BLY 202

Superclass Agnatha: Jawless Fishes

Objecves are to

• Gain understanding of the characteriscs of the jawless fishes

• Have knowledge of the forms and funcons of the two classes of the Superclass Agnatha • • Compare and contrast between Class Myxini and Class Cephalaspidomorphi Introducon • Living jawless fishes are represented by approximately 84 species

• Myxini (hagfishes) with about 43 species

• Cephalaspidomorphi (lampreys) with 41 species

• Members of both groups lack jaws, internal ossificaon, scales, and paired fins, and both groups share pore-like gill openings and an eel-like body form

• In other respects, the two groups are morphologically very different Characteriscs of Class Myxini

• Body slender, eel-like, rounded, with naked skin containing slime glands

• No paired appendages, no dorsal fin

• Fibrous and carlaginous skeleton; notochord persistent

• Bing mouth with two rows of eversible teeth

• Heart with sinus venosus, atrium, and ventricle; accessory hearts, aorc arches in gill region

• Five to 16 pairs of gills with a variable number of gill openings

Characteriscs of Class Myxini cont…

• Segmented mesonephric kidney; marine, body fluids isosmoc with seawater

• Digesve system without stomach; no spiral valve or cilia in intesnal tract

• Dorsal nerve cord with differenated brain; no cerebellum; 10 pairs of cranial nerves; dorsal and ventral nerve roots united

• Sense organs of taste, smell, and hearing; eyes degenerate

• Sexes separate (ovaries and testes in same individual but only one is funconal); external ferlizaon; large yolky eggs, no larval stage Class Myxini: Hagfishes • Hagfishes are an enrely marine group that feeds on annelids, molluscs, crustaceans, and dead or dying fishes

• They are not parasic like lampreys but are scavengers and predators

• The best known in North America are the Atlanc hagfish Myxine glunosa and the Pacific hagfish Eptatretus stou

• Almost completely blind, hagfish is quickly aracted to food, especially dead or dying fishes, by its developed senses of smell and touch

• The hagfish enters a dead or dying animal through an orifice or by digging inside. Using two toothed, keranized plates on the tongue that fold together in a pincer-like acon

• The hagfish rasps away bits of flesh from its prey Class Myxini: Hagfishes cont… • Hagfishes are renowned for their ability to generate enormous quanes of slime

• If disturbed or roughly handled, the hagfish exudes a milky fluid from special glands posioned along the body

• Unlike any other vertebrate, the body fluids of hagfishes are in osmoc equilibrium with seawater, as in most marine invertebrates

• Hagfishes have a low pressure circulatory system served by 3 accessory hearts in addion to the main heart posioned behind the gills

• There is no larval stage Characteriscs of Class Cephalaspidomorphi

• Body slender, eel-like, rounded with naked skin

• One or two median fins, no paired appendages

• Fibrous and carlaginous skeleton; notochord persistent

• Sucker like oral disc and tongue with well-developed keranized teeth

• Heart with sinus venosus, atrium, and ventricle; aorc arches in gill region

• Seven pairs of gills each with external gill opening

Characteriscs of Class Cephalaspidomorphi cont…

• Opisthonephric kidney; anadromous and fresh water; body fluids osmocally and ionically regulated

• Dorsal nerve cord with differenated brain, small cerebellum present; 10 pairs cranial nerves; dorsal and ventral nerve roots separated

• Digesve system without stomach; intesne with spiral fold

• Sense organs of taste, smell, hearing; eyes well developed in adult; two pairs semicircular canals

• Sexes separate; single gonad without duct; external ferlizaon; long larval stage (ammocoete) Class Cephalaspidomorphi (Petromyzontes): Lampreys

• All the lampreys of the Northern Hemisphere belong to the family Petromyzondae

• The group name refers to the lamprey’s habit of grasping a stone with its mouth to hold posion in a current

• There are 22 species of lampreys in North America

• About half of these belong to the – nonparasic type; – the others are parasic

• The genus Ichthyomyzon, includes three parasic and three nonparasic species. Class Cephalaspidomorphi (Petromyzontes): Lampreys cont…

• All lampreys ascend freshwater streams to breed

• The marine forms are anadromous, that is, they leave the sea where they spend their adult lives to swim up streams to spawn

• At spawning, with the female aached to a rock to maintain her posion over the nest, the male aaches to the dorsal side of her head. As eggs are shed into the nest, the male ferlizes them

• The adults die soon aer spawning. The eggs hatch in about 2 weeks, releasing small larvae (ammocoetes)

• Parasic lampreys either migrate to the sea, if marine

• Or remain in fresh water, where they aach themselves by their sucker-like mouth to a fish and, with their sharp keranized teeth, rasp away the flesh and suck out body fluids. Class Cephalaspidomorphi (Petromyzontes): Lampreys cont

• To promote the flow of blood, the lamprey injects an ancoagulant into the wound

• When gorged, the lamprey releases its hold but leaves the fish with a large, gaping wound that is somemes fatal

• The parasic freshwater adults live 1 to 2 years before spawning and then die

• The anadromous forms live 2 to 3 years

• Nonparasic lampreys do not feed aer emerging as adults and their alimentary canal degenerates to a nonfunconal strand of ssue

• Within a few months they also spawn and die.