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1 Introduction

Horseshoe crabs in the world are now represented by four extant species (Sekiguchi and Nakamura 1979). Two species namely Tachypleus gigas and Carcinoscorpius rotundicauda inhabit coastal waters of northeast coast of India along the Bay of Bengal (Fig. 1). Sekiguchi et al. (1976, 1988) had earlier reported the availability of the species along the coast of West Bengal. The very presence of these animals in a coastal zone indicates the health of the environment (Chen et al. 2004), as it indicates that these conditions are suitable for their survival, reproduction and development. But it is now a major concern that the population of Limulus polyphemus along the east coast of United States is in a declining trend (Rudloe 1982, Widener and Barlow 1999, Carmichael et al. 2003) as is also the case for Tachypleus tridentatus along the coast of Japan (Botton 2001). But the reasons for the population depletion for these two species and the degree of their exploitation are different. In the case of Limulus, exploitation is economically driven by fisheries for bait and LAL (Berkson and Shuster 1999, Carmichael et al. 2003), whereas the Japanese species lost its ground primarily because of the loss of spawning habitat (Botton 2001). But, in both the cases, the threat to the horseshoe crab population is man made and needs attention.

Fig. 1
figure 1

Location of study sites and distribution of T. gigas (Tg) and C. rotundicauda (Cr) along the northeast coast of India. Horseshoe crabs were observed at the three southernmost sites, and nesting was observed at the five northernmost sites. AP = Andhra Pradesh; WB = West Bengal

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In the case of two Indian species, T. gigas and C. rotundicauda, there is no known source of commercial exploitation for the production of amebocyte lysate or for bait as in the case of Limulus (Levin et al. 2003). But the animals are being used by traditional healers in the form of a liquified substance for curing joint pains (Fig. 2) and at times females are preyed upon by crows for their eggs but not to the extent that Limulus eggs are preyed upon by shorebirds along the east coast of United States (Botton and Shuster 2003). Though the above factors may not be the sole cause of population declines in Indian species, it has been observed during this study period that horseshoe crab populations along the Indian coast, particularly T. gigas, are declining very fast. Several studies have been carried out on Indian horseshoe crabs (Roonwall 1944, Sekiguchi et al. 1976, Chatterji et al. 1988, Debnath 1991, Mishra 1994) but they lack information on the population density. However, other studies suggest that there is a significant decline in the L. polyphemus population along east coast of United States (Widener and Barlow 1999, Berkson and Shuster 1999, Carmichael et al. 2003, 2004, Botton et al. 2006) and in T. tridentatus populations along the coast of Japan, Hong Kong, and Taiwan (Morton 1999, Botton 2001, Chen et al. 2004).

Fig. 2
figure 2

A Traditional healer selling medicine prepared out of T. gigas and C. rotundicauda on the streets of Kolkata (West Bengal), India

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The objective of this chapter is to discuss the eco-biological status of T. gigas and C. rotundicauda including their distribution pattern, nature of coexistence, nesting behavior, and the necessity for their ecological restoration along the northeast coast of India.

2 Materials and Methods

This chapter is based on the studies carried out by the author extending over a period of 19 years from 1988 to 2007. During this period horseshoe crab habitats along the northeast coast of India including West Bengal (WB), Orissa, and Andhra Pradesh (AP) were investigated (Fig. 1). An extensive survey was carried out in studying the natural habitat of T. gigas and C. rotundicauda. Simultaneously nesting behavior and ecological conditions of the breeding zone of both the species were also investigated. However, most of the studies were carried out with special reference to T. gigas due to easy accessibility to its natural habitat and the breeding beaches.

Spawning activity of the species was studied by egg laying behavior of the animal and the number of nests they make in each high tide to lay eggs. Egg densities were quantified by counting the number of eggs in each nest at Balaramgari (Lat. 21° 17' N; Long. 87° 00' E) in the vicinity of Budhabalanga estuary and at Kasafal in the vicinity of Haskura estuary (Lat. 21° 31' N; Long. 87° 09' E), where the congregation of breeding pairs of T. gigas were found to be highest (Mishra, 1991, 1994, Mishra et al. 1992).

