How do “fish of a feather” shoal together?

“Birds of a feather flock together,” as the old saying goes, and that simple axiom raises many fascinating questions. Do animals really choose to associate with conspecifics that closely resemble themselves? If so, how do they even determine, without the either aid of mirrors or cognitive abilities that enable self-recognition, who is “of a feather” and who is not? Why would once choose to be “just a face in the crowd,” or even a member of a crowd at all? These strategies must have adaptive advantages, as they are displayed in so many different species—from schooling fish to migrating geese to herds of ungulates on the African plains. But what are the mechanisms involved?

A study recently published in the journal Behavioral Ecology and Sociobiology sheds light on this issue (Ward & Currie 2013). The “flocking together” in this case is examined in two species of fish, the three-spined stickleback (Gasterosteus aculeatus) and the banded killifish (Fundulus diaphanus), both of which are known to move around in large shoals. Both species also exhibit extremely low variation in size between individuals comprising a given group, so Ward and Currie sought to examine how the fish manage to accomplish this size-sorting strategy.

Much research has shown that: 1) moving around in large groups reduces individual risk from predation, a sort of "safety in numbers" effect (Neill & Cullen 1974); and 2) often, the more similar your appearance to that of the other members of your group, the greater your risk reduction (Krause et al. 2000). This begs the question, though: how does an animal like a fish or a bird, or even a mammal, know what it looks like? With the exception of humans—and some dolphins and apes—few animals are capable of self-recognition even in the vanishingly rare instance of having access to a reflective surface.

Ward and Currie investigated this phenomenon in their study of sticklebacks and killifish. They hypothesized that there must be some sensory mechanism that allows fish to identify whether a conspecific is similar on an important aspect of phenotype, even in the absence of a visual concept of one's own appearance.

The authors note that previous work has shown that fish can visually assess whether they are within about 20% of the size of another member of their species (Ranta et al. 1992; Krause & Godin 1994). Many shoals, however, show much less than 20% variation in size. This strongly suggests that fish must have some other, more precise, form of size discrimination as well.  Fish use chemosensory cues for a wide variety of behaviors, and chemical cues have already been shown to help fish determine the body size of potential predators. In light of this, Ward and Currie hypothesized that a chemosensory stimuli might underlie their perception of body size when forming shoals.

The researchers collected individuals within two different size classes for each species. After an acclimation period in the lab, the fish were put into “flow channels” that circulated water from a reservoir bucket through the channel and then out a drain, creating an artificial current carrying water from two reservoirs. This created a “stream” of water from each reservoir on each side of the channel, with a neutral zone in the middle. The "focal fish” (the one in the channel) had the option to either swim in the stream with an individual of its own size class, a smaller or larger fish, or neither, based on chemical cues from the water streaming in from the reservoirs. A two-stream channel was also created to test whether fish would choose to swim in the stream of any conspecific or in a neutral zone.

The results? The fish “selected” the chemical cues taken from individuals that were of their own size class by a very significant margin, and they similarly preferred to swim in the stream of a conspecific than a neutral zone in the two-stream test. All of this strongly supports the hypothesis that fish use chemosensory information from the surrounding water to determine whether another member of their species is of a similar size. This is important, because it confirms that there is no need to visually assess one’s own size in order to determine the relative size of another individual.

All of the fish in this experiment were fed the same food items and kept in the same tank conditions during a control period leading up to the data collection, to avoid any bias based on dietary composition or other environmental factors. The authors point out, however, that size-based differences in diet (i.e., bigger fish can eat bigger and/or different prey) may enhance the difference in chemical cues, creating even stronger signals to inform fish about body size in natural settings.

By demonstrating that fish use chemical cues to determine the size of conspecifics relative to themselves, this study demonstrates the importance of chemical cues for a critical aspect of fish behavior. As water quality becomes more and more of a concern in many areas, awareness of whether or not these signaling mechanisms are impaired by changing temperature, pH, or chemical composition of the water will be critical. Future research on the precise chemicals involved in these interactions will also be interesting. This is definitely a line of inquiry to keep an eye on!

Krause J., Godin J. G. J., Brown B. (1998) Body length variation within multi-species fish shoals: the effects of shoal size and number of species. Oecologia 114:67–72.

Krause J., Hoare D.J., Croft D., Lawrence J., Ward A., Ruxton G.D., Godin J.G.J., James R. (2000) Fish shoal composition: mechanisms and constraints. Proc R Soc Lond B 267:2011–2017.

Neill S. R. S., Cullen J. M. (1974) Experiments on whether schooling by their prey affects hunting behavior of cephalopods and fish pred- ators. Journal of Zoology 172:549–569.

Ranta E., Lindstrom K., Peuhkuri N. (1992) Size matters when 3-spined sticklebacks go to school. Animal Behavior 43:160–162.
Ward, A., & Currie, S. (2013) Shoaling fish can size-assort by chemical cues alone. Behavioral Ecology and Sociobiology DOI: 10.1007/s00265-013-1486-9

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