USA

Some male fish use tail fin to ‘fan’ rival sperm off eggs: study

When a rival fish father sprays a pile of eggs with its semen, this species of male fish will use their tail fin to swish away the seed.

So says a new study by a group of Japanese scientists regarding the male dusky frillgoby fish. The dusky frillgoby is an “externally fertilizing species,” meaning that, to reproduce, the ladies lay piles of eggs which the men then ejaculate on top of, the authors explain in the journal Proceedings of the Royal Society B: Biological Sciences.

To reproduce, the bigger male frillgoby finds “spawning nests,” or underwater rock holes where they court and then mate with their dates. But “sneaker” males — which are physically smaller but equipped with larger testicles — will slink into nests after fish couples have had fish sex and “quickly ejaculate in the nest.” That prompts the original mating male to “aggressively chase sneaker males out” before fanning their tails in an attempt, researchers believe, to get the sneaker’s sperm off the egg pile.

By doing this, however, the fish also removes their own sperm from the pile and must ejaculate on the eggs for a second time. “Moreover, males who had removed sperm for a longer time spent more time releasing sperm after the removal, suggesting that the additional sperm release behavior compensated for the loss of their own sperm,” authors wrote.

The “sperm competition,” as the scientists call it, has never before been seen in externally fertilizing species.

To back up these findings, study authors set up an experiment where they injected seawater and semen into nests and observed how fish behavior changed based on which substance was used. When only seawater was injected, they found, males did not show tail-fanning behavior.

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