Some examples of the contribution of planktonic larval stages to the biology and ecology of polychaetes

There are many fields of research into adult benthic polychaetes where the study of their planktonic larvae is beneficial. These include the reproduct ive activity of adults from the larval period, the identification of hydrol ogical and climatic conditions for benthic life, the interannual fluctuatio ns of benthic populations, the transfer of energy between plankton and bent hos, settlement conditions at the benthic stage, variations in reproductive patterns with latitude, biogeographic implications of larval dispersion, l arvae and evolutionary processes. It is clear from this non-exhaustive list that, in dealing with planktonic larvae, the benthic part of the life cycl e associated with development in the sediment or on hard substrates of the benthos is also involved. Consequently, results can be obtained over the wh ole life cycle by studying larvae. Examples of such integration are provide d which also reflect a change in hitherto accepted thinking. Three points a re developed for polychaetes: (1) latitudinal variations in the pattern of reproduction; (2) contribution of planktonic larvae to the question of the existence of cosmopolitan species; (3) reproduction at great depth. A new p icture of the larval stage is evolving: the 'new' larva is not a passive ob ject, and it covers an area that is probably more limited than previously t hought; its characteristic features are connected with the other parts of t he life cycle. Finally, it is suggested that planktonic larvae, even those with a strong potential for dispersion, are not destined for widespread dis semination; they occupy a limited area, which results either from a reducti on in their dispersion potential, or from control by the physical structure s of the surrounding water masses.