Paedocypris progenetica

The world's smallest vertebrate. Below are pictures of live Paedocypris micromegethes, the Bornean species, by Mike Lo.

Just when I thought they couldn’t get any smaller, come not one but two little midgets that put strain on both the eyes and the lower reaches of evolutionary possibilities for chordates.

Paedocypris is the genus erected by Maurice Kottelat, Ralf Britz, Tan Heok Hui and Kai-Erik Witte for two minute cyprinids, relatives of goldfish, carp, zebra danios and tiger barbs, found in Southeast Asia’s dwindling peat swamps. As described in the Proceedings of the Royal Society, a British biology journal, the generic name refers to the fishes’ retention of larval characteristics during adulthood (Paideois is Greek for chewren) and their systematic affinity (cyprinus is Latin for carp).

P. progenetica (the specific name refers to the truncated development of the adult fish) is found in Sumatra and Bintan in Indonesia, while P. micromegethes (the specific name means small in size in Greek) hails from Sarawak. The former, with females maturing at just 7.9 mm) now holds the record for being the smallest fish (and vertebrate) ever, knocking off contenders such as an Australian marine gobioid (8 mm) and another cyprinid (Danionella translucida, 12.0 mm) from Burma. P. micromegethes matures at a humongous 8.8 mm.

An undescribed Danionella. Picture by Choy Heng Wah.

The tiny size of many larval fishes, as well as their rarity in the field, means that few laypeople are aware that vast morphological differences and transformations can exist through a fish’s life-cycle. The eye-moving metamorphosis from a normal bilaterally symmetrical body in a larval (and planktonic) flatfish to an adult flounder is probably one of the better known examples. Young eels (Order Anguilliformes) are leaf-shaped and transparent, changing to a ‘glass eel’ stage before reaching their robust and opaque adult forms. Larval swordfish (Xiphius gladius) don’t just have a sharp snout – their entire body is covered with spines for protection against predators. The fry of marine lantern fish (Myctophum spp.) are bizarre creatures with eyes on the end of long stalks and a filamentous gut that extends out of the body.

In developmental biology, neoteny or paedomorphism refers to the retention and exhibition of juvenile features in an adult organism. Icefishes (Salangidae) and certain needlefishes (Belonidae) are known to exhibit neotenal traits. Gobioids in the genus Schindleria from the Central Pacific Ocean are text-book exhibits for this phenotypic characteristic, with transparent adults that bear larval features such as a functional pronephros (a kidney-like organ that is functional in larval fishes and amphibians but subsequently degenerates), opercular gills and a nonfolded heart (in which the atrium is behind the ventricle instead of being folded over it).

In higher vertebrates (the adjective meaning simply more recently evolved, rather than any judgement of superiority), neoteny is also seen in many domesticated animals such as dogs, which some ethologists see as simply wolves that retain the playfulness, friendliness and submissiveness of overgrown cubs) and hogs (as evidenced by their relatively naked hide and continued accumulation of fatty deposits through adulthood, unlike their lean and mean cousins in the woods). In fact, some have postulated that man is merely a neonatal naked ape, and there is a rather B-movie like novella (I can’t recall the title now) which tells of an experiment where a man is hormonally induced to fully ‘mature’ into something reminiscent of a rabid Kong.

Habitat of P. micromegethes in Sarawak. The rare aquatic aroid Cryptocoryne pallidinervia also grows in the same swamp. Note the dark tea coloured water. Contrary to popular thought, these swamps do not harbour mosquitoes, due to their high acidity.

Miniaturisation appears to be a frequent phenotypic response by a number of fish groups (e.g. Cypriniformes, Anabantoidei, Channoidei) to the demands of surviving in peat swamps. These habitats in tropical Asia are in effect flooded forests, with a different terrain and plant-life from lowland tropical rainforests proper. Thick layers of peaty substrate act as a giant sponge that traps water in subterranean chambers. In the wet season, the waters rise and the swamp looks typically ‘swampy’, although the water retains a characteristic tea-like colour due to the high content of tannic and humic acids from the accumulated layers of leaf litter within. During the dry season, the surface water retreats, leaving behind the semblance of a dry forest. But pools and tiny puddles beneath and amongst the dry upper layer remain and shelter the swamp’s aquatic denizens until the rains come anew.

The highly acidic (with a pH of 3-4) environment makes for a microbe-, and thus plankton-unfriendly habitat, unlike typical lake or riverine systems. There are relatively few sources of aquatic invertebrate prey for fishes in these waters, and this scarcity of nourishment may be one factor that favours miniaturisation. The low mineral content of peat swamps also means there is very little calcium and magnesium available for skeletal development.

The cyprinid genus Boraras is probably one of the most well-known group of miniature fishes, owing to their attractive colouration and relative ease of care and breeding. In general appearance, they bear the hallmarks of an adult organism, but already certain features are absent: they lack a lateral line and have a reduced number of lateral line scales. Their even smaller relatives the Danionellas (see picture on the right by Choy Heng Wah) go further with adaptations such as a simplified or reduced skeleton, highly reduced pigmentation and scalelessness. 

The diminuation trend seems to have reached its extreme in Paedocypris (the picture above from the paper shows a living male: a, b, and a a female, c. Kottelat et al observes that Paedocypris is the only known cypriniform fish to retain as an adult a conspicuous skin fold along the lower edge of the caudal peduncle. Other progenetic attributes noted by the authors include the genus’s long caudal peduncle, translucent body and a brain that is not protected by frontals (in other words, the grey matter is partly exposed). Related to this is the absence of many bones and cartilages in the skull and a reduced number of branched dorsal and caudal fin rays.

