HOLOCEPHALI – GENERAL ORGANIZATION AND AFFINITIES​

HOLOCEPHALI – GENERAL ORGANIZATION AND AFFINITIES​

Introduction

• Holocephali (Gr. Holos = entire + kephale = head), is a very small ancient group of highly specialized marine fishes.
• It comprises of rat-tailed fishes. They appeared first in the lower Jurassic and at present, are represented by a few marine genera only.
• It includes the only cartilaginous fishes having fleshy opercular covering of the gills.
• Like the Acanthodians, they seem to represent divergent and specialized descendants of some primitive elasmobranch ancestor.

Key Characters of Holocephali

• Endoskeleton cartilaginous, often calcified.
• Persistent (unconstricted) notochord; poorly developed vertebrae.
• Holostylic jaw suspension, i.e., the upper jaw is immovably united with cranium, hence the name holocephali.
• Teeth united to form crushing plates, devoid of enamel. This is an  adaptation for crushing molluscs, crustaceans and sea urchins.
• The occipital condyles are well developed and are marked better than that of the sharks.
• The gill openings are only four in number and the spiracular cleft is absent.
• A fleshy operculum, supported by branchial rays, is attached to the hyoid arch and forms covering of the gill openings on each side.
• Median and paired fins are well developed.
• No spiracle; no air bladder; no cloaca.
• Absence of stomach and presence of spiral valve in the intestine.
• Kidney is opisthonephric corresponding fundamental pattern of other fishes.
• Sexual dimorphism is well marked. Females attain larger size than the males.
• Mature male with cephalic or frontal clasper on forehead, a pair of pelvic claspers and a pair of prepelvic tenacula.
• Oviparous.
• Fertilization internal and cleavage holoblastic.

General organization of Holocephali

External Features of Holocephali :-

• Body appearance is shark-like but the head is large and compressed, having a small mouth.
• Operculum is formed by a fold of skin to cover the gill slits so that a single branchial aperture is found.
• Spiracle and cloaca are absent. Two dorsal and a ventral fin present.
• Tail appears to be heterocercal, but in Chimaera, it is whip-like. The pectoral as well as pelvic fins are large in size.
• The anal fin is small. The urinogenital aperture is distinct from the anus.
• Sexes are separate and sexual dimorphism well mark. Females are larger than the males.
• Males having cephalic or frontal clasper on forehead covered with denticles.
• Skin is smooth and silvery.
• Lateral line system has open grooves, with many branches on head.
• The hyomandibular does not participate in the suspension of the jaw.
• Such suspension is often called holostylic, to emphasize, that it probably evolved independently of that in dipnoans and the tetrapods.
• There are 5 gill arches with four gill openings protected by cover.
• The spiracular cleft is completely closed.
• All the pterygiophores of the first dorsal fin are fused into a single plate, the remaining fins and the pectoral girdle are on elasmobranch pattern.
• Each pelvic half consists of a narrow iliac region and a broad pubo-ischial region.
• A fleshy operculum, supported by branchial rays is attached to the hyoid arch and forms covering of the gill openings on each side.

Fins and Locomotion :

• There are two dorsal fins. Usually the first dorsal has strong spines as in Chimaera. Anal fin is small.
• Tail is of ordinary heterocercal type but in Chimaera and Hydrolagus, it is of isocercal type.
• The paired fins are represented by a set of large pectoral and smaller pelvic fins. Pelvics are abdominal in position.
• The graceful flapping movement of large sized pectoral fins enables the holocephalians to swim by sculling the body in contrast to the swimming of the sharks.

Digestive System :

• The mouth in holocephali is small as compared to the wide mouth of elasmobranch.
• It is bounded by three lips-like folds and equipped with the tooth plates having irregular surfaces and sharp cutting edges.
• The tooth plates have reduced pulp cavities and the layer of enamel, replaced by vasodentine.
• Vomarine, palatine and mandibular teeth are present. The gut is straight and a simple tube.
• A short oesophagus opens behind directly into a broad intestine and the latter into a short rectum, opening to the exterior by an anal aperture.
• The true stomach is altogether absent and the intestine has a well-developed spiral valve.

Respiratory System :

• A mandibular pseudobranch is absent and the hyoid bears only a posterior hemibranch.
• The first, second, and third branchial arches have holobranchs but the fourth has a hemibranch.
• The fifth arch is gillless and lacks a cleft between it and its predecessor.

Circulatory System :

• Heart is built upon the pattern, similar to other fishes.
• The heart consists of a sinus venosus, atrium, ventricle and conus with three rows of valves.
• The holocepalians retain urea in the blood for osmoregulation as in sharks.

Nervous System :

• Cerebellum is small. Medulla oblongata is produced laterally into restiform bodies.
• The cerebral hemispheres are small and each is connected with an olfactory bulb by means of a narrow peduncle.
• Diencephalon is long and trough shaped.
• The small rounded pineal body is present at the end of a pineal stalk.

