![]() If an individual fish can reproduce as a female for the first portion of it’s life, then continue breeding as a male for the rest, it effectively doubles up on its reproductive output. Indeed, for many species such as the kobudai, the capacity to switch sex is highly adaptive. And where you get variation, you get diversity in all sorts of ways, including how they reproduce.” ![]() “There’s simply more diversity in fishes: thanks to evolutionary radiation, they are more widely adapted to a broad range of environmental niches. Compare this to the roughly 200 million years that mammals have been around, and the fact that there’s less than 6,000 species of us,” he says. “Think about it – fish have been around for 500 million years: today there are more than 26,000 described living species. The main reason for their extreme plasticity, he says, lies in the sheer number and diversity of species. “That is the £60 million question – if I had the answer I’d be in the world’s top journal Nature every week,” says Professor Charles Tyler, a reproductive physiologist at the University of Exeter who studies how environmental contaminants affect fish populations. Why should fish range so widely in their gender bending capabilities? …and if that wasn’t complicated enough: almost all fish have the capacity to switch sex at the embryonic stage if subjected to one or more of a number of environmental factors, including the acidity of the water, parasites, chemical pollutants, and temperature. And at least one species, the mangrove killifish, lives a fully hermaphroditic existence, self-fertilizing for their entire reproductive lives. Still others can switch back and forth depending on the circumstance, such as a variety of coral-dwelling gobies. Others, like the clownfish, do the opposite, from male to female. Some like the kobudai change routinely from female to male. The footage is remarkable – but the transformation is actually not terribly unusual. About two per cent of fish species display some kind of hermaphroditism: that’s 500 different species worldwide. ![]() Now even larger than the existing dominant male it had previously mated with when female, the new male defeats the aged alpha in a violent battle for dominance. After many months, the transformed male emerges from its lair larger than before, bearing testes, a huge bulbous forehead, and an aggressive nature. Millions of people saw a dramatic example of this in the first episode of Blue Planet II, in which a ten-year-old female kobudai (also known as an Asian sheepshead wrasse, Semicossyphus reticulatus) changes into a male.
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