Behavioural responses of fish groups exposed to a predatory threat under elevated CO2

Impacts of climate-related stressors on social group cohesion and individual sociability in fish

Group-living in animals comes with a number of benefits associated with predator avoidance, foraging, and reproduction. A large proportion of fish species display grouping behaviour. Fish may also be particularly vulnerable to climate-related stressors including thermal variation, hypoxia, and acidification. As climate-related stressors are expected to increase in magnitude and frequency, any effects on fish behaviour may be increased and affect the ability of fish species to cope with changing conditions. Here we conduct a systematic review of the effects of temperature, hypoxia, and acidification on individual sociability and group cohesion in shoaling and schooling fishes. Searches of the published and grey literature were carried out, and studies were included or excluded based on selection criteria. Data from studies were then included in a meta-analysis to examine broad patterns of effects of climate-related stressors in the literature. Evidence was found for a reduction in group cohesion at low oxygen levels, which was stronger in smaller groups. While several studies reported effects of temperature and acidification, there was no consistent effect of either stressor on sociability or cohesion. There was some evidence that marine fishes are more strongly negatively affected by acidification compared with freshwater species, but results are similarly inconsistent and more studies are required. Additional studies of two or more stressors in combination are also needed, although one study found reduced sociability following exposure to acidification and high temperatures. Overall, there is some evidence that hypoxia, and potentially other climate-related environmental changes, impact sociability and group cohesion in fishes. This may reduce survival and adaptability in shoaling and schooling species and have further ecological implications for aquatic systems. However, this synthesis mainly highlights the need for more empirical studies examining the effects of climate-related factors on social behaviour in fishes.

Response of CRH system in brain and gill of marine medaka to seawater acidification

Corticotropin-releasing hormone (CRH) is mainly secreted by the hypothalamus to regulate stress when environmental factors change. Gills contact with water directly and may also secrete CRH to maintain local homeostasis. Ocean acidification changes water chemical parameters and is becoming an important environmental stressor for marine fish. The response of brain and gill CRH systems to ocean acidification remains unclear. In this study, marine medaka were exposed to CO2-acidified seawater (440 ppm, 1000 ppm, and 1800 ppm CO2) for 2 h, 4 h, 24 h, and 7 d, respectively. At 2 h and 4 h, the expression of crh mRNA in gills increased with increasing CO2 concentration. Crh protein is expressed mainly in the lamellae cells. crhbp and crhr1 expression also increased significantly. However, at 2 h and 4 h, acidification caused little changes in these genes and Crh protein expression in the brain. At 7 d, Crh-positive cells were detected in the hypothalamus; moreover, Crh protein expression in the whole brain increased. It is suggested that CRH autocrine secretion in gills is responsible for local acid-base regulation rather than systemic mobilization after short-term acidification stress, which may help the rapid regulation of body damage caused by environmental stress.

Ocean acidification alters the acute stress response of a marine fish

The absorption of anthropogenic carbon dioxide from the atmosphere by oceans generates rapid changes in seawater carbonate system and pH, a process termed ocean acidification. Exposure to acidified water can impact the allostatic load of marine organism as the acclimation to suboptimal environments requires physiological adaptive responses that are energetically costly. As a consequence, fish facing ocean acidification may experience alterations of their stress response and a compromised ability to cope with additional stress, which may impact individuals' life traits and ultimately their fitness. In this context, we carried out an integrative study investigating the impact of ocean acidification on the physiological and behavioral stress responses to an acute stress in juvenile European sea bass. Fish were long term (11 months) exposed to present day pH/CO₂ condition or acidified water as predicted by IPCC "business as usual" (RCP8.5) scenario for 2100 and subjected to netting stress (fish transfer and confinement test). Fish acclimated to acidified condition showed slower post stress return to plasma basal concentrations of cortisol and glucose. We found no clear indication of regulation in the central and interrenal tissues of the expression levels of gluco- and mineralocorticoid receptors and corticoid releasing factor. At 120 min post stress, sea bass acclimated to acidified water had divergent neurotransmitters concentrations pattern in the hypothalamus (higher serotonin levels and lower GABA and dopamine levels) and a reduction in motor activity. Our experimental data indicate that ocean acidification alters the physiological response to acute stress in European sea bass via the neuroendocrine regulation of the corticotropic axis, a response associated to an alteration of the motor behavioral profile. Overall, this study suggests that behavioral and physiological adaptive response to climate changes related constraints may impact fish resilience to further stressful events.

