Carol T. Stuart

LARGE-SCALE BIOGEOGRAPHIC PATTERNS IN MARINE MOLLUSKS: A CONFLUENCE OF HISTORY AND PRODUCTIVITY?

Large-scale biogeographic patterns in marine systems are considerably less well documented and understood than those in terrestrial systems. Here, we synthesize recent evidence on latitudinal and bathymetric gradients of species diversity in benthic mollusks, one of the most diverse and intensively studied marine taxa. Latitudinal gradients in coastal faunas show poleward declines in diversity, but the patterns are highly asymmetrical between hemispheres, and irregular both within and among regions. The extensive fossil record of mollusks reveals that latitudinal gradients have become steeper during the Neogene, partly because of a rapid diversification in tropical coral reefs and their associated biotas. Much of the inter-regional variation in contemporary latitudinal trends depends on the longitudinal distribution of reefs and major Neogene vicariant events. Thus, coastal faunas reveal a strong evolutionary–historical legacy. Bathymetric and latitudinal gradients in the deep ocean suggest that molluscan diversity is a function of the rate of nutrient input from surface production. Diversity may be depressed at abyssal depths because of extremely low rates of organic carbon flux, and at upper bathyal depths and high latitudes by pulsed nutrient loading. While the deep-sea environment is not conducive to fossilization, relationships between local and regional diversity, and the distribution and age of higher taxa indicate an evolutionary signal in present-day diversity gradients. Marine invertebrate communities offer tremendous potential to determine the relative importance of history and ecological opportunity in shaping large-scale patterns of species diversity.

Species–energy relationships in deep-sea molluscs

Consensus is growing among ecologists that energy and the factors influencing its utilization can play overarching roles in regulating large-scale patterns of biodiversity. The deep sea—the worlds largest ecosystem—has simplified energetic inputs and thus provides an excellent opportunity to study how these processes structure spatial diversity patterns. Two factors influencing energy availability and use are chemical (productive) and thermal energy, here represented as seafloor particulate organic carbon (POC) flux and temperature. We related regional patterns of benthic molluscan diversity in the North Atlantic to these factors, to conduct an explicit test of species–energy relationships in the modern day fauna of the deep ocean. Spatial regression analyses in a model-averaging framework indicated that POC flux had a substantially higher relative importance than temperature for both gastropods and protobranch bivalves, although high correlations between variables prevented definitive interpretation. This contrasts with recent research on temporal variation in fossil diversity from deep-sea cores, where temperature is generally a more significant predictor. These differences may reflect the scales of time and space at which productivity and temperature operate, or differences in body size; but both lines of evidence implicate processes influencing energy utilization as major determinants of deep-sea species diversity.