TEMPERATURE AND PRESSURE TOLERANCES IN DISPERSAL STAGES OF THE GENUS ECHINUS (ECHINODERMATA, ECHINOIDEA) - PREREQUISITES FOR DEEP-SEA INVASION AND SPECIATION

The deep-sea fauna is generally thought to have arisen in shallow wate r, with species colonizing the depths during times when the water colu mn has been isothermal or nearly so. Whether these invasions took plac e slowly or by isolated events within single generations is currently a matter of speculation. To determine whether modern species have the physiological capacity to survive dispersal to bathyal and abyssal dep ths, we compared the temperature and pressure tolerances of embryos an d larvae of the sea urchin genus Echinus collected from subtidal to 20 00 m depths. Echinus esculentus, E. acutus and E, affinis all have sma ll egg sizes, indicative of planktotrophic development and have distin ct seasonal reproductive cycles. Tolerances in the early embryos off. esculentus and E. acutus from shallow water would limit their survival to waters less than 1000 m depth, whereas the larvae were capable of tolerating pressures down to 2000m. Early embryos of E. acutus from 90 0m depth tolerated higher pressures than did embryos of E. acutus from shallow water. The embryos of E, affinis are truly barophilic, develo ping only at pressures greater than 100 atm. Only in E, affinis does t he physiological pressure threshold of the embryo correlate with the d epth distribution of the adult. Our data show that shallow-water echin oid larvae could survive transport to the deep sea and suggest that at least one species, E. acutus, may be currently in the process of inva ding and speciating. We propose that the genus Echinus has invaded the deep sea along isotherms during glacial periods and that speciation m ay be driven either by geographic isolation or by depth-related differ ences in the timing of reproduction with respect to the spring phytopl ankton bloom. (C) 1998 Elsevier Science Ltd. All rights reserved.