CHEMOSYMBIOTIC LUCINID BIVALVES AS EXPERIMENTAL CONTROLS FOR TESTING THE BODY-SIZE/NUTRIENT-AVAILABILITY HYPOTHESIS FOR TROPICAL AMERICAN NEOGENE MOLLUSCS

Several marine invertebrate clades underwent turnover (speciation, migration, and extinction) during the Neogene in tropical America. A number of these clades also show remarkable morphologic trends (especially changes in size) during this time. Several authors have attributed these body-size and diversity trends to a post-Miocene decrease in nutrient availability and productivity in the Caribbean/western Atlantic.

To test whether a causal link underlies the correlation of body size and nutrient availability, additional types of data are required. For instance, clades examined thus far are all part of photosynthetically based food webs. Examining size trends in clades dependent on chemosynthesis provides an independent check of the size-nutrient correlation. If a chemosymbiotic clade does not show body size trends similar to those of its "normal" heterotrophic cohorts, the link between size and nutrients in photosynthetically based communities is supported. Alternatively, if trends in a chemosymbiotic clade mirror those of its non-chemosymbiotic cohorts, alternative causes that influence both types of food webs must be explored.

To perform such a test, we documented size trends in lucinid bivalves from the tropical American Neogene. Lucinids house sulfur-oxidizing bacteria, inhabit a variety of habitats ranging from hydrocarbon seeps to seagrass beds, and were very diverse in the tropical western Atlantic Neogene. We found that like non-chemosymbiotic corbulid and chionine bivalves, and strombinid gastropods, maximum size in lucinids increased from the Miocene to Holocene in the eastern Pacific. In the Caribbean/western Atlantic, however, patterns differ between trophic groups. Lucinids have a relatively constant maximum size through the Neogene with localized peaks in the early Miocene, "middle" Pliocene, and Holocene. In contrast, corbulids body size decreases dramatically from a middle/upper Miocene maximum, and a similar pattern is seen in the Pliocene for chionine bivalves. Therefore, a nutrient/body-size link in corbulids and chionines in the Caribbean/western Atlantic is supported. In the eastern Pacific, however, another dynamic, affecting both photosynthetic and chemosynthetic food webs, may be driving size trends.