L Journal of Experimental Marine Biology and Ecology, 223 (1998) 65–76 Thermal tolerances of embryos and planktotrophic larvae of Archaeopneustes hystrix (A. Agassiz) (Spatangoidea) and Stylocidaris lineata (Mortensen) (Cidaroidea), bathyal echinoids from the Bahamian Slope a, b c * Craig M. Young , Suki U.K. Ekaratne , J. Lane Cameron a Division of Marine Science, Harbor Branch Oceanographic Institution, 5600 US. Hwy.1 N., Ft. Pierce, FL 34946, USA b University of Colombo, P .O. Box 1490, Columbo 3, Sri Lanka c P .O. Box 3145, Friday Harbor, Washington 98250, USA Received 21 October 1996; received in revised form 5 May 1997; accepted 3 June 1997 Abstract Thermal tolerances of early embryos and echinopluteus larvae were studied for two common bathyal echinoids, Archaeopneustes hystrix (A. Agassiz) and Stylocidaris lineata Mortensen. Both species have adult distributions centered at a depth of 550 m on the slopes of the Bahamian Islands, though A. hystrix extends to a shallower depth than S. lineata. Early embryos incubated at 4 temperatures in laboratory incubators developed at the same rates as embryos transplanted by submersible to depths having comparable temperatures. Embryos of both species could tolerate temperatures as high as 258C. Stylocidaris lineata retained this ability during 2-arm and 4-arm echinopluteus stages, but larvae of A. hystrix died at temperatures above 208C. The lower depth limits of both species and the upper limit of A. hystrix corresponded closely to the thermal tolerances of larvae. Larvae of S. lineata could potentially migrate to the euphotic zone to feed, but larvae of A. hystrix probably must disperse and feed at greater depths.  1998 Elsevier Science B.V. Keywords: Deep-sea; Echinoid; Larva; Ontogenetic migration; Temperature tolerance; Vertical distribution 1. Introduction With few exceptions (e.g., Hansen, 1968), deep-sea echinoderms produce pelagic * Corresponding author. Tel.: 1 1 561 465 2400 ext. 303; fax: 1 1 561 468 0757; e-mail: youngc@hboi.edu 0022-0981 / 98 / $19.00  1998 Elsevier Science B.V. All rights reserved. PII S0022-0981(97)00149-4