Strange Ark

Sharks are on the menu of a number of peoples. In most cases, sharks can be eaten without any problems. A few species, however, are known to be dangerous. The Greenland shark (Somniosus microcephalus) contains trimethylamine oxide in its flesh, which breaks down to the toxin trimethylamine. Ciguatera (poisoning caused by the accumulation of ciguatoxins in the fish's flesh) is known in sharks in the family Carcharhinidae. One shark, Carcharhinus leucas, was responsible for hospitalizing 188 people in Madagascar (with a 30% mortality rate) due to two new toxins found in its flesh and liver. This particular species had been eaten previously with no effects.

• Anthoni, U., et al. 1991. Poisonings from flesh of the Greenland shark Somniosus microcephalus may be due to trimethylamine. Toxicon 29: 1205-1212.
• Boisier, P., et al. 1995. Fatal mass poisoning in Madagascar following ingestion of a shark (Carcharhinus leucas): clinical and epidemiological aspects and isolation of toxins. Toxicon 33(10): 1359-1364.

This is one of the more interesting feeding behaviors in a fish. Cookiecutter sharks (Isistius braziliensis) are primarily invertebrate feeders, but are known to attack larger fish and whales, leaving behind holes as they gouge out chunks of flesh. This study showed that the species primarily attacked by these sharks off the coast of Brazil include Makaira nigricans, Tetrapturus albidus, Thunnus thynnus, Epinephelus niveatus, Prionace glauca, and Megaptera novaeangliae. These animals may constitute an important protein source for the sharks.

• Gonzalez, M., and C. Cunha. 1997. Fish and cetacean mutilation by cookiecutter shark Isistius Braziliensis (Squaliformes: Squalidae) in Sao Paulo coast, southeastern Brazil. (sic) Abstract. Conference Proceedings, American Society of Ichthyologists and Herpetologists, University of Washington, Seattle, WA.

There are a number of blind cave fish, and it's fascinating to see how they have adapted to a cave environment. A study on the Mexican tetra, Astyanax fasciatus, showed that the tetras had three geographically separate origins. Even more interesting was that in the El Abra populations, different genes were responsible for eyelessness in different cave systems. The study was unable to determine whether the "regressive evolution" was accomplished through selection or drift.

• Borowsky, R., and L. Espinasa. 1997. Regressive evolution in the blind Mexican tetra, Astyanax fasciatus. Abstract. American Society of Ichthyologists and Herpetologists. Conference Proceedings, 77th Annual Meeting, University of Washington, Seattle, WA.