Dating in the dark, lessons from Cavefish

Originally published: August 2015

At the start of this week, the British public were greeted by the rather morbid news that Cilla Black, hallmark of Saturday night television, had sadly passed away. Cilla was undoubtedly a colossus of primetime entertainment and in light of this, I’d like to make this week’s article a tribute to one of Miss Black’s finest programmes: Blind Date.  For those who spent their Saturday nights productively, the concept of Blind Date was simple; three members of one gender, hidden from sight, attempt to woo a person of the opposite sex by answering a series of questions, using exclusively lewd and childish innuendos.  The show’s greatest success was to highlight the pitfalls of choosing potential partners without seeing them first, a problem that’s been encountered elsewhere in the world.

In the rivers of Mexico, a few species of rather unassuming fish hide a remarkable family secret.  As these rivers flow, they pass through naturally forming caves, creating pitch black habitats where life struggles to survive.  However, for the fish of these rivers, such as Mollies, Poecilia sp., and Tetra, Astyanax sp., these dark and dismal zones have provided new habitats to use and explore.  In order to do so, these fish have had to evolve to live in a world without light, and have done so with remarkable consistency, a fact that has made them a favourite among evolutionary scientists.  Eyes are as useless in the dark as a cocktail umbrella in a tsunami, and the resources used to make them are better spent elsewhere.  This is also true for the pigments in the skin, which the non-cave fish use to communicate and protect themselves from the sun’s rays.  The cave forms have therefore evolved pale, pasty skin, and are either almost or completely blind; some have gone so far as to lose their eyes entirely.

As unattractive as these fish may sound, this is the animal kingdom and blind, ghostly baby fish must be made!  But it begs the question of who should you mate with when you live in total darkness?  Understanding how cave fish choose mates has been a drive of evolutionary biologist Ingo Schlupp’s research group for a number of years.  For the non-cave forms, picking a mate is a simple choice of size, with females preferring bigger fish.  For fish, bigger is better because bigger fish have proven themselves successful at feeding and growing and so their genes are desirable for a female’s own offspring.  Scientists have observed this choice in the lab, but when non-cave fish were placed in darkness, their preference disappeared, as they were unable to judge males’ sizes in the dark.  The same was not true for their better adapted kin.  Female cave forms of the Atlantic Molly, Poecilia mexicanus, are still able to find their favourite ‘large-and-in-charge’ males even in complete darkness, suggesting that they’ve evolved a new way to judge the outward appearance of the opposite sex without their eyes, a discovery with tantalising prospects for the tabloid media.

But how is the question, and researchers suggested that the lateral line, a series of sense organs used to feel movement and vibrations around the fish, may be the solution.  The theory was a simple one, like Marvel’s ridiculous “Daredevil” superhero, a lawyer whose senses were heightened by his blindness, cave fishes’ lack of sight could have led to an increased ability to feel their surroundings using their lateral line.  Using this skill, cave fish may be able to choose mates by feeling the movements of larger males and may have evolved to do so in the absence of sight.

To test this, a group of scientists from the University of Oklahoma used antibiotics to disable the lateral line in a group of cave-dwelling Atlantic mollies to see whether they were still as choosy as before.  Even without their vibration sensing superpowers, cave mollies were still able to recognize the bigger males from the smaller ones, lining up a second possible sensing superpower, smell.  Whilst this is yet to be tested in cave fish, there’s evidence from other fish, such as swordtails, that the sensing of chemicals in the water can be a powerful tool for recognition.  If larger fish release more chemicals, cave fish may be able to follow the stronger scent to find the bigger and better males.

So, whilst the exact method they use remains unclear, fish that have evolved pale, blind cave-forms are truly the masters of judging books by their cover, even when they’re hidden from view.  The eyeless cave forms have evolved numerous times independently, stressing how significant the selection for the troglodyte features must be.  For evolution researchers, these fish represent a golden opportunity to understand how sensory organs evolve and function, but these fish are also useful for medical research.  By looking at the genes that have allowed cave fish to lose their eyesight, scientists can hope to understand which similar genes in humans might be responsible for lost eyesight and retina degradation.  In addition to this, as recently as last month, researchers at Harvard University found that changes to a gene that causes cave-dwelling Mexican tetras, Astynax mexicanus, to alter their metabolism and lose less fat is shared by some obese humans.

Whilst Cilla Black will always be remembered as the Queen of Blind Date, cave fish are undoubtedly the masters of dating in the dark.  Should Blind Date ever return, which if the “Catchphrase” remake is anything to go by would be a real shame, some cave fish inspired rounds could really help to avoid those awkward, mismatched couplings. The drive to understand how animals choose mates in the dark can provide us with a wealth of knowledge relating to ourselves and gives powerful and tediously linked meaning to one of Britain’s favourite Saturday night talismans.

References:

Plath, M., Parzefall, J., Körner, K. E., & Schlupp, I. (2004). Sexual selection in darkness? Female mating preferences in surface-and cave-dwelling Atlantic mollies, Poecilia mexicana (Poeciliidae, Teleostei). Behavioral Ecology and Sociobiology55(6), 596-601.

Plath, M., Heubel, K. U., De León, F. J. G., & Schlupp, I. (2005). Cave molly females (Poecilia mexicana, Poeciliidae, Teleostei) like well-fed males.Behavioral Ecology and Sociobiology58(2), 144-151.

Plath, M., Rohde, M., Schröder, T., Taebel-Hellwig, A., & Schlupp, I. (2006). Female mating preferences in blind cave tetras Astyanax fasciatus (Characidae, Teleostei). Behaviour143(1), 15-32.

Rüschenbaum, S., & Schlupp, I. (2013). Non‐Visual Mate Choice Ability in a Cavefish (Poecilia mexicana) is not Mechanosensory. Ethology119(5), 368-376.

Wong, B. B., Fisher, H. S., & Rosenthal, G. G. (2005). Species recognition by male swordtails via chemical cues. Behavioral Ecology16(4), 818-822.

Image Rights:

Scott, sourced from Flickr, CC BY-SA 2.0

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