The fish that can fly
Flying fish (Exocoetidae) are an extraordinary family of fish, which use their enlarged wing-like pectoral fins to glide above the surface of the ocean (Figure 1). Globally, there are 71 species which are found in large schools primarily in the open oceans of the Atlantic, Indo Pacific, and the Indian ocean. All species have a pair of enlarged wing-like pectoral fins and a distinctive forked tail. However, a few species such as the Atlantic and California flying fish have enlarged pectoral fins and pelvic fins and are known as four-winged flying fish. But, why have the species developed these unique adaptations? and how do they achieve their effective gliding?
The main reason for the adaptation is predator avoidance, of which they have numerous. Predators include tuna, mackerel, marlin, swordfish, dorado, dolphins and other large predators. However, the strategy is effective for escaping from oceanic predators but it makes them susceptible to predation from seabirds (Figure 2).
Figure 2 Predator avoidance of flying fish from both oceanic predators and birds. Source: Youtube, Author: BBC Earth
Another hypothesis for the gliding ability in flying fish is the migration between poor and rich food areas. One study from Baylor (1967) observed Mediterranean flying fish (Cheilopogon heterurus) deliberately landing in areas of weedlines. Weedlines also known as weedpatches are areas of seaweed aggregations. These areas provide shelter for concentration fish and crustacean larvae. Yet, apart from these observations there is little evidence to support this hypothesis.
Flying fish don’t have the real ability to fly but instead, they use their enlarged fins to glide. Prior to breaking the surface, they need to gather enough speed underwater. By having a torpedo body shape it increases their hydrodynamics and achieves a greater underwater speed, up to 37mph (Figure 3). As they increase speed towards the surface they then fold their fins close to their body making them more streamlined.
As they break the surface their large fins unfold (Figure 4a). Their tails remain within the water providing thrust through vigorously beating their tail (up to 50-70 strokes s-l) before achieving free flight (Figure 4a). To increase their gliding distances they tend to fly into the wind and use the updraft to increase their height. Studies revealed that the average taxi length (distance between breaking the surface and free flight) is 9m, with taxi length distances ranging from 5 to 25m (Figure 4b). Once in flight, they can reach heights of 8m and a distance of 180m in a single glide. With multiple glides, they can reach up to 400m before fully re entering the water. It is also believed that they are able to see in air due to it having a flat cornea. This may help the fish monitor its height and gauge when to dip its tail for the next taxiing. To re enter the water, the fish folds its fins back against its body and nose dives.
Two-wing vs four-wing fish
Gliding distances vary greatly between species, especially between two-winged and four-winged species (Figure 5). Where four winged species have the capability to glide further than two-winged species. Once gliding there is little evidence to show that flying can change direction, this is particularly true for the two-winged species. In four winged species, the enlarged pelvic fin can act as an air break allowing for more control when dipping its tail for the next taxiing.
Flying fish are a remarkable fish family which have adapted a unique and extreme physiological trait to effectively avoid predation from numerous oceanic predators. Additionally, as they are low trophic level species feeding on zooplankton they are important in maintaining a healthy ecosystem within the ocean zones. This is because they provide food for the many oceanic predators which are situated above them within the foodchain.