Tale of the tails
We can see that all of our favorite high-speed game fish – billfish, tunas, ono and mako sharks – share their sleek shape and their swimming mode, which relies on tails for thrust in addition to some posterior lateral push (in billfish and makos). But more adaptations than shape are common to fast fish, starting end first at the tail.
Considering how much these fish rely on their tails to get up and go, it is hardly surprising that the tail’s size and shape are critical. Think about the shape of the tails on these blue-water pelagics versus those of sea bass or snapper, for example. Fast pelagic predators all have a lunate form much like a crescent moon, tall and deeply forked. All these rocket-burners share what biologists call a high-aspect-ratio caudal fin.
That ration, the tail’s span or reach to its area, offers a pretty accurate indicator of top speed. It appears that the higher the ratio, the faster the fish. In addition, the aspect ratio, perhaps not surprising, is highest of all fish in marlin and swordfish (at 8-to-1 to 10-to-1). The long lobes of marlins’ tails jut far above and below much of the turbulence the body creates. That means the tail works more efficiently in water relatively undisturbed by the fish’s forward wake.
Tunas’ tails, by the way, have impressively high aspect ratios but less so than billfish. Why? Research suggests it’s because tunas school to hunt and feed. In other words, they typically swim in waters of high turbulence (of their own making). Very high-aspect tails, such as those of mostly solitary billfish, designed to function in an absence of turbulence, would prove inefficient. It’s worth noting that the fastest shark, the shortfin mako, also has the most lunate tail of any predatory shark.
Further adaptations include finlets on tunas and mackerels (ono). Finlets are the series of tiny flexible fins running along dorsal and ventral surfaces behind the dorsal and anal fins to the tail. These play a specific role in breaking up and directing water flow to the tail in a way that minimizes turbulence.
Keels, the small semi-rigid fins jutting out from either side of the narrow area before the tail, also on mako, may further reduce cross-flow turbulence near the tail and aid in maneuvering.
The bills of billfish and swordfish may also help cut through water and reduce friction by piercing the water to help ease the flow around the body.
So it’s clear that fast fish look fast for good reason. Their streamlined bodies and high, thin tails help them move with lightning speed. But more adaptations that we can’t easily see factor strongly into the speed equation. Some of these are not found in any species except fast-moving blue-water pelagics.