Drivers of biological diversity and responses to global changes in marine invertebrates

Abstract

Human activities, in particular global changes (e.g., ocean warming - OW and ocean acidification - OA) are projected to drive some marine species to extinction within the coming decades. Marine invertebrates are amongst the most vulnerable to these changes due to the increased energetic cost to maintain intracellular pH homeostasis. To mitigate extinction, organisms may migrate, acclimate or adapt genetically. While these mechanisms are increasingly documented, they are not fully understood. This knowledge is critical for assessment of extinction risks, an important index for effective conservation and management of marine biodiversity. This thesis aims to increase our understanding on the drivers of biological diversity and sensitivity of marine invertebrates to OW and OA. Specifically, I assess (1) the quality of inferences on adaptive evolution in recent publications on responses of marine invertebrates to OW or OA and summarize the current knowledge and identify the gaps (Paper I); (2) the drivers of genetic diversity, structure, connectivity among Acropora austera populations across Mozambique coral reefs (Paper II); (3) the sensitivity to low pH in larvae of the sea urchin, Tripneustes gratilla, from subtidal and intertidal seagrass meadows with contrasting pH variability at Inhaca Island, Mozambique (Paper III); (4) the role of natural fluctuation in pH on the response of larvae of the sea urchin Echinus esculentus to low pH (Paper IV). Field genome scans surveys, laboratory experiments and systematic literature review were used. My systematic literature review (Paper I) highlights that publication on adaptive responses of marine invertebrates to OW or OA used more frequently strong methods for inferences of genetic change, such as common garden experiments and molecular genetic analysis. Methods for weaker inferences, such as comparison to model prediction, were less frequently used. On the other hand, reciprocal transplants, the stronger method for inferring adaptive change was less used in comparison with weaker methods such as phenotypic and genotypic selection. I also showed different levels of genetic variability and connectivity between populations of corals along the Mozambique coast. These geographic differences in levels of genetic diversity and connectivity may be explained by oceanographic factors and mode of reproduction of the corals (Paper II). Larvae of the sea urchin T. gratilla from Inhaca Island had reduced fitness when exposed to low pH. Moreover, larvae from adults collected in an intertidal habitat were more sensitive to low pH as compared to larvae from adults collected in a subtidal population. This result reveals population specific responses to low pH and challenges current theories that predict higher tolerance in individuals living in habitats with higher pH range (Paper III). Under present day natural variability in pH, the extreme low pH does not appear to be the main driver of biological responses in larvae of the sea urchin E. esculentus and adaptation to such conditions might be associated with a cost of plasticity but not a cost of canalization (Paper IV). Overall, this thesis shows that oceanographic factors and natural variability in pH influence the levels of genetic diversity and biological sensitivity in populations of marine invertebrates. These parameters should be considered to better evaluate the ability of marine invertebrates to withstand environmental changes and to sustain the provision of ecological functions, and guide conservation strategies.