Imagine you’re on a date—not just any date, the dreaded first date. It’s going reasonably well, but then the lights in the restaurant go out. You think maybe this isn’t a bad thing, candle-lit dinners are romantic right? The waiters are rushing around trying to get the candles lit, so in the mean time you have three choices. 1) Try harder to make yourself heard over the commotion. 2) Call the date a dud and go home. 3) Make a fool of yourself by spilling your wine all over the table, yourself, and your date, then go home.
Now let’s reimagine this scenario. Instead of a first date, there are two fish courting, and instead of the lights going out, a ton of mud has been dumped on their heads. This is the reality that many freshwater fish face. Turbidity, or the murkiness of the water, is increasing in many aquatic ecosystems due to high nutrient inputs increasing algal growth or greater inputs of soil. To visualize what fish courting might look like, check out the video of a male cichlid trying to woo a receptive female using a move appropriately termed a quiver. Fish in turbid waters can increase the time and energy they spend courting in hopes of attracting a mate, but this might not lead to increased reproduction for the male (the fish equivalent of making a fool of yourself?) (Candolin et al., 2007). In other words, it can be a waste of time. In colorful species like cichlids, turbid water leads to duller fish, fewer color varieties, and lower species diversity (Seehausen et al., 1997). Even when species do manage to choose a proper mate, turbidity can still hamper hatching success because the eggs become smothered and cannot obtain sufficient oxygen. Beyond reproduction, turbidity can also affect community structure, predator-prey dynamics, and cause infections by damaging gills (Gray et al., 2012a). In species that choose their mates based on visual cues, the inability to successfully choose a suitable mate could reduce population viability (Gray et al., 2012b).
References
Candolin U, Salesto T, Evers M (2007) Changed environmental conditions weaken sexual selection in sticklebacks. J Evol Biol 20: 233-239
Gray SM, Chapman LJ, Mandrak NE (2012a) Turbidity reduces hatching success in threatened spotted gar (lepisosteus oculatus). Environ Biol Fishes 94: 689-694
Gray SM, McDonnell LH, Cinquemani FG, Chapman LJ (2012b) As clear as mud: Turbidity induces behavioral changes in the african cichlid pseudocrenilabrus multicolor. Curr Zool 58: 146-157
Seehausen O, van Alphen JJM, Witte F (1997) Cichlid fish diversity threatened by eutrophication that curbs sexual selection. Science 277: 1808-1811