Lampreys and Hagfishes

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Lampreys and Hagfishes

Lampreys and hagfishes are unusual, jawless fish that comprise the orders Myxiniformes (hagfishes) and Petromysoniformes (lampreys). There are approximately 40 species of lampreys and approximately 35 species of hagfishes and slime hags.

Lampreys and hagfishes lack the scales typical of most fish, and are covered with a slimy mucous. These animals have an elongated, eel-like shape, and do not have any paired fins on their sides. Lampreys and hag-fishes have gill pouches for ventilation, connected to the external environment by numerous holes or slits on the sides of the body and back of the head. These animals have a simple, cartilaginous skeleton. However, lampreys and hagfishes are divergent in various anatomical characteristics, and each represents an ancient and distinct evolutionary lineage.

Lampreys and hagfishes are living representatives of an ancient order of jawless, fish like animals that comprise the class Agnatha. Almost all of these jawless vertebrates became extinct by the end of the Devonian period about 365 million years ago. This dieback probably occurred because of competition from and predation by the more efficient, jawed fishes that evolved at that time. However, the ancestors of lampreys and hagfishes survived this evolutionary and

ecological change. These distinctive creatures first appear in the fossil record during the Carboniferous period (365-290 million years ago), and are considered to be relatively recently evolved jawless fish. The lack of scales and paired fins in lampreys and hagfishes are traits that evolved secondarily, and are atypical of fossil members of their class, Agnatha.

Lampreys

Adult lampreys have relatively large dorsal and ventral fins on the latter half of their bodies, and have a well-developed visual sense. These animals have a circular mouth that can be used to attach sucker like to the body of a fish. Parasitic species of lamprey then rasp a hole in the body wall of their victim, using rows of keratinized, epidermal structures that function like teeth, and a tongue that can protrude beyond the mouth. The lamprey then feeds on the ground-up tissues and bloody discharges of its prey. If the victim is a large fish, it will generally survive the lamprey attack, and perhaps several attacks during its lifetime. The victim is seriously weakened, however, and may fail to reproduce, or may eventually succumb to environmental stresses a more vigorous animal could tolerate. Because lampreys do not usually kill their victims directly, they are generally considered to be a parasite, rather than a predator.

Lampreys also use their disk-mouths to hold onto rocks to stabilize themselves in moving water, and to move pebbles while digging their nests in a stream. The name of the common genus of lampreys, Petromyzon, translates as “stone sucker” from the Greek.

Lampreys can pump water directly into and out of their seven gill cavities through separate gill slits. This ability allows lampreys to ventilate water over their gills, even though their mouth may be actively used for feeding or sucking on rocks.

Most species of lampreys are anadromous, spending their adult life at sea or in a large lake, and swimming upstream in rivers to their breeding sites in gravelly substrate. The larvae of lampreys, known as an ammocoete, look very unlike the adult and were once believed to be a different species. The ammocoete larvae live in muddy sediment, and are a filter-feeder on suspended aquatic debris and algae. The larval stages can last for more than four years, finally transforming into the adult stage, when they reenter the oceans on lakes.

Hagfishes

Hagfishes are entirely marine animals, living in burrows dug into the sediment of the sea floor of the temperate waters of the continental shelves. They have degenerate, non-functional eyes, and appear to rely mostly on short, sensory tentacles around their mouth for detecting their food. Hagfishes have a single nostril, through which water is taken in and used to ventilate the gas-exchange surfaces of their gill pouches before being discharged back to the ambient environment through gill slits. Hagfishes have four distinct blood-pumping regions in their circulatory system, which represent four functional hearts.

A single, elongated gonad on the front part of hag-fishes develops into an ovary in females, and the back part into a testis in males. In 1864, the Copenhagen Academy of Sciences offered a prize for the first zoologist to describe the method by which hagfish eggs get fertilized. This prize has yet to be claimed! Hagfish feed largely on invertebrates, especially polychaete worms. They also feed on dead fish, which they enter through the mouth, and then eat from the inside out. Hatchlings of hagfishes resemble the adults, and therefore are not, strictly speaking, larvae.

Hagfishes are extremely slimy creatures. Their mucous may protect them from some types of predators, but it can also represent a problem to the hagfish, by potentially plugging its nostril. Hagfishes periodically de-slime themselves by literally tying themselves into a tight knot, which is skillfully slid along the body, pushing a slime-ball ahead of itself. This unusual, knotty behavior is also used by hagfishes to escape from predators, and to tear into the flesh of dead fish.

Interactions with humans

Marine fishers sometimes find hagfishes to be a minor nuisance because they swim into deep-set nets and eat some of the catch. They are extremely messy and unpleasant to remove from nets, because of the copious, thick, sticky slime that covers their body.

The sea lamprey (Petromyzon marinus ) is an important parasite of economically important species of fish in the Great Lakes of North America. This species was probably native to Lake Ontario, but it spread to the other Great Lakes after the construction of the Welland Canal in 1829 allowed the sea lamprey to get around Niagara Falls, which had been an insurmountable barrier to its movement up-river. The sea lamprey was similarly able to colonize Lake Champlain after a transportation canal was built to link that large water body to the sea. The sea lamprey now occurs in all the Great Lakes and in other large lakes in North America, where it parasitizes all of the larger species of fish and greatly reduces their productivity. The lake trout (Salvelinus namaycush ) was virtually eliminated from some lakes by the sea lamprey, and probably would have been extirpated if not for the release of young trout raised in hatcheries.

The deleterious effects of the sea lamprey are now controlled to a significant degree by the treatment of its spawning streams with a larvicidal chemical (TFM, or 3-trifluoromethyl-4-nitrophenol) that is applied to

KEY TERMS

Anadromous —Refers to fish that migrate from salt-water to freshwater, in order to breed.

the water, killing the ammocoetes. However, TFM cannot be applied to all of the breeding habitats of the sea lamprey, and this parasite continues to cause important damage to commercial and sport fisheries, especially on the Great Lakes.