Category Archives: Fish

Fish do not have fingers?

Although actinopterygian radiation seems to have made the most significant footprint in vertebrate diversity, with more than 30 000 Recent species, there are also scientists who looking into the origin of the less successful group, the sarcopterygians, among which the tetrapods are the terrestrial ones – except that some escaped back to the water (sirenians, seals, cetaceans, and the like).

Attending Catherine Boisvert’s defence of her dissertation The origin of tetrapod limbs and girdles: fossil and developmental evidence recently, I came to realize that the evolutionary quirk with tetrapods is not the possession of limbs – lots of actinopterygians have limbs, and many tetrapods lack them – but the presence of fingers. Catherine’s thesis is about the evolution of fingers and the pelvic girdle. The digits (fingers) sit at the end of the autopod (the hand and foot of tetrapods) and are homologous with the distal radials of the dermatotrichs (fin-rays) attached to fish proximal radials (the pectoral-fin base, homologous with the sarcopterygian/tetrapod limbs). Interestingly, the finrays and the digits form before the autopod and proximal radials in both sarcopterygians and actinopterygians. This was not well understood before, but Catherine and co-workers used fascinating evo-devo methods, catscan, recent lungfishes and salamanders, and fossils of early tetrapods to demonstrate this. Perhaps even more interesting, the front and hind limbs of tetrapods are remarkably similar, despite that the pelvic girdle of actinopterygians is a very tiny little plate of bone compared to the complex pectoral fin base. Catherine has advanced a theory about how the pelvic girdle developed into a support for the hind limb in tetrapods (making them four-legged). The real mystery, however, for me at least, is how come the hind foot is so similar to the front foot, and in particular, how come they both have five digits? Is this really the magic number? A handy come in pleiotropy?

Most fish that walk about use only one pair of limbs, either the pectoral fins, or, rarely (as in skates) the pelvic fins. It seems likely that early tetrapods were more like mudskippers than skates, but who knows.

My big question, which I never got to ask Catherine, is: what forced the fish up on land? I know the answer is out there somewhere, but I am always surprised at my prejudicial assumption that fish evolved into tetrapods. Or tetrapods evolved from fish. (Or aquatic animals have an intrinsic evolutionary tendency to become terrestrial). An obvious explanation would be that water was first, land later, and seemingly all fish ancestors were aquatic. It is still possible for a protist to have become terrestrial and evolved into a tetrapod without actinopterygian intervention (or they became insects?). So, the conclusion must be that those early fish that took on fingers were right away driven out of the water. Today’s terrestrial fishes, and our close relatives the lungfish, live in shallow water, swamps, and mangroves, in very particular habitats with strong seasonal or diel changes in water level. Maybe the mudskipper will take over the planet after us. It looks sympathetic anyway. Still has to do something about its hind limbs.

Image by Catherive Boisvert: Devonian landscape and fish actors making evolution.

Sturgeon fever


A sturgeon (Acipenser oxyrinchus) once owned by King Adolf Fredrik, studied by Linnaeus,
now housed in the Swedish Museum of Natural History collections.Photo A. Silfvergrip. Data . Image CC-BY.

Once upon a time there was in the Baltic Sea a fish known as the sturgeon. Its existence in Swedish waters were known to Linnaeus and Artedi, and Linnaeus named it Acipenser sturio in 1758, based on two small specimens in alcohol, and a skin, plus literature records. These two alcohol specimens are still in the collection of the Swedish Museum of Natural History. One comes from Amsterdam apothecary Albertus Seba, bought by King Adolf Fredrik probably 1752, the origin of the other one is more obscure.

From 1758 till recently the sturgeon was studied, fished, eaten and extinguished from European waters, save for a declining population in the French river Garonne, which defies rapid extinction despite cadmium poisoning and sexually asynchronous breeding (males and females do not breed at the same time …). Then a German-US team in 2002 (Arne Ludwig et al.) could show that the northern European and Baltic Sturgeon is genetically the same as the North American Atlantic sturgeon Acipenser oxyrinchus Mitchill, 1815, and not at all the same as the sturgeon in France, the Mediterranean, and the Black Sea.

