A thought about the terms "living fossil" and the tree of life

I every now and then come across people referring to species as living fossils, lower, primitive or ancient species. I personally feel that these terms shall be used with caution. The lineages encompassing all organisms alive today (the year is 2012 at the time of writing if you find this blogpost in an archeological dig in the future) have evolved from a common ancestor some billion years ago. The thing with that all organisms, at least all known, on earth share a common ancestor is that all lineages you can imagine have evolved for the same amount of time. A coelacanth, often reffered to as a living fossil or ancient species, have evolved for just the same amount of time as us humans. The major differences however is that the only two extant coelacanth species are very similar in shape to their fossilized relatives. The reason for this is not that the extant species is the same as the ones in the fossils, no these coelacanths have continued to change however at a slower rate in some characteristics making them appear "old" in their look. The situation in sort of the same when it comes to chimeras, sharks and rays. They appear to have stood still during the course of time, however the reason is not that the species are old but rather a slower change in characters that are beneficial for the life of these fish. 

The term "lower" in front of any chosen lineage is also often seen in both the literature and mentioned in various presentations by scientists. What do they really mean with this? The thing is that many still for some reason regard humans as sort of the perfected endpoint of an evolutionary tree with earlier (lower-) branching species groups, clades, regarded as less advanced than later branching groups (higher) and hence are they called lower vertebrates for example. However the tree of life is not anthropocentric, humans are not the end of a line of perfected selection. Rather the tree of life is a tumbleweed of life with branches going out in all directions. All extant species, or rather all living individual organisms, including you, me and the tiny ants on your lawn, are the endpoints of countless branches in the "tumbleweed of life", hence no life form is higher or lower, we are all at the same time point in the era of life on this planet. 

Animal of the week: The Dracula fish (Danionella dracula)

No this is not, as you might think from its name, a fish from recent TV-series and movies such as True blood or Twilight that spends its days by sucking blood, having complicated relationship dramas and fighting werewolves. No this is a amazingly tiny cyprinid found in freshwaters of Burma with relatively massive fangs. I write relatively since this is a really tiny fish, so there is no need to panic if you encounter one of these river monsters reaching 17mm in total length. 

The funny thing with the Dracula fish is not surprisingly their fangs. Actually teethes were lost early in the cyprinid lineage and one would then be surprised to discover fangs in a cyprinid. The thing is, the fangs of these fish are not actually true teeth. The teeth of the dracula fish have actually evolved from the jawbones. This is a sexually dimorphic feature so female fish have less prominent fangs. Another interesting thing with this fish, as I learned last year at a cyprinid seminar arranged by the Swedish part of FishBase, is that most of its "bone" in its body hasn't been ossified but is still cartilage. It seems as if the tininess of this species is due to them being sort of neotenic. This means that they become sexually mature earlier when they still are developing, and that they now never grow up so to say. Actually it seems as if their developmental status, as adults, corresponds to a stage in the early life of small sub adult zebrafish of a similar size as the adult dracula fish. Zebrafish, another cyrprinid, is actually a rather close relative of the dracula fish. 

So to summarize, if you ever meet a dracula fish don't be afraid unless you are a tiny crustacean larva swimming around in the rivers of Burma. If you happen to be just one of those crustaceans, be afraid, be very afraid...

Swim bladders, lungs and a whole new way of life.

Hello again, it was a while since I last made a post on this blog. However now I'm back again and continues to deal with the rise of the tetrapods. 

In my previous blog entry I discussed a paper dealing with the origin of tetrapod gait styles, and that the gait style most tetrapods have already was present in the common ancestor of tetrapods and lungfishes. in this post i will deal with another thing lungfishes and tetrapods have in common, namely lungs! A lung is useful when you want to extract oxygen from air rather from water. Simply speaking, a lung is an organ which purpose is to oxygenate the blood and remove carbon dioxide.

Lungs are present in all extant tetrapods, although amphibians use gills during their aquatic larval stages. The closest extant relatives of tetrapods is, as you should know now if you have been following this blog, the lungfishes. As their name implies they also have lungs, either a pair as one can see in the Lepidosireniformes or one lung as in the Australian lungfish (Neoceratodus foster). However they also have gills. There is no doubt that the lungs of the lungfishes are homologous to their tetrapod counterparts. However it has long been suggested that the swim bladder of ray-finned fish (Actinoptherygians) has a common origin (is homologous) to the lung. A swim bladder is an organ present in most ray-finned fish, both teleost and non-teleost. It usually consists of two connected sacs which holds gases. This organ is mainly used for buoyancy control (like a BCD for you divers out there). In some lineages the swim bladder is connected to the intestines throughout life and in some species this connection is closed some time after hatching. The swim bladder can either be filled by inhaling air from the surface sort of how we fill our lungs. It can also be filled using a special organ that extracts gasses from the blood.

Developmentally the swim bladder and lungs seem to have the same origin. However genetic evidence for a common origin, which would strengthen the hypothesis of homology of the lung and swim bladder, have been scarce. Not all to long ago a paper on this subject was published in PlosOne (Zheng et al. 2011). What they did was to take a look at what types of genes were expressed in the swim bladder tissue of zebrafish (Danio rerio) and compare the swim bladder gene expression profile with the ones of mouse and human lungs. The interesting thing they could see was that the lung and swim bladder expression profiles overlapped which would provide further evidence for a shared origin of lungs and swim bladders.

It is therefore possible that the common ancestor of ray-finned fish and lobe-finned fish had a swim bladder type organ that in shore/shallow water dwelling lobe-finned fish adapted to also serve as a air breathing organ and then later into "real" lungs, while it in more pelagic species remained a buoyancy control device that in even more specialized species (many teleost fish) lost its connection to the intestine all together.

Reference

Zheng W, Wang Z, Collins JE, Andrews RM, Stemple D, & Gong Z (2011). Comparative transcriptome analyses indicate molecular homology of zebrafish swimbladder and mammalian lung. PloS one, 6 (8) PMID: 21887364