Although it is debatable whether hagfish are vertebrates current classification places the hagfish within the Subphylum Vertebrata. Hagfish and Lampreys are placed in different classes of the Superclass Agnatha. Phylum Chordata Subphylum Vertebrata Superclass Agnatha Class Myxini - hagfishes Class Cephalaspidomorphi - lampreys Although they are superficially similar they differ in many ways
Characteristic Lampreys Hagfishes Dorsal fin 1 or 2 None Preanal fin Present Present Eyes Well developed Rudimentary Oral disk Present Absent Lateral line system Well developed Absent Barbels Absent 3 pairs Spiral valve intestine Present Absent Nostril location On top of head On snout External gill openings 7 1-14 Cranium Well developed Rudimentary Vertebrae Present Absent Pronephric kidney Present Absent Osmoregulation Hyper- or hypoosmotic isomotic Eggs Small, no hooks Large, with hooks Embryonic cleavage Holoblastic Meroblastic
Hagfishes - 43 spp. - worldwide - marine in cool waters - inshore at high latitudes - osmoconformers - 30-80 mm in length Live in association with the bottom, in burrows, in colonies, scavengers on carrion, life history poorly known, commercially impt. Produce large hooked eggs 2-3 cm in length
Lampreys - 40 spp. - in cool waters of northern and southern hemispheres - either anadromous, or entirely freshwater Two different lifestyles - parasitic and nonparasitic (brook lampreys) Parasitic forms feed on large prey, mostly fishes, feeding on blood or tissue fluids, or muscle Have oral disk with keratinized teeth, action of the tongue rasps hole and tears flesh
Respiration with gill pouches - allows respiration while attached to host. Parasitic forms are anadromous - they spawn in clear flowing streams in gravel Like salmon - they are semelparous - they invest heavily in reproduction and die after spawning Produce small eggs (1 mm) - one female may produce 250,000 eggs
Lamprey larvae and juveniles are called ammocoetes - they live in gravel and filter feed - parasitic forms they metamorphose as they migrate downstream and adopt a parasitic lifestyle
Nonparasitic lampreys are called brook lampreys - retain the juvenile form, filter feed, and mature - mature earlier and spawn fewer eggs. There are many species pairs of brook and parasitic lampreys - with the brook lamprey apparently derived from the parasitic form by loss of the parasitic phase Neoteny - retention of juvenile characteristics in the adult - reproductive maturation while in the juvenile body form
Grade Telostomi - bony vertebrates (formerly C. Osteichthyes) Class Acanthodii - spiny sharks Class Sarcopterygii - lobe-finned fishes Subclass Dipnoi - lungfishes Subclass Coelocanthomorpha - coelacanths Subclass Osteolipimorpha - rhipidistians - extinct Subclass "Tetrapoda -(amphibians, reptiles, mammals, birds) Class Actinopterygii - ray-finned fishes Subclass Chondrostei - sturgeons, paddlefish, birchirs Subclass Neopterygii - "modern" ray-finned fishes - the most primitive neopterygians are gars and the bowfin Lungfishes, coelacanths, sturgeons, paddlefishes, birchirs, gars, and bowfin are remnants of early fish groups. Many of the ancestral groups were widespread. Today, the few species remaining are often distributed among different continents (relictual distributions) and many have lost characteristics commonly seen in fossil species.
All non-tetrapod Teleostomes have lungs (or swimbladders), bone, bony scales or scutes, bony gill covers, bony fin rays (lepidotrichia) Lungs likely originated in species living in oxygen poor waters. A bony skeleton has been nearly lost in some living Chondrosteans and Dipnoans.
Subclass Dipnoi - lungfishes - 6 spp. - 1 in Australia, 1 in South America, and 4 in Africa - a relictual distribution Australian South American The Australian lungfish is most similar to early lungfishes African
Characteristics: lobed paired fins, upper jaw fused to cranium, internal nostrils, cloaca, dermal tooth plates (blade-like in modern forms), ventral connection to one or two dorsal lungs, spiral valve intestine In their evolution, lungfishes have lost or reduced many characteristics seen in the earliest forms: loss of bone, reduction in thickness of scales, loss of separation between dorsal, anal, and caudal fins, loss of rays on paired fins, and reduction of lobes to filaments
Neoceratodus forsteri - found in Australia - omnivorous heavy scales and large lobed fins with rays 2m in length - heavy bodied, lives only in permanent bodies of water, incapable of aestivation, gills used for respiration Lepidosiren paradoxa - found in South American - omnivorous lung is primary respiratory organ can live in mud burrow during dry periods Protoperus (4 species) found in Africa - predators lung primary respiratory organ undergoes true aestivation with reduction in metabolic rate
Mud burrow of Protopterus sp. lined with a mucous cocoon where they can live for months or years. Protopterus spp. and Lepidosiren some have external gills in larvae (like amphibians) All lungfish lay eggs.
The last gill arch of lungfishes has no gill filaments or capillary beds. The blood flows directly through the gill arch to the lungs. Blood returns to the heart from the lungs well oxygenated. Oxygenated blood partially mixes with deoxygenated blood and is then pumped back through gill arches and on to the body.
