Distribution

1. C. rotundicauda and T. gigas in the Indo-Pacific, from the Bay of Bengal across the South China Sea to the Philippines
2. Indo-Pacific
3. Islands of the Pacific

Fig. 4.4 Crab (Zozymus sp.)

Fig. 4.5 Horseshoe crab (Tachypleus gigas)

Biology

Of the marine arthropods (segmented animals), there are poisonous species among the Xiphosura order (Horseshoe crabs) and the Decapoda order (Decapods).

There are only 5 Horseshoe crab species that still exist today. They are included in the larger grouping Chelicerata, which also includes spiders and scorpions, among other animals. The entire body is surrounded by a large, concave shell (up to approx. 30 cm in length), at the rear of which there protrudes a (non-toxic) caudal spine (Fig. 4.5). They live in sandy and muddy coastal stretches, and during the reproductive period they migrate to the intertidal zone to lay their eggs. It is only during this period that the flesh, viscera and eggs are poisonous. A substance similar to saxitoxin, as well as tetrodotoxin, have been isolated from Horseshoe crabs.

The crab species listed above are only sporadically poisonous and are popularly eaten in particular in the Indo-Pacific region. Saxitoxin and gonyautoxins, but also tetrodotoxin and palytoxin, have been found distributed throughout the body in several species. These toxins are ingested in high doses over the food chain during certain periods, and are connected in particular with shellfish, Puffer and ciguatera poisoning.

Coconut crabs (Birgus latro), which may be up to 30 cm long, live primarily on land and only enter the water for reproductive purposes. They feed on plants, including coconuts that they cut down from the palms themselves. The toxins found in poisonous specimens appear to be plant toxins that have been ingested with their food.

Risk

The incidence of Horseshoe crab and crab poisoning is not known.

Eggs from the Horseshoe crabs named above, although periodically poisonous, are a popular delicacy in the Asian region. Serious cases of crab poisoning occur sporadically in the Indo-Pacific region. Poisoning may also occur after the consumption of soups in which crabs have been cooked.

The Coconut crab is eaten on several islands of the Tuamotu Archipelago. Isolated cases of poisoning have been reported from this region, a few of which are believed to have ended fatally (Bagnis 1970). On the Ryukyu and Amami Islands, 19 cases have been described, with 4 deaths (Hashimoto et al. 1968, cited in Bagnis 1970).

Signs and symptoms

Depending on the concentration of poison ingested, the same signs and symptoms may occur as seen in paralytic shellfish poisoning (saxitoxin) or tetrodotoxin poisoning.

Treatment

Symptomatic (see Diagnosis & Treatment: Poisonous animals).

Literature

Bagnis 1970, Halstead 1988, 2001b, Kungsuwan et al. 1987, Mebs 1992