Petromyzon (Lamprey): General Characters
Introduction: Lampreys are Anadromous or freshwater, Eel-shaped jawless fishes. They can be readily recognized by the large, rounded sucker which surrounds their mouth and by their single nostril on the top of their head.
Systemic position of Petromyzon:
- Phylum: Chordata
- Sub Phylum: Vertebrata
- Group: Agnatha
- Class: Cyclostomata
- Order: Petromyzontiformes
- Family: Petromyzontdae
- Type: Petromyzon (Lamprey)
Habit and habitats of Petromyzon
It has been known for a long time that the sea lamprey breeds in freshwater. However, it does not enter all the streams within its range indiscriminately.
As an illustration, we may cite outer Nova Scotia and the Bay of Fundy, where lampreys run in St. Marys, Sackville, Annapolis, Shubenacadie, Petit Codiac, and St. Johns Rivers, but not in the Moser or Apple Rivers, although these last also are “salmon” rivers. Their requirements are a gravelly bottom in rapid water for their spawning beds, with a muddy or sandy bottom in quiet water nearby, for the larvae.
External features of Petromyzon
Lampreys are Eel-like in appearance but possess a soft cartilaginous skeleton. They lack paired fins. these organisms possess well-developed dorsal and ventral fin-folds.
The jaws are rudimentary in adults, the mouth is a longitudinal slit when closed, but forms an elliptical disc at the tip of the snout when open and is armed with numerous horny hooked teeth arranged in 11 to 12 rows, the innermost largest.
The lampreys contain two dorsal fin folds and seven open gill slits on each side.
The length at the time of transformation from the larval stage is about 4 to 8 inches (100-200 mm.). Sexually mature individuals in American rivers, average 2 to 2½ feet long, up to a maximum of about 3 feet. One of 33 inches weighed 2¼ pounds
The Lampreys possesses the following organs:
- External gill slits: Lamprey consists of 7-gill slits that are opening that leads to the internal gills that are used to extract oxygen from the water.
- Buccal cavity: possess teeth in adult lamprey. It is surrounded and supported by oral disc.
- Buccal papillae: These are finger-like projections that surround the buccal funnel.
- Lateral line system: The system consists of lines of pores that sense water currents, water pressure changes, movements, and vibration in the water. The visible external pores of the lateral line system lead to an internal canal, which connects with specialized sensory cells. This system is believed to be related to the sense of hearing in other vertebrates
- The Median Nostril: The median nostril is the primitive unique feature of lamprey. Other vertebrates consist of paired nostrils. The nostril is responsible for detecting scents and leads to a nasal tube in the dorsal region of the head.
- A lamprey can “smell” by perceiving chemicals in the water. These scent particles can be detected from great distances
- Pineal Organ– It is located under the skin immediately posterior to the medial nostril. Evidence of its presence is shown by a generalized round bump on the dorsal side of the head. the pineal organ is sometimes referred to as a “Third eye” as it plays role in perceiving light and dark.
- Eye– in Lamprey the eye is a sensory organ responsible for visual input. It leads to optic nerves that send visual impulses to the brain. The adult lamprey eye is structurally very similar to the eyes of other vertebrates consisting of a cornea iris, lens, and retina. The eyelids are absent in the lamprey.
- Anterior Dorsal Fin – A fin used to maintain an upright orientation in the water while moving about.
- Posterior Dorsal fin– There functions the same as the anterior dorsal fin.
- Caudal fin– these are powerful, used to thrust the lamprey’s body through the water.
- Cloaca– It is the common opening of the urinary and reproductive system. It receives waste from the kidneys and fluids from the reproductive organs and transfers them to the external environment via the opening of the cloaca.
- The urogenital papilla is a protrusion that may be extending from the cloaca.
- Anus– it is located immediately anterior to the cloaca, the solid waste is expelled from the body through it.
The digestive system in Petromyzon (Lampreys)
The digestive system consists of the alimentary canal, which runs from the mouth to the anus. The food enters through the mouth and moves through the pharynx into oesophagus.
- The adults are ectoparasite and their food is in the form of fish blood.
- The stomach is not found in lampreys, food passes directly from the oesophagus to the intestine, which absorbs the bulk of the nutrients.
- The intestine becomes the site of the emulsification, digestion, and absorption of nutrients.
- The last section of the intestine narrows to form the anus, by which the waste removes from the body.
The nervous system in Petromyzon
The lampreys consist primitive nervous system. The brain structure is fairly simple compared to other vertebrates. The nervous system consists of the brain and hollow spinal cord, situated above the alimentary canal.
The circulatory system in Petromyzon
In these organisms, the blood flows through a series of vessels to supply oxygen and nutrients and remove carbon dioxide and other wastes.
- Arteries and arterioles carry blood away from the heart
- Veins and venules carry the blood back towards the heart
- Capillaries are the smallest vessels where the gases are exchanged with the cells of the body
Respiratory System in Petromyzon (lampreys)
The lampreys breaths by extracting oxygen present in water. Seven-gill pouches are found in respiratory tubes, each containing fine feather-like gill lamellae.
- The gill lamellae increase the surface area of the respiratory structures and contain the small capillary beds that extract oxygen Problem, when a lamprey is feeding and attached to a fish the mouth serves as an attachment organ, it is no longer available for use in respiration. water is taken directly into the respiratory tube through external gill slits.
- Muscular contractions change the volume of the respiratory tube and thus control the movement of water over the gill lamellae.
Excretory system in Lamprey
In lampreys, the kidney filters out waste from the blood in the form of ions, water, and other nitrogenous wastes. The kidney is responsible for maintaining osmoregulation.
- Extremely dilute urine is excreted by the kidney to maintain the ionic balance in the body.
- Kidneys excrete highly concentrated urine.
- Lamprey relies on the gills to get rid of the body of excess salt.
Life Cycle and Reproduction in Petromyzon (Lampreys)
These are anadromous with naked and slimy skin. The organism, whether marine or freshwater, always spawns and lays eggs in brooks and rivers.
- The larval stage is during most of their life, about seven years and after that, they undergo a metamorphosis and become an adult.
- Anadromous lampreys live in freshwater, adults return to the sea, where they become mature and then they return to rivers to reproduce and then generally die.
- Lamprey eggs generally hatch into small larvae, known as “ammocoetes”, these are not predators.
- The larvae lack the suckermouth (found in adults), feed producing strands of mucus and trapping food particles.
- The ammocoetes stage lasts up to seven years, before its metamorphosis into an adult.
- The adult lampreys live for a year or two before spawning and die soon after spawning.
You can also read:
- Characteristics and Classification of Phylum- Chordata
- Animal kingdom – Classification
- Phylum – Porifera
- Phylum – Coelenterata (Cnidaria)
- Phylum- Ctenophora
- Phylum- Platyhelminthes
- Amphioxus – Lancelet
- Herdmania – Sea Squirts
- Parental care in amphibians
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