The research in my laboratory falls into three main areas:
Evolution of reproduction and mode of development in marine invertebrates.
Mode of development in benthic marine invertebrates (e.g., planktonic vs. benthic, feeding vs. non-feeding) has numerous evolutionary implications. Species with feeding larvae often spend weeks, months, or sometimes even years in the water-column. During this time they are subject to dispersal by oceanic currents and may travel vast distances. Species with encapsulated benthic development or short-lived, non-feeding larvae generally do not have much chance for dispersal in the water column before metamorphosis into a benthic juvenile. As a result of these differences in dispersal, species with planktotrophic development are believed to have higher levels of gene flow, less population structure, and larger geographic ranges than similar species with direct or lecithotrophic development. These emergent species-level characters have also been shown to result in lower rates of speciation and extinction in planktotrophic species than in direct developing species during the Mesozoic and Cenozoic.
I have used a comparative phylogenetic approach to understand the evolution of mode of development in marine invertebrates. My comparative data for calyptraeid gastropods show that evolutionary changes in mode of development may be large and rapid. I am currently interested in developing similar comparative datasets for other groups of marine gastropods. I am also beginning a project to understand population genetics of variation in development. By examining the heritability of and intraspecific variation in developmental features I hope to gain insight into the development of interspecific variation in life history characters.
On the beach in Patagonia
Systematics and evolution in marine gastropods
In order to address many of the most interesting evolutionary questions about patterns in mode of development it is necessary to have some idea of how species with different development are related. Unfortunately little information is available on relationships within many groups of marine invertebrates. To provide a phylogenetic context for my comparative data on development, I have been working for several years on the molecular and morphological systematics of calyptraeid gastropods. Calyptraeids (Crepidula, Calyptraea, and Crucibulum) are all sedentary, protandrous, filter-feeding marine limpets that brood their eggs. Due to the ease with which the embryos and larvae can be obtained calyptraeid gastropods have played a large part in our understanding of reproductive strategies in marine molluscs. Some aspects of mode of development have been documented for 78 species in the family. Despite being easily collected, ecologically important, and the subject of considerable scientific study, the taxonomy of calyptraeids is still uncertain in many cases. For example, in the last few years I have described two new species from collections near marine laboratories in Florida and numerous other species still await description. Systematic analyses of 90 calyptraeid species has shown that taxonomic divisions based on shell characters are often misleading and that the group displays pervasive parallelisms and convergences in morphology, development, and general lifestyle. In addition to continuing this work with calyptraeids, I am beginning systematic work on several other families of marine gastropods.
Crepidula capensis from South Africa
Evolutionary Ecology of Sex Change
Life history theory predicts that animals should change sex at a size or age that maximizes their lifetime reproductive output. In the few species of fish and molluscs that have been examined, the general predictions of these theories are supported. However the details of the timing of sex change and the mechanisms responsible for the decision to change at a specific time have not been investigated in many animals.
Calyptraeid gastropods are protandrous (males change to females) sex changers. Some species are solitary while others form pairs or large stacks of individuals. Preliminary data on a few species suggests that animals change sex in response to their associations with conspecifics. I am currently conducting laboratory experiments to determine if species with different social structures in the field respond differently to associations with conspecifics.