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Fish fight breaks out over tiny catch

January 31, 2006 By Michael Hopkin This article courtesy of Nature News.

Contenders line up to net credit for smallest vertebrate.

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You might be more used to arguments about who has caught the biggest fish. But this week a squabble has broken out among zoologists, each of whom is claiming to have found the smallest.

And now, in attempting to settle the argument, has stumbled upon an unlikely third contender for the small-vertebrate prize.

The debate began when Ralf Britz, of London's Natural History Museum, and his colleagues announced the discovery of Paedocypris progenetica, a fish that lives in acidic peat swamps of southeast Asia. With females measuring just 7.9 millimetres long, and males just a tad bigger, the species is truly a tiddler. The researchers claimed it should be recognized as the smallest backboned animal in the world1 (see Go fish!).

It's not just millimetres that count - it's how you use those millimetres.
David Wake
University of California, Berkeley
But this prompted a challenger to emerge. Ted Pietsch, of the University of Washington in Seattle, points out that last year he described an even smaller fish, which he claims should be recognized as first (or perhaps last) in the size stakes.

"When I saw the paper I thought 'hey!'," Pietsch recalls. He was surprised to see that the researchers made no mention of the deep-sea anglerfish Photocorynus spiniceps, males of which are just 6.2 millimetres long.

My fish is smaller than your fish

Pietsch discovered his fish, which lives in waters off the Philippines, by trawling through a museum collection. He described the species at a meeting in Taiwan in May 2005, and later published the details2.

So is his the true king of the mini fish? Like all the best arguments, it's not that simple. Photocorynus spiniceps females are much larger, and the tiny males hitch a ride on their backs as permanent breeding partners. Once attached, the fish's blood systems become fused, so a sceptic might argue that the males are not free-living.

That argument holds no water for Pietsch, however. "To me it doesn't matter if it's a breeding parasite," he says. It is still a sexually mature adult with a backbone, he argues.

Britz counters that because the male's jaws are latched on to the female, it may in fact be longer than Pietsch's measurement. "How do you measure a fish that is fused to its partner? Where's the snout?" he asks.

It's all relative

In a bid to end the stalemate, contacted an outside adjudicator, who came up with a surprise third viewpoint. "Salamanders are the smallest vertebrates - there's not even any question," says David Wake of the University of California, Berkeley.

These amphibians are typically at least several centimetres long. But that is tiny in relation to the size of their genome, Wake points out. Of the 500 different salamander species, many have well over ten times as much genetic material in each cell as humans do. This makes for big, cumbersome cells, which means that adopting a complex body form is more of an achievement given the same body size, Wake argues.

"I'm always surprised that biologists who want to make sophisticated arguments resort to using a metre stick," he says. "It's not just millimetres that count - it's how you use those millimetres."

Fella feeling

But how should one resolve the fish dispute? Wake suggests judging on the basis of the larger of the two sexes rather than the smaller, meaning that P. progenetica, championed by Britz's team, would emerge victorious.

Either way, it does not look as if either of the camps will hold a grudge against the other. "These are fellas I've known for 30 years," says Pietsch. "It's a very friendly debate."

Britz agrees: "There are no hard feelings. It's not really a very scientific issue. These are just very interesting fish."

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  1. Kottelat M., Britz R., Hui T.H.& Witte K.-E. . Proc. R. Soc. B, doi:10.1098/rspb.2005.3419 (2006).
  2. Pietsch T. W., et al. Ichthyol. Res., 52 . 207 - 236 (2006).


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