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

Picture inspired by the previous blog entry.

The origin of walking?

We, as tetrapods, are constitute the largest group of animals in the sarcopterygian clade. Sarcopterygians include, as you may know if you have followed this blog, lungfishes, tetrapods and coelacanths. The common denominator for the animals of this clade are their lobed fins that in tetrapods have become the arms and legs. 

In this group lungfishes are considered to be the closest extant relatives of all tetrapods. A common denominator of tetrapods is, as the name implies, the four paired legs used to propel the body around in the environment. This is a bit different from the coelacanths and lungfishes which still have a set of fins. However the base of their fins are more similar to our arms than you might think, as some of the bones present in their paired fins are equivalent to some of the bones in our arms. The feet is however unique to the tetrapods, believed to be a necessity to be successful in moving about on land. 

Recently a paper was published by King et al. (2011) titled "Behavioral evidence for the evolution of walking and bounding before terestriality in sarcopterygian fishes". In their paper they analyzed the movement of a african lungfish (Protopterus annectens) using video recordings. Their very interesting finding was that this particular species of lungfish uses its pelvic fins as "legs" to move about on the substrate. The fins moved in a synchronized gait as you see in the gaits of tetrapods. So the lungfishes can be in a way bipedal when they move about on the bottom. They can even lift up their body with their pelvic fins, which has previously suggested to be hard without the pelvis connected to the spinal cord as all extant tetrapods.

Link to one of their videos: http://www.youtube.com/watch?v=DgqBzCOfOdE

They conclude that bottom living (benthic) sarcopterygian fishes can lift their bodies up from the substrate and possibly create tracks that later have been fossilized and that the origin of the type of gait used by tetrapods has an earlier origin already present in the water living ancestors of tetrapods.

The lungfish and the tetrapod lineage separated, according to molecular data, approximately 428 million years ago (timetree.org) which would mean that the gait that seem to be common between these two lineages is somewhat older than that. The earliest known fossil tracks was produced by a tetrapod with feets in the Eifedian (early middle devonian, about 395 million years ago) period in todays poland (Niedzwiedzki et al. 2010). This mean that feets have their origin earlier than 395 million years ago, worth noting is that the earliest known tetrapod body fossil is 18 million years younger than these tracks. 

Could the gait style of tetrapods and lungfishes be even older or could it even have been reinvented in several lineages (convergent evolution)? After seeing the videos of the lungfish from the paper described above I couldn't stop thinking of several other non-sarcopterygian vertebrates that also have a similar way of moving about on the bottom substrate using their paired fins as "legs" and "feets". One very striking example is the epaulette shark (Hemiscyllium ocellatum) and its close relatives of the benthic bamboo shark group. Just as the modern lungfishes these sharks are benthic and live in shallow water. These sharks have a habit of crawling about on the bottom substrate and in the coral reef structures using their paired fins (pectoral and pelvic fins) in a similar way as a tetrapod (Goto et al. 1999). This would either suggest that the way tetrapods move have a origin way earlier than split between tetrapods and lungfishes, an origin before the bony fish lineage and the chondrichthyes about 462 million years ago (timetree.org). However since most vertebrate species are not using this type of locomotion but rather swim, just as normal fish, its possible that the jawed vertebrate body plan with its paired fins pave the way for evolving this type of gaits in benthic species that need to move efficiently over the bottom substrate, like the tetrapod ancestors, lungfish ancestors and epaulette shark ancestors. This would also make it difficult to say for sure that the gait style observed in lungfish has the same origin as the tetrapod style, even thought it is likely due to the close relationship between these lineages. It is also possible that the gait style of these two lineages evolved separately in both lineages after the divergence and that the way movement is controlled in vertebrates facilitate evolution of these very similar gait styles.  

Link to video of epaulette crawling about on the substrate: http://www.youtube.com/watch?v=nHJWqYw3IIM

References:

King HM, Shubin NH, Coates MI, & Hale ME (2011). Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes. Proceedings of the National Academy of Sciences of the United States of America PMID: 22160688


Niedźwiedzki G, Szrek P, Narkiewicz K, Narkiewicz M, & Ahlberg PE (2010). Tetrapod trackways from the early Middle Devonian period of Poland. Nature, 463 (7277), 43-8 PMID: 20054388


Tomoaki Goto, Kiyonori Nishida, & Kazuhiro Nakaya (1999). Internal morphology and function of paired fins in the epaulette shark, Hemiscyllium ocellatum Ichthyological Research, 46 (3)

