I saw on Muton, and several readers have mentioned it to me, this article about the world’s smallest vertebrate, fish of the genus Paedocypris. It’s a gorgeous translucent cyprinid, so is somewhat related to my favorite fish, Danio rerio. They live in cool, slow moving water in peat swamp forests of Southeast Asia. One female, only 7.9mm long, contained about 50 eggs, so they know it was sexually mature.

Living Paedocypris progenetica, CMK 18496, (a, b) male, ca 9 mm; (c) female, ca 8.8 mm.

That size isn’t at all shocking—my zebrafish larvae at about that size are active hunters with functioning visual systems, capable of coordinated bouts of swimming, and they’re also very impressive animals…but they don’t have sex. It takes about 6 months for zebrafish to reach sexual maturity, and they are several centimeters long at that point. I would love to know how old these fertile Paedocypris were, but they were captured in the wild and virtually nothing is reported about their behavior or lifecycle. Ah, to have a fish colony that could be raised in a set of beakers, and could produce a couple of generations of crosses in a single semester…

One other clue that these are fully functioning, sexually mature adults are the presence of some pelvic specializations. Males have a hook and flange widget on their pelvic fins, and an odd prepelvic knob. Again, though, without knowing anything about their behavior, we don’t know how these are used in mating and courtship. Wouldn’t it be cool to put a pair under my Wild M3 scope and watch courtship and mating?

(a) Paedocypris micromegethes, paratype male, ZRC 49869, 10.4 mm; pelvic fins, anteroventral view, showing hook and flange on anterior ray. (b) Paedocypris micromegethes, paratype, male, BMNH 2004.11.16.1-40, 10.9 mm, ventrolateral view on hypertrophied pelvic arrector and abductor muscles marked by asterisk symbols. (c) Paedocypris progenetica, paratype male, ZRC 43199, 8.5 mm, scanning electronic micrograph of pelvic region in ventrolateral view, arrow points to keratinized prepelvic knob.

Of course, in addition to not knowing their generation time yet, these fish have another drawback relative to zebrafish: tiny eggs. They extracted a range of sizes from the ovaries, but assuming the smallest are immature, they max out at around 0.3mm diameter. That’s respectable, but Danio eggs are about 1mm in diameter.

Can you tell I’d love to get my hands on a bunch of these little fish? Unfortunately, I’ve heard from fish importers that it is agonizingly expensive and time consuming to bring wild tropical fish into the country, and for good reason: to block invasive species, to prevent the spread of new fish diseases, and also to discourage the plundering of native populations. I may not ever see one of these animals, short of making a trip to Malaysia, and even then I won’t be bringing any home.


  1. says

    You may want to get to Malaysia fast. Their peat swamp habitat sounds like its days are numbered.

    The authors of the paper write:

    “The structurally complex peat swamp forests are disappearing quickly in Southeast Asia, due to logging, urbanization and conversion for agricultural use, especially oil palm plantations and shrimp farms. Peat swamp forests paid a high toll to the forest fires of Sumatra and Borneo in 1997, which lasted for several months. Many of the peat swamps we surveyed throughout Southeast Asia no longer exist and their fauna is eradicated. Populations of all the highly endemic and stenotopic miniature fishes of peat swamps have decreased or collapsed or are extirpated. Besides those listed as type material, we have collected or observed Paedocypr is at numerous localities; many populations have disappeared, potentially dooming any efforts to elucidate the enigmatic reproductive biology of these species.

  2. Hai~Ren says

    Extremely cool stuff, I’m quite sure sooner or later some enthusiastic aquarist will find a way to breed them.

  3. SEF says

    And I was about to be another one to alert you to Paedocypris! It’s interesting that they seem to be another lineage to have gone down that neoteny route once again.

  4. Great White Wonder says

    Smallest *known* vertebrate.

    I’d put the certainty that there are smaller vertebrate species out there that we haven’t yet discovered at close to 100%.

  5. fyreflye says

    Extremely cool stuff, I’m quite sure sooner or later some enthusiastic aquarist will find a way to breed them.

    No aquarist has ever had any trouble breeding guppies.

  6. says

    if you look at figure c the right way it looks like a caped superhero accidentally flew directly into the prepelvic knob..

  7. Torbjorn Larsson says

    “I’d put the certainty that there are smaller vertebrate species out there that we haven’t yet discovered at close to 100%.”

    Yes. Would be interesting to if there are a smallest theoretical size of a vertebrate. My uninformed guess is, there aren’t any good way of telling.

    “No aquarist has ever had any trouble breeding guppies.”

    Yes. But these guys are definitely going to clog the aquarium filter system. :-)

  8. ledge says

    This is one of those conundrums that always seems to affect biologists. (And botanists. I’m an enthusiast of many unusual plants and we have the same problems.) You discover some new, interesting species and as soon as you discover it, you discover that it’s at least threatened if not highly endangered. Not just new species, but existing species. Only they are protected. So you can’t go out and collect them to bring them into cultivation or breeding programs to save them from extinction, or to artificially try to expand wild populations. Yet at the same time, it takes no effort (or permission) at all to drain a swamp, fill it in and build a hotel or strip mall and totally eliminate an entire wild population and potentially decimate the genetic pool of something we know nothing about.

    What’s an ethical scientist supposed to do? It makes me cry just thinking about it.

  9. darkymac says

    What’s an ethical scientist supposed to do?

    Right…which is a reason to not hit the swamps with a net and a big bucket, I’m afraid.

    That’s an exemplary decision from the Prof., ledge.

  10. Torbjorn Larsson says

    “Definitely clooging” – yes, “definitely” must be dropped. I realised my mistake too late. Definitely too late.

  11. says

    Dear Dr. Myers,

    I have posted my personal (illustrated) take on the fish at my blog. Do you have any ideas on the genetic mechanisms that lead to the retention of neotenal traits?

  12. Kitty says

    I know I’m late to the party here, but these guys live at a pH of 3. Dude! That’s like living in vinegar! Since they are so itty, pretty much every cell in their body is bathed in acid – I’d be interested to learn more about their metabolic processes.

    Seriously, these fish are insane. Insane!

  13. Pete Liptrot says

    This species, or a congener, has already been imported into the UK on at least two occasions, unfortunately in insufficient numbers to establish them. The first time was around 2 years ago when they arrived as an undescribed species along with a species of Danionella. Although the ova may appear small while in the ovary, many fish eggs swell significantly post-fertilisation so they may not be too problematic to raise.