Subclass Coelacanthamorpha - fringe-finned fishes early forms originated in and diversified in freshwater have sculptured enamel on teeth, upper jaw fused to skull, no cloaca, fossil forms had cosmine layer on scales )but this is not seen in the living species), spiral valve intestine, hollow dorsal spines, lobed fins - both paired and unpaired, and a jointed skull,
one living species - Latimeria chalumnae - lives in relatively deep water in the Indian Ocean off the coast of Madagascar - discovered in 1938 - recently discovered in Indonesia also Latimeria has a nonfunctional lung - filled with fat - for buoyancy Like elasmobranchs it osmoregulates using urea to raise tonicity It is livebearing with a 9 cm egg Lives over rocky bottoms - slow moving - lie-in-wait predator
Class Actinopterygii - ray-finned fishes originated in Devonian - 325 million years ago and were rare until about 200 million years ago today the dominant fishes in freshwater and the sea fins lack fleshy lobes and are supported by lepidotrichia have branchiostegal rays early and primitive forms have ganoid scales no external gills in larvae (with one problematic exception) dorsal lung or swimbladder (usually with a dorsal connection) external nostrils The Class Actinopterygii is divided into two subclasses - the Subclass Chondrostei - sturgeons, paddlefish, bichirs Subclass Neopterygii - gars, bowfin, teleosts
Chondrosteans - originated in lower Devonian (325 mya) - became dominant group, most died out in Mesozoic (100 mya) survived today by two orders - O. Acipenseriformes- sturgeons (23 spp.) and paddlefish (2 spp.) O. Polypteriformes - 11 spp.- bichirs and reedfish Chondrosteans usually have a spiracle, heterocercal tail, no premaxilla, upper jaw (maxilla) united with skull, spiral valve intestine, reduction in bone, usually lack vertebral centra, well developed notochord, one or no branchiostegal rays
Order Acipensiformes - with 2 very different families F. Acipenseridae - sturgeons (26 spp.) - all northern hemisphere - anadromous and with some entirely freshwater - five rows of bony scutes on body, four barbels, - inferior and protrusible mouth, no teeth in adults - includes largest freshwater fishes - beluga sturgeon, over 1 ton - feed on invertebrates and smaller fish - sluggish swimmers - source of caviar, and edible flesh - require clear fast flowing water over gravel for spawning - several species threatened with extinction because damming and siltation
F. Polyodontidae - paddlefishes - 2 spp. - one in Mississippi drainage - 2m, non-protrusible mouth one in Yangtze drainage - 3m, protrusible mouth both have paddlelike snout that is an electrical sense organ with ampullary organs, and with minute barbels very long gill covers American species is a plankton feeder with 100s of gill rakers Asian species is a piscivore
O. Polypteriformes - bichirs and reedfishes 11 spp. only in Africa, predators, relatively common appear relatively late in fossil record but have many primitive characteristics similar to Sarcopterygians - lobed pectoral fins, ventrally connected lungs, larvae have external gills (like lungfishes) Lobes do not have a skeleton similar to Sarcopterygians and tetrapods.
Similar to Actinopterygians in possession of ganoid scales (like gars) unique in possession of 5 to 18 dorsal finlets - each with a single spine, and rays attached to the spine lungs used in low oxygen situations, and some species require access to air at the surface Common in the aquarium trade - often sold as ropefish or congo eels.
Within the Subclass Neopterygii historically there have been two Divisions with a variety of names 1. Holostei (gars and bowfin) and Teleostei or 2. Ginglymodi (gars) and Halecostomi - bowfin and teleosts the second classification reflects greater evolutionary similarity of bowfins to the teleosts each has a maxilla that is more moveable
Division Ginglymodi, Order Semionotiformes, Family Lepisosteidae - gars - 2 genera, 7 species. most primitive of the Subclass Neopterygii finned fishes good fossil record - back to permian-triassic-jurassic (200 to 150 mya) fossil forms known throughout N. Hemisphere Modern Gars fresh to brackish water alligator gar (to 3 m) sometimes found in sea water all in North and Central America - Costa Rica and Cuba
elongate jaws and body, modified heteroceral tail (dorsal lobe extension) heavy ganoid scales, 3 branchiostegals, swimbladder highly vascularized - can be used as lung unique opisthocoelous vertebrae (anterior convex, posterior concave) classic lie-in-wait predators attack from the side with lateral movement of head eggs and larvae are toxic
Division Halecostomi, Subdivision Halecomorphi, Order Amiiformes Family Amiidae - 1 sp. - Amia calva - bowfin freshwater throughout eastern North America fossil relatives known worldwide in Mesozoic - 150-300 mya more advanced than gars 10-13 branchiostegals, round relatively thin cycloid scales, very long dorsal fin
exhibit some primitive characteristics tail appears homocercal - modified heteroceral with upturned vertebral column and larger dorsal lobe modified spiral valve intestine highly vascularized swimbladder that can serve as lung used as lung in warm water, and can live through dry spells in burrows The most primitive fish with sexually dimorphic coloration - only males have a spot at the base of the caudal fin extensive male parental care, with aggressive guarding of young A relatively intelligent fish, can be easily trained for hand feeding.