Molidae
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The Molidae comprise the family of the molas or ocean sunfishes, unusual fish whose bodies come to an end just behind the
dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage The fus ...
and
anal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported o ...
s, giving them a "half-fish" appearance. They are also the largest of the ray-finned bony fish, with the southern sunfish, ''Mola alexandrini'', recorded at in length and in weight. The family name comes from the
ocean sunfish The ocean sunfish (''Mola mola''), also known as the common mola, is one of the largest bony fish in the world. It is the type species of the genus ''Mola'', and one of five extant species in the family Molidae. It was once misidentified as th ...
's scientific name ''Mola mola'', both its genus name and epithet come from the Latin word ''mola'' for "
millstone Millstones or mill stones are stones used in gristmills, used for triturating, crushing or, more specifically, grinding wheat or other grains. They are sometimes referred to as grindstones or grinding stones. Millstones come in pairs: a s ...
" because of its circular shape.


Description

Molidae have the fewest
vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...
e of any fish, with only 16 in ''Mola mola''. The axial musculature, caudal and pelvic fins are completely lost during development, and most of their
skeleton A skeleton is the structural frame that supports the body of most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal fra ...
is made of
cartilage Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...
. They also lack
swim bladder The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled organ (anatomy), organ in bony fish that functions to modulate buoyancy, and thus allowing the fish to stay at desired water depth without having to maintain lift ...
s. No bony plates occur in the skin, which is, however, thick and dense like cartilage and is fairly rough. Under the skin there is a stiff and gelatinous layer consisting of 89.8% water with a collagen and elastin meshwork, acting like an exoskeleton. Due to its greasy texture, it may also contain lipids. This layer, which is horizontally separated by a septum, provides the fish with buoyancy, and makes up a larger part of the animal's total mass the bigger the individual is. Molids mostly swim by using their anal and dorsal fins; the pectoral fins are probably just stabilizers. To steer, they squirt a strong jet of water out of their mouths or gills. They can also make minor adjustments in the orientation of the anal fin or the dorsal fin so as to control the amount of force it produces and the angle at which the force is produced. In this respect, they use their fins much like a bird uses its wings. Molids are said to be able to produce sound by grinding their
pharyngeal teeth Pharyngeal teeth are teeth in the pharyngeal arch of the throat of cyprinids, suckers, and a number of other fish species otherwise lacking teeth.Tetraodontiformes Tetraodontiformes (), also known as the Plectognathi, is an order of ray-finned fishes which includes the pufferfishes and related taxa. This order has been classified as a suborder of the order Perciformes, although recent studies have found ...
, their teeth are fused into a beak-like structure, making it impossible for them to close their mouths. Despite this, they feed mainly on soft-bodied animals, such as jellyfish and
salp A salp (: salps, also known colloquially as “sea grape”) or salpa (: salpae or salpas) is a barrel-shaped, Plankton, planktonic tunicate in the family Salpidae. The salp moves by contracting its gelatinous body in order to pump water thro ...
s, although they also take small fish or crustaceans.


Behavior

Molids have been filmed interacting with other species. Since molids are susceptible to skin
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...
s, they make use of
cleaner fish Cleaner fish are fish that show a specialist feeding strategy by providing a service to other species, referred to as clients, by removing dead skin, ectoparasites, and infected tissue from the surface or gill chambers. This example of cleaning ...
. A molid in need of cleaning will locate a patch of floating
algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
or flotsam that is home to halfmoons. The molid signals a readiness for cleaning by swimming almost vertically with its head near the surface of the water, and waits for the smaller cleaner fish to feed on the parasite worms. Similarly, the molid may break the surface of the water with its dorsal fin and beak to attract the attention of a
gull Gulls, or colloquially seagulls, are seabirds of the subfamily Larinae. They are most closely related to terns and skimmers, distantly related to auks, and even more distantly related to waders. Until the 21st century, most gulls were placed ...
or similar
seabird Seabirds (also known as marine birds) are birds that are adaptation, adapted to life within the marine ecosystem, marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent ...
. The seabird will then dig worms and other stubborn parasites out of the molid's skin.


