Synergic effects of ocean acidification and shading on behavior and mortality of hermit crabs
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1
Instituto Oceanográfico, Universidade de São Paulo (IOUSP), Brazil
Anthropogenic activities release high carbon dioxide concentrations in the atmosphere, which is absorbed by the oceans. Such absorption causes ocean acidification (OA) due to a pH reduction and changes in seawater carbonate system. OA process affects the dynamic and the biodiversity of marine ecosystems by changing the distribution and abundance of species and predator-prey interactions. Calcifying organisms, as crustaceans, suffer several physiological and ecological impacts due to OA, which can increase mortality due to changes in post molt calcification. In hermit crabs, reduced pH may affect frequency of shell selection and chemo-responsiveness, implying difficulty in chemical cues detection in seawater. Along with that, enterprises (e.g. harbors, docks and aquaculture systems) induce several impacts in coastal ecosystems, as environmental shading that may originate negative effects due to changes in light availability to the organisms. Visual orientation is important to avoidance and attraction responses to chemical cues and it has an important role in exploratory behavior of new shells by hermit crabs. However, the effects of multiple stressors have been poorly studied in crustaceans and the effects of OA have been only evaluated combined with variation in abiotic factors such as temperature, oxygen and salinity. In this context, the aim of this study is to evaluate potential acute and synergic effects of OA and shading in mortality and behavioral responses of hermit crabs to odors of predators (danger signal of predation risk) and dead gastropods (possibility of new shell available to selection). The hermit crab Pagurus criniticornis was used as model and individuals were collected in Araçá Bay (north coast of São Paulo state, Brazil). This species is abundant and ecologically important to this region, which is threatened due to expansion proposal of the Port of São Sebastião that will cause environmental shading. Crabs were reared for 14 weeks with artificial seawater in 25ºC and salinity 32. Experimental design was set considering as factors: pH (two levels, 8.1 and 7.6) and shading (two levels, 12:12h and 0:24h). Thus, four treatments were used: pH=7.6 and photoperiod of 12:12h (reduced pH and control photoperiod); pH=7.6 and photoperiod of 0:24h (reduced pH and shading); pH=8.1 and 0:24h (control pH and shading); and pH=8.1 and 12:12h (control), with 48 hermit crabs per treatment (n=48). For the reduced pH system, pH was maintained with CO2 bubbling controlled by a pH computer system. Mortality was recorded every week for all treatments and Factorial ANOVA was applied to evaluate average cumulative mortality by comparison of four treatments among the 14 weeks of experiment. At the end of the experiment, we randomly selected 20 hermit crabs of each treatment to assays of behavioral responses, with 48h of interval between the assays of both type of odors. 48h after feeding, crabs were placed in a tank with 500mL of seawater of their respective treatment and 1mL of odor was added. A paper with drawn lines of 1x1cm was fixed in the tank’s bottom and the behavior response was evaluated by counting the number of crossed lines in 3 minutes. The predator odor was prepared placing three specimens of the blue crab Callinectes danae in 10L of seawater for 2 hours. To prepare gastropod odor, 30 frozen specimens of Cerithium atratum were macerated and mixed in 1L of seawater. A factorial ANOVA was applied to compare the response behavior between the treatments for each type of odor. Mortality analysis showed interaction between treatment and time (F=6.88; DF=39; p<0.001) and there was a significant difference only for the treatment 7.6/0:24h after the sixth week. Individuals of this treatment represented 67,4% of total deaths over the experiment. Before the sixth week, there was no difference among treatments. The treatments 7.6/12:12h, 8.1/0:24h and 8.1/12:12h had no difference in mortality over the 14 weeks of experiment, except for the 7.6/12:12h that was different from treatments 8.1/12:12h and 7.6/0:24h at the last week. These results show the potential effect of reduced pH in mortality of hermit crabs in long-term experiment. When crabs were exposed to only one factor of stress (reduced pH or shading), they showed a similar pattern in mortality in the last week of experiment, but it is clear that shading do not represent a significant mortality compared to control when it acts as a single stressor over the 14 weeks. Results of response to predator odor indicated no interaction between pH and photoperiod (F=1.39; DF=1; p=0.24). Only photoperiod affected the number of crossed lines by hermit crabs (13.32; DF=1; p<0.001); there was no difference between pH treatments (F=0.01; DF=1; p=0.92). Therefore, only shading condition presented influence in locomotory behavior when crabs were exposed to predator signal, which suggests that visual orientation is important to hermit crabs to detect potential risk of predation. Regarding the response to odor of dead gastropods, results showed interaction between pH and photoperiod (F=6,46; DF=1; p=0.01) and hermit crabs reared in 7.6/0:24h treatment presented displacement significantly lower. These results represent a synergic effect of reduced pH and shading in hermit crabs in detecting new and potentially adequate shells for occupation. When crabs were exposed to both stressors, exploratory behavior to seek new shells was lower, which indicates that crabs were not motivated to find and select new shells. Thus, these results contribute to the understanding of potential effects of climate changes associated with local anthropic impacts in marine organisms.
Acknowledgements
We are grateful to Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for support to this research financial (Proc. 2015/02727-0).
Keywords:
pH,
photoperiod,
Pagurus criniticornis,
Coastal impacts,
Araçá Bay.
Conference:
XIX Iberian Symposium on Marine Biology Studies, Porto, Portugal, 5 Sep - 9 Sep, 2016.
Presentation Type:
Oral Presentation
Topic:
2. GLOBAL CHANGES, INVASIVE SPECIES AND CONSERVATION
Citation:
Nagata Ragagnin
M and
Turra
A
(2016). Synergic effects of ocean acidification and shading on behavior and mortality of hermit crabs.
Front. Mar. Sci.
Conference Abstract:
XIX Iberian Symposium on Marine Biology Studies.
doi: 10.3389/conf.FMARS.2016.05.00023
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Received:
30 Apr 2016;
Published Online:
02 Sep 2016.
*
Correspondence:
Miss. Marilia Nagata Ragagnin, Instituto Oceanográfico, Universidade de São Paulo (IOUSP), São Paulo, Brazil, marilia-nr@hotmail.com