Philip M. Gravinese

Throughout this century coastal marine organisms will be subjected to increasing environmental stress, primarily due to warming and acidification. Many of these organisms have complex life histories, including dispersal through an obligate pelagic larval phase. The impacts of environmental stressors on early life history stages of marine organisms are poorly understood; however, the importance of these life stages to demographic and metapopulation processes suggest they are potentially far-reaching. Through combined laboratory experimentation and simulation modeling, we demonstrated that end-of-century conditions have the potential to dramatically reshape Florida stone crab (Menippe mercenaria) metapopulations as larvae experienced rapid development, reduced survivorship, and stage-specific reversed vertical geotactic swimming responses under these conditions. These swimming responses drive the positioning of larvae within the water column and, therefore, influence larval exposure to depth-stratified currents that can augment or reduce their dispersal. Thus, through biophysical interactions, climate change conditions are expected to alter the connectivity of populations, the resilience of the species, and the stability of the multimillion-dollar Florida stone crab fishery. These findings suggest that marine species with complex life-histories and fragmented populations may be especially vulnerable to environmental change.

The combined effects of coastal acidification and warming seawater temperatures are changing species' behaviors, altering their physiology, and reducing their survival. One species that may be affected by these stressors is the Florida stone crab, Menippe mercenaria, which is a $30 million per year fishery. This study determined the impacts of reduced seawater pH and warmer seawater temperatures on the larval swimming response to gravity (geotaxis). We measured the swimming speed and direction of larval stone crabs when they were raised in treatment combinations of reduced seawater pH (pH = 8.0 and 7.6) and elevated temperature (28℃ and 32℃). Larvae were harvested at each larval stage (stages 1-5, n = 13 independent broods for stages 1-3; n= 6 for stage 4; n= 3 for stage 5) for geotaxis experiments, which were performed in a darkroom room.  Our results show that there was no difference in larval upward swimming speeds. The majority (> 65%) of stage 1 and 2 larvae swam upwards in all treatments; however, there was a significant change in the swimming direction for stage 3 larvae in the combined temperature and pH treatment (only 28% swam upwards). Later-stage larvae also oriented downward. The change in swimming direction in the combined temperature and pH treatment (stage 3) suggests that stone crab populations may have difficulty migrating beyond their current range as coastal waters become more acidic and continue to warm.