POTENTIAL FOR DEEP-SEA INVASION BY MEDITERRANEAN SHALLOW-WATER ECHINOIDS - PRESSURE AND TEMPERATURE AS STAGE-SPECIFIC DISPERSAL BARRIERS

Hypotheses about the origin of the deep sea fauna often assume that th e deep sea was first colonized by cold water animals migrating through isothermal water columns in polar seas. Deep water in the Mediterrane an Sea has much warmer temperatures than comparable depths in the larg er ocean basins. Moreover, the entire water column may be virtually is othermal during the winter months, making oceanographic conditions in the Mediterranean analogous to those prevailing throughout most of the world ocean during the Mesozoic and Cenozoic. We investigated the phy siological potential for deep sea invasion through a warm water column by studying the pressure and temperature tolerances of embryos and la rvae of 3 species of shallow water Mediterranean echinoids, Paracentro tus lividus, Arbacia lixula, and Sphaerechinus granularis. Early life history stages of all 3 species tolerated pressures (to 150 atm) much higher than those experienced in the adult environment. Cold temperatu res (<10 degrees C) exacerbated the adverse effects of pressure; larva e were more likely to survive at deep sea pressures and warm temperatu res than at shallow water pressures and cold temperatures. Tolerances to high pressures and low temperatures increased with ontogeny and var ied with species. In the Mediterranean, high pressures should be a mor e important limiting factor than low temperatures. Nevertheless, some species have physiological tolerances that should allow them to coloni ze bathyal depths. Absence of these shallow water species from such de pths must be attributed to factors other than pressure and temperature .