A paper from some work I did during my dissertation was recently accepted and published online in Functional Ecology. (click on the citation to get to the lay summary and accepted article).
This study was done by Kristine the summer I was finishing, and I had the time here at Towson to analyze, write and submit it. We examined the kinemtics of the tail fin during steady swimming among three swordtail species (X. alvarezi, X. clemenciae, X. nigrensis) and a platyfish (X. meyeri). We also surgically removed the sword in X. alvarezi, similar to previous studies examining the cost of the sword on whole-organismal performances.
We swam each fish at various speeds, representing a percentage of their critical swimming speed, a measure of their endurance; and recorded them at 120 frames per second. We later digitized the videos to obtain several tail beat kinematics, including tail beat amplitude (how much the tail moves), tail beat frequency (how fast it moves) and total tail displacement (how much and fast the tail moves). The latter trait was recommended Jay Nelson.
We found that among the four species, there was little difference in their tail beat kinematics, dispute large differences in sword length. In particular, the species with the longest swords (X. alvarezi and X. nigrensis) did not differ substantially in their tail beat kinematics from the species with no sword, X. meyeri. This suggests that the presence of the sword does not hinder how the fish swim.
However, when we removed the sword, the fish had an increase in their tail beat amplitudes, but not frequencies. This suggests that the sword may hinder how much the tail can move, but not affect the contractile velocity of the muscles.
These results may help explain some of the discrepancies in the literature. Natural variation in sword length does not alter tail beat kinematics, but removing the sword increases tail beat amplitude, potentially increasing thrust and endurance, which has been seen in other studies.
This study is only on four species and only on males. It would be interesting to examine these traits in other species with varying length swords, as well as compare males to females to see how fin variation through sexual selection affects the way the fish move.
Below are some videos of fish used in the study.
Click on each picture to see an example the fish swimming at 100% critical swimming speed (slowed down to 1/4 original speed) with digitization marks.