CHIMAERA : RAT FISH

Commonly known as ‘Rat Fish’ or ‘King of Herrings’. Body divisible into head, trunk and tail.
Body is shark-like with compressed head and blunt snout. Mouth is bounded by lip-like folds. Mouth and nostril ventral in position.
Gill-slits communicate with the exterior by a single branchial opening below the operculum which is not bony and covers the gills. Spiracles absent.
There are two large dorsal fins and a small ventral fin. First dorsal fin bears an immensely bony anterior erectile dermal spine while the second dorsal fin is non-erectile. The paired ectoral and pelvic fins are enlarged. Ventral fin small.
Caudal fin is like a folded tail, is produced into whip-like filament and is of diphyceral type.
Sexes are separate. Male Chimaera contains five claspers, a frontal or cephalic clasper on the head, a pair of anterior claspers in front of pelvic fins and a pair of ordinary claspers near pelvic fins.
Lateral line consists of open groove and body surface is smooth and scaleless. Male fish contains claspers.
Jaw suspension autostylic.
Eggs are surrounded by egg shell. Placoid scales are found in young rat fish.

SPECIAL FEATURES

Chimaera is important because it shows selachian and certain specialized characters. Selachian features comprise of cartilaginous endoskeleton, selachian limb girdles, dermal denticles, absence of air bladder, presence of spiral valve in the intestine, conus arteriosus and claspers in male. The specialized characters include autostylic skull, firmly fused palato-qnadrate with the base of cranium, teeth in the form of crushing dental plates and five claspers in male. The fish also reveals tendencies towards the bony fishes in having crowded gill-slits beneath the head covered by operculum, suppressed spiracles and absence of cloaca. Chimaera is an interesting fish, intermediate between sharks and bony fishes.

Like sharks they have a skeleton of cartilage but unlike them Chimaera has small fleshy lipped mouth with tooth plates attached to jaw. Spiracles absent. Chimaera further resembles the bony fishes in lacking cloaca and in having urinogenital aperture behind anus.