Life cycles in brackish habitats: adaptive strategies of some polychaetes from the Venice lagoon

The life cycle of five species of polychaetes living in the Venice lagoon is analysed with the aim of finding out if some traits can be interpreted as an adaptation to brackish environments. The studied species are characterized by different sizes and reproductive strategies. Marphysa sanguinea is a large-sized, annual iteroparous species belonging to the Eunicidae family that reproduces without epitokal modification. Perinereis cultrifera and P. rullieri are large-sized semelparous species belonging to the Nereididae family: the first reproduce with epitokal metamorphosis, the second do not. Ophryotrocha labronica and Dinophilus gyrociliatus are small iteroparous species with semi-continuous reproduction belonging to the Dorvilleidae and Dinophilidae families, respectively. All the species are gonochoric. The larger species often have a sex ratio close to 1:1, while in the smaller ones the sex ratio is biased toward the female sex (2:1 in O. labronica and 3:1 in D. gyrociliatus). Moreover, D. gyrociliatus exhibits strong sexual dimorphism: the males are dwarf and short living. All the large-sized species have external fertilization, while the small-sized O. labronica and D. gyrociliatus exhibit external fertilization with pseudo-copulation and internal fertilization, respectively, that ensure a high reproductive success. All the species produce lecitotrophic eggs, protect young stages from at least some environmental stresses and reduce larval dispersion. The role of the suppression of epitoky and protection structure for developing eggs in limiting the dispersal phase is discussed. All species, independent of reproductive strategies, are characterized by high fecundity and reproductive effort. In the small-sized species, the marked bias of the sex ratio in favour of the female sex and, in D. gyrociliatus, the reduction in the size of the male eggs, are significant factors in the increasing rate of the populations, enabling them to tolerate sharp variation in density due to environmental unpredictability. (C) 2003 Editions scientifiques et medicales Elsevier SAS and Ifremer/CNRS/IRD. All rights reserved.