Temperature and pressure tolerances in dispersal stages of the genus Echinus (Echinodermata: Echinoidea): prerequisites for deep-sea invasion and speciation

Abstract

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. Tolerances in the early embryos of E. 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 2000 m. Early embryos of E. acutus from 900 m depth tolerated higher pressures than did embryos of E. acutus from shallow water. The embryos of E. affinis are truly barophilic, developing only at pressures greater than 100 atm. Only in E. affinis does the physiological pressure threshold of the embryo correlate with the depth distribution of the adult. Our data show that shallow-water echinoid larvae could survive transport to the deep sea and suggest that at least one species, E. acutus, may be currently in the process of invading and speciating. We propose that the genus Echinus has invaded the deep sea along isotherms during glacial periods and that speciation may be driven either by geographic isolation or by depth-related differences in the timing of reproduction with respect to the spring phytoplankton bloom.