Molidae

The Molidae are a special group of fish known as molas or ocean sunfishes. These fish look quite unusual because their bodies end right behind their dorsal and anal fins, making them appear like only half a fish. Because of this shape, they are sometimes called "half-fish."

These fish are also the largest of the extant ray-finned bony fish. One type, called the southern sunfish, Mola alexandrini, can grow up to 4.6 meters (15 feet) long and weigh as much as 2,744 kilograms (6,049 pounds)!

The family name Molidae comes from the scientific name of the well-known ocean sunfish, Mola mola. This name has a tautonymous style and comes from the Latin word mola, meaning "millstone"—a reference to the fish’s round, stone-like shape.

Description

Molidae, also known as ocean sunfishes, have very few vertebrae, only 16 in Mola mola. During their growth, they lose their caudal and pelvic fins, and most of their skeleton is made of flexible cartilage. They also do not have swim bladders. Their skin is thick and rough, and under it, there is a special layer that helps them stay afloat in the water. This layer is made of 89.8% water and has a mesh of collagen and elastin, acting like a protective cover.

These fish mainly move by using their anal and dorsal fins, with their pectoral fins helping to keep steady. They can steer by pushing a strong jet of water out of their mouths or gills. They can also adjust the angle of their fins to control movement. Molids can make sounds by grinding their long, claw-like pharyngeal teeth. Their teeth are fused into a beak-like structure, so they cannot close their mouths fully. They eat mainly soft-bodied animals like jellyfish and salps, but they also eat small fish or crustaceans.

Behavior

Molids, also known as ocean sunfish, have been seen interacting with other sea creatures. Because their skin can get pests called parasites, they use special fish called cleaner fish to help them. When a molid needs cleaning, it finds a spot with floating seaweed or other objects where small fish called halfmoons live. The molid shows it’s ready by swimming almost straight up with its head near the water’s surface, waiting for the cleaner fish to eat the pests off its skin.

Sometimes, the molid will also break the water’s surface with its fin and beak to get the attention of a seagull or other seabird. The bird then helps by picking out stubborn pests from the molid’s skin.

Fossil record

The fossil history of Molidae, or ocean sunfish, goes back to a time called the Eocene. One ancient group, called Eomola, lived in the mid to late Eocene in a place known as the North Caucasus. Another group, Austromola, lived during a time called the Lower Miocene in a place called the Ebelsberg Formation near Pucking, Austria. This species was thought to be about 320 cm (130 in) long, making it one of the biggest fish fossils from a time called the Cenozoic.

We also know about fossils of a fish called Mola from a time called the Upper and Middle Miocene in Europe, and maybe from the Lower Miocene in North Carolina, United States. Another group, Ranzania, has five known fossil species found in places like Virginia, Italy, Japan, and Algeria. One fossil from a time called the Early Miocene was found in Chubut, Argentina, showing that these fish once lived farther south than they do today. The warmer climate of that time may have allowed them to live there, but today the colder waters make it less common to find them in that area.

Species

There are five living species of ocean sunfish, grouped into three different groups. These special fish have a round body shape that looks a bit like a millstone, which is why they got their name.

The family tree showing how these species are related comes from a study done in 2002 that looked at their physical features.

Main article: cladogram
Main articles: morphological, phylogenetic tree

Rank	Scientific name	Common name	Year described	Mass	Total length	Clavus (pseudo-tail) appearance	Shape of head and chin	Image
1	Mola mola	Ocean sunfish	1758	2,300 kg (5,100 lb)	3.33 m (10.9 ft)	Scalloped	Less bumpy and less protruding
2	Mola alexandrini	Southern sunfish	1839	2,744 kg (6,049 lb)	3.0 m (9.8 ft)	Rounded	Bumps on head and chin
3	Mola tecta	Hoodwinker sunfish	2017	1,870 kg (4,120 lb)	2.42 m (7 ft 11 in)	Rounded with medial indentation	No lumps and bumps on head and chin
4	Masturus lanceolatus	Sharptail mola	1840	2,000 kg (4,400 lb)	3.37 m (11.1 ft)	Pointed projection	Convex forehead
5	Ranzania laevis	Slender sunfish	1776	N/A	1.0 m (3 ft 3 in)	Slant and flat	Pointed and bump-less