The Finless Fish
The finless fish, Exocoetus sinfina, belongs to the Exocoetidae, commonly known as the flying flish. Unlike his relatives, who have unusally developed pectoral fins which allow them to glide over the water to escape predators, the finless fish is characterized by an absence of lateral fins. Only the caudal fin (the tail fin) remains.
Like all other fish, the finless fish moves forward by undulating its body. However, due to the lack of lateral fins, it uses its gill openings to change direction, opening and closing them to go right or left. In addition to its natural swimming motion, the finless fish can accelerate to speeds of up to 60 km/h (~37 mph) by violently expelling water through its anus. However, due to the speed and the lack of lateral fins, it can only slightly change direction while maintaining its top speed.
Biologists have emitted a number of hypotheses as to why the finless fish has lost its lateral fins in the course of its evolution, but none have been proven to this day. One of the most prominent ones, formulated by Dr. Marcus Pescus, of the South-Asian Institute for Fishy Studies (SAIFS), stipulates that the absence of lateral fins allows the finless fish to explore the coral caves found in the tropical Asian oceans, its natural habitat. The lack of lateral fins makes it easier for the fish to extricate itself in case it gets stuck, and allows it to go through smaller tunnels. However, it has recently been shown that finless fish prefer open waters to coral reefs, and as such coral-cave exploration would not exert any selective pressure on them. Dr. Marcus now insists that Sinfina used to frequent said caves, and only recently changed its habitat.
Another theory stipulates that the absence of lateral fins makes its body more hydrodynamic, allowing it to escape predators at greater speeds. But it seems unlikely that the small gain in speed would warrant the considerable decrease in mobility, as mobility is a much more important factor in escaping predators than pure speed. Besides, a number of finless fish die each season because of said lack of mobility, when, trying to escape from danger, they propulse themselves into rocks or boats. The force of this impact can be quite substantial, and it is believed that the finless fish crashing into their boats is what gave the Japanese the inspiration for their Kamikaze fighters.