3. Integrated impacts of temperature increase and ocean acidification on larval shell development in the American Lobster, Homerus americanus

Christine San Antonio (1)*, Michael Tlusty (2), Robyn Hannigan (3)

1 School for the Environment, University of Massachusetts Boston, Boston MA, 02125, USA
2 New England Aquarium, Central Wharf, Boston MA, 02110, USA
3 School for the Environment, University of Massachusetts Boston, Boston MA, 02125, USA

Background
To date, research on the American lobster primarily describes mechanistic consequences of OA for calcification, growth and survival rates. Here we explore the interactive effects of temperature and CO2/pH on shell development, and how these effects translate to whole animal responses to external stressor such as disease. American lobster in the Gulf of Maine provide an ideal model for studying multi-factor OA effects.

Methods
This laboratory-based study examined responses of larval (hatch to post-larval Stage IV; ~ 30 per tank, 3 replicate tanks per treatment) lobsters to elevated pCO2 and temperature in a 3 x 3 factoral experiment. We assessed shell morphology and chemistry using scanning electron microscopy (mineralogy), and attenuated reflectance fourier-transform infrared spectroscopy (inorganic and organic composition). We analysed the data as a nested (larval rearing container) two way MANOVA. We tested shells of bacterially exposed lobsters for evidence of disease and shell composition as well. Data from lobsters exhibiting shell disease were analyzed by a 2 way ANOVA.

Findings
Larvae reared at low pH showed loss of shell integrity with a coincident change in mineralogy from rhomboidal low-Ca calcite to prismatic low-Ca and low-Mg calcite. Temperature increases ameliorated the change in crystal morphology but did not influence growth rate which was also reduced under low pH. Bacterially challenged animals reared at lower pH and at higher temperatures showed the most significant impacts on shell chemistry and mineralogy due to the increased incidence of epizootic shell disease under the most extreme temperature and pH condition.

Conclusions
We hypothesized that ocean acidification and increased temperature would result in decreased shell growth and changes in mineral composition of larval lobster shell. We found that shell microstructure development is largely pH dependent and that OA and increased temperature alter the onset and prevalence of the shell disease.