Effect of Ocean Acidification on Predator-Prey Interaction

To predict future effects of ocean acidification (OA) on marine ecosystems we need increased understanding of how OA affects interaction between species. In this project we have performed laboratory experiment to find out if ocean acidification (OA) affects predator - prey interaction for a selection of benthic marine invertebrates. The main focus has been on the effects of OA on the response to predation cues. In WP1, we investigated if OA (pH 7.6) affected how blue mussels (Mytilus edulis) responded to predation cues from the common starfish (Asterias rubens). There were no effects of OA on physiological or behavioural parameters for mussels and starfish, but there was an effect of predation cues on aggregation of mussels and production of byssus threads. A higher percentage of the mussels aggregated and 65% more byssus threads were produced by mussels kept in tanks with caged starfish. The mussels responded to predation cues in the same way at high and low pH. In WP2, we investigated if OA (pH 7.6) affected how green sea urchins (Strongylocentrotus droebachiensis) responded to predation cues from the edible crab (Cancer pagurus). There was no significant effects of OA on survival, feeding (kelp grazing), growth or behaviour for the sea urchins. There was, however significant responses to predation cues. In tanks where a crab was kept below the sea urchin cage, 80% of the urchins stayed on the walls of the cage compared to 43% in tanks without crab. A high percentage of sea urchins on the walls of the cage, further away from the crab, is considered an escape response. Crab cues caused a 40% reduction of kelp grazing. Reduced grazing as a response to crab cues was documented after many weeks exposure and after one day exposure to crab cues. The response disappeared when the exposure to crab cues ended, and the sea urchins increased their grazing rate to the level of sea urchins that had not been exposed to cues. Therefore, the response to predation cues was rapidly induced, lasted for many weeks, and rapidly disappeared when the exposure to cues ended. The sea urchin response to predation cues - escape and reduced grazing - was the same at high and low pH. The weight of small sea urchins increased 40% in ten weeks in tanks without crabs, but in tanks with crab there was only a 20% increase in weight at pH 8.1 and 10% at pH 7.6. OA alone did not affect growth of the sea urchins, but OA increased the negative effect of predation cues on growth of small sea urchins. Predation cues also caused reduced growth of large urchins, but the effect was the same at high and low pH. In WP3, the interaction between the red sea urchin (Strongylocentrotus franciscanus) and the sunflower star Pycnopodia helianthoides was investigated at pH 8,0 and pH 7,7. The experiment lasted 22 weeks. The sunflower star had higher feeding and growth rate at low than high pH. The sea urchins had a higher feeding rate when exposed to predation cues, but the growth rate was lower. OA and predation cues caused reduced growth and it was an additive negative effect of OA and predation cues on growth of red sea urchins. The positive effect of OA on the sunflower star, and the additive negative effect of OA and predation cues on sea urchin growth could increase the strength of the trophic interaction for this predator-prey pair and lead to stronger top-down control in the future. In WP4, the escape response for the common seastar Asterias rubens exposed to cues from its predator the common sunstar (Crossaster papposus) was investigated after four weeks acclimation to different pH levels in the range 7.3 - 8.0. A. rubens did not have an escape response when exposed to predator cues, regardless of pH in the acclimation period. Presence of the predator had no significant impact on the preys behaviour. The plan for WP5 was to model the response of mussels to OA and predation cues, but since there were no clear effects of neither OA nor predation cues on any of the tested physiological parameters there was nothing to model. It was decided to rather use energy budgets (Dynamic Energy Budget, DEB) and literature data to model how OA affects sea urchin larvae (S. droebachiensis). Data on growth, feeding and respiration for sea urchin larvae was used in DEB modelling to improve our understanding of the effects of OA. The results are best explained by the hypothesis that OA increases the maintenance costs of the larvae. The literature show that OA affected the behaviour of larval coral fish; they became attracted to predator cues leading to increased predation rate. OA did not change the way prey responded to predation cues in the OAPPI project. Generally, the response to predation cues was similar at high and low pH. The only exception is the reduction of sea urchin growth that was even more pronounced at low than high pH (control).

Different species can benefit or suffer from OA, and the effect at the ecosystem level will depend on the relative fitness of interacting species. We will study the effects of OA on predator-prey interaction (PPI) for a selection of benthic marine inverte brates. Predators can affect prey populations via consumption (lethal effects) or via non-lethal effects on prey traits (behaviour, morphology), or trait-mediated interactions. We will study two aspects of the effects of OA on PPI; 1) indirect PPI; how OA affects the development of defensive traits (behaviour, physiology, growth) in prey exposed to chemical cues from their predator, and 2) direct PPI; how long-term exposure of the interacting species to OA will affect predation rate. Four well-known preda tor-prey pairs from the Atlantic and Pacific Ocean haven been selected; 1) Asterias rubens vs. Mytilus edulis, 2) Cancer pagurus vs. Strongylocentrotus droebachiensis, 3) Pycnopodia helianthoides vs. Strongylocentrotus franciscanus), and 4) Crossaster pap posus) vs. Asterias rubens. The use of multiple predator-prey pairs will allow us to determine the extent to which results are broadly generalizable across taxa and geographic regions, vs. specific to certain species pairs and localities. Echinoderms, whi ch are keystone species in many benthic systems, will be examined in their roles as both predator and prey, which will more effectively allow us to disentangle the effects of OA on prey-detection vs. predator-avoidance independent of species identity. Eff ects of OA on direct or indirect PPI could have consequences for the structure and function of marine communities. Data from the experiments will be used for modelling effects of OA at the population level, and mussel data will be used to model the effect of OA on the Dynamic Energy Budget (DEB) for mussels.