This study describes the egg membrane structures of angelfish (Pterophyllum scalare), morpho-physiological changes during angelfish embryogenesis from activation to hatching under optimal conditions (28°C; pH 6.8), the developing larvae and fry, the effect of alkaline pH on the early developmental stages of the species, the relationship between food item size and fry survival. Egg membranes (thin, transparent, 1.67-2.18 µm thick) are covered by a sticky substance. The amber-coloured angelfish eggs were oval in shape, with average diameters of 1.436 and 1.171 mm, i.e., a mean volume of 1.033 ± 0.095 mm 3 . The survival rate of embryos and larvae kept in water with an elevated, slightly alkaline pH was very low: as few as 2% of the embryos survived, while in the batch kept in optimal water conditions very few eggs died. The first larvae hatched after 1288 h of embryonic development. The newly hatched larvae measured on average 2.60 ± 0.093 mm and had large (0.64 ± 0.077 mm³) yolk sacs. They attached themselves to the substrate with a secretion of thin, viscous threads, which was released from glands situated on the top of the head. The glands vanished on day 5. The 1-day-old larvae showed the first pigment cells on the body and the eyes of the 2-dayolds were already fully pigmented. Between day 4 and 5 of larval life, the larvae began feeding on live food. The 23-day-old fry looked like a miniature versions of the adults. Mortality of the angelfish larvae during their first days after hatching was higher in those fed brine shrimp (Artemia salina) nauplii than those fed protozoans and rotifers.En este trabajo se ha descrito la estructura de las túnicas ovulares del escalar o pez ángel (Pterophyllum scalare), las modificaciones morfo-fisiológicas que transcurren durante la embriogénesis del escalar desde el momento de activización para el desove en condiciones ambientales óptimas (28ºC y pH 6,8) y, se han caracterizado las larvas y los alevines desarrollándose. Adicionalmente, se ha estudiado el efecto del pH básico del agua sobre los primeros estadios de desarrollo y la dependencia entre la cantidad de alimento y la supervivencia de los alevines. Las túnicas ovulares del escalar son finas (1,67-2,18 mm), transparentes, cubiertas de una sustancia viscosa. Los huevos de color ámbar tienen forma ovalada de diámetros medios 1,436 y 1,171 mm y de un volumen medio de 1,033 ± 0,095 mm 3 . El porcentaje de supervivencia de embriones y de larvas en agua con pH aumentado, ligeramente básico, fue muy bajo, ya que sólo el 2% de los embriones sobrevivió, mientras que en agua de parámetros óptimos sólo algunos huevos palidecieron. Las larvas recién salidas del huevo medían 2,60 ± 0,093 mm por término medio y poseían grandes (0,64 ± 0,077 mm³) sacos vitelinos. Se han adherido al substrato mediante una secreción en forma de filamentos finos y viscosos. Esta secreción se ha desprendido de las glándulas ubicadas en la cumbre de la cabeza. Las glándulas desaparecieron al 5º día de vida de las larvas. Las larvas de un día poseían ya l...

Magnetoreception is the ability of organisms to perceive magnetic fields in the surrounding environment and changes in its properties such as field direction, intensity and gradient, where the effect on organisms can manifest as an array of reactions. As the magnetic sense is found in many taxa, both evolutionarily young and old, it can be assumed that magnetoreception came into existence as one of the first sensory systems. Many studies on the effect of magnetic fields on fishes have considered both fishes that migrate for long distances and those that are more or less sedentary. Research has focused on tracing the perception of the geomagnetic field by fishes and understanding magnetic fields that are smaller and larger than the ambient Earth's geomagnetic field. The question of the effect of magnetic fields of values higher than the Earth's is gaining importance with the increasing effect of anthropogenic magnetic and electromagnetic fields in aquatic ecosystems. This review draws together the results of studies on the effect and reception of natural and human‐generated magnetic fields on fishes at various stages of ontogeny, chronologically arranged from gametes, through embryonic development, embryonic and larval motor function, directional reactions of embryos and larvae, orientation of fishes, to the mechanisms of magnetic field reception. The present state of knowledge indicates a common nature of effect on various ontogenetic stages of fishes. However, understanding of the mechanisms of magnetic sense in fishes and its relevance for ecological outcomes highlights that further progress requires more detailed research.

