The Effects of Elevated pCO₂ and Temperature on the Embryonic Development, Larval Survivorship, Condition, Calcification, Morphology, and Behavior of the Florida Stone Crab, Menippe Mercenaria
Gravinese, Philip Michael
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The emission of greenhouse gases into the atmosphere over the last century has increased atmospheric and oceanic temperatures, and has led to a decrease in oceanic pH. The increased ocean temperatures and reduced pH have been detrimental to marine life. In addition, Florida’s watersheds have suffered from decades of disrupted hydrology and diverted runoff, which has increased nutrients and changed coastal carbonate chemistry. Although the adult stages of many marine species are capable of tolerating fluctuations in environmental conditions (i.e., elevated pCO₂ tolerances to such conditions improve. and temperature), marine larvae that hatch in coastal habitats may not have the ability to detect, respond to, or even tolerate such conditions. This study examined the response of the Florida stone crab, Menippe mercenaria, as a model for coastal crustaceans, to determine the impacts of elevated temperature and ocean acidification (OA) on embryonic development and hatching success. The study determined the impacts of simultaneous exposure to both elevated temperature and OA on stone crab survival, larval growth, larval condition, and larval morphology throughout development. The study also examined whether changes in environmental conditions affected the ability of larvae to orient vertically. More specifically, the geotactic swimming behavior of larval stone crabs was tested to determine if elevated temperature and pCO₂ impacted geotaxis orientation, and if geotactic responses change throughout larval development. The impacts of OA on stone crab embryonic development and hatching success was determined by maintaining ovigerous females in conditions that mimicked present-day (pCO₂ ~ 360 ppm, pH = 8.1) and future carbonate conditions (pCO₂ ~ 1500 ppm, pH = 7.5). To determine the effects of simultaneous exposure of stone-crab larvae to elevated temperature and pCO₂, larvae were raised in a fully crossed experiment with two treatments, temperature, and pCO₂, each with two levels. The two temperature levels were 30°C and 32°C, and the two pCO₂ levels were ~450 ppm and ~1100 ppm. This study also synthesized the experimental work by parameterizing a matrix-population model using the larval survivorship data to predict the population densities of larval stone crabs under future temperature and OA scenarios. The elevated pCO₂ treatment (1000 ppm) significantly reduced the rate of embryonic development (i.e., time to hatching) by ~32%, but had no effect on the size of developing embryos (i.e., embryonic volume). Larvae that successfully hatched were not morphologically different among treatments, although hatching success was reduced by 38% in the elevated pCO₂ treatment. Exposure to elevated pCO₂ and ambient temperatures significantly reduced larval survivorship, which was exacerbated by elevated temperature. Indeed, exposure to elevated temperatures had the greatest effect on larval survivorship and development. Larvae raised in the combined treatments (i.e., both elevated pCO₂, and temperature) exhibited a ~79% decrease in survivorship relative to the control. The molt-stage duration was ~1.2 days shorter when larvae were exposed to elevated temperatures. However, stage V larvae reared in the elevated pCO₂ and ambient temperature exhibited a significantly longer molt-stage duration than stage V larvae in controls. Larval condition (i.e., ash free dry weight), Ca and Mg content, Mg/Ca ratios, and larval morphology showed no significant differences among treatments. Geotactic responses varied throughout ontogeny and directional movement was dependent on pCO₂ level, rather than on temperature. Stage III larvae, which swam upwards under ambient pCO₂ conditions, showed a significant reversal of their swimming orientation (i.e., downward swimming), when exposed to elevated pCO₂. These results indicate that future changes in seawater pCO₂ may reduce the reproductive success of stone crabs. The significant decrease in survival at elevated temperatures may also have a negative effect on larval supply, which will be detrimental to the stone-crab populations. Reductions in reproductive output and larval supply could have potential socioeconomic implications for the stone-crab fishery, unless the crabs are able to acclimatize or adapt to future seawater conditions. Geotactic responses were more adversely affected by OA rather than by temperature. Typically, early to mid-stage larvae of brachyuran crab larvae elicit swimming behaviors that position them at or near the surface, where currents facilitate transport offshore. When exposed to elevated pCO₂, the swimming behavior of stage III larvae changed, suggesting that mid-stage stone crab larvae may be positioned deeper in the water column, which may reduce their advection potential to offshore areas. Reducing advection may result in individuals being exposed to different conditions and different predators than typically experienced when larvae are transported offshore. The population model, using the stage-specific larval survivorship data from the experimental treatments, showed that ~14.8% of the control population survived to post-larval stage. Elevated pCO₂, elevated temperature, and the combined treatment (elevated pCO₂ and elevated temperature) resulted in 11%, 6.5%, and 6.8% survival to the post-larval stage, respectively. The sensitivity analysis indicated that earlystage larvae (stage-II) were most sensitive to mortality, particularly in control simulations. Elevated pCO₂ and temperature shifted the sensitivities to late-larval stages (i.e., stages IV and V). This shift was potentially a consequence of greater effects of ocean acidification on more developed larval stages, with more pronounced anatomical features. The sensitivity analyses indicated that elevated pCO₂ and temperature had the greatest impact on late-stage larvae. These results suggest that new recruits into the stone-crab fishery will be affected by both exposure to elevated pCO₂ and temperature, which are expected by the end of the century, unless larvae tolerances to such conditions improve.