SPECIAL ICHTHYOLOGY - R. S. Pentyliuk - 2011

1. ORIGIN AND EVOLUTION OF CYCLOSTOMES AND FISHES. SYSTEMATICS OF MODERN CYCLOSTOMES AND FISHES. SUBPHYLUM CRANIATA. INFRAPHYLUM JAWLESS FISHES (AGNATA). CLASS CYCLOSTOMATA

1.3. Subphylum Craniata. Infraphylum Jawless Vertebrates (Agnata). Class Cyclostomes (Cyclostomata)

1.3.1. Subclass Lampreys (Petromyzones)

Modern lampreys, represented by 6–7 genera, inhabit the seas and freshwaters of temperate latitudes in both hemispheres. Lamprey species differ in their life strategies: the majority are anadromous, spending their adult lives in the sea. However, some species never leave rivers and lakes. Lampreys reproduce exclusively in fresh Water. A distinctive feature of lamprey biology is their development through metamorphosis—their eggs hatch into larvae that differ significantly from adult lampreys in external appearance, internal Structure, and feeding habits. These larvae are known as ammocoetes, and were previously mistaken for independent species.

We can trace The life cycle of anadromous lampreys using the European river lamprey (Lampetra fluviatilis) as an example. It reaches a length of 40 cm and a weight of about 200 g. The back and sides of this lamprey are dark gray with a subtle metallic luster, while the belly is pale yellow or white. The European river lamprey ranges from Italy, England, and Northern Norway, and is also found in the Baltic Sea. It inhabits coastal marine waters, and in late summer to early autumn, it gathers in large schools and heads upstream into rivers. From this moment on, lampreys cease feeding, and profound changes occur in their bodies. The intestine degenerates, becoming a thin thread as it is no longer needed; the Teeth become blunt; and the Salivary Glands located in the oral disc cease functioning. Most remarkably, lampreys decrease not only in weight but also in body length—spawning lampreys are significantly shorter than their conspecifics that have just migrated from the sea into the rivers. Within the bodies of these migrating animals, eggs and milt actively mature. All these transformations take several months, with spawning occurring in spring or early summer. They congregate in areas with sandy or pebbly bottoms, where the males begin to construct nests for their future offspring—fairly large depressions in the substrate. A male lamprey building a nest frantically twists and lashes its tail,

stirring up and scattering the sand while removing pebbles and gravel by grasping them with its oral disc. Later, the females begin to assist the male. Once the depression reaches about half a meter across, the lampreys begin spawning. Having found a firm anchor point within the nest (such as a pebble or solid substrate), the female attaches to it with her Mouth. The male approaches from behind, gently attaches to her side, and then glides his oral disc along her body until reaching her HEAD. Meanwhile, the male wraps his tail around his «partner» just ahead of the dorsal fin, and by sliding this «loop» from front to back, helps the female release her eggs while simultaneously releasing milt into the water. Quite frequently, several pairs of lampreys intertwine into a single «spawning ball» and deposit their eggs in the same nest. Each female yields an average of about 20,000 eggs. These sink to the bottom of the nest, after which the lampreys bury them in sand with sweeps of their tails. The adults subsequently weaken, seek shelter in crevices, and soon die. Within 11–14 days, the eggs hatch into larvae that initially resemble tiny (about 3 mm), pale-yellow worms. Approximately 20 days later, having reached a length of about 6 mm, the larvae leave the nest and drift downstream in search of areas with a weak current and a silty bottom. Upon finding a suitable spot, the larva burrows into the substrate, cementing its burrow with mucus. Then, protruding its head from the shelter, it begins to feed by filtering diatoms and various organic detritus from the surrounding water.