Fossil record

The known fossil history of Molidae extends back to the
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
, with the genus '' Eomola'' containing the species '' E. bimaxillaria'', known from the mid/late
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
(
Bartonian The Bartonian is, in the International Commission on Stratigraphy's (ICS) geologic time scale, a stage or age in the middle of the Eocene Epoch or Series. The Bartonian Age spans the time between . It is preceded by the Lutetian and is follow ...
) of the
North Caucasus The North Caucasus, or Ciscaucasia, is a subregion in Eastern Europe governed by Russia. It constitutes the northern part of the wider Caucasus region, which separates Europe and Asia. The North Caucasus is bordered by the Sea of Azov and the B ...
. The fossil genus '' Austromola'' containing one species, '' A. angerhoferi'' Gregorova, Schultz, Harzhauser & Kroh, 2009, is known from the Lower
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
Ebelsberg Formation near Pucking, Austria. This species was a resident of the Paratethys Sea and is estimated to have reached a length around , making it the largest Cenozoic fossil
teleost Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...
known. At least one fossil species of ''Mola'', '' M. pileata'' (van Beneden, 1881), is known from the Upper and Middle Miocene of
Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...
with a possible second species known from the Lower Miocene of
North Carolina North Carolina ( ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It is bordered by Virginia to the north, the Atlantic Ocean to the east, South Carolina to the south, Georgia (U.S. stat ...
, United States. The genus ''Ranzania'' has five known fossil species: '' R. grahami'' Weems, 1985 and '' R. tenneyorum'' Weems, 1985, both from the Middle Miocene
Calvert Formation The Calvert Formation is a Formation (geology), geologic formation in Maryland, Virginia, and Delaware. It preserves fossils dating back to the early to middle Miocene, Miocene epoch of the Neogene Period (geology), period. It is one of the three ...
of
Virginia Virginia, officially the Commonwealth of Virginia, is a U.S. state, state in the Southeastern United States, Southeastern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States between the East Coast of the United States ...
, USA; '' R. zappai'' Carnevale, 2007 from the Middle Miocene of Italy; '' R. ogaii'' Uyeno & Sakamoto, 1994 from the Middle Miocene of Japan; and an as yet unnamed species from the Upper Miocene of
Algeria Algeria, officially the People's Democratic Republic of Algeria, is a country in the Maghreb region of North Africa. It is bordered to Algeria–Tunisia border, the northeast by Tunisia; to Algeria–Libya border, the east by Libya; to Alger ...
. An
Early Miocene The Early Miocene (also known as Lower Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages: the Aquitanian age, Aquitanian and Burdigalian stages. The sub-epoch lasted from 23.03 ± 0.05 annum, Ma to ...
-aged fossil molid beak (potentially of '' Ranzania'') from the Gaiman Formation of
Chubut Chubut may refer to: * Chubut Province Chubut ( from Tehuelche language, Tehuelche 'transparent'; ) is a provinces of Argentina, province in southern Argentina, situated between the 42nd parallel south (the border with Río Negro Province), ...
, Argentina represents one of the southernmost known fossil records of tetraodontiform fish. Although it may have been a stray to the region, it is also possible that the warmer climate of the Miocene allowed molids to inhabit the seas that then covered
Patagonia Patagonia () is a geographical region that includes parts of Argentina and Chile at the southern end of South America. The region includes the southern section of the Andes mountain chain with lakes, fjords, temperate rainforests, and glaciers ...
. In modern times, molids are only occasional strays to the region due to its cold climate.


Species

Only five extant species in three extant genera are described: * Ocean sunfish ('' Mola mola'') * Southern ocean sunfish ('' Mola alexandrini''), has been recognized as a senior synonym of ''Mola ramsayi'' (Gaglioli 1889), the "bump-head sunfish" * Hoodwinker sunfish ('' Mola tecta'') * Slender sunfish ('' Ranzania laevis'') * Sharptail mola ('' Masturus lanceolatus'')


References

{{Authority control Taxa named by Charles Lucien Bonaparte Bartonian first appearances Extant Eocene first appearances