Minnows, catfish and their relatives (the ostariophysan fishes) make up a very large proportion – about 64% – of all freshwater fish species. When their skin is broken as a result of a predatory attack, many ostariophysans secrete a substance from specialised “club” cells that is detected by fellow shoalmates and warns them of potential danger. from predators. The evolution of this alarm system has been explained in terms of kin selection, where individuals that share genes with an attack victim are alerted and gain indirect fitness benefits. However, this would only work if fish prefer to shoal with their close relatives. To check this idea, German researchers collected DNA by clipping the fins of minnows taken from the Lenne and Sieg rivers. In both rivers, six shoals were sampled at each of three sites, and the fins of 20 fish from each shoal were clipped. Analysis showed that the fish making up a minnow shoal were essentially unrelated, the average level of relatedness being a hundred times lower than that of sibling individuals. In other words, there was no evidence that the specialised secretory cells in the skin of ostariophysans are the product of kin selection. Therefore, other evolutionary possibilities need to be explored. Foremost among these is the recent suggestion that predator avoidance is only a secondary function of club cells, which have been shown to increase in abundance after exposure to pathogens, parasites and ultraviolet radiation, but not to increased predation pressure. It seems that the alarm response may be a by-product of selection for fish that are sensitive to the sudden release of chemicals whose fundamental role is to strengthen the fish’s immune response to generalized damage to the skin.
References:
Bernhardt, B., Lampert, K.P., Leese, F., Mayer, C. & Tollrian, R. 2012. Are shoals of minnow Phoxinus phoxinus formed by close kin? Journal of Fish Biology 80, 713–721.
Chivers, D.P. et al. 2007. Epidermal ‘alarm substance’ cells of fishes maintained by non-alarm functions: possible defence against pathogens, parasites and UVB radiation. Proceedings of the Royal Society B 274, 2611–2619.
Freshwater Research News 
Posted on September 23, 2013