Seahorses are interesting creatures, unlike most others in the animal kingdom. After debates over what exactly seahorses are and where they come from, it was decided that they are indeed fish. They are classified as a bony fish due to the fact that they have gills, a swim bladder to control their buoyancy, interlocking plates on the outside of their body covering a spine made of bone (similar to fish scales), and fins, albeit in odd locations.
Seahorses are notoriously bad swimmers that cannot move quickly yet are very maneuverable, moving up, down, forward and backward quite easily. Luckily, they are very good at camouflage and can change colors to blend in to their surroundings.
One of the most interesting and unique traits seahorses possess is the ability for male seahorses to give birth. Once the female inserts eggs into the male's brood pouch, the male waits out gestation for several weeks by hanging on to nearby coral or seaweed. The males then give birth to baby seahorses that look just like miniature versions of their parents.
So, how did the seahorse evolve these diverse and extremely unique traits and behaviors? A collaboration between groups in Germany, China and Singapore set out to find the answer.
Published in Nature, the groups were interested in understanding the genetic basis of the specialized morphology and reproductive system of seahorses. In order to learn more about these exceptional creatures, the team sequenced the genome of the tiger tail seahorse, Hippocampus comes, and performed comparative genomic analyses with the genome sequences of other bony fish.
The full genome and total RNA were sequenced and assembled, comprising of an estimated 502 Mb or about 1/6 the size of the human genome. The groups then calculated different mutation rates across certain gene families, finding a very high rate of exchange. Further analysis of gene family evolution identified an expansion of 25 gene families and contraction of 54 families. Loss of function or duplication in genes can contribute to novel evolutionary traits and can be positively selected for, as is seen within the seahorse genome.
Estimated divergence times of H. comes in comparison to other fish suggest that seahorses diverged from other bony fish about 103.8 million years ago. Notably, the branch length of H. comes on the evolutionary tree is longer than that of others in its class, which suggests a higher protein mutation rate compared to the other fish analyzed.
Researchers conclude that the highly specialized morphology and behavioral characteristics of the seahorse are due to these high evolutionary rates. The increased mutation rates for both proteins and nucleotides could cause loss of function and changes in expression patterns that create unique animals that are very different from even their closest relatives.