Sex in the dark isn't for everyone!

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Utter darkness can be such a turn off for some fishes, according to a study that sheds further light on how cave fishes evolved from surface-living relatives.

The paper by Rüdiger Riesch, Martin Plath and Ingo Schlupp demonstrated that surface populations of the Atlantic molly (Poecilia mexicana) had trouble reproducing in the dark; this made the darkness an effective reproductive isolating mechanism that did not allow surface and cave populations of the molly to interbreed.

In the study, which is to be published in a forthcoming issue of the journal Biology Letters, the authors raised offspring from gravid females collected from the Cueva de Villa Luz in southern Mexico and two neighbouring surface populations to sexual maturity.

The fry were separated into eight groups and one group each of the cave- and surface-dwelling populations were raised in one of four treatment regimes over a period of 12 months.

In the first, the fish were raised in a regular cycle of 12 hours’ darkness and 12 hours’ light, but given little food; in the second, the fish were raised in the same light regime, but given plenty of food.

In the third treatment, the fish were raised in complete darkness and given little food, while the fourth treatment had the fish raised in complete darkness and given plenty of food.

The authors found that even though male fish from both surface- and cave-dwelling populations were able to mature sexually and reproduce successfully irrespective of whether they were raised in the light or in the dark, the same cannot be said of the female fish.

They found that the surface-dwelling female mollies were unable to successfully reproduce when maintained in the dark, irrespective of the amount of food they were fed (although the fish fed little food generally fared more poorly than those fed plenty of food).

By contrast, the cave-dwelling fishes of both sexes had no trouble reaching sexual maturity and reproducing whether or not they were raised in complete darkness.

This implies that the surface-dwelling fish had trouble reproducing in caves, but the cave-dwelling fishes had no trouble reproducing outside of them.

This is supported by previous research that shows that low rates of gene flow exist in the Cueva del Villa Luz system but are unidirectional from the inside of the caves towards the outside.

According to the authors, one possible explanation for the results is that living in the darkness induces enough stress in the surface-dwelling females fish that they divert resources otherwise used for reproduction into re-establishing a stable, constant internal environment.

This study thus provides evidence that darkness also acts as a strong selective force that effectively prevents gene flow between diverging (the cave- and surface-dwelling) populations and thus has the potential to establish reproductive isolation between diverging populations.

For more information, see the paper: Riesch, R, M Plath and I Schlupp (2011) Speciation in caves: experimental evidence that permanent darkness promotes reproductive isolation. Biology Letters doi:10.1098/rsbl.2011.0237

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