Download
1 / 135
1.4k likes | 1.77k Vues
Living fishes. The living fishes (not a monophyletic group) include: the jawless fishes (e.g. lampeys), cartilaginous fishes (e.g. sharks and rays), bony, ray-finned fishes (most of the bony fishes such as trout, perch, pike, carp, etc) and
E N D
Living fishes • The living fishes (not a monophyletic group) include: • the jawless fishes (e.g. lampeys), • cartilaginous fishes (e.g. sharks and rays), • bony, ray-finned fishes (most of the bony fishes such as trout, perch, pike, carp, etc) and • the bony, lobe-finned fishes (e.g. lungfishes, coelacanth).
Living jawless fishes • The living jawless fish once were included in the “Agnatha” along with ostracoderms because they lack the gnathostome characters of jaws and two sets of paired fins. • Today it is apparanet that the extinct ostracoderms are more closely related to the gnathostomes than are the living agnathans.
Living agnathans • There are slightly more than 100 species of living jawless fishes or Agnathans (the term agnathan does not represent a monophyletic group). • These belong to two classes the Myxini (hagfishes) and the Cephalaspidomorphi (lampreys).
Characteristics of living agnathans • Lack jaws (duh!) • Keratinized plates and teeth used for rasping • Vertebrae absent or reduced • Notochord present • Dorsal nerve cord and brain • Sense organs include taste, smell, hearing, vision.
Hagfishes: class Myxini • Hagfishes are a marine group of deep-sea, cold-water scavengers. • They use their keen sense of smell to find dead or dying fish and invertebrates and rasp off flesh using their toothed tongue. • As they lack jaws, they gain leverage by knotting themselves and bracing themselves against whatever they’re pulling.
Hagfishes • Hagfishes feed using two horny plates located either side of their tongue that are covered in sharp tooth-like structures. • When the tongue is everted the plates are spread apart and when the tongue is retracted the plates come together and mesh.
Hagfishes • Hagfishes are considered the sister group of all vertebrates because they lack any trace of vertebrae. • They also have many other primitive characteristics including simple kidneys and only one semicircular canal on each side of the head.
Hagfishes • Hagfishes are unusual in that they have body fluids, which are in osmotic equilibrium with the surrounding sea. This is unknown in other vertebrates, but common in invertebrates. • They are also unusual in having a low pressure circulatory system that has three accessory hearts in addition to a main heart.
Hagfishes • Hagfishes have a remarkable (and revolting) ability to generate enormous quantities of slime, which they do to defend themselves from predators. • A single individual can fill a bucket with slime.
Lampreys: Class Cephalaspidomorphi • Lampreys are similar in general size and shape to hagfishes, but are more closely related to gnathostomes than are hagfishes. • Lampreys possess vertebral structures called arcualia, tiny cartilaginous skeletal elements that are homologous with the neural arches of vertebrates.
Lampreys • Unlike hagfishes, lampreys possess large well developed eyes and have two semicircular canals. • They also are not isosmotic. Instead well-developed kidneys and chloride cells in the gills regulate the concentration of body fluids and allow lampreys to live in a wide range of salinities.
Lampreys • The lamprey’s mouth is located at the base of the oral hood (a fleshy suction cup lined with teeth). • The oral hood allows the lamprey to latch on tight to its prey and once attached the lamprey is very hard to dislodge.
Lampreys • Lampreys occur in both marine and fresh waters and about half of all species are ectoparasites of fish (the others are non-feeding as adults and live only a few months). • Lampreys spawn in streams and the larvae (ammocoetes) live and grow as filter feeders in the stream for 3-7 years before maturing into an adult. Feeding adults live a year or so before spawning and dying.
Lampreys • Parasitic lampreys have a sucker-like mouth with which they attach to fish and rasp away at them with their keratinized teeth. • The lamprey produces an anticoagulant as it feeds to maintain blood flow. When it is full the lamprey detaches, but the open wound on the fish may kill it. At best the wound is unsightly and largely destroys the fish’s commercial value.
Lampreys • Because attached lampreys cannot have a through-flow of water they have to ventilate their gills in a tidal fashion. • Water is drawn in and pumped out of the gill slits, which is not very efficient, but is a necessary compromise.
Introduced sea lampreys • Landlocked sea lampreys made their way into the Great Lakes around 1918 and caused the complete collapse of the lake trout fishery by the 1950’s. • Lamprey numbers fell as their prey base collapsed and control efforts were introduced. Trout numbers have since recovered somewhat, but wounding rates are still high.
Sea lampreys in Lake Champlain • Lake Champlain also has large populations of sea lampreys which spawn in the creeks that empty into the lake. • Until recently, lampreys were believed to have been introduced into Lake Champlain, but genetic analyses indicate the population was established perhaps as much as 11,500 years ago by lampreys that migrated up the St. Lawrence.
Sea lampreys in Lake Champlain • As is the case elsewhere there has been a campaign to control lamprey numbers primarily by using lampricides in steams. • Controls do reduce lamprey wounding rates and after control rates have fallen from 60-70 wounds per 100 fish examined to as low as 30 wounds/fish.
Early jawed vertebrates • The origin of jaws was a hugely significant event in the evolution of the vertebrates and the success of the Gnathostomes [the jawed vertebrates, “jaw mouth”] is obvious. • The first jawed vertebrates were the placoderms heavily armored fish which arose in the early Devonian (about 400mya). • They also possessed paired pelvic and pectoral fins that gave them much better control while swimming.
