Monday, July 25, 2011

River monsters and a really old citation...

Airing on Animal planet is a show featuring Jeremy Wade called River Monsters. Jeremy Wade is an angler and biologist traveling all around the world fishing for what they call river monsters. A common denominator for these fish is that they are often large and look weird. Some of them are venomous and have caused rare deaths. Often the species he targets isn't very new to science and often common in the aquarium trade. However the show never seem to acknowledge the knowledge present on these species in the scientific literature or in the aquarium hobby. They present each species as if it was new and almost unknown, probably due to dramaturgic effects...

In the episode 'Silent Assassin' (S03E05) we follow Jeremy to the Parana river in Argentina where he follow up stories and victim accounts of stingray "attacks" in the river. According to the locals theres a mysterious stingray living in the river attacking and killing locals and their animals. Instead of doing research in the field of stingrays he sets out on a mission to catch this 'river monster'. Most of the people a bit interested in Elasmobranchs would be aware of the family of stingrays inhabiting the river systems of South America, Potamotrygonidae. However the episode overlook this fact and attribute the attacks to a mysterious ray. This mysterious ray however is a rather well known member of the Potamotrygon genus, actually one of the bigger species if not even the biggest. The species of freshwater stingray in this case is Potamotrygon brachyura. This species was described by Albert Günther (back then named Trygon brachyurus) in the article "A contribution to the knowledge of the fish fauna of the Rio de la Plata" in 1880 so it is not totally new to science. In the episode they wrongly assign this species into the family Dasyatidae and shows a picture of a general whiptail stingray which is the common name of the family. However the Potamotrygon genus is one of four genuses in the family Potamotrygonidae or river stingrays. They are thought this mistake not completely wrong due to the closest extant sea living relatives of river stingrays are thought to be stingrays of the genus Himantura in the Dasyatidae family. 

River stingrays are generally docile and generally only sting victims in self defense. Thats what their poisonous spine are for, self defense, and nothing else. So I think its a overstatement to talk about attacks when often the rays are being stepped on.

Albert C L G Günther (1880). A contribution to the knowledge of the fish fauna of the Rio de la Plata Annals and Magazine of Natural History, 6, 7-13

Tuesday, July 5, 2011

And the lungfish said: "oh man, not again..."

The other day a college of mine at the blog Ego sum Daniel made a nice picture and wrote  a post on his blog about the totally awesome fish Coelocanth (Latimeria chalumnae). He also showes a estimated timetree of the divergence times between the different bony fish lineages based on an nice site and book called TimeTree. If one excludes the ray finned fish (Actinopterygii) and only look at the so called lobe finned fish (Sarcopterygii) we see the Coelocanths, tetrapods and lungfishes. If one excludes the 21000 tetrapod species the Sarcopterygii clade in the tree of life have only eight extant species. The Coelocanth genus (Latimeria) includes two of these species. Lungfishes (Dipnoi) are divided into two orders, Ceratodontiformes and Lepidosireniformes, that are further subdivided into three genuses. Ceratodontiformes with one extant genus Neoceratodus includes one extant species the australian lungfish. Lepidosireniformes on the other hand includes two genuses Lepidosirenidae and Protopteridae, south american lungfish and african lungfishes respectively. While the south american lungfish is the single species in its genus the african lungfishes consists of four species with a few subspecies. 

Clearly the tetrapod lineage of this clade made great success after populating land, but what have happened to the lungfishes since then, what is the evolutionary history of todays six species? Several fossils have been found and it seems that the lungfishes once was a quite large group of fish. Todays lungfishes are a monophyletic group where the Ceradontiformes lineage separated from Lepidosireniformes around 277 million years ago (figure 1). 

Figure 1. Rough phylogenetic tree over the divergence times between the three extant genuses of lungfish.

The South American and the African lungfishes diverged somewhere around 120 million years ago. Since a major force in speciation is separation of two populations by a barrier one suddenly realize that theres actually a barrier between the South American and African lungfish populations, the little puddle the Atlantic ocean! Geological studies have suggested that the Gondwana continent split between whats now South America and Africa about 120 million years ago. Isn't it beautiful? Molecular and geological data seem together to explain in part when they formed and why we today have the South American and African populations of lungfishes.
When the separation had occurred the African population diverged further finally resulting into what we see today with four extant species. Tokita et al. set out to sort out the evolutionary history of the African lungfishes which in previous studies had been suggested to be paraphyletic. What they did was to look at a part of the mitochondrial 16S rRNA gene and to use that to calculate the divergence of these four species by molecular clock. Their phylogeny of the four species roughly look like what i have drawn in figure 2. 

Figure 2. Rough phylogeny of the four extant African lungfish species with the respective divergence times.  

In their calculations they conclude that the P. dolloi and P. amphibicus diverged from the P. annectens and P. aethiopicus lineages between 76 and 53 million years ago. The P. annectens and P. aethiopicus species diverged from each other between 42 and 26 million years ago. These two species shares a lot of morphological characters and can sometimes be hard to distinguish therefore they have previously been proposed to be closely related to each other. These two species inhabits different types of habitats. P. aethiopicus is found in the large lakes and the nile river of eastern parts of Africa while P. annectens inhabits river systems of western Africa. The authors of the paper suggest that these two species diverged during the tertriary period. During this period, they say, there was substantial landrifting in central and eastern Africa accompanied by volcanic activity. This lead to a separation  of two populations which later resulted in the two species we see today. So once again, it seem, a major geological event have helped the lungfish to diverge further...

Tokita M, Okamoto T, & Hikida T (2005). Evolutionary history of African lungfish: a hypothesis from molecular phylogeny. Molecular phylogenetics and evolution, 35 (1), 281-6 PMID: 15737597