THE EVOLUTION AND LOSS OF FEEDING LARVAL STAGES OF MARINE INVERTEBRATES

The feeding larvae of marine invertebrates have interested evolutionists for three reasons. First, the long pelagic larval stage often associated with larval feeding permits dispersal over unusually large distances. This dispersal of larvae affects gene flow and the geographic distribution of species (Mileikovsky, 1971; Scheltema, 1971). Second, many traits of feeding larvae are extremely conservative, so larval forms have played a large part in phylogenetic theories (Garstang, 1951; Hatschek, 1878; Jagersten, 1972; Zimmer, 1973). Third, homologous stages of development can be either planktotrophic (feeding) or lecithotrophic (dependent on reserves in the egg) within many genera or families, and this variation has promoted several models of adaptive strategies concerning parental investment per offspring, dispersal, and other consequences of planktotrophy versus lecithotrophy (Crisp, 1974a; 1974b; Strathmann, 1974a; 1977; Vance, 1973). The conservatism of many traits of feeding larvae within a class or phylum contrasts with the independent loss of a feeding larval stage in many genera and families, yet patterns of evolution and loss of feeding larvae have not been reviewed, and differences in these patterns among higher taxa have not been recognized. The following survey of ten phyla presents a scheme for the evolution and loss of feeding larval stages which is consistent with recent studies on larval feeding mechanisms. These phyla fall into two groups, the oligomera and the spiralia (Table 1). Comparisons among the phyla indicate potential or realized differences in flexibility in reacquiring a planktotrophic larval stage once it is lost. I will present evidence for the oligomera indicating that loss of a planktotrophic larval stage is usually irreversible, that feeding larvae have originated in the remote past, and that feeding larvae have been lost by all members of some orders and classes. A survey of larvae in spiralian phyla indicates that for several reasons the spiralians have been more flexible in acquiring new planktotrophic larval forms. This survey therefore indicates trends and constraints in the evolution of larval planktotrophy which differ among major groups of invertebrates, and some of the reasons for these differences. The survey also can serve as a guide which will prevent erroneous extensions of predictions from optimization models to groups with little variation or evolutionary flexibility. The paper which follows extends these results to indicate progressive vacating of adaptive types in some clades. The acceptance here of a particular set of relationships among phyla may disturb readers who are either unfamiliar with construction of phylogenies or advocate an alternative phylogenetic scheme. The criterion for deciding among phylogenetic schemes, as for any set of alternative hypotheses, is parsimony, and under available evidence other phylogenetic schemes are less parsimonious. (See Garstang to Zimmer, cited above.) New evidence may change views on phylogeny, but phylogenetic schemes could change substantially without affecting the major conclusions of this review. I have therefore not discussed the effect which this or that change in inferred re-