Not Exactly Rocket Science

Blogging on Peer-Reviewed ResearchYou’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.

i-d873808d1d38710a8dc9dab615abcff5-Danionella_dracula_mouth.jpg

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.

i-673e13bd913fa447f3e37c2633570ffd-Danionella_dracula_female.jpgSome 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 fish inadvertently freed certain parts of its skeleton to develop new adaptations, free of past restrictions.

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

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Comments

  1. #1 Zach Miller
    March 10, 2009

    I was only vaguely aware of Dollo’s Law, so it’s good to see an example of it. Very strange fish, I’ll give you that, and I wonder why Dollo’s Law holds. If perfectly good gene network are available, why rebuild the wheel?

  2. #2 Jim Thomerson
    March 10, 2009

    Obviously not your typical minnow. The basic minnow method of feeding is pipette feeding. The jaws and mouth form a slender tube leading to the bulb, the pharyngobranchial cavity. Expansion of the bulb draws food in through the pipette tube into the bulb. There it is chewed, so to speak, by teeth on the modified lower branches of the last gill arch. The food is crushed, speared, cut, etc, against a cartilage pad on the basoccipital process of the skull. Teeth in the jaws would just intefere with the pipette function. So clearly our little fellow is not a pipette feeder!

  3. #3 Ed Yong
    March 10, 2009

    Indeed! The authors say, “The biological function of these jaw modifications, which can be easily mistaken for true teeth, is at present unknown.”

    I humbly suggest “biting shit up.”

  4. #4 lewis thomason
    March 10, 2009

    Is this just a newly discovered species or a newly evolved species,and how would you be able to tell .

  5. #5 Christopher Taylor
    March 11, 2009

    “Newly discovered” is the base assumption. The vast majority of the world’s species remain undescribed, and tropical areas like south-east Asia have the highest proportions of undescribed species.

  6. #6 Paper Hand
    March 11, 2009

    Lewis:

    What do you mean by “newly evolved”? What counts as “new”?

    At any rate, it is possible to determine when a species branched off from its nearby relatives by measuring the amount of genetic difference between the two, especially in areas of the genome that are not subject to selection.

  7. #7 scicurious
    March 11, 2009

    Cool!! I want to see a sci-fi movie where we accidentally mutate them to humongous size, and they arise out of the oceans to wreak havoc on the world of man. Could be good. What would you aim for, though, if you can’t aim for the whites of their eyes?

    And yeah, “biting shit up”. What shit do they bite? Inquiring mind want to know.

  8. #8 MattK
    March 11, 2009

    I notice that all the pictures are of a male individual. I’m going to take a wild guess and suggest that these little cyprinids are sexually dimorphic and that the teeth play a role either in courtship or territory/mate defense against rival males. Many male plethodontid salamanders develop enlarged teeth during the breeding season and use them to attack rivals and in some species they also bite females, presumably to allow pheromones from the mental gland (under the chin) into the bloodstream of a potential mate.

    Is there any info on whether females have this too?

  9. #9 Ed Yong
    March 11, 2009

    And the gold star goes to MattK. Yes, the teeth are a sexual dimorphism – only the males have them (or at least to that extent). Courtship/defence is therefore a really good guess.

    (The colour photo is a female).

    Hmm, memo to self: should have included that detail in the write-up.

  10. #10 Ferdi Akbayrak
    March 12, 2009

    At first glance, it does not seem like a new one, but just evolved from its ancestors. In Asia, the are alike fishes that are described earlier.I think, its teeth are adopted to hunt flies above the surface of the water.

  11. #11 MattK
    March 12, 2009

    At first glance, it does not seem like a new one, but just evolved from its ancestors.

    Every species is evolved from its ancestors.

  12. #12 gillt
    March 16, 2009

    wait! Is Danio rario the British spelling or a typo? These little guys are my research model, and I’ve always seen it Danio rerio.