Ooops … And this had already been demonstrated by morphological studies, that somehow did not draw the right conclusions.

Whereas there are still technical issues over which name goes to which species, the preliminary conclusion is that in the Garonne swims Acipenser sturio, and in the North Sea and the Baltic Acipenser oxyrhinchus was as much at home as in the United States.

The Baltic sturgeon is extinct in Europe. The last specimen identified as A. sturio in the Baltic died in 1996, and was an A. oxyrinchus.

Nevertheless, Swedish media the last two days have enthusiastically declared the return of the sturgeon! Based on a fish taken outside the island of Öland in the Baltic Sea, 10 of April by fisherman Ulf Åkerlund.

From the local newspaper scoop we learn that the fisherman is excited (as he should be; this is an uncommon fish whatever it is), and the Fisheries Board expert is more interested in tasting the roe than getting it properly identified.

From the published image, you can see the mutilated pectoral fin indicating a pond cultured fish. And what more, it looks not like a sturgeon but resembles more the Russian sturgeon Acipenser gueldenstaedti, and could also represent Siberian sturgeon Acipenser baeri. Although there are efforts to reintroduce Acipenser oxyrhinchus into the Baltic, I am not informed of any releases having taking place yet. If and when it happens it will be a great waste of money on a lost case.

The “sturgeons” we now have in the Baltic are escapes from cultures of Siberian and Russian sturgeon and hybrids between different species. They look like sturgeon because all 20+ species of sturgeons look about the same. In May 2000 a small “sturgeon” was caught in Kalmarsund strait and given to the Swedish Museum of Natural History (NRM). The specimen was sequenced and blasted as Acipenser stellatus, which it is not, and morphologically identified as A. baeri.

Other more or less recent Swedish “sturgeons” include a Siberian sturgeon from the Stockholm Archipelago in 1969, a Russian sturgeon from Lysekil in 1970, and one “sturgeon” from Skåne in 2007 that, like one from Kummelbank in 1991, has not been identified yet. Browse sturgeon data in the NRM fish collection.

Will we know which species was caught this time? It would be a good thing to build up knowledge as early as possible about potential “sturgeon” invasions in the near future, and we could learn more how to identify the fishes, and using molecular and morphological markers track the movements of these aliens in our waters. Let’s see what tomorrow has in store.


Fish from nowhere

Yesterday I mentioned briefly the leopard danio, a small golden fish full of dark spots, apparently a color mutation in the zebrafish Danio rerio, but described in 1963 as a species on its own with the name Brachydanio frankei. The leopard danio is a popular aquarium fish in its own right, which keeps its colors but does not differ in behaviour or size from the zebrafish, and hybridises freely with zebrafish. When it first appeared in the aquarium trade, its origin in the wild was unknown. That should have called for some caution … On the other hand, who could believe other than that the differences in colour pattern was a strong indicator of species distinctness?

Spotted danios are known from the wild, however. There is Danio kyathit from northern Myanmar, in which the spots are more or less irregularly arranged in rows, and many morphological characters distinguish it from other species of Danio. Described by Fang in 1998, based on four specimens she collected herself, she also included two specimens collected in the 1920s that were not spotted but striped. There was simply no way of distinguishing the striped and spotted kyathit other than by colour pattern, and because the rows of spots are merely broken up horizontal stripes, there was room for considering intraspecific variation. How different conclusions can be once you know a little more about the group you are working with!

Here is an image of Danio kyathit, photographed by Fang Fang.

The other spotted danio has no scientific name yet. It is a small fish, similar to Danio rerio but with large spots on the side. It is already available in the aquarium trade where it is called Danio sp. Burma. Will anyone dare to describe it? Is there a striped counterpart already available among the many supposed synonyms of Danio rerio??

A real sunshine story is the that of the “Odessa barb”, one of the major aquarium fish species, and belonging to the large family of cyprinid fishes. Males are marked by a stunning, glowing, deep, exquisitely brilliant red band along the side, and contrasting black spots in the dorsal, anal, and pelvic fins. Females are less colourful. It is or relatively small size, less than 5 cm, and easy to reproduce in aquarium.