Amidst these extreme adaptations for life in the small lane, the authors point out that one feature runs counter to overall simplification of the phenotype. They write of the males: “Paedocypris possesses one feature unique among fishes – a complex pelvic girdle, in which the first pelvic-fin ray and its abductor muscles are hypertrophied and highly modified.”

The authors suggest that this is an adaptation for reproductive advantage. The pelvic fin could form a clasping device in conjunction with the prepelvic keratinised pad. Or it may be used to grab females during mating. Another hypothesis is that Paedocypris uses the pelvic girdle and fin to manipulate spawned eggs onto suitable substrates. The paper offers no observations of the fish’s behaviour and breeding pattern, which remains a subject for further research, perhaps with the aid of data from aquarists. Mike Lo from Sarawak notes that the fish is very fragile, dying quickly when removed from water. And given its adaptation to waters with a pH of 3-4, the fish is not likely to be a suitable aquarist subject except for peat swamp specialists.

Systematic ducks are also interested in the evolutionary affinities of Paedocypris with other miniature cyprinids such as Boraras, Sundadanio, Danionella and Microrasbora. However, the authors remark that the fish’s “numerous paedormorphic features make its precise affinities unclear and will require further morphological and molecular investigation.” The biogeography of the genus seems to point to a lineage that far predates the submergence of the Sunda shelf.

A tiny fish in hand, by Mike Lo.

Further investigation, unfortunately, is an endeavour threatened by the fast disappearing act being performed by many peat swamps in this region. The authors comment that “many of the peat swamps we surveyed throughout Southeast Asia no longer exist and their fauna is eradicated.” Peat swamps, sadly, have so far been poor cousins to tropical rainforests and their charismatic fauna (think orang-utans, elephants, tigers, proboscis monkeys and other noisy primates). Only in recent decades have the tremendous biodiversity endemic to peat swamps been discovered, only to vanish once again in the annals of extinction. The conversion of peat swamps (which I suspect is classified as degraded land) to oil palm plantations and shrimp farms is rampant in Borneo and Sumatra, and there are precious few peat swamps that lie in designated reserves (e.g. Berbak in Sumatra and Tanjung Puting in Kalimantan).

The latest news report from Malaysia reveals that Paedocypris populations in peninsular Malaysia have since been converted into more productive land, leaving only one remaining known locale in Selangor.   

Can one put forth an economic case for the preservation of peat swamps, akin to how researchers are placing a measure of utility to coral reefs and coastal mangroves for their role in storm protection and supporting fisheries and tourism? So far, apart from their obvious interest to aquarists, scientists and other silly monkeys ducks who care about tiny, low-class fishes, I can only think of a negative incentive: the unintended creation of firesheds as a result of the draining of peat swamps for agricultural or residential development. The dry peat beneath becomes a gigantic tinderbox that ignites and consumes those who drew away the water. Perhaps, if developers factor in the cost of draining the swamp as well as removing the deep layers of dry peat to prevent infernal disasters, the total cost of ‘developing’ such lands might be deemed uneconomic. In this task of putting forth a case for the preservation of peat swamps, more ideas are certainly needed before it is too late.

Sources:
Bond, Carl E., Biology of Fishes, 2nd edition, Thomson Learning, 1996.
Brittan, Martin, Rasboras: Keeping and Breeding Them in Captivity, T.F.H. Publications, 1998.
Britz, Ralf, Danionella mirifica, a new species of miniature fish from Upper Myanmar (Ostariophysi: Cyprinidae), Ichthyological Exploration of Freshwaters, Vol. 12, No. 3, October 2003
Dr. Tan Heok Hui, personal communication.

Comments

[Hai~Ren]

Absolutely fascinating. Saw it on the news, and it really is a pretty little thing.

I wonder how it was discovered. And I do wonder if there will ever be a bunch of aquarists daring enough to take the risks to keep this species in captivity?

[Otterman]

I did tell Heok you'd be fascinated!

[budak]

Ivan, I hear that Japanese aquarists have been keeping this fish for some time. Unfortunately, a lot of biological research published in Japan doesn't reach the non-Japanese world readily. For instance, the moss known as Java moss was clarified by Japanese bryologists to be Taxiphyllum barbieri rather than Vescicularia dubyana (as explained by Benito Tan of NUS DBS). V. dubyana turns out to be very different plant and is actually quite common locally in suitable habitats (even coconut plantations!). So I would imagine there could well be useful ethological and reproductive observations being done in Japanese journals or even aquarist publications (of which there are several) but it's not widely circulated outside the country.

Collecting fishes in peat swamps already entails using very fine mesh nets (given the abundance of tiny fishes like Boraras and Parosphremenous). From the reports, it seems the fish was initially thought to be a juvenile, until gravid females were found. I hear that it's also very fragile out of water (not surprisingly).

[tscd]

Did they drag the peat swamps expecting to find these tiny fish? Or was it by accident?

[budak]

I think it was by accident. The description paper notes that the 1 mm mesh nets used for fishing peat swamps rarely catches specimens smaller than 7 mm. And due to their size, they frequently get mistaken for fries of other species, until somebody decides to take a closer look.

[slurp!]

wah ... so small, i read on straits times today as well. v. interesting. if really so small, must we have aquarium with magnifying glass to fully enjoy this tiny fish huh? :)