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Lampreys and Hagfish

The vertebrates are normally divided into two groups based on the presence or absence of jaws. The vast majority possess jaws and are termed Gnathostomata; jawless forms are termed Agnatha. The Agnatha are represented today by the lampreys and hagfish, organisms of little economic importance but of great interest to biologists as the sole survivors of an extensive group of Paleozoic (245 to 570 million-year-old) agnathans, which includes the earliest known vertebrates. The extant forms are often referred to as the cyclostomes, the fossil forms as ostracoderms; however, these do not constitute monophyletic groups (groups including the common ancestor and all descendants), and some fossil Agnatha are closely related to cyclostomes.

The Physiology of Jawless Fishes
Lampreys and hagfish are similar in general appearance despite far-reaching differences in internal organization. Both are elongated fish, without paired fins, that swim with an eel-like motion and reach lengths of one meter and weights of up to one kilogram. They possess an entirely cartilaginous skeleton; however, the organization of the body conforms to the basic vertebrate pattern. The lamprey has more active life habits and, therefore, has a more developed median fin. The propulsive section of the tail is longer than that of the hagfish. The lamprey’s mouth is surrounded by a suckerlike oral disc covered by small teeth on its internal surface. This rasping organ allows the lamprey to attach itself to prey, on which it feeds by rasping away surface tissues and ingesting blood. Above and behind the oral disc is the nasohypophysial opening, which leads into an olfactory pouch and ends blindly in a dilated sac above the anterior gill pouches. Lampreys have seven gill pouches opening through seven gill ports. In hagfish, the mouth is ventral and overhung by a rostrum, and it contains a dental plate that can be rapidly retracted, enabling the animal to bite off fragments of tissue from the dead and decaying fish on which it feeds. The nasohypophysial duct passes backward below the brain to join the pharynx, and the respiratory current passes along it. Hagfish are quite variable in their number of gill pouches and openings. Pacific hagfish have twelve pouches with separate openings (this number may vary up to fifteen), while Atlantic hagfish have five pouches with one common opening. The eye is vestigial and covered by surface tissue, so it is not normally visible.

There are about thirty-five species of lampreys, and all breed in freshwater and spend the major part of their life cycle in a freshwater larval stage before transformation to an adult stage. The eggs develop into a blind larval form, the ammocoete, which burrows into silt banks and remains for several years filtering microscopic particles from the water. The ammocoete is very unlike the adult, as no suctorial disc is present; a ciliated groove on the floor of the pharynx, together with a tubular endostyle, forms the feeding apparatus. After metamorphosis, three different types of adult lamprey are known: nonparasitic brook lampreys; freshwater parasitic forms; and anadromous forms (living in marine conditions but ascending rivers to spawn). The nonparasitic forms, which constitute about half the known genera, have the mechanisms for parasitic feeding but do not use them. The gonads mature immediately after metamorphosis, the intestine atrophies, and the foregut remains as a solid rod. These nonparasitic forms remain dwarf as they cannot feed, and six to nine months after metamorphosis they reach sexual maturity, spawn, and die. The parasitic freshwater forms live entirely in river systems, where they feed on fish. The anadromous forms move downstream to the estuary or sea after metamorphosis. There, they feed voraciously and grow rapidly, eventually returning to the river on an upstream spawning migration after one to three years. As in all lampreys, death follows shortly after spawning. In some cases, it appears that populations of originally anadromous lampreys may have become landlocked and are now entirely freshwater. That is what appears to have happened with the lampreys (Petromyzon marinus) that now inhabit the Great Lakes. They are capable of causing significant damage to fish populations and almost eradicated trout and whitefish from the Great Lakes between 1950 and 1960, when they peaked in that system, resulting in the collapse of a flourishing fishery.

Hagfish are purely marine fish, in contrast, and as they are benthonic (live on the bottom) and often inhabit deep water, little is known of their reproductive biology beyond the fact that eggs may be laid at any time of year and there appears to be no larval stage. They remain buried in mud during the day, emerging at night to feed. It appears that they normally attack only dead or dying fish, lacking the suction apparatus that makes the lamprey so successful in its attacks on living organisms; however, they will consume small invertebrates if they are available. When feeding, they may show “knotting behavior,” during which the flexible body ties into a knot in order to gain a better purchase for the tearing off of food fragments. One characteristic of all twenty species of hagfish is the development of mucus pores along the body. If roughly handled or irritated, these pores produce copious quantities of mucus, resulting in the name “slime eels” for these animals. Neither hagfish nor lampreys are of more than local economic importance.

