Free Amino Acids Mediate Association Preferences in Fish

Abstract

Chemical information is used in a variety of contexts including habitat recognition, foraging and predator avoidance, and a plethora of studies have shown that an individual's distinct chemical profile can mediate interactions with conspecifics. Interestingly, recent work has revealed that diet alone is sufficient to change the chemical profile of individuals, and with it, the way other individuals perceive and interact with them. Free amino acids are known to be utilised across species in a range of contexts, including during predator–prey and host–parasite interactions, and have been widely postulated to underpin diet‐mediated social interactions, especially in fish. However, so far no empirical evidence has been presented to support this suggestion. Using an established behavioural assay in three‐spined sticklebacks (Gasterosteus aculeatus), we aimed to assess association preferences in groups of fish fed experimental diets which differed by a single free amino acid. Our results demonstrate that free amino acids alone are sufficient to mediate interindividual association preferences, raising the possibility that such a mechanism may be widespread among aquatic animals.

The impact of water pH on association preferences in fish

“…It has been suggested that behavioural impairment towards chemical cues caused by freshwater acidification can either be mediated by the molecular change of the chemical cues themselves (Brown, Adrian, Lewis, & Tower, ; Leduc et al, ) or caused by a disruption of the fishes’ chemosensory receptor systems (Brown et al, ; Moore, ; Royce‐Malmgren & Watson, ; Tierney & Atema, ). Free amino acids have been postulated to underpin diet‐mediated interactions within shoaling fish (Atton et al, ; Kleinhappel, Burman, et al, ; Kleinhappel et al, ) and, as the isoelectric point of different free amino acids lies at different pH values (Lehninger, Nelson, & Cox, ), pH could change the charge of the amino acid which could then interfere with the amino acid receptor interaction (Royce‐Malmgren & Watson, ). An artificial change in pH has indeed been shown to change the behaviour, such as attraction or avoidance, of fish towards free amino acids (Royce‐Malmgren & Watson, ).…”

“…During data collection, each group consisted of two fish fed with the same diet and one fish fed on a different one, counterbalanced over diet conditions. Shoals were randomly assigned to one of two treatment groups: (a) the low pH condition in which fish were tested in water with a pH ranging between 6.2 and 6.4 (measured using a Benchtop digital pH meter) and (b) the high pH condition in which fish were tested in a pH ranging between 8.3 and 8.5, which was comparable to their normal housing water and the water used in previous experiments (pH range 8.2–8.4, unpublished data) (Kleinhappel, Burman, et al, ; Kleinhappel et al, ). Experiments were conducted in artificial freshwater, the preparation of which was varied in order to manipulate the water pH.…”

“…Water in the testing tanks was maintained at a constant 11°C by placing the experimental tanks within a water bath containing circulating chilled water. A webcam (Microsoft LifeCam) was mounted at the top of each tank and used to collect images every 16 s (Kleinhappel, Burman, et al, ; Kleinhappel et al, ) for one hour, using custom written MATLAB (MathWorks, Natick, MA, USA) code. Image collection started immediately after releasing the groups.…”

“…Previous studies have demonstrated consistent diet‐mediated association preferences in this species (Atton et al, ; Kleinhappel, Burman, et al, ; Ward, Hart, & Krause, ), with individuals within a shoal associating more frequently with others receiving the same diet, likely mediated by chemical cues. At least some of these studies (Kleinhappel, Burman, et al, ; Kleinhappel et al, ) were conducted in water with a relatively alkaline pH, and so we predict that low water pH will disrupt the chemically mediated association preferences induced by different diet treatments, while these will be unaffected at higher pH.…”

“…Sticklebacks naturally inhabit freshwater with a range of different pH values, from relatively low (e.g., down to around pH 6.0) to relatively high (e.g., up to approximately pH 8.5) (MacColl, El Nagar, & de Roij, ). Using an established behavioural assay (Kleinhappel, Burman, et al, ; Kleinhappel et al, ), we analysed diet‐mediated inter‐individual association preferences within groups of fish exposed to pH levels towards the extremes of their natural range by conducting two experiments: fish were tested in (a) experimentally manipulated water pH and (b) in their natural water, from varying pH habitats. In the first experiment, we manipulated water pH to test the impact of a short‐term reduction in pH levels which can, for instance, occur as a result of anthropogenic acidification (Leduc et al, ).…”