| dc.description.abstract | 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. | en_US |