Urinogenital System :

• The kidney of holocephalians is opisthonephric having a large number of uriniferous tubules and built upon the basic pattern of other fishes.
• The peritoneal funnels are absent in holocephalians but the abdominal pores are present.
• Unisexual. Clasping organs are remarkable in males only.
• Females attain larger size than the males.
• The male reproductive system consists of testes, vas deferens, epididymis and vesicular seminalis.
• The testes are large oval bodies but contain only immature sperms.
• The sperms become mature in the epididymis and form spermatophores.
• Vesicula seminalis is divided internally into several chambers by means of transverse septa.
• The spermatophores are stored in these compartments and are finally released into the urinogenital sinus.
• The female reproductive organs resemble those of the elasmobranchs and consist of a pair of ovaries, shell glands and uteri.

Fertilization :

• The fertilization is internal and the cleavage holoblastic type.
• The incubation period is fairly long, for example, in Callorhynchus, it extends from 9-12 months.

 Eggs and Development :

• The holocephalians are oviparous.
• Their eggs are characteristically spindle shaped and are surrounded by horny egg capsules secreted by the shell glands.
• The capsules of Hydrolagus measure about 15 cm and those of Callorhynchus collei, about 25 cm, in length.
• The eggs are laid in pairs, and contain three compartments.

Sensory Organs :

• Holocephalian’s eyes are large in relation to the body size, presumably as an adaptation to their total, dark habitat in the deep sea.
• The lateral-line canals are open and are specially developed on the head and on the underside of snout.
• They differ from those of the elasmobranchs possibly because of a change in the snout’s structure or the style of their food detection.
• The membranous labyrinth typically consists of three semicircular canals.
• The vertical canals forming the crus, also receive the horizontal canal.
• The endolymphatic duct opens externally by a pore.

Affinities of Holocephali

• Holocephali occupies a position in between the cartilaginous and bony fishes and have conserved certain of the primitive characteristics from their so called placoderm ancestors.
• They show resemblances with the elasmobranchs on one hand and teleosts on the other.
• They also possess a number of well-defined characters that are peculiar to the group and entitle them to be a separate class.

Resemblances with the Elasmobranchs :

• Holocephalians show both, the characters of palaeozoic elasmobranchs as well as those of modern living sharks.

Primitive Shark’s Features of Holocephali :

• The tail is heterocercal.
• A stiff spine along the anterior edge of the first dorsal spine is retained from the ancient sharks.
• Mouth placed ventrally as also in the modern sharks.

Modern Shark’s Features of Holocephali:

• Skin smooth and silvery.
• Cartilaginous endoskeleton, devoid of any replacing or dermal bones.
• The vertebral column is ancestrous and ribless consisting of a persistent notochord with cartilaginous arches.
• Lack of bony jaws.
• Simplified brain (Diencephalon only elongated due to large sized eyes).
• The general development of chondrocranium.
•  A pair of claspers present posterior to the pelvic fins in the male, as in elasmobranchs. Besides these, anterior claspers and the frontal claspers are also present in holocephali.
• The paired fins and girdles are built upon the elasmobranch pattern.
• The tail is heterocercal.
• The nasal and labial cartilages along with the orbital margins of cartilages are present.
• Air bladder is absent.
• A spiral valve is present in the intestine.
• A separate posterior superficial ophthalmic foramen is present.
• The reproductive organs on the elasmobranch pattern.
• The conus arteriosus is present in the heart and contains three rows of valves.
• Urea is retained in the blood for osmoregulation.
• Presence of egg capsule.
• The lateral line canals are distinct and suggestive of the sharks.
• Excretory system on the elasmobranch pattern

Resemblances with Dipnoi :

• Nature of skull.
• Unconstricted notochord.
• Presence of crushing tooth plates.

Resemblances with Teleosts :

• An operculum is present so that the gills do not open directly to the exterior but into a common chamber situated beneath the operculum.
• Single external branchial chamber is present.
• Reduced interbranchial septum, allowing gill filament to project beyond it.
• Spiracle is absent.
• Cloaca absent.
• Four complete aortic arches serving the four gill openings.

Primitive Characteristics : 

• Cartilaginous endoskeleton.
• Persistent notochord with cartilaginous arches.
• Separate external openings for each gill slit.
• Large opercular covering of the gill slits.

Specialized Features:

• Jaw suspension holostylic.
• Presence of unusual frontal claspers.
• Absence of scales.

Holocephalians show close relationship with some placoderms like- Rhamphodopsis and Ctenurella in the following points:

• Presence of a large labial cartilage.
• Presence of large tooth plates.
• Presence of long tail.
• Presence of a pair of rostral process to support fleshy snout.
• Presence of pre-pelvic claspers in male.
• Presence of short and deep platoquadrate.