Early life stages under ocean acidifcation : direct effects, parental influence, and adaptation

Abstract

Anthropogenic carbon dioxide (CO2) emissions are forcing shifts in seawater carbonate chemistry. Having already caused the global average oceanic pH to drop from from 8.2 to 8.1, the trend is expected to continue in the coming centuries resulting in pH levels as low or lower than 7.4. This anthropogenically driven process is called ocean acidification, or OA. Reduced calcium carbonate due to OA threatens calcifiers as it is the building block used to calcify. For most species the process of OA will encompass many generations, and biological responses to OA will be shaped by evolutionary adaptation as the centuries progress. Adaptations to new environments can occur over tens of generations, and as such, evolution may provide an escape for some ostensibly vulnerable animals. So it is important to consider factors like multigenerational change and the potential for evolutionary adaptation when estimating the likelihood of success for any given species.

In this thesis I present 1 study on the early life stages of the barnacle Balanus amphitrite and 4 studies on the early life stages of the calcifying tube worm Hydroides elegans and how they are affected by OA. By combining several controlled experiments that include quantitative genetics, fertilization kinetics, and multigenerational responses I present an in-depth, multi-perspective study of how an economically important biofouling species may fare as conditions shift over a relatively long period of time. (1) Larval survival, metamorphosis, and post-larvae calcification in response to multiple levels of seawater pH in both species.

(2) Parental influence on fertilization success under future OA conditions in the broadcast spawning tube worm H. elegans. (3) Quantitative genetic analysis of multiple life history parameters, using offspring from a total of 48 mated pairs from 20 males and 13 females of H. elegans. (4) Influence of parental pH environment on performance of offspring in H. elegans. Major findings: (1) Observable effects of OA on the early life stages were only apparent in the tube worm H. elegans where growth at low pH (7.9) was reduced. This contrasted with B. amphitrite where low pH responses were only observed at the lowest, most extreme, pH (7.4). (2) Fertilization success in H. elegans was not affected by conservative OA scenarios, however, fertilization rates in different male/female combinations responded differently to different pH’s. (3) H. elegans performance is significantly affected by their parents’ pH environment, both maternal and paternal, however the effects of low pH were only apparent when offspring were raised at high (ambient) pH. Additionally, parental low pH experience effect on offspring revealed that maternal and paternal influence were independent, opposite and additive. (4) Lastly, metamorphosis success and growth rates in H. elegans are heritable in low pH conditions. Survival was not. Interestingly, in the lower pH environment, heritability of metamorphosis increased while heritability of growth decreased.

These two ecologically and economically important biofouling species show distinct responses to low pH conditions, the barnacle being ostensibly robust to low pH while the tube worm proved sensitive to relatively conservative low pH scenarios. Using a multi-generational approach and estimating the genetic link to variation in pH response this series of experiments makes it clear that pH response may change over time. Species that are considered sensitive to OA may adapt over time, and only by acknowledging this potential through experimentation can thorough predictions for species responses be established.