Female Crucian carp (Carassius carassius) ignore predators when they are ready to mate, according a study published in the most recent issue of the Journal of Chemical Ecology.
Stine Lastein and coworkers maintained wild-caught Crucian carp in aquaria, and induced them to breeding condition (onset of ovulation in females, increased milt volume in males) by altering the daylength and temperature.
The fish were then exposed to a small amount of carp skin extract injected through a plastic tube into the water and their reactions to the extract noted.
Skin extract contains alarm substance, which is an olfactory cue released upon mechanical damage to the skin and is used to warn conspecifics of a predator threat; exposure to this cue stimulates a stereotypic avoidance behavior (fright reaction), which consists of rapid tail movements and swimming to the bottom in Crucian carp.
Twelve minutes after the introduction of the skin extract, blood samples were taken from the fish and measured for plasma levels of sex steroid hormones.
The authors found that female Crucian carp that were on the verge of mating (ie. had already ovulated) comprised the majority of the female fish that showed no fright reaction in response to skin extract (verified by the plasma levels of sex steroids).
By contrast, male fish that were ready to breed did not display any significant difference in predator avoidance upon exposure to alarm substance compared to male fish that were not ready.
The authors surmise that the female carp fail to respond to alarm substance in order to prevent interruption of spawning. The reduction of predator avoidance in favour of reproduction may be necessary, since the fright reaction includes hiding and freezing, which are behaviours that are incompatible with typical spawning behaviour.
For more information, see the paper: Lastein, S, E Hglund, I Mayer, ~ ~verli and KB Dving (2008) - Female crucian carp, Carassius carassius, lose predator avoidance behavior when getting ready to mate. Journal of Chemical Ecology 34, pp. 1487"1491.