Note
The deep-sea fauna is generally thought to have arisen in shallow water, with species colonizing the depths during times when the water column has been isothermal or nearly so. Whether these invasions took place 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 depths, we compared the temperature and pressure tolerances of embryos and larvae of the sea urchin genus Echinus collected from subtidal to 2000 m depths. Echinus esculentus, E. acutus and E. affinis all have small egg sizes, indicative of planktotrophic development and have distinct seasonal reproductive cycles.