Toxicity in Holothurians: A Geographical Pattern

Experiments on the toxicity of holothurians to fishes were conducted at Cocos Island (eastern Pacific), in Mexico, California, and Washington. Six of seven species of holothurians tested at Cocos Island are toxic to fishes. Five species in Mexico are toxic, one of two species in California is mildly toxic, and three of twelve species in Washington are mildly toxic to fishes. Feeding experiments confirm laboratory tests on toxicity. A summary of information is presented on the current status of our knowledge of toxicity in holothurians. The evidence supports the hypothesis that toxicity in tropical holothurians probably has evolved in part as a chemical defense mechanism against predation by fishes. THE PURPOSE of this investigation was to evaluate the hypothesis that toxicity in holothurians is inversely related with latitude and that this phenomenon is in part a reflection of fish predation, which differs in intensity with latitude and habitat (Bakus 1968, 1969). Studies were conducted at the Instituto Technologico de Monterrey, Escuela de Ciencias Maritimas y Technologia de Alimentos, Bahia de Bacochibampo, Guaymas, Sonora, Mexico (28? N), 30 December 1968 to 1 January 1969 and 2-4 April 1969; the University of Southern California Santa Catalina Marine Biological Laboratory (330 N) from 4 May to 11 May 1970; the Allan Hancock Foundation, University of Southern California, during 1969 and 1970; Cocos Island, eastern Pacific (6? N), from 29 March to 8 April 1972 and the University of Washington Friday Harbor Laboratories (48? N) from 24 July to 16 August 1972. MATERIALS AND METHODS Marine fishes and holothurians were collected by skindiving and with SCUBA, using the narcotizer quinaldine, dip-nets, and diving bags. They were maintained in aquaria. Tests were conducted to determine which holothurians are toxic to fishes and to obtain a rough approximation of the level of toxicity. Techniques used to study holothurians generally follow those of Bakus (1968) except where noted. Briefly, pieces of the animals are weighed (2 g) then placed in a beaker. A solvent (usually 95% ethanol) is added to the beaker and the solution is evaporated by boiling for 20 minutes. The extracted residue is dissolved in 100 ml of fresh seawater, distilled water, or tapwater in a fingerbowl. A fish is placed in the fingerbowl, and the time at which it dies is recorded. Experiments were terminated when fish showed consistently normal behavior or had died. Violent escape behavior, paralysis, and loss of equilibrium indicated the presence of a toxin. A strongly toxic holothurian is defined as causing death in fishes within 15 minutes (often < 10 minutes); a weakly toxic holothurian causes death in fishes from 20 to 45 minutes. Temperatures under which experiments were conducted were at or near the ambient for the site of capture, unless otherwise indicated, in order to simulate natural responses to toxins by fishes. Control experiments were conducted simultaneously with every test experiment. They consisted of placing fishes in the same volume of clean seawater or freshwater as that used in the experiments.