The ocean sunfish or common mola (Mola mola) is an animal with an unusual appearance, that from the beginning of the spring begins to be seen more often near the surface of the oceans and seas, and which we usually find flapping its dorsal fin above the sea surface while swimming slowly or lying on their sides to bask in the sun.
Photo: C. Schmidt.
Unique appearance
The ocean sunfish has an oval shape and laterally compressed body. It is usually grey/brown, although the colour becomes lighter on the sides and ventral area. Its body is taller than long. The ocean sunfishes have rough, scale-less skin, covered with a layer of mucus. The pectoral fins are small and rounded, oriented upwards and have no ventral fins. The caudal fin is not a real tail but is a tail-like structure that consists of expansions of the dorsal and anal fin rays, called clavus, that is used as a rudder. The dorsal and anal fins are large and are used for propulsion.
The mouth also gives it its characteristic appearance as the teeth in both jaws are fused into beak-like plates.
Its name
The sunfish gets its scientific name from its appearance, since Mola in Latin means “millstone” and describes the ocean sunfish’s circular shape and grey colour. It gets its common name “sunfish” from its habit of floating on its side, at the sea surface, warming itself in the sun.
A record size
It is one of the world’s heaviest bony fishes, with an average size of 1.8 m long and 2.4 m high (from dorsal and anal fins tip to tip). And an average weight of 1000 kg.
Until recently, it was considered to be the world’s heaviest bony fish, with a record on an individual of 2300 kg and 2,72 m long, but it was recently confirmed that the animal was misidentified, and actually was a southern sunfish (Mola alexandrini).
Active swimmers
Photo: SUBMON.
Diet
Although it was believed that sunfishes feed mostly on jellyfish, recent studies have shown that their diet is probably much more varied by feeding on crustaceans, molluscs and fish. Ocean sunfishes undergo an ontogenetic shift in diet, where smaller individuals feed broadly on high energy-density benthic prey in coastal environments, while larger fishes are found offshore, where they shift to a diet of predominantly gelatinous (jellyfish) organisms.
These new discoveries suggest that ocean sunfishes play a far more important and diverse trophic role than previously thought.
Reproduction
There is little information about the reproduction of this species, but it is known to be one of the most fecund of all vertebrates since a 1.4 m long female was found with 300 million eggs of about 1.3 mm in diameter in the ovary.
At birth, the larvae have a head-pin size and three developmental stages will be needed before reaching adulthood, which may require a 60 million times mass increase.
Threats
Photo: NOAA.
Considering the ongoing mass catch of sunfishes, the removal of this wide-ranging mesopredator may have far-reaching implications for a variety of marine systems and therefore it is imperative to work for its conservation.
