New information on size composition, feeding, and spawning of Siberian stone loach Barbatula toni in a large watercourse in the northeast of Sakhalin Island is reported. The recorded maximal absolute body length and weight comprised 200 mm and 53.3 g, respectively. Predation and cannibalism in large fish specimens have been revealed.
In most fish species, the transition to a schooling lifestyle occurs soon after the start of exogenous feeding. The age at which juveniles begin to show schooling behavior differs in different species and is not related to the level of schooling in adults, i.e., their belonging to obligate or facultative schooling fish. In marine fish passing through the metamorphosis phase, schooling develops, as a rule, after the completion of this process. Among freshwater fish, juveniles living in rivers begin to show schooling behavior earlier than juveniles in stagnant waters. By the time a school is formed, the level of development of sensory systems and locomotion in juveniles is sufficient for intra-school contacts and schooling swimming. As juveniles grow, the coordination of schooling reactions and the ability to maintain the unity of a school during maneuvering increase. Compared to adult fish, schools of juveniles are less uniform in size and species composition.
The problems and prospects of studying schooling behavior of fish have been considered. Areas that remain little developed or have controversial and contradictory results have been noted: the hydrodynamics and energetics of fish swimming in a school, the mechanisms of fish interaction and the dissemination of information within a school, the principles of forming a coordinated school response to external stimuli and the rapid decay of the reaction, interaction between different schools during their collision or when being part of large aggregations of many schools, patterns of rapid change in the forms of a school. It has been shown that there are no clear ideas about the formation of mechanisms in the ontogeny of fish that underlie coordinated schooling behavior. The sensory base of schooling behavior requires further study. The origin and evolution of schooling behavior and the formation of emergent properties of a school based on individual actions of fish remain at the level of assumptions and hypotheses. The interspecies differences in the schooling behavior of fish, the interaction of schooling fish with fishing gear and adaptation to them are poorly studied. Attention has been drawn to the need for verification in nature of information obtained in laboratory conditions and on aquarium fish that have undergone selection. The necessity of using new technologies, devices, methods of mathematical modeling and other approaches for the intensification of experimental research has been emphasized. Knowledge of the schooling behavior of fish is important for elucidating the general patterns of social behavior of large associations of animals. The development of research is hampered by the lack of generally accepted terminology and quantitative criteria for schooling behavior, which would make it possible to adequately assess, compare, and analyze it. An exhaustive definition of a fish school has been given.
Ideas about the hydrodynamic and energetic benefits of swimming in a school have been considered. The reduction in energy cost for locomotion in a school is achieved due to the interference of microvortices generated by fish moving or staying in the current (hydrodynamic hypothesis of schooling swimming). The effect is most pronounced if the fish are of the same size, have similar locomotor capabilities, and show consistency when changing the mode or direction of swimming. It is also believed that the hydrodynamic effect of a school is realized only when partners in intra-school subgroups interact. Increased endurance of fish when in a school compared to single individuals has been experimentally confirmed. Energy costs for swimming while in a school can be lower by about 10–20%. Most of the fish that make long migrations are schooling fish, or form schools for the period of migration. Combining small schools into larger schools and school aggregations (mega-schools) facilitates finding optimal paths and improves the accuracy of migrations. Migration in schools reduces the vulnerability of fish to predators.
The article opens a thematic issue of the Journal of Ichthyology, dedicated to Dmitry Viktorovich Radakov, a researcher who made an outstanding contribution to the knowledge of schooling behavior of fish. The article deals with many terms and concepts that are widely used in scientific publications on the schooling behavior of fish, but still do not have a generally accepted definition—crowd, aggregation, shoal, school, flock. Attention is drawn to the difficulties caused by the fact that the use of these terms is most often based not on objective criteria, but on the preferences, views or beliefs of individual researchers. General ideas about the prevalence of schooling behavior in fish of different taxonomy, lifestyle, condition and age; about the importance of the visual structuring of the environment for the manifestation of schooling, about the difficulty of dividing fish into facultative and obligate schooling, about the importance of knowledge about schooling behavior for solving applied problems have been considered.
Small freshwater (resident) populations of the European river lamprey Lampetra fluviatilis from an isolated from the sea upper part of a river system with numerous lakes (the upper reaches of tributaries of the Daugava River and Lake Peipus, Pskov Oblast, Russia) have been studied. Adult individuals of the lamprey have been attributed to common and large size groups; adults are similar to other resident lamprey from the Baltic Sea basin. The presence of larvae of different size and age groups indicates regular spawning of the European river lamprey in the studied watersheds. Analysis of the modern and historical (before the isolation) distribution of lamprey revealed a reduction in habitats and a decrease in the number of individuals. Dam-associated disruption of migration routes led to the disappearance of anadromous form, and consequently, disappearance of resident lamprey from most of the studied waterbodies. We attribute the decrease in the number of rivers inhabited by lamprey and abundance of the lamprey both to anthropogenic factors (direct blocking of an access of anadromous form of the lamprey to spawning grounds by hydraulic construction) and to the characteristics of the studied area. The richness of the river system with lentic waterbodies makes it suitable for lamprey inhabitation with an influx of large anadromous individuals capable of crossing such reservoirs in search of spawning sites.
