A COMPARATIVE-STUDY OF REPRODUCTION AND DEVELOPMENT IN THE POLYCHAETEFAMILY TEREBELLIDAE

The reproduction and development of four species of terebellid polycha etes from the west coast of North America were studied and compared wi th several other terebellid species to reveal the covariation of life history traits in the group, and assess any limitations on terebellid life history evolution that may be imposed by ancestry or body design. The four species in the present study span the range of reproductive and developmental modes known for the family Terebellidae. Eupolymnia crescentis and Neoamphitrite robusta are both free spawners that repro duce during discrete 3-month breeding periods. In E. crescentis, oogen esis takes from 5 to 8 months and spawning occurs from July to Septemb er, maximum oocyte diameter is 210 mum, and fecundity reaches approxim ately 128,500 during a single breeding period. The E. crescentis larva develops near the bottom for about 7 days before settling as a five-s etiger juvenile. Neoamphitrite robusta reproduces from April to July a fter a 12-month oogenic cycle; oocytes in this species measure up to 1 80 mum, and fecundity reaches approximately 830,000. The two brooders in the study, Ramex californiensis and Thelepus crispus, brood their l arvae in the maternal tube. T. crispus reproduces continuously for at least 6 months, and has up to 51,500 larvae in a single brood. The ooc ytes in this species (400 mum) give rise to larvae that are brooded to the one-setiger stage and then emerge to undergo a one-day planktonic period before the larvae settle and become juveniles at eight setiger s. Ramex californiensis reproduces continuously year round; larvae are brooded in cocoons that are laid sequentially in the tube, with up to 44 larvae in a single cocoon. Development from the 4 1 0 mum oocytes is direct, and juveniles have 11 setigers. Unlike E. crescentis and N. robusta, in which oogenesis is synchronized within individuals to pro duce a peak of large oocytes during the discrete spawning period, R. c aliforniensis and T. crispus females have a wide range of oocyte sizes throughout the year. Correlation analysis and analysis of variance of reproductive and developmental traits of these and several other tere bellid species revealed some expected trends. For example, egg size va ries according to the mode of reproduction (free spawning, extratubula r brooding, or intratubular brooding), and is also correlated with juv enile size. However, egg size does not predict fecundity in terebellid s when body size is held constant, and brooding is not restricted to s mall-bodied species. Indeed, the largest and smallest species in the s tudy brood their larvae intratubularly, suggesting that allometric con straints may not be important in determining mode of reproduction in t hese polychaetes. The Terebellidae is a diverse family found in all ma rine habitats, yet all known terebellid larvae are non-feeding; this c ontrasts with the occurrence of both planktotrophy and lecithotrophy i n other polychaete families, and leads to the proposal that larval dev elopment in terebellids has been constrained during the evolution of t he lineage. The results of this study demonstrate that generalizations regarding complex relationships among life history traits are often i nappropriate. The need for more comparative studies of marine inverteb rate reproduction and development, and the integration of phylogenetic analyses into the study of life history evolution in marine invertebr ates is highlighted.