Welcome to a new series we’re starting here on the blog! The aim is to have a new entry in this series every fortnight, but more often if circumstances are permitting. We’re going to be exploring together a new paper from the previous two weeks that has taken my interest – this could be because it’s groundbreaking new work, highly relevant to my current reading or, as in this week’s case, is just really damn cool. The aim is to communicate awesome science in such a way that it’s accessible to all, but in a more detailed way than seen in popular media.
First things first, today’s paper can be found here. All research articles published in the Journal of Experimental Biology are open access six months after publication – so be sure to check back then if you want to read the full article and don’t have access currently.
Let’s get stuck in! The authors make observations that hammerhead sharks show a variety of cross species morphological (shape) changes, though this hasn’t been extensively looked at previously. It stands to reason that these changes in shape may have an impact on how the sharks swim, following the classic moniker “form ever follows function”. The phrase was actually coined in the 18th Century by American architect Louis Sullivan and used to describe a variety of natural instances of form-function associations such as a “branching oak” and “toiling work-horse”. It has since been considered integral to functional morphology in both living and fossil creatures.
The two species investigated are the scalloped hammerhead (Sphyrna lewini) and the bonnethead (Sphyrna tiburo). It’s clearly from the images below that these two species have very different morphology of the T-bar at the head of the animal, which is called the cephalofoil – cephalo meaning head. Previous studies have shown an inverse relationship between cephalofoil pectoral fin area, such as one increases the other decreases. The authors decided to investigate how body shape and cephalofoil area contribute to the swimming style of the sharks.
Four of each species, for a total of eight sharks, were captured and filmed in large swimming pools using a GoPro video camera situated above the water. From these videos, several points across the animal (seen below) were tracked through steady swimming and the areas of the cephalofoil and pectoral fins were calculated. Any lengths were calculated using Total Body Length (TL) to standardise for body size. One specimen of each species was CT scanned so that the 3D shape of the body could be better analysed (see this post for more on CT scanning and how it works!). Parameters such as swimming speed, tail beat frequency and amplitude were measured and analysed.
So what were the findings? Firstly, the relationship between cephalofoil and pectoral fin area proportions was recovered as in the historic studies. This is always reassuring to see, since peer-reviewed science should always be sound and reproducible. Secondly, the two sharks were found to have different body shapes in the anterior region of the post-cranial body, so the equivalent of the “torso” region in humans. The scalloped hammerheads measured in this study had a more laterally compressed “torso”, so had relatively taller and thin bodies compared to the dorso-ventrally (top-bottom) compressed “torsos” of the bonnethead sharks. Interestingly, standardised velocity and tail beat amplitude didn’t differ significantly between the two species, suggesting each is performing at relatively the same rate. Further analysis showed that the tail segment had the greatest flexion amplitude, as would be expected of tail based propulsion.
Tl;dr: bonnethead and scalloped hammerheads differ greatly in their morphology, particularly in the anterior region of the body. Differences are observed in the morphology of the cepahlofoil (spade-like in bonnetheads vs straight T-bar in scalloped hammerheads) and the anterior body shape (flatter bonnetheads vs taller scalloped hammerheads). Despite these morphological changes, no relative differences are seen in swim speed or undulation frequency, but do differ in undulation amplitude.
This was a fantastic, well-constructed study to read about some really cool animals. It also alludes to there being a “standard” relative speed for shark swimming, similar to how tetrapods move at similar relative speeds (Froude numbers).
Any thoughts on the format for these posts would be appreciate. We can make them snappier, maybe even a bit more visual if I dedicate some more time to developing my own graphics aside from those in the papers.