3 Results

It was found that the horseshoe crabs are distributed along the northeast coast of India, extending from the Sunderbans (estuarine mangroves of Ganges) in the extreme north of West Bengal (WB) coast through the coasts of Orissa and northern Andhra Pradesh (AP) in the south of the Bay of Bengal (Table 1). Sekiguchi et al. (1976) reported that T. gigas are distributed along the coast of WB. During this study, it was observed that C. rotundicauda are available particularly along the coast of WB including the Sunderban mangroves of River Ganges up to Digha in concurrence with other reports (Chatterji et al. 1988, Itow et al. 2004). T. gigas are distributed along the coast of Orissa up to the coast of Visakhapatnam along northern AP in the south at a depth of up to 20–30 m. Sekiguchi (1988) reported that at Digha and Junput (coast of WB), both T. gigas and C. rotundicauda are available. Similarly along the coast of Orissa, coexistence of these two species were observed at Hukitola Bay (Lat. 20° 24' N; Long. 86° 48' E), which was found to be a unique habitat of sandy estuarine beaches and mudflats associated with mangrove forests suitable for nesting by both species. Although the distribution of horseshoe crabs stretches along a vast coast line of northeast Bay of Bengal with varying coastal geomorphology, the breeding migration and nesting behavior of the species were unique to the individual species and they navigate to particular breeding zones of their preference for egg laying.

Table 1 Distribution and nesting sites of T. gigas and C. rotundicauda along the northeast coast of India. ‘O’ stands for availability and ‘–’ for non-availability of eggs and juveniles
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It was found that T. gigas prefers the sandy estuarine zones for laying eggs, whereas C. rotundicauda lays eggs in the mangrove mud flats (Sekiguchi et al. 1976, 1978, Chatterji and Parulekar 1992, Mishra 1994, Itow et al. 2004). Also the nesting pattern and number of eggs in each nest varies in these two species (Table 2). In case of T. gigas, breeding is found to be perennial with maximum during the months of July to February. The intensity of nesting was closely related with the textural properties (mean grain size) of the sediments of the estuarine breeding beach at Balaramgari. The species preferred a range of 63–125 μm mean grain size for nesting and any deviation from this affected their nesting behavior (Mishra, 1991). This phenomenon was in confirmation with the shifting of the nesting site from one place to another accompanied by the changes in sand grain size driven by the monsoon pattern at this breeding beach. Similarly eggs and juveniles were found in the mangrove mud flats at Hukitola. Though the breeding time of C. rotundicauda in Indian waters is not yet studied, in the waters of Thailand they breed throughout the year (Sekiguchi and Nakamura 1979).

Table 2 Nesting behavior of T. gigas and C. rotundicauda
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The most important observation was on the destruction of breeding beaches, which is very important for the egg-laying activity of both the species. Along the coast of Orissa at Balaramgari, there is a tremendous change in the intertidal sands due to the shrinkage of nesting area caused by human activities such as maintenance work by fishing trawlers on the beach itself (Fig. 3) and shifting of beach sands for construction purposes at times. These may be the major cause of damage to the main nesting sites (about 200 m2) of T. gigas at Balaramgari and there is a sharp decline in the number of nests (identified by their concave shape) (Fig. 4).

Fig. 3
figure 3

Breeding beaches at Balaramgari in the vicinity of Budhabalanga Estuary, showing the sand bar adjacent to creek where nesting by Tachypleus gigas occurs

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Fig. 4
figure 4

Number of fresh horseshoe crab nests found at Balaramgari, northeast coast of India

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4 Discussion and Conclusions

Breeding migrations of horseshoe crabs are found throughout the year, which is dependent upon the preparedness of the breeding beaches with a maximum percentage of sand with grain size between 63 and 125 μm as in case of T. gigas. This phenomena was observed in one of the much studied site at Balaramgari, where breeding pairs navigate to either site of the estuary sensing the availability of the preferable sands in that area (Mishra 1991) at different times of the year. However, the mechanism behind these environmental phenomena needs a detailed study.

Several anthropogenic activities, mainly related to fisheries and construction works along the coastline, are posing serious threats by destroying the natural breeding beaches. This restricts the migrating pairs from finding a suitable nesting site for spawning. Unlike their American counterpart (Rudloe 1982, Berkson and Shuster 1999, Widener and Barlow 1999, Carmichael et al. 2003, Levin et al. 2003) horseshoe crabs in India are not subjected to commercial exploitation for biomedical, fertilizer, and bait purposes. The real threat comes from the human population, which plays an important role in the coastal ecology (Botton et al. 2006). In addition, natural calamities such as super cyclones and tsunamis cause serious damage by shifting the sands from the beaches.

A constructive approach is needed to protect the intact, undisturbed breeding zones in India to help honeymooning horseshoe crab couples to come ashore and lay eggs. As it is evident from the present status of breeding beaches at Balarmgari, it is a matter of concern that the day is not far away when we will not be able to encounter breeding pairs. Efforts should be made by creating public awareness, educating fisher folks, providing alternative sites for boat building and repairing, and if required, enacting laws to protect these precious guests from the brink of extinction from our coast.