Intermediate modes of larval development: bridging the gap between planktotrophy and lecithotrophy.

The extraordinary diversity of larval form and function in marine invertebrates has motivated many studies of development, ecology, and evolution. Among organisms with pelagic development via a larval stage, this diversity is often reduced to a dichotomy between two broad nutritional categories: planktotrophy and lecithotrophy. Despite the clear utility of the planktotrophy-lecithotrophy dichotomy to those interested in the history or consequences of life history patterns, it is also clear that a number of larval forms do not fit neatly into either of these general categories. Here we review studies of these intermediate larval forms, focusing on descriptions of larvae known as facultative feeders. Recent descriptions of larval development suggest that facultative feeders and other intermediate larval forms are not as rare as commonly assumed. We assess the importance of these forms for models of life-history evolution and call for a more-detailed and nuanced view of larval biology to account for their existence. Clearer knowledge of the phylogenetic distribution and frequency of occurrence of larvae that exhibit intermediate nutritional requirements is also essential for evaluating current ideas on evolutionary transitions between planktotrophy and lecithotrophy. Finally, intermediate larval types provide valuable and underutilized opportunities for testing hypotheses in the fields of larval ecology and the evolution of development.