Boldness and reproductive success in zebrafish

Animals in general react in similar ways to stressful stimuli. There are two basic mechanisms in which animals react to stress. some animals react by being aggressive and actively try to attack the thing that is perceived stressful by the individual while other animals react by being passive and show low levels of aggression. The aggressive individuals have a so called proactive coping style while the inactive non-aggressive individuals have a reactive coping style. Proactive animals often are dominant and reactive animals subordinate in a social interaction. The coping style an animal will show can be determined by many factors for example genotype or environmental factors. The differences between proactive and reactive individuals is not only visible in the behavior but also in their neuroendocrinology. In vertebrates stress is regulated through the so called Hypothalamic-Pituitary-Adrenal Axis (HPA-Axis) or as it is called in fish the HPI axis (where I stands for interrenal, which have the same function as the Adrenals in mammals). The regulation of this axis differ between proactive and reactive individuals where proactive individuals show a lower activity of the axis than reactive individuals. 

The zebrafish, which is a commonly used experimental animal, is a fish that lives in shoals of approximately 10-20 individuals. Zebrafish has its native range in India, Myanmar and Bangladesh where it lives in small streams and flooded plains. Being a social fish species they interact between each other a lot. However how the boldness of an individual zebrafish contribute to the social hierarchy in the group had until recently not been investigated. At the day of my birthday this year my former supervisor published a paper with me among others as a co author (Dahlbom et al. 2011) where we investigated if one could predict social status by testing for boldness in individual zebrafish i.e. if boldness correlates with coping style.

To test this we first tested the boldness of individual zebrafish and then put the fish in a tournament where they met conspecifics in two rounds to isolate the extremes in dominant and subordinate individuals (see Dahlbom et al. 2011 for details). What we could see was that individuals that proved to be bold in the initial boldness screen actually more often ended up dominant (proactive) in a encounter with another zebrafish and thus boldness seem to correlate with coping style shown in the social encounter. 

Recently the first paper citing our paper (Ariyomo and Watt, 2011) was published. They further investigated the effects of boldness and aggressiveness in zebrafish but in the area of reproductive success of male zebrafish. Could it be that bold and proactive males had higher reproductive fitness than shy subordinate males? They saw that if a female was paired with a male it didn't make any difference between bold and shy males on the number of laid eggs by the female. However bold and aggressive males succeeded better with fertilizing eggs. Thus, more eggs in the tanks with a bold aggressive male were fertilized than in thanks with a shy non-aggressive male. 

So to conclude bold zebrafish seem to be more likely to become dominant and be proactive than shy zebrafish. Bold zebrafish males also have a higher reproductive success than shy males by being more successful in fertilizing eggs. Then you might ask, why isn't all zebrafish bold and proactive, why is there still reactive shy individuals? This could be explained by the fact that the different coping styles actually both have their benefits, however in different environments, and thus are both types maintained in the population. 

References

Dahlbom SJ, Lagman D, Lundstedt-Enkel K, Sundström LF, & Winberg S (2011). Boldness predicts social status in zebrafish (Danio rerio). PloS one, 6 (8) PMID: 21858168


Ariyomo, T., & Watt, P. (2011). The effect of variation in boldness and aggressiveness on the reproductive success of zebrafish Animal Behaviour DOI: 10.1016/j.anbehav.2011.10.004

Animal of the week: The yeti crab

"...imagine theres no sun..."

This is probably something John Lennon would have written in the lyrics of imagine if he were a Yeti crab. Fortunately enough he wasn't a yeti crab, because if he was e would probably not have had the cognitive skills needed to write such master pieces. 

The Yeti crabs are (not very skilled in terrestrial music culture), as their name implies, white and hairy. They live in deep water with the first species discovered off the coast of Easter island in 2005 (Macpherson et al. 2005) and the second species the following year (Thurber et al. 2011). The two species have been assigned a genus, Kiwa. 

The interesting thing about these crabs is not that they are white and hairy, although thats pretty awesome in a way, but that they actually culture bacteria on their hairs and use these as their main food source. But the coolness about these critters does not even stop here! The bacteria they culture produces energy from the inorganic gases on the sea floor were these crabs roam about. This mean that the crabs actually live a Sun isolated life fueled purely by the gases of the deep sea floor!  

Some references: 

Macpherson, E., Jones, W. & Segonzac, M. Zoosystema 27, 709–723 (2005). 

Thurber, A. R., Jones, W. J. & Schnabel, K. PLoS ONE. 6, e26243 (2011).