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Hypotheses about the origin of the deep sea fauna often assume that the deep sea was first colonized by cold water animals migrating through isothermal water columns in polar seas. Deep water in the Mediterranean Sea has much warmer temperatures than comparable depths in the larger ocean baslns. Moreover, the entire water column may be virtually isothermal 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 physiological potential for deep sea invasion through a warm water column by studying the pressure and temperature tolerances of embryos and larvae of 3 species of shallow water Mediterranean echinoids, Paracentrotus 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 temperatures (<1O0C) exacerbated the adverse effects of pressure; larvae were more likely to survive at deep sea pressures and warm temperatures than at shallow water pressures and cold temperatures. Tolerances to high pressures and low temperatures increased with ontogeny and varied with species. In the Mediterranean, high pressures should be a more important limiting factor than low temperatures. Nevertheless, some species have physiological tolerances that should allow them to colonize bathyal depths. Absence of these shallow water species from such depths must be attributed to factors other than pressure and temperature. K E Y WORDS: Mediterranean . Echinoids Pressure . Barophysiology

Potential for deep sea invasion by Mediterranean shallow water echinoids:pressure and temperature as stage-specific dispersal barriers