The Fossil Record
The fossil record of hagfish and lampreys is sparse, presumably because of the lack of an ossified endoskeleton. Both are known only from the Pennsylvanian period (280 million years ago) of Illinois and show close similarity to modern forms. These sediments also include two other agnathans of unknown affinities, indicating a wide diversity of forms of this type. There is, however, an extensive record of armored agnathans (ostracoderms) fromrocks of Ordovician to Devonian age (360 to 470 million years old). These have been divided into two main groups, the Osteostraci and the Heterostraci, together with some smaller groups. The Osteostraci were a group of fish that lived in the Silurian and Devonian periods (370 to 425 million years ago) and were characterized by the presence of an armored head-shield, the rest of the body being covered by large bony scales.The large eyes were dorsally placed, and between them were the pineal foramen (an opening for a light-sensitive organ) and the nasohypophysial opening. Located laterally on the head-shield were sensory fields that probably were sensitive to pressure waves in the surrounding water. The ventrally placed mouth and dorsoventrally flattened head indicate that the Osteostraci were benthonic fish, possibly feeding by sucking organic debris or small organisms into the mouth. Paired pectoral fins may have acted to move the animals by rhythmic undulations. Because the head-shield surrounded the brain, a considerable amount of detail of brain structure and cranial nerve pattern has been preserved. It has shown that the general pattern was very similar to that of the lamprey, implying a close relationship. The Heterostraci were a long-ranging group whose earliest representatives occurred in the Middle Ordovician period (470 million years ago) and represent the first known record of vertebrates. Typical Heterostraci were armored over the anterior part of the body by variable numbers of bony plates and were further characterized by having only one pair of external gill openings. They were common in shallow marine and freshwater environments during the Upper Silurian and Devonian periods (360 to 425 million years ago) but became extinct at the end of the Devonian. They seem to have been adapted to a variety of modes of life, from benthonic detritus feeding to cropping algae and filter feeding. The Ordovician forms are known from rocks in North and South America and Australia and are united with later Heterostraci by the presence of the same type of acellular bone, aspidin, in the armor. Their exact position is uncertain, however, as they do not appear to have a series of branchial (gill) openings on each side. Although it is clear that the Osteostraci were closely related to modern lampreys, no relationship has been determined yet between hagfish and any ostracoderm group. The relationship of the fossil and modern agnathans to gnathostomes is also still the subject of considerable debate, though in broad terms it is accepted that lampreys are the sister group of gnathostomes and hagfish the sister group of lampreys and gnathostomes. Much further information is needed before the details of their phylogeny can be elucidated, and continuing work on these organisms will aid understanding of both the origin of vertebrates and the early development of major vertebrate groups.

The Ichthyology of Jawless Fishes
Fish are generally excellent subjects for study and for the demonstration of anatomy, physiology, ecology, evolution, and other aspects of science. The detailed anatomy of the cyclostomes has been known for many years, and knowledge in this area is dependent on careful dissection of specimens. General knowledge of the life cycle, ecology, and feeding habits of the modern forms is based on observation either in the natural habitat or in aquariums. As lampreys spend most of their life cycle in freshwater, understanding of their development is fairly complete. Hagfish, however, are marine benthonic fish that move and feed mostly at night, and hence the ability to observe them is somewhat limited and understanding of their life cycle is incomplete. Sophisticated laboratory techniques now make it possible to analyze the biochemistry and physiology of these organisms and compare them to other chordates. Studies have also been made of their swimming methods, using highspeed cameras and electromyography, a technique that allows the tracing of the electrical changes that take place in muscles when they are active. Fossil Agnatha cannot be studied as completely, because only the hard parts are preserved in the sediments. Techniques for studying these remains have changed very little in the past one hundred years. The bones are removed from the rock by chipping or dissolving the surrounding sediment away. Bone is composed of calcium phosphate and thus will resist some acids that can break down carbonate rocks. The acids most commonly used are acetic acid and formic acid, and the specimen to be dissolved out is often backed with plastic so that it does not disintegrate when the supporting matrix (surrounding sediment) is removed. The bone from these ancient agnathans is often so well preserved that thin ground sections can be made and viewed through a microscope using transmitted light. The characters determined by these means are used to determine relationships. Studies on the relationships of the fossil and recent Agnatha have relied in recent years on the methodology termed “phylogenetic systematics,” or “cladistics.” Cladistics is distinguished from other taxonomic methods (taxonomy is the study of interrelationships) by the fact that it is a rigorous system in which only shared advanced characters are used to show relationships. These relationships are expressed as branching diagrams termed cladograms (from the Greek klados, “branch”), hence the name cladistics. Although studies using this methodology have improved understanding of the relationship between modern Agnatha and the gnathostomes, the relationships of the fossil forms are still poorly understood.

Modern Jawless Fishes
Modern Agnatha, or cyclostomes, are relatively rare and unimportant organisms, although they are representatives of a group that was important in the early history of vertebrate evolution. For example, hagfish can be a nuisance to fishermen, attacking and destroying bait and even the catch itself. Lampreys, because of their active parasitic mode of life, can be a serious menace to fisheries, as evidenced by the sea lamprey depredations in the Great Lakes. The construction of a canal bypassing Niagara Falls unfortunately allowed lampreys to enter the upper Great Lakes from Lake Ontario, where they had been established. Once in the upper lakes, they underwent a population explosion, probably as a result of the abundance of prey fishes, lack of predators, and suitability of the system for spawning and maintenance of larvae. The establishment of the lamprey resulted in a serious decline of a number of fish species and the collapse of a flourishing commercial fishery that has only been reversed by the establishment of control measures and the use of larvicides. The fossil Agnatha, or ostracoderms, are generally poorly known as a result of their incomplete preservation. Yet they do throw some light on the earliest stages of vertebrate evolution, indicating that, as far back as 470 million years ago, humankind’s earliest ancestors were small, rather tadpolelike fish with an external armor of bony plates. Although they appear in the fossil record before the earliest jawed vertebrates, or gnathostomes, it appears that the separation into jawed and jawless forms had already occurred. Many gaps remain, but it is to be hoped that further discoveries will enable humankind to develop a clearer picture of its earliest vertebrate ancestry.

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Principal Terms

Agnatha: a class of vertebrates that includes all forms in which jaws are not developed; the group to which the earliest vertebrates belong
ammocoete: the larval form of lamprey, which lives in river silts
cladistics: a method of determining relationships in which shared derived (advanced) characters exhibited by the organism are used
cyclostomes: the modern agnathans, comprising lampreys and hagfish
Gnathostomata: all vertebrates in which jaws are developed
nasohypophysial opening: an opening in the head of modern agnathans leading to a sac that aids in olfaction
ostracoderms: armored fossil agnathans that flourished during the Paleozoic
Paleozoic era: time period from 570 to 245 million years ago, which comprises the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods

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