The study of the effect of thyroid hormones on the development of the pigment pattern, including the expression of sexual dichromatism, contributes to our understanding of the role of endocrine signaling in the evolution of cichlid fishes, one of the most diverse groups of teleosts. This work shows the effect of reduced thyroid hormone signaling on the development of reversed sexual dichromatism in Amatitlania nigrofasciata, a Neotropical cichlid in which females, unlike males, have carotenoid coloration. In hypothyroid fishes, there was a slowdown in the rate of metamorphic transformations of the pigment pattern and an increase in phenotypic variability. The adult pattern based on carotenoids began to develop in females only after the completion of treatment of thiourea, which suppresses the synthesis of endogenous thyroid hormones. The data obtained indicate a potentially important role of thyroid hormone-mediated developmental plasticity in the diversification of carotenoid coloration in Neotropical cichlids.
The results of the study of growth, ossification sequence and peculiarities of the development of skeletal elements, body proportions and meristic characters of laboratory-reared larvae and fry of two dwarf forms of Arctic charr Salvelinus alpinus сomplex spawning at the shore slope (Lake Tokko) and in the profundal zone (Lake Bol’shoe Leprindo) (Transbaikalia) are presented. Charr from Lake Bol’shoe Leprindo demonstrated slower rates of growth and morphogenesis, retardation of the ossification of their predorsal bones series with its displacement from larval to juvenile period of the ontogeny, but the acceleration of the development of the vertebral column. High mortality of pre-larvae of charr from this lake connected with the transfer to exogenous feeding was observed. For the first time the problem of the reorganization of early ontogeny of charr of the genus Salvelinus in connection with the colonization of deepwater habitat is discussed. In larvae of charr from Lake Tokko, the ossification of vertebral centra was often accompanied by the formation of anomalous bony structures in the notochord. The phenomenon of the penetration of skeletogenic cells into the notochord, which has been described in hybrids of sympatric charr forms, was found in a pure charr form for the first time. The development of characteristic morphological features of the two charr forms in the ontogeny was traced. Differences in body proportions between their fry corresponding to those between adult fish were revealed.
The balance between phenotypic plasticity and adaptive specialization in response to environmental pressures remains a hot topic in evolutionary biology. In fish, one of the strongest impact factors is the chemical pollution of habitats. In an attempt to assess the consequences of heavy pollution of fresh waters for resident fishes, we studied Kamchatkan charr, which undergo paedomorphosis in the case of isolation in streams of volcanic areas contaminated with heavy metals. Experiments were carried out on the resistance of charr to metal mixtures during normal development and in six experimental groups with therapeutically altered intensity of metabolism and the secretory activity of thyroid gland. Water from volcanically contaminated streams was found to be lethally toxic for embryos and early juveniles of unadapted charr. The success of acclimation to toxic exposure was correlated with an increase in thyroid status. In experiments, the group with significantly elevated thyroid status showed a significant decrease in mortality and attenuation of oxidative stress in solutions of heavy metals. Under natural conditions, hyperthyroidism provokes a redistribution of the charr’s organism resources from somatic growth and morphological differentiation to stress counteracting and accelerated maturation, which is necessary for the long-term survival of the population under conditions of increased risk of individual mortality. Our experiments highlight the role of thyroid hormones in the rapid response to habitat pollution and the subsequent adaptation of fish populations to chronic deterioration.
The basic mechanisms of schooling behavior of fish, which is a genetically fixed species character, have been considered. The intention of schooling fish to unite with individuals of their own species or with fish that are similar in shape, color and motor activity pattern (schooling reaction) is an innate reflex that manifests itself in natural and artificial environments in individuals with different individual experiences. The intention to unite is expressed the stronger, the more schooling behavior is characteristic of fish. The larger the school, the more attractive it is for fish. To choose a school for association, it is enough that it be 2–3 times larger than the rest. This difference decreases with an increase in the number of fish in schools, with the threat of a predator attack and other stresses. In juveniles, the intention to unite with larger schools is more pronounced than in adult fish. Given a choice, fish prefer individuals of their own species and fish similar in size and color. Imitation is an unconditioned reflex, which is another important mechanism of school behavior. Imitative reactions are most pronounced in schooling fish, but their manifestation is possible if the imitated and imitating fish are conspecific and close in size. An innate optomotor reaction (following reflex) ensures that fish maintain a single school during movements and rapid maneuvering. Schooling coordination is achieved by focusing on the actions of one of the closest partners, and the parallel arrangement of fish is achieved by accurately following the leading partner. Recognition of individuals of their own species, mutual orientation and coordination of actions of fish is facilitated by schooling coloration—spots, stripes and patterns on the body, head and fins, differing in position, size, shape, color, brightness and other details. Important visual landmarks are the contrasting eyes of school partners. Schooling coloration is not present in all schooling fish. In many species, schooling coloration changes as the fish grow and develop.
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