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
Dorsal fin
Preanal fin
Eyes
Oral disk
Lateral line system
Barbels
Spiral valve intestine
Nostril location
External gill openings
Cranium
Vertebrae
Pronephric kidney
Osmoregulation
Eggs
Embryonic cleavage
Lampreys
1 or 2
Present
Well developed
Present
Well developed
Absent
Present
On top of head
7
Well developed
Present
Present
Hyper- or hypoosmotic
Small, no hooks
Holoblastic
Hagfishes
None
Present
Rudimentary
Absent
Absent
3 pairs
Absent
On snout
1-14
Rudimentary
Absent
Absent
isomotic
Large, with hooks
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
The Australian lungfish
is most similar to early
lungfishes
South American
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.