The early aquarium history of the “Odessa barb” is not well documented. In 1973, Russian aquarist Dazkewitsch wrote that it originated from a market, not stated where, and arrived in Odessa, Ukraine, in 1971, soon being cultivated in Moscow. So it was named “Odessa barb” although quite evidently a South Asian species. It was a confusing time, a time for much speculation among American and European aquarists, and no information from Ukraine. And how come a small aquarium fish, soon of world fame could first be found in Ukraine, at the time part of the Soviet Union and with no aquarium fish import? Maybe it was also a “form” of some kind, like the leopard danio? Strangely, nobody got the idea this time to describe the new fish as a new species!

So, things remained for 30 years till Frank Schäfer in 2001, working for the German wholesale aquarium fish importer Glaser reported having found the “Odessa barb” in an importation from Myanmar.

Wow! And I and my colleague Ralf Britz (yes, that’s him with the vampire fish), missed it totally on our collecting trip in Myanmar just a little before, in 1998, crossing the country from Yangon in the south north to the foothills of the Himalayas in Putao.

Well, there was still no precise locality. And Ralf then found the fish in 2003, near Mandalay. So, it exists in nature, we know where, and last year we got it described. We named it Puntius padamya. Padamya is the Burmese word for ruby. The ruby barb of aquarists, however, is Puntius nigrofasciatus from Sri Lanka. The description is available online as an Open Access resource from the Electronic Journal of Ichthyology.

The “Odessa barb” freshly collected in the wild, a bit pale in the photo tank. Photograph by Ritva Roesler, from Kullander & Britz (2008: Electronic Journal of Ichthyology, 4: 56-66):

And the stately preserved holotype of Puntius padamya:

The lessons, if any, are: Don’t name fish known only from the aquarium trade. Be patient. And, give nice names to nice fish.

Zebrafish in spirits

Zebrafish (Danio rerio) are probably the most important fish for understanding humans. They are small fish, 2-3 cm long and native to India, Bangladesh, and Pakistan. Most conspicuous about them is the contrasted coloration with alternating blue and white horizontal stripes, even extending onto the caudal fin. That means they are horizontal where zebras are vertical. Otherwise there are no similarities 🙂

Zebrafish occur in many places in India, in the north in the Indus, Ganga, and Brahmaputra as well as in Kerala in the south, but appear uniform in color pattern and other morphology. They live in schools in different habitats, and are often collected in large numbers. As aquarium fish they are hardy, easy to breed, and prolific.

The distinctive colour pattern, which can be genetically altered, ease of keeping, and the fast generation time contribute to zebrafish status as a so-called model organism, which developmental biologists use to study the development and inheritance of various structures. Recently also, zebrafish researchers have been helped by improved understanding of the systematics of the group of fishes to which zebrafish belong, so that structures can be studied comparatively in closely related species.

Here is a dead zebrafish in alcohol from Assam, India. Not very colourful, but useful for taxonomic studies.

I just spent two days writing a description of a new species that I and my student Te Yu Liao collected last year (about this time) in Myanmar. There are many species of fishes closely related to the zebrafish. Thirteen species have been named in the genus Danio, and at least ten more species remain to be described. Most fascinating among the zebrafish relatives, is the leopard danio, which turned up in aquarium circles in Czechoslovakia in the 1960s and was described as a new species with the name Brachydanio frankei. This form, with small dark spots all over the body and fins, has never been found in the wild. It is probably a mutant of zebrafish, differing only in the irregular colour pattern. The genus, however, includes species that are spotted for real.

I will come back to fish species that only exist in the aquarium trade later. Let us round off the day with some zebrafish entertainment.

If you need just an overview of zebrafish, with the basic data, try Fishbase.

Wikipedia insists on being very technical about zebrafish

These developing zebrafish embryos are just irresistible:

The Wolfgang Driever Lab at the University of Freiburg has still images so you can track all the details at different stages. Click the image (from their website) to enter the zebrafish anatomy:

And then there is an enthusiasts’s website with images of various zebrafish-like species, Pete Cottle’s Danios and Devarios.