Nontraditional life-history choices: what can "intermediates" tell us about evolutionary transitions between modes of invertebrate development?

Mode of development in marine invertebrates has been largely viewed as a dichotomy between small eggs that develop into free-living planktotrophic larvae and large eggs that bypass the larval stage and develop directly into juveniles. Modes of development that could be categorized as "intermediate" between these two extremes include facultative feeding larvae, short-lived planktotrophic larvae, dispersal dimorphisms, and poecilogony (in which nutritional mode varies within a species). These intermediates are rare. The few species-level phylogenies available that include them do not generally support the interpretation of intermediates as necessary, ephemeral transitional forms. Instead, they support the idea that intermediates are well adapted to their environments but that either these environments are short-lived relative to the frequency of speciation, or speciation events are associated with shifts in the mode of development. Each of the different intermediate forms could have evolved in response to variable environments. The phenotypically plastic intermediates could be a response to predictable environmental variation. Facultative feeding larvae and short-lived planktotrophic larvae could reflect conservative bet-hedging in response to unpredictable environmental variation, whereas poecilogony with mixed clutches could represent the alternative: diversifying bet-hedging. Since environmental variability is common, it remains an enigma why these intermediates are so rare. Discovery of more intermediates, and their careful description in terms of the level of variation expressed within, and among, clutches, and among females and populations, as well as determination of the genetic and environmental influences on this variation, will provide valuable test-cases for theories of the evolution of alternative phenotypes.