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Newly discovered fish crosses Peter Pan with Dracula

Not Exactly Rocket Science
By Ed Yong
Mar 11, 2009 5:00 AMNov 5, 2019 2:00 AM


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You're looking at the face of a new species of fish and judging by the two fearsome fangs, you'll probably understand how it got its scientific name - Danionella dracula. The teeth do look terrifying but fortunately, their owner is a tiny animal just 15 millimetres long. Ralf Britz from London's Natural History Museum discovered the fanged fish in a small stream in northern Burma, just two years ago. The more he studied them, the more he realised that they are physically extraordinary in many ways.

For a start, those are no ordinary teeth - they are actually just part of the fish's jawbone. True teeth are separate from the jaws that house them and are made of several tissues including enamel and dentine. Those of D.dracula are protrusions of the jaw itself and are made of solid bone. The fish has rows of them in both its upper and lower jaw that look very convincingly like actual teeth. Even though it comes from a long line of fish that have lost their teeth, D.dracula has managed to re-evolve them through a completely unique route.

Secondly, D.dracula seems to be missing several bones, with 44 fewer than close relatives like the zebrafish, Danio rario. They haven't disappeared - they never formed in the first place. Compared to other related fish, D.dracula stops developing at a much earlier point and retains the abridged skeleton of a larva throughout its adult life. It's the Peter Pan of the carp family.

The new species is part of a large family called the Cypriniformes, which includes carps and minnows. The lineage include over 3,600 species and among their number are 36 miniaturised ones that are noticeably smaller than their relatives.

Some appear to be shrunken versions of typical species, smaller but identical in proportions. Others have achieved a tiny size by stunting their growth so they effectively become sexually mature larvae. It's a phenomenon called "paedomorphosis" and species that have taken this route include the world's smallest vertebrate - Paedocypris progenetica, just 10 millimetres at its largest size. D.dracula, while larger, has taken this developmental stunting to an extreme and ends up with fewer bones than any of its close relatives.

This general theme of loss and reduction is balanced by extreme growth in certain parts of its body, particularly in its mouth. All the cypriniform fish have lost their teeth, which puts them on common ground with many other groups of back-boned animals including birds and turtles. Even so, toothless vertebrates still carry the genetic networks that are needed to produce teeth - they've just been switched off through millions of years of evolution. Through genetic engineering experiments, scientists have shown that these dormant networks are still viable - activate them and you can induce teeth in otherwise toothless fish like the zebrafish.

However, D.dracula shows that in reality, it's not so easy to reacquire teeth once they have been lost through evolution. It's not a simple matter of switching on inactive genetic pathways; this fish has had to start from scratch.

Britz can't be sure if the genes that produce its bony teeth haven't been co-opted from those responsible for the true teeth of its distant ancestors. Either way, the fish is living support for Dollo's Law, which states that evolution is not reversible, and that body parts or organs that are lost do not re-evolve with their previous complexity.

It seems that in becoming extremely small, miniature cyprinids have become hotspots for the evolution of unique physical features. D.dracula is no exception to that rule. An animal's development follows a very precise timetable and set of instructions, but Britz suggests that those rules became more lax as D.dracula became smaller. By prematurely stopping its development, the fishinadvertently freed certain parts of its skeleton to develop new adaptations, free of past restrictions.

Reference: Proc Roy Soc B doi:10.1098/rspb.2009.0141

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