Shoaling

Atmospheric CO2 is expected to more than double by the end of the century. The resulting changes in ocean chemistry
will affect the behaviour, sensory systems and physiology of a range of fish species. Although a number of past studies
have examined effects of CO2 in gregarious fishes, most have assessed individuals in social isolation, which can alter individual
behaviour and metabolism in social species. Within social groups, a learned familiarity can develop following a prolonged
period of interaction between individuals, with fishes preferentially associating with familiar conspecifics because
of benefits such as improved social learning and greater foraging opportunities. However, social recognition occurs
through detection of shoal-mate cues; hence, it may be disrupted by near-future CO2 conditions. In the present study, we
examined the influence of elevated CO2 on shoal familiarity and the metabolic benefits of group living in the gregarious
damselfish species the blue-green puller (Chromis viridis). Shoals were acclimated to one of three nominal CO2 treatments:
control (450 μatm), mid-CO2 (750 μatm) or high-CO2 (1000 μatm). After a 4–7 day acclimation period, familiarity was examined
using a choice test, in which individuals were given the choice to associate with familiar shoal-mates or unfamiliar
conspecifics. In control conditions, individuals preferentially associated with familiar shoal-mates. However, this association
was lost in both elevated-CO2 treatments. Elevated CO2 did not impact the calming effect of shoaling on metabolism, as
measured using an intermittent-flow respirometry methodology for social species following a 17–20 day acclimation period
to CO2 treatment. In all CO2 treatments, individuals exhibited a significantly lower metabolic rate when measured in a
shoal vs. alone, highlighting the complexity of shoal dynamics and the processes that influence the benefits of shoaling.

Living with relatives can be beneficial to individuals via the evolution of kin-directed altruism, but this is tempered by the increased risk of inbreeding. Therefore, in social species, the ability to recognise relatives can be highly advantageous. This study focuses on kin discrimination in the Lake Eacham rainbowfish, Melanotaenia eachamensis, an endangered freshwater species from north-east Queensland, Australia. First, I examined kin recognition abilities when a combination of both chemical and visual recognition cues was available. When given a choice of shoaling with same-sex groups, females spent significantly longer with full-sibs rather than half-sibs, full-sibs rather than non-relatives and half-sibs rather than non-relatives. Males spent significantly longer shoaling with full-brothers versus half-brothers, but showed no other shoalmate preferences. Second, in the presence of only chemical cues, females did not discriminate among groups of different levels of relatedness, but males showed a non-significant tendency to associate with full-sibs rather than non-relatives. Male shoaling behaviour seemed to be more influenced by factors other than relatedness, e.g. intra-sexual competition. Finally, I found that the shoaling preferences of females changed when exposed to groups of males. Females preferred to associate with non-relatives rather than half-brothers and non-relatives rather than full-brothers. There was no significant difference in the time spent with half-brothers versus full-brothers. Taken together, my results suggest that females have very good kin recognition abilities. They prefer to shoal with female relatives but avoid male relatives, and so are able to balance the benefits of nepotism and the costs of incest.

Grouping behaviour is widespread across the animal kingdom, and is known to reduce an individual's risk of predation, for example through predator confusion. Theory predicts that individuals that are different in appearance to the rest of the group are at a greater risk of predation because they are more conspicuous to predators (the ‘oddity’ effect). Thus, animals should choose group mates that are the most similar in appearance to themselves. Another common antipredator tactic is crypsis (camouflage). Fishes are capable of changing colour to match their visual background, but few studies have examined how this might influence shoaling decisions, particularly in the context of the oddity effect. We induced colour pattern changes in a colourful species of freshwater fish, the western rainbowfish, Melanotaenia australis, by maintaining fish in dark and pale aquaria for 2 weeks. Analysis of the proportion of black body pigmentation confirmed that rainbowfish in dark environments developed darker colour patterns than those held in pale environments. We then conducted behavioural observations to determine whether fish subsequently based their shoaling decisions on body coloration. We found that rainbowfish preferred to shoal with similar individuals; fish that had been held in dark aquaria preferred to shoal with other dark fish and fish from pale aquaria preferred other pale fish. Our findings are consistent with the predictions of the oddity effect and demonstrate how morphological colour pattern changes and behavioural decisions interact to mediate antipredator tactics in fish.