Marine organisms that inhabit the Southern Ocean surrounding the continent of Antarctica live in an extremely cold and nutrient-poor environment. How do these extreme conditions change the “optimum” shape and size of an organism? In collaboration with Dr. Art Woods at the University of Montana, I am using invertebrate embryos and nudibranch egg masses as a model system for understanding how limits on oxygen supply to internal tissues affect organismal design. Using artificial egg masses, we experimentally manipulate the size and shape of masses and density of embryos in masses to test models of 02 diffusion and metabolic demand at different temperatures. Using natural nudibranch egg masses, we are investigating how form and function of these structures differs between the Antarctic and warmer climates.
Science team on the sea ice, Antarctica.
Larval Energetic Strategies:
Most marine organisms have a larval stage that is morphologically and ecologically distinct from the adult. Across all animal phyla, a striking pattern exists in larval developmental mode; species tend to fall into one of two categories based on larval feeding strategy. In "planktotrophic" species, larvae develop from small eggs, must feed in the plankton to reach metamorphosis, and have elaborate feeding morphology. "Lecithotrophic" species have large eggs and simplified, non-feeding larvae. As one focus of research we are investigating the energetics of development of lecithotrophic larvae to understand how egg energy reserves fuel larval development. We are also working with planktotrophic larvae of oysters, abalone, sea urchins, and polychaete worms to describe energy utilization during the prefeeding phase of planktotrophic larval development, and to understand the extent to which larvae rely on either endogenous egg reserves or exogenous food sources (phytoplankton and dissolved organics) to fuel metabolism.
Pluteus larva of Arbacia punctulatus.
Evolution of Egg Size:
Planktotrophy is widely held to be the ancestral condition with respect to lecithotrophy, and one important step in the evolutionary transition from planktotrophy to lecithotrophy is the acquisition of large eggs. The rise of the Central American Isthmus ~3.5 million years ago isolated populations of many marine species in the newly separated Pacific and Atlantic oceans, forming 'geminate' or twin species pairs in many taxa that have been evolving in different oceans since closure. Among planktotrophic marine species of echinoderms, many Atlantic species have larger eggs than their Pacific geminates; this pattern has been attributed to an adaptive response to lowered planktonic productivity and a poor larval feeding environment in the Caribbean. I have demonstrated that this pattern occurs among tropical American molluscs in the bivalve family Arcidae. With arcids, I am able to infer egg size from fossil shells to study the evolution of egg size in relation to the paleoenvironmental changes in productivity associated with the rise of the Central American Isthmus.
Arcid bivalves under coral rubble, Panama.
Larval Dispersal and Population Connectivity:
How, and to what extent, are populations of benthic marine organisms connected by dispersal of free-living larvae? This question is particularly difficult to address in marine systems because larvae are tiny and very difficult to study in the field. With Dr. Peter Marko at Clemson University, I am developing methods to directly measure dispersal of larvae of the Bay Scallop, Argopecten irradians. We have developed a larval-shell-marking technique and performed large-scale releases of marked larvae into Bogue Sound, North Carolina. By deploying spat collectors throughout the sound, we hope to capture settlement of marked larvae and determine how far they have traveled since release.