15.13 Early jawed fishes of the Devonian (400 mya).
Evolution of Jaws • Vertebrate jaws are made of cartilage derived from the neural crest, the same material as the gill arches (which support the gills). • Jaws appear to have arisen by modification of the first cartilaginous gill arches, which aid in gill support and ventilation.
Evolution of Jaws • The advantages of possessing jaws are obvious. • However, structures must benefit the organism at all times or they will not be selected for. • What use would a proto-jaw have been before being fully transformed?
Evolution of Jaws • Mallatt (1996,1998) has suggested that jaws were originally important for gill ventilation, not grasping prey. • Gnathostomes have much higher energy demands than agnathans. They also possess a series of powerful muscles in the pharynx. These muscles allow them to both pump water across the gills and suck water into the pharynx.
Evolution of Jaws • It is likely that selection initially favored enlargement of the gill arches and the development of new muscles that enabled them to be moved and so pump water more efficiently. • Once enlarged and equipped with muscles it would have been relatively easy for the arches to have been modified into jaws.
Evolution of Jaws • Being able to close the mouth would have enabled the muscles of the pharynx to squeeze water forcefully across the gills. • Selection would have favored any change in gill arches and musculature that enhanced water movement over the gills. • Thus, Mallatt suggested that the mandibular branchial arch enlarged into protojaws because it allowed the entrance to the pharynx to be rapidly opened and closed.
Evolution of Jaws • Selection would have favored enlargement and strengthening of the mandibular arch to tolerate the forces exerted on it by the strong pharyngeal muscles. • Once the proto-jaws can be rapidly closed they can also take on a grasping function and new selective forces would quickly have driven jaw elaboration.
15.12 Note resemblance between upper jaw (palatoquadrate cartilage) and lower jaw (Meckel’s cartilage) and gill supports immediately behind in this Carboniferous shark
Evolution of Jaws • Equipped with jaws for grabbing and holding prey and powerful pharyngeal muscles that could suck in prey gnathostomes could attack moving prey. • An enormous diversification of gnathostomes followed.
Four major groups of fish are present in the Devonian, two now extinct groups (the placoderms and acanthodians) and two living (the Chondrichthyians, [sharks and relatives] and the Osteichthyians, [bony fishes]).
Placoderms • The Placoderms are armored fishes that appear to be basal to other gnathostomes. • The oldest known are from the early Silurian. Large, heavy plates of dermal bone covered the front half of the body and small bony scales covered the rest.
http://universe-review.ca/I10-29-placoderm.jpg http://tea.armadaproject.org/Images/deaton/deaton_5placoderm.JPG.jpg
Placoderms • Most placoderms did not possess true teeth (although late forms do, evolved independently of the other gnathostomes). • Instead they had toothlike structures called tooth plates that were extensions of the dermal jawbones.
Arthrodires • More than half of all known placoderms are arthrodires (“jointed necks”). • Arthrodires had modified joints between the head shield and trunk shield, which gave them an enormous gape and made them ferocious predators
http://www.palaeos.com/Vertebrates/Units/050Thelodonti/Images/Gnathostomata1.jpghttp://www.palaeos.com/Vertebrates/Units/050Thelodonti/Images/Gnathostomata1.jpg Dunkleosteus upper Devonian. 10 meters long.
Placoderms • Placoderms like ostracoderms declined rapidly in the mass extinctions of the late Devonian. • A few forms survived for about 5 million years beyond the last ostracoderms, but the group was extinct by the end of the Devonian.
Acanthodians • The other extinct group of fishes is the acanthodians, which appear closely related to the bony fishes. • Acanthodians (from the Greek acantha meaning a spine) are named for the spines they had in front of their numerous fins (as many as six pairs in addition to the pelvic and pectoral pairs).
http://higheredbcs.wiley.com/legacy/college/levin/0471697435/http://higheredbcs.wiley.com/legacy/college/levin/0471697435/ chap_tut/images/nw0273-nn.jpg Acanthodians http://people.eku.edu/ritchisong/RITCHISO//Acanthodian.gifo
Acanthodians • Acanthodians had fusiform bodies and heterocercal tails and so were likely midwater fishes. • Acanthodians became extinct in the early Permian.
Chapter 4. The challenges of living in water • All vertebrates inhabit one or other of two fluid media: air and water. • These differ greatly in their physical characteristics.
Air vs. water • Density: water is 800 times denser than air. • Because water is dense, aquatic animals don’t need strong weight bearing skeletons. Gravity has little impact on their body structure. • In contrast, gravity is a constant challenge for terrestrial animals.
Air vs. water • Viscosity: water is 18 times more viscous than air. Viscosity measures how easily a fluid moves across a surface. • Because of this difference aquatic animals have to be much more streamlined than those that live in the air. • Because air flows easily, tidal ventilation is possible in lungs. In water, it is difficult and very rare.
Air vs. water • Oxygen content: Oxygen makes up about 20.9% of the volume of air (209ml of O2 in a liter of air). Water is never more than 50ml per liter and is often 10ml or less. • Low O2 content is another reason fish don’t use tidal ventilation. Because of the low O2 content of water, fish gills have evolved to be very efficient at extracting O2.
