Thursday, December 15, 2011

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:

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 ( 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 ( 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:

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)

Tuesday, December 6, 2011

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. 

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

Saturday, December 3, 2011

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).