The silver spinyfin (''Diretmus argenteus'') is a

spinyfin Spinyfins are a family, Diretmidae, of trachichthyform fishes. The family name is derived from the type genus, '' Diretmus'', from Greek, ''di'' meaning "two" and ''eretmos'' meaning "oar". They are found worldwide in deep waters, as deep as . ...

of the genus ''Diretmus'', found around the world except the

Mediterranean The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the e ...

, at depths down to 2,000 m. It belongs to the

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

genus ''Diretmus''. Their length is between 30 and 40 cm. The silver spinyfin is a widely distributed but rare species with a flattened disc-shaped body with a sharp

scute A scute or scutum (Latin: ''scutum''; plural: ''scuta'' "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterior po ...

-covered edge on the belly. A large upturned mouth and huge

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

dominate the front half of the disc and a small truncate caudal fin projects from the other. They are

oviparous Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and ...

and the

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. The ...

e spend time in the

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

. ''D. argenteus'' has 38 opsin genes for the rods in its retinas, suggesting that they can see in color in very low light conditions. The silver spinyfin has the highest number of visual

opsin Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become Retinylidene proteins, but are usually still called opsins regardless. Most pro ...

s of any vertebrate according to a research study. It has small spiny scales but no

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...

and is a uniform silvery colour, the top of the back being black.

References

* * * Tony Ayling & Geoffrey Cox, ''Collins Guide to the Sea Fishes of New Zealand'', (William Collins Publishers Ltd, Auckland, New Zealand 1982) silver spinyfin Cosmopolitan fish silver spinyfin silver spinyfin {{Beryciformes-stub