Callorhinchus, the plough-nosed chimaeras or elephantfish, are the only living genus in the family Callorhinchidae (sometimes spelled Callorhynchidae).[2] A few extinct genera only known from fossil remains are recognized.[3] Callorhinchus spp, are similar in form and habits to other chimaeras, but are distinguished by the presence of an elongated, flexible, fleshy snout, with a vague resemblance to a ploughshare. They are only found in the oceans of the Southern Hemispherealong the ocean bottom on muddy and sandy substrates. They filter feed, with small shellfish making up the bulk of their diet. The plough-nosed chimaera lays eggs on the ocean floor that hatch at around 8 months. They are currently not a target of conservation efforts; however, they may be susceptible to overfishing and trawling.
Plough-nose chimaeras range from about 70 to 125 cm (2.30 to 4.10 ft) in total length.[4] Their usual color is black or brown, and, often a mixture between the two. While the club-like snout makes elephantfish easy to recognize, they have several other distinctive features. They possess large pectoral fins, believed to aid in moving swiftly through the water. They also have two dorsal fins spaced widely apart, which help identify the species in the open ocean.[5] In front of each pectoral fin is one single gill opening. Between the two dorsal fins is a spine, and the second dorsal fin is significantly smaller than the more anterior one. The caudal fin is divided into two lobes, the top one being larger.[6]The eyes, set high on the head, are often green in color.
The snout is used to probe the sea bottom in search of the invertebrates and small fishes on which it preys.[2] The remainder of the body is flat and compressed, often described as elongated. The mouth is just under this snout and the eyes are located high on top of the head. They have broad, flat teeth that have adapted for this eating habit, two pairs that reside in the upper jaw and one pair in the lower jaw. In addition to its use for feeding, the "trunks" of the Callorhinchus fish can sense movement and electric fields, allowing them to locate their prey.
Phylogenetically, they are the oldest group of living jawed Chondrichthyes. They possess the same cartilaginous skeleton seen in sharks, but are considered holocephali to distinguish them from the shark and ray categorization.[7] Because of this, they provide a useful research organism for studying the early development of the jawed characteristic. Among the Chondrichthyes, Callorhinchus has the smallest genome. Because of this, it has been proposed to be used for entire genome sequencing to represent the cartilaginous fish class.[7] They are considered to resemble a cross between a shark and a ray or skate, but can be distinguished from sharks because they possess an operculum over their gill slits. Additionally, their skin is smooth, not covered in tough scales, characteristic of the shark. While the shark's jaw is loosely attached to the skull, the family Callorhinchidae differ in that their jaws are fused to their skulls. Many classify the Callorhinchidae as a chimeric species due to their shared characteristics of both the sharks and rays.[8]
The brain-to-body weight ratio is higher than that of a human[citation needed]. Compared to humans, it has a larger cerebellum than forebrain. Its vision is very poor and the electrical sensing capabilities of the snout are predominantly used to find food. Both its circulatory and endocrine systems are similar to similar vertebrates, likely due to the early homologous structures the Callorhinchidae possess relative to the other Chondrichthyes.[11]