Stone crab larvae reverse their phototaxis swimming response in reduced seawater pH 

Shallow coastal habitats are experiencing accelerated changes in seawater pH and ocean warming which may alter behavior and pose physiological challenges for planktonic larvae. Stone crabs are a $35 million per year fishery in Florida but populations are in decline. Stone crab larvae hatch in coastal waters and their swimming behavior promotes advection offshore habitats to more favorable conditions where they complete their larval development. However, we are unsure how simultaneous climate stressors (like elevated temperature and reduced seawater pH) impact larval vertical swimming behaviors which are critical for transport to settlement habitats.. Here we raised stone crab larvae in four fully-crossed treatments of elevated temperature (28℃, 32℃) and reduced seawater pH (8.0, 7.6). Larvae were then harvested at each stage and their phototactic swimming behavior was determined within each treatment. At ambient temperatures and present day pH, larvae elicited a neutral phototactic response, however, this shifted to a negative phototaxis whenever larvae were exposed to reduced seawater pH. These changes in larval phototactic swimming direction indicate that reduced pH could be impairing some physiological mechanisms associated with orientation which could increase larval mortality if individuals are advected into unfavorable habitats (i.e., retained nearshore where conditions are not favorable and predators abundance is high) for completing larval development. Furthermore, a deeper depth distribution will expose larvae to slower currents which may also disrupt the connectivity among stone crab metapopulations along Florida’s western shelf.

Acknowledgments: We thank T. McNelly,  E. Garland, & Z. Loreant for assistance in data collection and analysis. This research is funded by a National Science Foundation Biological Oceanography Grant (award number OCE-2049047). 

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