Pharyngula

Pay special attention to those orange spots on the anal fin.

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Astatotilapia burtoni

(via Kevin Bauman)

Those spots help this fish get oral sex.

As is the case in many fish species, the sight of a brightly coloured male somehow triggers females with ripe eggs to start releasing them. But in cichlids, there is a twist. Females hold their eggs in their mouths and incubate them there after fertilisation – a behaviour that is thought to have evolved to protect the eggs from predators.

As soon as a female has spawned her eggs, she collects them up in her mouth. Normally, sperm released into the water by a male nearby will then fertilise the eggs.

But males of certain cichlid species in east Africa have evolved a way to increase the odds that females take up their sperm. Oval yellow markings resembling the eggs are found on the anal or pelvic fins. When a female approaches the male, she thinks she sees an egg on its fin, so tries to vacuum it up in her mouth – and get a mouthful of sperm from the canny male in the process.

That fish may think it’s come up with a clever trick, but the Catholics know it’s going to fry.