Hong Kong's Horseshoe Crabs

by Dr. Huang Qin

The only four extant species of horseshoe crabs (Xiphosura), i.e. Limulus polyphemus Linnaeus 1758, Tachypleus gigas Muller 1785, T. tridentatus Leach 1819, and Carcinoscorpius rotundicauda Latreille 1802, are sometimes called "living fossils" because they have changed little morphologically compared with their fossil relatives, of which the oldest species was identified from the Upper Silurian (~450 million years B.P.).

There are other things which make horseshoe crabs of value, but the most persistent study on these animals has focused on the properties of their blue blood. In 1977, The Food and Drug Administration of the United States approved a new test for identifying endotoxins using Limulus Amoebocyte Lysate (LAL) purified from horseshoe crab blood, and the LAL test has since developed for commercial use by the Associates of Cape Cod.

In spite of their commercial and medical importance, horseshoe crabs are threatened by the loss of living and breeding habitats and the populations have declined rapidly in only a few decades. In Hong Kong, according to monthly reports related to a marine ecological survey by the Provisional Airport Authority (1995), no horseshoe crabs were collected from the Sha Chau area, adjacent to Hong Kong's new airport at Chek Lap Kok, although the area was once said to be a thriving habitat of the animals.

Alarmed by the dramatic population decline in horseshoe crabs worldwide, scientists from different countries have appealed for their protection. In order to ensure a source of material for biochemical work, a colony of horseshoe crabs was established in the mariculture laboratory of Israel Oceanographic and Limnological Research Ltd. in Elat in 1977. In Japan, the Association for Conservation of Horseshoe Crabs was established in 1979. In China, one of eighteen Marine Conservation Areas for Rare and Endangered Species has been reserved for the horseshoe crab Tachypleus tridentatus population that returns to Pingtan Island, Fujian Province, to breed.

Little, however, is known about the biology and ecology of Hong Kong's horseshoe crabs, except for a few taxonomic and distributional records in which two species, T. tridentatus and T. gigas, were reported. According to Hill & Phillipps (1981), an individual of T. tridentatus "measures about 4-6cm across the carapace and can be found on certain sandy shores at low tide. The relatively enormous T. gigas measures up to 40cm across the carapace and ... is often caught in fishermen's nets". It was a mistake to identify T. tridentatus as the "small species". In fact, T. tridentatus is the largest of the three Asiatic species of horseshoe crabs and has been reported to have a maximum length of 70cm from Japan and 81cm from China. The largest specimen of this species is recorded to have a total length of 92cm. In contrast, the other two species, i.e. T. gigas and Carcinoscorpius rotundi-cauda, have average lengths of 35cm and 30cm respectively. The maximum recorded total length of T. gigas is 50cm.

Nevertheless, the occurrence in Hong Kong of fully-grown individuals of T. tridentatus and small horseshoe crabs with lengths of <12.2cm, which is close to the size Hill ascribed to T. tridentatus, have been confirmed in recent investigations by researchers at the Swire Institute of Marine Science. T. tridentatus, recognizable by its large size, was said to be common on many local sand-mud beaches during the summer, but has been seen less frequently in recent years. In spite of the noticeable decline in the numbers of animals coming onto local beaches, individuals of this species can still be collected by commercial trawlers and are seen regularly in the tanks of seafood restaurants.

It is unlikely that the small horseshoe crabs mentioned above deserve a new name other than the three extant Asiatic species, the taxonomy of which has undergone careful revision. In fact there is evidence suggesting that those small individuals could be juveniles of one or other of the horseshoe crabs recorded from Hong Kong.

Born in summer, the trilobite larvae of Tachypleus tridentatus stay on the natal sandy beach for the first winter and move to a nearby mud flat in the spring (Mikkelsen, 1988). Ecdysis occurs in the second summer and the total body length of juveniles increases at a rate of 1.4 times after each moult. The young animals dwell on the mud for about seven to nine years until a total body length of over 15cm has been reached before they migrate to deeper waters in the sea. Individuals over 22cm in length can be sexed morphologically, but are rarely found on beaches except during the breeding season. Such size variations in juveniles of T. tridentatus overlap with those recorded for the local small horseshoe crabs.

Tachypleus gigas has also been reported to be common in Hong Kong's waters, and individuals of this species co-occur with T. tridentatus in such areas as Deep Bay, Tai Po (Tolo Harbour) and the east coast of Peng Chau Island. The other Asiatic species of horseshoe crab, Carcinoscorpius rotundicauda, was not previously recorded from Hong Kong but was identified by the author in April 1997, from the near-shore mud flat on the Ma Wang Chung side of Tung Chung Bay, Lantau Island.

Like T. tridentatus, juveniles of T. gigas and C. rotundicauda live on mud flats for their early life. There are, therefore, practical questions regarding the identity of the local small horseshoe crabs described by Hill & Phillipps (1981) and by Morton & Morton (1983), and the extent of sympatric use of local habitats by the juveniles of different horseshoe crabs. A solution to these questions will not only help stabilise the taxonomy of horseshoe crabs in Hong Kong but, more importantly, aid identification of their breeding and growing beaches, a critical step in conserving these long-lived animals.

Although adults, especially males, of the four species of horseshoe crabs are morphologically distinguishable, it is difficult to identify juveniles or even adolescents, particularly of females. In contrast to the morphological similarity between animals, their genetic divergence has been shown to be constant at both the protein and DNA levels, without an age effect. Using protein electrophoretic techniques, it was shown that the larval hybrids, both of T. tridentatus (F) x C. rotundicauda (M) and of T. gigas (F) x C. rotundicauda (M), had both parental patterns present in the monomeric haemocyanins. Protein electrophoresis has also demonstrated that the level of protein polymorphism in horseshoe crabs is comparable to that reported for other organisms which have evolved more rapidly in terms of morphology.

It is, therefore, possible to employ genetic techniques to identify juveniles of horseshoe crabs which occur on local beaches. In a present study at SWIMS, allozyme electrophoresis has been considered more attractive for this purpose than other molecular methods because it is much faster and less costly. It is also anticipated that this work will provide information on genetic variation in the three Asiatic species of horseshoe crabs, and that such knowledge can be applied to the study of population genetics of horseshoe crabs from Hong Kong and other Asian areas.

References
Hill, D.S. & K. Phillipps, 1981. A Colour Guide to Hong Kong's Animals . Hong Kong: Government Printer.
Mikkelsen, T., 1988. The Secret in The Blue Blood. Science Press, Beijing, 124 pp.
Morton, B. & J. Morton, 1983. The Sea Shore Ecology of Hong Kong. Hong Kong University Press, H.K., 350 pp.

WE NEED YOUR HELP
We would be grateful if any person encountering dead horseshoe crab specimens on Hong Kong beaches could collect them for our identification and measurement purposes. Sending in sightings (photos if possible) of living animals with information on the date of sighting and precise location would be invaluable to the study.

We, Dr. Huang Qin and Helen Chiu (pager 73321611), can be contacted at the Swire Institute of Marine Science

P.17-18

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