essential component of the pharmaceutical industry. The fact that the population has dwindled so significantly over the years along the coasts of West Bengal is a glaring pointer to the declining ecological balance in these areas. Being a resilient organism, the signs are even more worrying and something needs to be done on a war footing.

"Beach restoration and stopping anthropogenic activities at the breeding grounds are the need of the hour. We have to earmark the breeding grounds and constructions must stop in these regions if we at all want to save an ecological disaster," said Prof Mandal.