Effects of Ocean Acidification on physiology, behaviour and ecology of fish

CO2 concentration in the atmosphere is increasing at unprecedented rate since the last 800,000 years due to rising fossil fuel combustion, industrial processes and land use by humans. By absorbing part of this gas from the atmosphere, Oceans contribute to the mitigation of climatic changes, but at the cost of greater modifications of their physical and chemical characteristics. CO2 dissolved in the seawater leads to increased bicarbonate (HCO3 -) and hydrogen ions (H+) concentrations with a consequent pH drop, a phenomenon known as Ocean Acidification (OA). If global policy will not put in force mitigation measures to reduce CO2 emissions, it is projected that partial pressure of dissolved CO2 (pCO2) will increase up to 600-700 μatm by 2050 and up to ~1000 μatm by 2100. At the same time, ocean surface pH is predicted to further drop by 0.3- 0.4 units by the end of this century. OA is considered one of the most serious threat for marine organisms and ecosystems, and this topic was the most studied field of investigation between 2000 and 2013. Several quantitative reviews based on meta-analytic approaches allowed some generalizations on the sensitivity of marine species to elevated pCO2 levels and lowered pH showing that OA expected for next decades would negatively affect survival, calcification, growth and development of a wide variety of taxa. Calcifying organisms are considered the most threatened group, as increase of hydrogen ions concentration may also determine the decrease of saturation states with respect to aragonite and calcite, two forms of calcium carbonate forms commonly used for building up organisms’ shells and skeletons. However, expected CO2 levels in the ocean may also affect several biological processes of non-calcifying organisms. In the last decade, a growing number of studies focused on the impact of elevated CO2 concentration on fish. The vast majority of these studies was conducted under controlled laboratory conditions and showed highly variable sensitivities among species, making difficult to reach some generalization on the possible future impact of this environmental change. The general aim of this thesis was to assess how elevated CO2 may affect different biological processes of marine teleost fish through field-based and laboratory experiments. For this purpose, I investigated the response of tropical and temperate species during different life stages (from embryos to adults) and at different organization level (from individuals to community). The first aim of my thesis was to depict the state of the art on ocean acidification effects on fish worldwide. This goal was achieved through a quantitative review of papers published until June 2016 (chapter 2). Specifically, through a mixed-effects meta-analytic approach, I provide the first comprehensive global analysis of eco-physiological, developmental, and behavioural responses of fish to expected CO2 levels in the near-future ocean. To assess if such responses may be mediated by fish characteristics, I evaluated the role of different life history traits (e.g. climatic zone, life stage, physiology, and habitat) in modifying fish sensitivity to OA. Results showed that, if anthropogenic CO2 emissions continue to rise, significant effects of elevated CO2 levels on fish mortality, calcification, and behavioural performances are expected to occur in the next decades, particularly for larvae. Some effects may be more subtle than expected, as mortality rates per se may be enhanced by observed decreasing growth of larvae and higher predation riskunder elevated CO2. Despite some specific fish traits might mediate the responses of fish to OA (e.g. I found differences in behavioural impairments levels between tropical and temperate species), my analyses failed to find a single life history characteristic potentially confering fish tolerance to elevated CO2. A strong need to expand the number of OA studies to multi-generational, multi-stressor and species interactions experiments is also discussed. The following three chapters dealt with the effects of OA on embryos, juveniles and adults of a temperate wrasse species (Symphodus ocellatus) living off a volcanic CO2 seep site (Vulcano island, Italy). It has been widely demonstrated that fish reared at elevated CO2 concentrations display severe behavioural disruptions, such as altered lateralization, increased activity rates, unability to recognize olfactory cues. These effects have been mainly investigated in laboratory conditions and only two studies evaluated the impact of OA on behaviour of fish chronically exposed to elevated CO2 in the wild. Moreover, no previous studies focused on the possible effects of OA on fish reproductive behaviour. Chapter 3 provided the first evidence of the effects of ocean acidification on reproduction of fish in the wild. Specifically, I assessed how