Exploring Marine Larvae
Zoology professor Larry McEdward discusses his recent work with marine larval development.
This article was originally published in the May 1999 issue of CLASnotes.
My interests are in the evolution of larval development patterns
in marine organisms. Marine larvae are radically different from their adult
counterparts, in morphology, habitat, and mode of nutrition. Marine larvae
are particularly interesting because they must function as planktonic organisms
and, at the same time, undergo very rapid and extensive developmental changes.
What fascinates me and motivates my research program, is the fact that for
all of the morphological, taxonomic, and ecological diversity that exists among
the more than 30 phyla of marine animals, there are only three major patterns
of larval development. What evolutionary factors have led to the origin and
persistence of these patterns? What drives evolutionary transitions among these
patterns? Through my research, I seek a deeper understanding of the evolution
of life cycles by integrating the fields of evolutionary developmental biology,
larval ecology, and life-history theory.
One of the most striking patterns in larval ecology is the existence of only two modes of larval nutrition. In planktotrophy, eggs are very small and larvae require exogenous food (e.g., phytoplankton or other small food items) for development. In the alternative strategy (lecithotrophy), eggs are very large and development is fueled entirely by endogenous reserves originally provided in the egg. The current paradigm for larval ecology is based on theory that predicts two distinct modes of larval nutrition (i.e., complete reliance on external food items, or complete reliance on internal energy stores). This theory has proven difficult to test empirically. In collaboration with colleagues at the Friday Harbor Laboratories (University of Washington), we experimentally reduced egg size by isolating single cells from early embryos of sea urchins to mimic the effects of an evolutionary modification of egg size. Our work provided the first definitive demonstration of the causal relationship between egg size and early life-history traits in marine benthic invertebrates. Using manipulations of egg size, controlled feeding experiments, and analyses of larval growth, here at the University of Florida and at the Keys Marine Laboratory, we discovered a wide range of larval feeding requirements in subtropical sea urchins and sand dollars. We have shown a direct link between the level of resources provided by the parent in the egg and the degree of dependence on food by the larvae. Furthermore, the degree of dependence on food seems to be inversely correlated with the capacity for facultative feeding (i.e., the ability to acquire food before it is necessary for continued development).
I have developed computer models of larval evolution that include facultative larval feeding, based on our experimental results. The new models show that maximal reproductive success can be achieved at intermediate egg sizes, depending on food supply and mortality rate. A diversity of nutritional strategies, such as we observe in subtropical sand dollars, is predicted. We have recently begun a new project to measure the effects of facultative feeding on larval development and growth and to develop new models that link adult reproductive strategies, larval energetics, and postmetamorphic juvenile success. This represents a first step towards our long-term goal of building whole life cycle models.
Credits
Writer
Larry McEdward, Zoology professor
