Vertical distribution correlates with pressure tolerances of early embryos in the deep-seaasteroid Plutonaster bifrons

The astropectinid asteroid Plutonaster bifrons (Wyville Thomson) occurs on the continental slope of the north-east Atlantic between 1000 and 2500 m depths. As in most deepsea animals, the factors limiting bathymetric distribution of this species are unknown. Eggs were fertilized in vitro and incubated through the early embryonic cleavage stages at pressures that correspond to depths from 0 to 3000 m. The highest percentage of normal development occurred near the peak of the species distribution (2000 m), and virtually no normal development occurred at a pressure corresponding to 3000 m depth. Develop-mental rate was retarded at pressures higher and lower than those found near 2000 m. These experiments indicate that embryonic pressure tolerances could determine both the upper and lower bathymetric limits of distribution for this species.