Jawless Fish

Jawless Fish

The first fish to evolve were fairly basic, with spinal cords and only partial skeletons. However, they set the blueprint for vertebrates, so that all later fish, amphibians, reptiles, birds and mammals, including humans, evolved from them. The class of fish Agnatha- the jawless fish contains the lampreys and the hagfish. Both groups are eel like in form, although true eels belong to the bony fish super-class. As their common name indicates, these fish do not have structured jaws as other fish do. Instead they have sucker mouths, lined with teeth with which they abrade the surface of their food. In the case of lampreys they attach themselves to other fish and feed on their blood. In the case of hagfish, the decomposing flesh of other animals is their meal.

Lampreys

Lampreys are parasitic fish, as they feed on the blood of other living fish. Their mouths are lined with concentric circles of teeth designed to saw their way through the scales of their hosts and then rupture the blood vessels in their skin. The lamprey then gorges itself on blood until it is satisfied and goes on its way. Host fish can die from blood loss, but they are usually much larger than lampreys and are able to recover. The name lamprey is derived from the Latin Lambere for ‘lick’ and petra for ‘stone’ meaning ‘stone licker’ because the fish also use their sucker mouths to hold onto stones in fast flowing rivers, thus enabling them to save on energy when travelling upstream.

Hagfish

There is some disagreement about the classification of hagfish as they happen to be the only chordates that possess skulls but have no vertebral column, making them somewhat more primitive than lampreys. Consequently, they are given their own class Myxini. Hagfish are part of the ‘clean- up squad’ as they play their part in the decomposition and recycling of dead animal bodies lying on the seabed. They are designed to puncture holes in the skin of dead animals and then drill their way inside, consuming the decomposing flesh as they go. To assist in this way of life, they are able to produce large quantities of slime as lubrication and they are also able to literally tie themselves in knots, which they can then move along their bodies, thereby enabling them to extract themselves from tight holes that generate a vacuum. The slime may also serve to deter predatory fish, as it gels with water and clogs the mouth and gills, assisting in escape. Hagfish will also attack dying animals, burrowing inside their bodies and finishing them off by devouring their internal organs. Due to the sporadic availability of food, hagfish can survive for months without feeding, but feed keenly when the chance arises.

Hag fish have barbells around their mouths and simple eyes, but relatively little is known of their sensory equipment and how they detect their food. Similarly, much detail of their reproduction and life cycle awaits scientific investigation. They typically inhabit deep water environments, where behavioural observations are difficult and expensive to conduct. They are known to lay eggs, from which young hagfish hatch, whereas lampreys have a larval stage before adults develop.

Reference

1. Britannica Reference Encyclopaedia 

2. Knowledge Encyclopedia Animal!: The Animal Kingdom as you've Never Seen it Before

3. Images are taken from Zoo-Que and Google.

4. In Defense of Plants: An Exploration Into the Wonder of Plants

5. A Naturalist’s Guide to the Birds of India

6. Earth: A Sustainable Planet

Also See

• Migration & Navigation of Birds

• Archaeopteryx – The First Bird`s Fossil

• Parrots 

• Bats

• Bears

• Woodpecker 

• Birds

• Flightless Birds 

• Corals, Jellyfish and Sea Anemones

• Butterflies, Bees and Ants 

• Starfish and member of Echinoderm 

• Jawless Fish

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