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Petromyzon : The Lamprey

Lampieys are worldwide in distribution found in both marine and freshwater

forms.

Systematic position.

The life cycle of Petromyzon includes two quite different phases. The larval

phase, called ammocoete is a freshwater, sedentary, filter feeding and

microphagus creature and the fish-like adult that lives in the sea and is

parasitic on fishes.

FUNDAMENTALS. The Cyclostomata is a group of agnathans that comprises the living jawless

fishes: the lampreys and hagfishes. Both groups have jawless mouths. The

class cyclostomata derived its name by having a round or circular mouth.

The class Cyclostomata includes two orders:

Order 1: Petromyzontia. e. g. lampreys - Petromyzon marinus.

Order 2: Myxinoidea e.g. hagfishes- Bdellostoma and Myxine spp.

Phylum : Chordata

Subphylum : Vertebrata

Class : Cyclostomata

Order : Petromyzontiformes

Family : Petromyzonidae

Type : Petromyzon

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The lampreys, with the exception of some small freshwater forms, attach

themselves to other fishes using their suctorial mouth and then rasp off the

flesh by means of the horny teeth carried by the highly-developed tongue.

In the river system, lampreys do riot eat anything, but utilize accumulated

subcutaneous fat that provides nourishment. They migrate to the rivers during

autumn after attaining maturity in winter. However, breeding takes place in

spring. Some lampreys (Lampetra) have peculiar habit of nest building, at the

bottom of the river, where sand and stone both are present.

Note: Nearly all lampreys are anadromous; that is, they live as adults in oceans or

big lakes and ascend rivers and streams to breed.

External Features Shape, size and colour. The adult lamprey has an elongated eel-like body made of three regions: head,

trunk and tail, which are not clearly demarcated. Head and trunk are

cylindrical, where as the tail is laterally compressed. Body surface or skin is

mottled greenish-brown in colour. Upper body surface is usually dark and

lower surface islight. It is without exoskeleton, soft and made slimy by

secretions from epidermal glands.

Fins. Paired appendages are absent. Two unequal median dorsal fins, first and

second, are located near the posterior end. Around the tail there is a caudal fin,

the upper lobe of which is continuous with the second dorsal fin. The fins are

supported by thin cartilaginous rods, the fin rays. In some lampreys (Lampetra

planeri), the female possesses an anal fin, but in males, it is reduced to a

copulatory papilla.

Buccal funnel. The anterior body end or head bears a ventrally directed large cup-like

depression, the sucker or buccal funnel. It is surrounded by a marginal

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membrane, beset with numerous soft small projections, the oral fimbriae or

papillae which help in attachment to a fish. In between the papillae project out

longer sensory processes, the cirri. The inside of buccal funnel is beset with

radiating rows of conical yellow, horny, epidermal teeth which have a very

definite arrangement. The teeth in the upper and lower sides of the mouth fuse

to form large tooth plates called supra oral and infra oral tooth plates

respectively. Teeth are not homologous with true vertebrate teeth. Immediately

below and behind the mouth projects the so-called tongue. It also bears large

horny teeth.

Eyes.

On each lateral side of the head is a large prominent eye. The two eyes lack

eyelid sand are covered by a transparent area of skin.

Fig. 1. Sea lamprey - Petromyzon marinus. Apertures. (i) Mouth →A narrow aperture lying at the apex of the buccal funnel and held

open by a ring of cartilage.

(ii) Nostril or nasohypophyseal aperture → Lampreys are unique among living

vertebrates in having a single nasal opening situated on the top of the head,

combined with a duct leading to the hypophysis (pituitary) and known as a

nasohypophysial opening.

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(iii) External gill slits → Just behind each eye, on each lateral side of the head,

a longitudinal row of Seven small rounded openings of external gill slits

present.

(iv) cloaca → On the ventral side, at the junction of trunk and tail, is a slit-like

depression, the cloaca.

(v) Numerous small sensory pores of the lateral line system extend along each

lateral side of the body and below the head.

Fig. 2- Sea Lamprey.