Friday, May 6, 2011

Callorhincidae, the Plough-Nose Chimaeras

A juvenile Callorhinchus milii.

Callorhincidae is a family of Chimaeriforms more commonly known as Plough-Nose Chiaeras, the Elephant Sharks and even occasionally the Ghost Shark (a name which is frowned upon outside of colloquial use as this term is generally used to describe the closely related family Chimaeridea). While all Chimaeriforms are known to be a bit odd in appearance, members of this family of deep water fish have taken things just a step further. As their common name sake suggests, they have evolved a unique proboscis (known as a rostrum) that functions much like a metal detector. But in this case, the bits of treasure it is seeking are hidden morsals of food buryed within the sea floor.

The Chimaeras, are among the oldest group of living fish with known fossils dating back as early as the Devonian. A member of the class Chondrichthyes (the cartilaginous fish) and sub-class Holocephali (of which the order Chimaeriformes is the only extant order), the family Callorhincidae contains only one genus, Callorhinchus which in turn is composed of three known species: Callorhinchus callorynchus, Callorhinchus capensis and Callorhinchus milii.

These fish are often found around 200-500 meters (656-1640 ft), although they have been known to come into shallower water to lay their eggs. They range in size from 90 cm to 125 cm (3 to 4.10 ft). But as I have said, their most prominent feature is their rostrum that is filled with sensory nerves. It functions much like the broadened head of the various species of hammerhead sharks by detecting the electrical signals given off by other organisms. With this tool, the Plough-Nose can simply swim along the sea floor until its 'nose' picks up the faint signals of previously hidden critters.

A close up of the sensitive rostrum of the Plough-Nose Chimera.

While the unique nose of this fish has granted them an easy way to find a meal, it has also attracted many deep sea fishers who are hoping for a unique catch and for its flesh (which supposedly, has a taste simimilar to Cod and has become quite popular in many restaurants in Australia and New Zealand). Though unwary fishers and researchers may encounter a defensive feature shared by all extant Chimaeriforms, a spine at that base of the dorsal fin. Thankfully this family does not seem to be threatened by such human actions.

Another odd feature about the Plough-Nose and other Chimaeriforms can be found in their reproduction. They share many features with their closest relatives, the sharks and rays, internal fertilization and the laying of protected eggs known as 'mermaid purses' (though some sharks are viviparous). However, the Chimaeriforms method of fertilization differs greatly. While male sharks and rays have a pair of claspers located near the pelvic fin, Chimaeriforms have a pair of split claspers as well as a retractable structure on its forehead. Known as the tentaculum, this structure is a spiny knob like protrusion that is believed to be used to 'stimulate' the female during mating. There is an obvious joke here, but it is far to blatant to mention.

A final interesting feature about these fish has been revealed by the Elephant Shark Genome Project. The project was proposed due to the fact that Chimaeriforms have the smallest genome of any cartilaginous fish and because cartilainous fish are the oldest jawed vertebrates. Using the Australian Ghost Shark (Callorhinchus milii, the project hopes to be able to use these fish as a model which could help shed light on vertebrate evolution. One unexpected result of the inital findings is that this species has a three cone color vision system, something shared by humans.
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