DATING THE ORIGINS OF MARINE INVERTEBRATE LARVAE WITH A MOLECULAR CLOCK

Many marine invertebrates have a complex life cycle whereby the egg, rather than developing directly to the juvenile stage, develops instead into an intermediate larval form with a unique morphology when compared to the final adult stage. Larvae may spend weeks to months feeding in the plankton before they become competent to undergo metamorphosis into the benthic juvenile form. Because marine larvae offer few, if any, evolutionary advantages, their origin and subsequent maintenance over geological time is mysterious. To better understand the evolution of marine larvae, I employ both a molecular clock and the fossil record to date the origin of several different larval forms. The molecular clock dates the origin of the cnidarian planula larva to ~590-525 Ma, the deuterostome dipleurula to ~577-537 Ma, and the spiralian trochophore to ~557-545 Ma. In addition, the presence of an ascidian in the ~520 Ma Doushantuo Formation indicates that the chordate tadpole larva evolved by the end of the Early Cambrian as well. Importantly, at least three of these larval forms were non-feeding, indicating that dispersal was not the primary factor underlying the origins of these larvae. Feeding larvae appear to have evolved no earlier than the latest Cambrian. The fossil record indicates that the maximum age for the ectoproct cyphonautes larva, the two molluscan veliger larvae, and the echinoderm feeding dipleurula larva is latest Cambrian to Early Ordovician. In addition, the molecular clock indicates that the maximum age for the evolution of the nemertean pilidium larva, and all annelid feeding larval forms, is ~495 Ma. The minimum age for most of these larval forms is Early to Middle Ordovician; only the cyphonautes (~370 Ma) and the pilidium (~270 Ma) could have arisen substantially later. Because the origin of planktotrophy correlates with the dramatic rise in the number of epifaunal suspension feeders, it seems likely that benthic predation selected for the origin of plankotrophy, allowing for the evolution of a rapid metamorphosis. Therefore, non-feeding larval forms arose in the late precambrian-Early Cambrian, most likely as a result of benthic grazing; these larval forms then spurred the evolution of suspension feeding which in turn triggered the evolution of planktotrophic larvae in multiple animal clades by the Middle Ordovician.