The redmouth whalefishes are two species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of deep-sea whalefishes in the genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Rondeletia, the only genus in the family Rondeletiidae. They are apparently close to the velvet whalefish

Velvet whalefish

The velvet whalefish, Barbourisia rufa, is a deep-sea whalefish, the sole known member of its family Barbourisiidae. It is found throughout the tropical and temperate parts of the world's oceans, mainly in the Pacific near Japan and New Zealand, at depths of 300 to 2,000 m...

 (Barbourisia rufa), and apparently also to the gibberfishes (Gibberichthyidae). The latter and the Rondeletiidae are the only known living fishes which have the mysterious Tominaga's organ.

Like the plant genus of the same name

Rondeletia (plant)

Rondeletia is a genus of flowering plants in the family Rubiaceae. It is endemic to the neotropics. There are around 160 species.Rondeletia odorata is widely grown as an ornamental. Several other species are also known in cultivation....

, this genus is named after Guillaume Rondelet

Guillaume Rondelet

Guillaume Rondelet , known also as Rondeletus , was Regus Professor of medicine at the University of Montpellier in southern France and Chancellor of the University between 1556 and his death in 1566. He achieved renown as an anatomist and a naturalist with a particular interest in botany and zoology...

.

Redmouth whalefishes are small fishes. Rondeletia bicolor grows to about 6 cm long or so, perhaps more. It is most commonly found in the northern Atlantic, but has been documented from the eastern Pacific as well and presumably also occurs in the rest of the Atlantic as well. The better-known Rondeletia loricata is known from all oceans between 47°N and S; it has been recorded to reach a length of more than 9 cm when adult and can reach over 10 cm.

Usually found in deep waters of tropical and temperate oceans around the world, down to 3,000 m or more, they make migrations up to shallower waters (around 100 m) at night to feed on crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s and amphipods. The larva

Larva

A larva 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...

e occur in the surface waters, down to 50 m; previous to the onset of metamorphosis they can be found in the uppermost few meters, descending as they mature. Notochord

Notochord

The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...

 flexion occurs at around 4 mm standard length in R. loricata.