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.
HOLOCEPHALI - GENERAL ORGANIZATION AND AFFINITIES​

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.

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