Evolution has left us with many mementos from our fishy past, most of which vanish early in development, and the notochord is one of them. All vertebrates develop a notochord, a form of rigid hydrostatic skeleton, early in gestation. The notochord soon becomes enclosed by short sections of bone that develop into individual vertebrae that eventually form the vertebral column. But hagfish didn't evolve a mineralised skeleton. John Long explains that hagfish are a `mosaic of ancestral and derived features', with the features that remain from the past providing a link to our ancestors. He adds that their undulating swimming style could also be related to the way our aquatic ancestors swam. Long wondered whether the hagfish could tell us anything about how a notochord functioned in swimming from the past through to the present. Together with his colleagues, Long has discovered that the hagfish's notochord has an amazing mechanical property; the notochord is mechanically tuned for maximum swimming efficiency with minimum effort(p. 3819).
Long knew that if he wanted to analyse how the fish's hydrostatic skeleton contributed to its undulating swimming stroke, he would have to understand the fish's undulating swimming motion first. Fortunately, Long didn't have to go trawling around the ocean floor to catch specimens, as the fish are caught in fisheries off the North American coast. Working with Magdalena Koob-Emunds and Thomas Koob, they began videoing the fish in the lab, as they wiggled through a tank. Despite the fish's unpleasant habit of `sliming' on anyone that touched them, they were remarkably cooperative swimmers, and the team were able to get accurate measurements for the fish's natural flexibility at cruising speeds. But measuring the fish's stiffness was more tricky.
The team clamped the slippery fish in a specially designed bending machine,and reproduced the fish's swimming movements while measuring the body's stiffness. Then they looked at how the stiffness varied as they stripped the fish down to its notochord. Long was amazed when he realised that the narrow notochord contributed 80% of the body's stiffness, even though it only contributed 20% of the body's diameter.
Knowing that biological material's mechanical properties are highly variable, Long and his colleagues began testing how the notochord's flexibility changed as the fish swam over a variety of speeds. He explains that biological materials have complex viscoelastic properties, and the notochord stiffness varied enormously over the fish's natural range of swimming speeds. Long wondered if this variable stiffness could some how be tuned to help the fish swim efficiently.
One way to improve a fish's swimming efficiency, is for the fish to undulate close to the resonant frequency of its body, taking advantage of the way that the resonance naturally amplifies each movement, at no extra cost. When he looked at the amount that each undulation of the body was amplified as the fish swam, Long realised that the notochord's mechanical properties were tuned to give each sweep a sevenfold amplification, no matter what speed it is swimming at.
Long believes that other species could be using this trick to power themselves along. He explains that although notochord material is every bit as rigid as the pads in vertebral columns, fish that have stuck to notochords have the mechanical bonus of an internal amplification system that takes the effort out of swimming.