What are Lamprey and their Evolution?

Introduction:

Lampreys have almost  43 species of primitive fish-like, jawless vertebrates ich are placed with hagfishes in the class Agnatha. Because both the lampreys and hag-fishes are jawless vertebrates.  Lampreys belong to the family, “Petromyzonidae”. They are found mostly in temperate regions in coastal and fresh waters around the world, except in Africa. These eel-like and scaleless animals range from about 15 to 100 centimeters long in length. They have prominent and well-developed eyes having one or two dorsal fins, a tail fin, a nostril (single)  on top of their head, and seven-gill openings on each side of their bodies. Just like the hag-fishes, they lack bones, jaws, and paired fins. Their skeleton is made up of cartilage and the mouth is a round sucking. Which is just like a cleft surrounded by sharp and horny teeth. They are predatory in nature, using their mouth by eating the prey from inside out and placing their mouth on the body, and start sucking the blood and body fluids.

 They mostly live on seas, not all lampreys spend time in the sea. Some are land-locked and also live in freshwater. A prominent example is a land-locked race of the sea lamprey. This type of lampreys entered the “Great Lakes of North America” and, because of its parasitic habits, had a catastrophic killing effect on lake trout and other commercially important fishes before control measures were adopted.  Some other lampreys, like the “Brook lamprey”, also spend their whole lives in freshwater. They are non-parasitic in nature and do not feed after they become adults instead, they reproduce give off-springs and die.

Lampreys also have long been used to some extent as food in some countries. They are but, of no great economic value.

They begin life as larvae in burrowing freshwater and are called ammocoetes. At this stage of their life, they have no teeth, have elementary eyes, and feed mostly on crustaceans, and paramecium. and other microorganisms. They live in freshwater at this stage of their life. After some years, they change into adults and generally move into the sea to start a parasitic life like their parents,  by attaching to a fish by their mouths and feeding on the blood, body fluids, and tissues. At the stage of spawning and for the purpose of reproduction, lampreys return to freshwater and build a nest, then lay their eggs and finally die.

Classification:

Kingdom: Animalia

Phylum: Chordata

Infraphylum: Agnatha

Class: Hyperoaritia

Order: Petromyzontiformes

Evolution:

“Synapomorphies’ are specific characteristics that are the same over the evolutionary history of organisms.  These characteristics of chordates are having a notochord, dorsal (hollow) nerve cord, pharyngeal slits, and a muscular post-anal tail during the process of their development are believed to be Chordates. Lampreys possess all these characteristics that describe them as chordates.

  The anatomy of Lamprey is very different based on other fishes in which stage of development they are in. The notochord emanated from the mesoderm and mesodermal tissues and is one of the explanatory features of a chordate. The notochord provides signaling and mechanical cues to help the organism when swimming. The dorsal hollow nerve cord is another characteristic of lampreys that describes them as chordates. During the stage of development, this part of the ectodermal tissues and ectoderm folds and forms a hollow tube. That is why it is typically directed as the dorsal "hollow" nerve cord. 

The third feature of chordate is having the pharyngeal slits, which are openings found mostly between the pharynx. These are filter-feeding organs that aid in the movement of water through the mouth and out of the body by these slits when they are feeding. During the larval stage of lamprey, they totally depend on filter feeding for the purpose of obtaining their food.

But when lampreys reach their adult stage then they become parasitic in nature on other fish, and these gill slits become very important helping in the respiration of the organism. The lat synapomorphic feature of chordates is the possession of a muscular post-anal tail which is made up of muscles that extend behind the anus.

Sometimes adult and lamprey larvae are compared by anatomists due to their high level of similarities.  The similarities between adult amphioxus and larvae of lamprey include all the derived characteristics of chordates i.e. a pharynx having pharyngeal slits, a notochord, a dorsal( hollow nerve cord), and post anal muscular tail.

The lamprey has rare fossils because they are made up of cartilage and, cartilage does not fossilize completely as bone.  But the exceedingly well-preserved fossil showed a well-developed and sucking oral disk, a relatively long branchial apparatus which showed a branchial basket, gill arches, seven-gill pouches, and also the impressions of gill filaments, and almost 80 myomeres of its musculature.