Hybridization in the sea: gametic and developmental constraints on fertilization in sympatric species of Pseudechinus (Echinodermata: Echinoidea) Daniel J. McClary a,1 , Mary A. Sewell b, * a Department of Zoology and Portobello Marine Laboratory, University of Otago, P.O. Box 8, Portobello, New Zealand b School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand Received 15 April 2002; received in revised form 10 October 2002; accepted 17 October 2002 Abstract The constraints of gametic compatibility and developmental competence on hybrid production were examined in three sympatric species of the echinoid Pseudechinus in southern New Zealand. Differences were noted in embryonic temperature sensitivity between the species. Pseudechinus novaezealandiae embryos appeared to be better adapted to cooler temperatures than those of Pseudechinus huttoni, while Pseudechinus albocinctus embryos tolerated a wider range of temperatures than either of its sympatric congeners. Such differences were mirrored by the kinetics of gamete interaction between all possible reciprocal crosses of the three species. There appeared to be some physiological incompatibility between the eggs of P. novaezealandiae and the spermatozoa of the other two species. P. novaezealandiae spermatozoa were, however, capable of fertilizing both P. huttoni and, to a lesser extent, P. albocinctus ova. Hybrid echinoplutei resulting from the P. huttoni (female)  P. novaezealandiae (male) cross did not, however, become competent to metamorphose. The probabilities of settlement of echinoplutei resulting from the other reciprocal crosses involving the gametes of P. novaezealandiae were also low. It appears that a combination of physiological (gametic incompatibility) and genetic (developmental incompetence) factors contribute to maintaining the integrity of the P. novaezealandiae lineage. These factors were not useful in understanding how P. albocinctus and P. huttoni remain distinct species. This species-pair was characterized by reciprocal gamete compatibility as well as developmental competence to settlement. 0022-0981/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0022-0981(02)00487-2 * Corresponding author. Tel.: +64-9-373-7599x83758; fax: +64-9-373-7414. E-mail address: m.sewell@auckland.ac.nz (M.A. Sewell). 1 Present address: Kingett Mitchell Limited, P.O. Box 33-849, Takapuna, Auckland, New Zealand. www.elsevier.com/locate/jembe Journal of Experimental Marine Biology and Ecology 284 (2003) 51 – 70