Molidae
The Molidae family, encompassing the molas or ocean sunfishes, consists of unique marine species characterized by their bodies terminating just behind the dorsal and anal fins, resulting in a distinctive "half-fish" appearance. Notably, they represent the largest group of ray-finned bony fish, with the southern sunfish, Mola alexandrini, achieving recorded lengths of 4.6 meters (15 feet) and weights of 2,744 kilograms (6,049 pounds). The nomenclature of the family derives from the ocean sunfish's scientific designation, Mola mola, with both the genus and species names originating from the Latin term "mola," meaning "millstone," a reference to its circular shape.
Molidae possess the fewest vertebrae among fish, with only 16 present in the species Mola mola. Additionally, they are devoid of caudal bones, and much of their skeletal structure is composed of cartilage. Notably, their skin lacks bony plates but is robust and dense, resembling cartilage, and has a relatively rough texture. These fish are also characterized by the absence of swim bladders.
Their primary mode of locomotion involves the utilization of their anal and dorsal fins, while the pectoral fins likely serve as stabilizers. For navigation, molids employ a method of expelling a strong jet of water from their mouths or gills. They are capable of making minor alterations in the orientation of the anal and dorsal fins to adjust the force produced and its angle, akin to how birds maneuver using their wings.
Molids can produce sound by grinding their pharyngeal teeth, which are elongated and claw-like. As typical members of Tetraodontiformes, their teeth are fused into a beak-like structure, rendering them unable to close their mouths. Nevertheless, their diet primarily consists of soft-bodied organisms, such as jellyfish and salps, although they occasionally consume small fish or crustaceans.
Molids have been observed engaging in interactions with other species due to their susceptibility to skin parasites. They take advantage of cleaner fish by approaching areas of floating algae or debris inhabited by halfmoons. By adopting a nearly vertical swimming posture with their heads near the water's surface, they signal readiness for cleaning, allowing smaller fish to remove parasitic worms. Similarly, molids may breach the water's surface with their dorsal fin and beak to draw the attention of seabirds like gulls, which then remove worms and other persistent parasites from their skin.
