Philip M. Gravinese

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.

Many coral reefs have been degraded over the past several decades due to the

combination of human and natural disturbances. In the Florida Keys, coral habitats have been in

decline since the 1970s due to multiple stressors including coral bleaching, disease outbreaks,

hurricanes, and cold snaps. As a result, once coral-dominated communities are now becoming

increasingly dominated by macroalgae, which are associated with lower economic, ecological,

and aesthetic value. Herbivory, or the grazing of macroalgae, is a key process on coral reefs that

helps maintain coral-dominated states and can potentially reverse macroalgal shifts on degraded

reefs. The Caribbean king crab is a noted voracious algae consumer with natural physiological

resilience to stressful environmental conditions, making them a promising coral restoration

candidate. Therefore, understanding the environmental factors which promote or discourage

settlement may serve as a management tool for local restoration efforts. In this experiment,

larvae were exposed to five different treatment cues that include: a) control (no cue, purified

seawater), b) chemically defended algae cue seawater (Dictoyata spp.), c) conspecific cue

seawater (Mithrax spinosissimus), and d) spiny lobster predator cue seawater (Panulirus

guttatus). Caribbean king crab larvae were more likely to metamorphose and settle when

exposed to the control and conspecific cues, whereas there was a higher mortality rate and lower

settlement among larvae that experienced predator odors and the algae cue seawater.