Jawless Fish

Jawless fish, belonging to the infraphylum Agnatha, are unique among vertebrates due to their lack of jaws. They possess long, cylindrical bodies devoid of paired fins and scales and feature seven or more paired gill pouches. These fish feed by suction, utilizing their round, muscular mouths to attach to and consume their food. Both the larva and the adult stages are found to retain their notochord. Like other fish, they are ectothermic and can regulate their body temperature based on their external surroundings.

Lampreys and hagfishes, collectively called cyclostomes, are the sole surviving members of the agnathans. Cyclostomes are now considered a sister group to jawed vertebrates (gnathostomes).

Description

Jawless fish are easily recognized by their smooth, nearly cylindrical bodies lacking jaws and paired fins (though most have a caudal or dorsal fin). They possess seven or more paired gill pouches and a light-sensitive third or pineal eye.

These vertebrates have a cartilaginous skeleton and a two-chambered heart for pumping oxygen and nutrients throughout the body. They possess a long and uniform gut with no distinct stomach.

Lampreys

These cyclostomes resemble eels in appearance and have well-developed dorsal and caudal fins. Their funnel-like mouths are surrounded by a suctorial disc comprising a row of sharp, horny teeth. They also have well-developed eyes and a single nostril on top of their head.

Hagfish

They are scaleless, soft-skinned agnathans, having a single nostril at the anterior end of their bodies. In most species, the gills open to a common duct, but some open to the surface through separate pores.

Taxonomy

The only extant members of Agnatha are the cyclostomes (Superclass Cyclostomi), comprising the lampreys (Order Petromyzontiformes) and the hagfish (Family Myxinidae).

The classification of hagfish has remained controversial because it is uncertain whether this cyclostome is a degenerate type of vertebrate fish or represents a preceding stage in the evolution of the vertebral column. They are traditionally grouped with lampreys under Cyclostomi, the oldest surviving group of vertebrates alongside gnathostomes. 

Lampreys are classified under the class Hyperoartia, but this grouping also remains disputed. Although recently, it has been proposed that the Myxini are more basal among the skull-bearing chordates, lampreys have always been grouped with hagfish and represent one of the oldest divergences of the vertebrate lineage. Some paleontologists suggest that lampreys be placed under Ostracoderms in Cephalaspidomorphi,  Pteraspidomorphi, or Conodonta.

According to an alternative classification scheme, jawed vertebrates are more closely related to lampreys than hagfish, while vertebrates are closer to hagfish and are thus grouped under the proposed clade, Craniata.

• Superclass: Cyclostomi (Cyclostomes)
  • Class: Myxini
    • Order: Myxiniformes
      • Family: Myxinidae (Hagfish)
  • Class: Hyperoartia
    • Order: Petromyzontiformes (Lampreys)

The other agnathan subgroups Myllokunmingiida, Conodonta, and Ostracoderms (including Pteraspidomorphi, Thelodonti, Anaspida, and Cephalaspidomorphi) are extinct.

Evolution

• Although agnathans constitute only a minor part of the marine fauna today, they were once abundant and thriving as one of the earliest fish forms in the Paleozoic Era (around 538 to 251 million years ago).
• Although Early Cambrian (538 million years ago) animals, like Haikouichthys, Haikouella, and Myllokunmingia, apparently had fins, they are considered early agnathans. Conodonts (Class Conodonta) also originated around the same time and remained abundant until they became extinct in the Triassic Period (251 to 201 million years ago).
• Ostracoderms from the middle Ordovician Period (485 to 443 million years ago) were the first agnathans to develop bony and spiky armor. By the Late Silurian Period (443 to 419 million years ago), they had diversified into thelodonts, osteostracans, and galeaspids.
• Cyclostomes probably split from other agnathans before groups like conodonts developed dentine and bone, thus explaining their cartilaginous endoskeleton.
• The race of agnathans started declining in the Devonian Period (419 to 358 million years ago), and their numbers kept crashing ever since.

Distribution and Habitat

While Pacific and black hagfish inhabit the waters of the North Pacific Ocean coasts and are mostly restricted to the bottom waters, sea lampreys are native to the Atlantic Ocean. The latter is also found in the Baltic, Adriatic, and Mediterranean seas and often spawns in freshwater environments.

Diet

While lampreys consume fish and small mammals, hagfish are scavengers, primarily feeding on decaying animals (carrion).

Reproduction and Life Cycle

Male lampreys dig a nest-like depression in gravel beds of freshwater streams for mating with the females. Multiple eggs, each measuring about 1 mm (0.04 inch) in diameter, are released outside and fertilized externally by the sperm. The fertilized eggs are then lodged in the gravel around the nest. All lampreys die after mating as their cloacas remain open, allowing the entry of a deadly fungus into their intestines that eventually kills them.

The eggs hatch into blind, worm-like ammonocoete larvae, with their mouths overhung by a hood-like upper lip. For about three to four years, the larvae feed on microscopic plants and grow up to 10 cm (about 4 inches) in length, after which they undergo metamorphosis. Their upper lip is modified to the suctorial oral disc, and the eyes are completely developed.

After the completion of metamorphosis, lampreys migrate to the sea, where they attach themselves to the bodies of bony fish using suckers.

Adaptations

• Hagfish have numerous slime glands in their skin that help clog the gills of an attacking enemy. 
• The saliva of lampreys contains anticoagulants that facilitate the ingestion of blood and muscle tissues of the bony fish they consume.
• Agnathans do not eat much food and thus maintain very low metabolic rates to thrive in cold waters.