The ammocoete bears little resemblance to the adult lamprey, possessing tiny, rudimentary eyes barely visible beneath the Skin. The mouth opening is quadrangular, covered dorsally by a well-developed upper lip. Numerous branched villi are distributed along the ventral surface of the lip and around the mouth. Due to the movement of these villi and the beating of cilia located in the anterior part of the Pharynx, a mucous cord is generated in the ammocoete's mouth, moving from the external environment into the gut. Food particles captured in the water current adhere to this cord and enter the intestine, while the filtered water exits through the gill pouches. The larval Development of the European river lamprey lasts about 5 years. During this time, it grows to a length of 15–18 cm, after which it undergoes metamorphosis, transforming into an adult lamprey. Sharp cornified teeth appear on its oral disc, The connection between The Liver and the digestive tract is lost (from then on, the lamprey's liver Functions exclusively as an endocrine gland), and an outgrowth develops in the pharynx, separating the entrance of the Esophagus from the branchial cavity. The eyes develop—in adult lampreys, these are quite well-formed, and In addition to the two regular eyes, There is a third eye located anteriorly near the single nasal opening. This «third eye» is a very ancient organ inherited from the lamprey's ancestors; it lacks a lens but is capable of detecting light.

Metamorphosis takes about 6 months, after which the adult lampreys migrate from the river to the sea. Following a six-month fast, they are ravenous and immediately begin searching for food. In terms of feeding habits, the adult lamprey differs significantly from the filter-feeding ammocoete. It preys on fish (typically sick or weakened individuals), using its powerful teeth to pierce the skin and feed on the victim's Blood and Tissues. The lamprey's salivary glands contain substances that not only prevent blood clotting but also destroy erythrocytes and hydrolyze Proteins. Thus, partial extraintestinal Digestion occurs. While this phenomenon is widespread among invertebrates, among vertebrates it is known only in cyclostomes. Once attached to a fish, a lamprey may remain fastened for weeks; even if it eventually detaches without completely draining the host, the weakened victim quickly falls prey to other predators or parasites. Notoriously voracious and attacking A wide variety of Fishes—including herrings, salmon, cod, and flounders—lampreys can inflict substantial damage on fish populations. One year after descending to the sea, a lamprey doubles in size, and after two years, it loses its appetite and begins migrating toward river mouths to reproduce.

A similar life cycle characterizes other anadromous lampreys—for instance, the sea lamprey (Petromys marinus) inhabiting the North Atlantic, the Pacific lamprey (Entosphenus tridentatus) living in the coastal waters of the North Pacific, the Caspian lamprey (Caspiomyzon wagneri) inhabiting the Caspian Sea, and A number of other European, Asian, American, and Australian species.

Interestingly, the largest of the lampreys, the sea lamprey (which reaches a meter in length and a weight of 3 kg), can feed not only in the marine environment during its adult stage. In the Great Lakes system of North America, it has formed a freshwater population. Initially, this population was confined to Lake Ontario, from which lampreys migrated to spawn in the St. Lawrence River. Their spread to other lakes was prevented by Niagara Falls. However, when a bypass canal was constructed around the falls in the 1920s, adult lampreys rapidly colonized the entire Great Lakes system, resulting in catastrophic consequences for many valuable native fish species (primarily salmonids). Lampreys now began migrating to spawn in other lake-connected rivers as well, and their numbers surged. Americans had to expend considerable effort and resources to mitigate the Damage caused by the lampreys. Among lampreys, There are also species whose life cycle is devoid of Migrations—they spawn in the exact same streams and rivers where they live. A prime example is the European brook lamprey (Lampetra planeri). It is much smaller than the anadromous European river lamprey, yet it is frequently found in the same rivers, albeit further upstream and in shallower waters. Spawning of the brook lamprey, development,

and larval metamorphosis proceed much like those of anadromous lampreys. The primary difference in their life cycle is that the adult lamprey, having completed metamorphosis, does not attack fish and does not feed at all. It rapidly proceeds to reproduction and subsequently dies. Therefore, non-anadromous lampreys are also referred to as non-parasitic. Neoteny—reproduction at the larval stage—has even been documented in some of them. Unlike anadromous river lampreys, brook lampreys are not edible. Certain glands in their bodies produce a potent toxin that induces inflammation of the gastrointestinal tract. This poison is not destroyed even by heating, rendering any culinary preparation of brook lampreys useless for making them edible.