different CO2 concentrations may alter key mating behaviours and spawning ability in a species with different typologies of breeding males (dominant nesting, satellite and sneaker S. ocellatus males). Results showed that dominant male mating behaviour was unaffected, as courtship and nest defence did not differ between sites under ambient and elevated CO2 concentrations. However, dominant males experienced significantly lower rates of pair spawning at elevated CO2 levels, despite maintaining a trend of higher paternity than sneaker and satellite males. The chapter 4 assessed the influence of increasing CO2 concentrations on the metabolic rate (oxygen consumption) and the development of ocellated wrasse early life stages. For this purpose, I ran translocational experiments in the field, by collecting embryos from nesting sites with different pCO2 levels and transplanting embryos from ambient- to high-CO2 sites for 30 h. Results showed that ocellated wrasse offspring brooded in different CO2 conditions had similar metabolic responses, but embryos from parents that spawned in ambient conditions had higher metabolic rates and a smaller size at hatching when transplanted in the high-CO2 site. However, the adverse effects of increased CO2 on metabolism and development did not occur when embryos from the high-CO2 nesting site were exposed to ambient conditions, suggesting potential parental effects as offspring from the high-CO2 nesting site could be resilient to a wider range of pCO2 values than those belonging to the site with present-day pCO2 levels. The chapter 5 focused on the ability of S. ocellatus recruits to recognize their predator under elevated CO2. Several studies showed that fish exposed to CO2 concentrations expected for the next decades are potentially more sensitive to predation, as they show bolder behaviour and appear to be attracted from cues they normally avoid. However, treatment with gabazine - a neuroreceptor GABA antagonist – restore the normal behaviour. These evidences have been described mostly through short-term laboratory trials on tropical fish species, whilst experiments on temperate fish from populations chronically exposed to elevated CO2 in the field have been seldom carried out. I investigated whether recruits (i.e. settled larvae) of the ocellated wrasse S. ocellatus from high CO2 recruitment sites off a volcanic seep lost their ability to recognize the odour of a common predator. Results showed that fish, treated or untreated with gabazine, both from ambient and high CO2 sites, displayed a similar avoidance of predator odour, suggesting that ocellated wrasse recruits could be tolerant to rising CO2. This study adds on previous evidence that temperate fish species might exhibit higher resilience to OA than tropical species. The chapter 6 evaluated how short-term exposure to high CO2 concentrations may influence the escape response and the shelter use under predation risk of juveniles of the tropical damselfish Chromis viridis. For this purpose, I ran two different experiments in aquaria, by measuring the escape response kinematic and the use of shelter of groups of damselfishes reared for 5 days under different CO2 conditions (ambient and high CO2 concentrations) in presence/absence of their predator, the black tip grouper Epinephelus fasciatus. Whereas high CO2 had no effect on the escape performance of C. viridis, their use of shelter was significantly impaired, as juveniles reared at acidified conditions spent more time outside and ventured farther from the refuge than those reared at control conditions. The chapter 7 documented how OA may indirectly affect the composition and structure of fish communities via biogenic habitat shifts in CO2 seeps off Japan. Many recent studies suggested that elevated CO2 may affect the structural complexity of biogenic habitats, altering the species composition of the associated fauna and amplifying the indirect negative effects of OA on the biodiversity they support. By now, only two studies empirically estimated the indirect effects (via biogenic habitat change) of ocean acidification on fish assemblages, showing contrasting results. Long-term elevated CO2 did not lead to changes in reef fish community composition and structure in Papua New Guinea, whereas community effects were recorded on temperate coastal fish assemblages in CO2 seeps off Italy and New Zealand. Here, I coupled fish and benthic community surveys (complexity, canopy, % cover of main biogenic habitats) at and around natural CO2 seeps off Shikine Island (NW Pacific, Japan) to assess how OA-mediated changes of biogenic habitat may alter structure and composition of reef fish communities. Results showed shift in biogenic habitats from zooxanthellate scleractinian corals and high profile macroalgae in ambient CO2 sites to small turf algae in elevated CO2 sites, and rather consistent changes in the associated fish communities with a clear diversity loss