Limited behavioural effects of ocean acidification on a Mediterranean anemone goby (Gobius incognitus) chronically exposed to elevated CO2 levels

An in situ reciprocal transplant experiment was carried around a volcanic CO2 vent to evaluate the anti-predator responses of an anemone goby species exposed to ambient (∼380 μatm) and high (∼850 μatm) CO2 sites. Overall, the anemone gobies displayed largely unaffected behaviors under high-CO2 conditions suggesting an adaptive potential of Gobius incognitus to ocean acidification (OA) conditions. This is also supported by its 3-fold higher density recorded in the field under high CO2. However, while fish exposed to ambient conditions showed an expected reduction in the swimming activity in the proximity of the predator between the pre- and post-exposure period, no such changes were detected in any of the other treatments where fish experienced acute and long-term high CO2. This may suggest an OA effect on the goby antipredator strategy. Our findings contribute to the ongoing debate over the need for realistic predictions of the impacts of expected increased CO2 concentration on fish, providing evidence from a natural high CO2 system.

Gross morphology, histology, and ultrastructure of the olfactory rosette of a critically endangered indicator species, the Delta Smelt, Hypomesus transpacificus

The Delta Smelt (Hypomesus transpacificus) is a small, semi-anadromous fish native to the San Francisco Bay-Delta Estuary and has been declared as critically endangered. Their olfactory biology, in particular, is poorly understood and a basic description of their sensory anatomy is needed to advance our understanding of the sensory ecology of species to inform conservation efforts to manage and protect them. We provide a description of the gross morphology, histological, immunohistochemical, and ultrastructural features of the olfactory rosette in this fish and discuss some of the functional implications in relation to olfactory ability. We show that Delta Smelt have a multilamellar olfactory rosette with allometric growth. Calretinin immunohistochemistry revealed a diffuse distribution of olfactory receptor neurons within the epithelium. Ciliated, microvillous and crypt neurons were clearly identified using morphological and immunohistochemical features. The olfactory neurons were supported by robust ciliated and secretory sustentacular cells. Although the sense of smell has been overlooked in Delta Smelt, we conclude that the olfactory epithelium has many characteristics of macrosmatic fish. With this study, we provide a foundation for future research into the sensory ecology of this imperiled fish.

Boosted fish abundance associated with Posidonia oceanica meadows in temperate shallow CO2 vents

Ocean acidification (OA) may induce major shifts in the structure and function of coastal marine ecosystems. Studies in volcanic CO₂ vents, where seawater is naturally acidified, have reported an overall simplification of fish assemblages structure, while some primary producers are likely to increase their biomass under elevated concentration of CO₂. Here we used temperate shallow CO₂ vents located around the coast of Ischia island (Italy) to assess the effects of OA on necto-benthic fish assemblages associated with the foundation seagrass species Posidonia oceanica in the Mediterranean Sea. We compared P. oceanica meadow structure, its epiphytic community and the associated fish assemblage structure and diversity at vents with low pH sites and reference sites with ambient pH using underwater visual census strip transects, in two seasons (fall 2018 and summer 2019). Data were analysed using both univariate and multivariate statistical techniques. Results showed greater P. oceanica habitat complexity (i.e. shoot density) and lower abundance of epiphytic calcareous species (e.g. coralline algae) at the vents than reference sites. Total abundance of adult and juvenile fish was higher at vents than reference sites, while no differences were found for species richness and composition. Overall, the herbivore Sarpa salpa stands out among the species contributing the most to dissimilarity between vents and reference sites, showing higher abundances under OA conditions. This pattern could be explained by the combined effect of a positive response to the higher structural meadows complexity and the greater seagrasses palatability/nutritional value occurring at the vents, which may help herbivores to withstand the higher energetic cost to live under high pCO₂/low pH conditions. Our results indicate that necto-benthic fish assemblages associated with the Mediterranean P. oceanica ecosystem may cope with OA under the CO₂ emission scenarios forecasted for the end of this century.

Fish assemblages cope with ocean acidification in a shallow volcanic CO2 vent benefiting from an adjacent recovery area

Shallow CO₂ vents are used to test ecological hypotheses about the effects of ocean acidification (OA). Here, we studied fish assemblages associated with Cymodocea nodosa meadows exposed to high pCO₂/low pH conditions at a natural CO₂ vent in the Mediterranean Sea. Using underwater visual census, we assessed fish community structure and biodiversity in a low pH site (close to the CO₂ vent), a close control site and a far control site, hypothesising a decline in biodiversity and a homogenization of fish assemblages under OA conditions. Our findings revealed that fish diversity did not show a unique spatial pattern, or even significant relationships with pH, but correlated with seagrass leaf canopy. Among-site similarity was found in the abundance of juveniles, contrary to the expected impacts of OA on early life stages. However, pH seems an important driver in structuring fish assemblage in the low pH site, despite its high similarity with the close control site. This unexpected pattern may represent a combined response of fish mobility, enhanced food resources in the acidified site, and a 'recovery area' effect of the adjacent control site.