Background. The stinging catfish, Heteropneustes fossilis (Bloch, 1794), has recently raised interest among fish farmers, ornamental fish keepers, and pathologists. Its natural populations are threatened due to habitat loss and high fishing pressure. A number of factors may influence the reproductive success of this. The aim of this study was to assess the effect of one of such factors-the water hardness-on the course of the embryogenesis, the structure of the egg shell, the general morphology, and the behaviour of the hatched larvae. Materials and Methods. The fertilised eggs were incubated at a constant temperature of 23 ± 0.2°C in water of different hardness: 0ºGH (soft), 9ºGH (moderately soft), 18ºGH (moderately hard). Egg membranes of activated eggs were viewed under a scanning electron microscope. Also egg membranes strength and egg deformations were determined 1.5 h after fertilisation. Images of eggs and newly hatched larvae, recorded with the observation sets described above, were measured and analysed. Results. The study showed the eggs were surrounded by thin, translucent,~5 µm thick membranes equipped with numerous outgrowths on the external surface and porous on the inner side. The following observations were made within 828 h°(degree-hour) at 23 ± 0.2°C: the highest volume of eggs (1.16 ± 0.092 mm 3 ) was typical for eggs incubated in soft water (0°GH), while the lowest egg volume (0.99 ± 0.113 mm 3 ) was recorded in eggs incubated in moderately soft water (9°GH); yolks spheres were greenish in colour, the embryonic disc and the embryo itself being reddish; the embryo performed diverse movements (quasi-peristaltic, cardiac muscle contractions); after few hours of hatching, the pigments developed resulting in colour appearance; club-like primordial barbels were formed 24 h after hatching; the larvae commenced feeding on day 3 post hatching. Conclusion. Water hardness influences embryonic and larval development and the effects may be diverse. Low water hardness is recommended for egg incubation. However, the situation changed when the egg membrane protection ceased to exist at hatching. Increasing water hardness a few hours prior to the expected hatching time prevents larval deformation.Keywords: stinging catfish, Heteropneustes fossilis, embryogenesis, eggs, embryonic motorics, embryonic morphometry, water hardnessIn addition to the species' value for fish farmers, the stinging catfish-due to some structural and physiological peculiarities-has become interesting also for the ornamental fish trade. Recently, the species has become a study object for pathologists focusing on transmittable zoonoses.The stinging catfish occurs naturally in tropical waters of south Asia, general in bodies of water abundant in vegetation, oxygen-depleted due to decomposition of the plant material, and heated by the sun throughout the water column (Munshi et al. 1976, Pandey 1978, Pethiyagoda 1991, Kahl et al. 1997, Jayaram 1999.In its native habitat, the stinging catfish attains 70 cm of total length. The sexual dimorphi...

Effects of a constant magnetic field on the selection of swimming direction by trout (Salmo trutta L.) larvae and fry were investigated in an experimental facility consisting of chambers equipped with magnets as well as magnet-free chambers placed at the entrance. The experiments showed a close relationship between the direction selected by the larvae and fry and the presence or absence of ferritic magnets generating a constant magnetic field. The results obtained indicate during early ontogenesis, when a newly hatched individual is no longer an embryo but also not a mature form, that the larva is sensitive to a magnetic field. This sensitivity seems to be related to an exteroreception system that develops at that time involving magnetoreceptors containing magnetite particles.

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Summary The objective of the study was to investigate the effects of a static magnetic field on the trout sperm motility parameters (CASA), DNA integrity (comet assay) and morphology (SEM). The fertilisation rate was also assessed. Exposing the trout spermatozoa to a static magnetic field (1 mT, 5 mT, 10 mT) prolonged the sperm motility to 12 days (288 h), while the control sperm remained motile for only 3 days (72 h). Analysis of fragmentation of genetic material in the spermatozoon head revealed no effects from the studied magnetic fields. After a 30‐min exposure of spermatozoa to the magnetic field, the proportion of fertilised eggs was already 12.7% (1 mT) to 15% (10 mT), higher than that in the control variant. After a 24‐h storage in the magnetic field the difference was even larger – <1% of eggs were fertilised in the control; in the magnetic field the proportion of fertilised eggs increased to 11.4 at 3 mT and 22.1 at 1 mT. The results demonstrate that short‐term storage of sperm in a static magnetic field of low intensity has a positive effect on the sperm motility parameters (not causing DNA fragmentation), which in turn improves the effectiveness of fertilisation and hatching. This suggests that this method, after more comprehensive studies, might be used for short‐term storage of trout sperm.

. Effects of the magnetic field on different forms of embryonic locomotor activity of northern pike, Esox lucius L. Acta Ichthyol. Piscat. 34 (2): 193-203.Background. The magnetic field is known to exert its influence on both adult and juvenile fishes, as well as the embryos. The effects are diversified and pertain to, practically all, life aspects of an individual. Another aspect of this issue can be the existing and still growing magnetic pollution of the natural environment. In view of the above we decided to study the embryonic motorics of fish, exemplified by pike, affected by constant-and alternating magnetic fields. Material and methods. Eggs of pike (Esox lucius L.) after period of adaptation (control) were subjected to 30-min action of the magnetic field (constant 4 mT and alternating 15 mT). The inflicted changes in the embryonic motorics were observed under a microscope and recorded on videotape. Results. The following aspects of the embryogenesis were affected:• Quasi-peristalsis (increased angular velocity and the amplitude of ectoplasmic waves-at the stages of blastula end gastrula).• Heart muscle action (in alternating field-short, small acceleration in the heartbeat is followed by a sudden rise of the number of contractions).• Somatic motorics (no visible effect of neither constant-nor alternate field). Conclusions. Three different mechanism of the magnetic field are taken into consideration: a) directly on actomyosin structures in the quasi-peristalsis, b) on automatic centres of the developing heart, and c) on developing structures of the central nervous system (no effect presently observed).

aspects of the embryonic development of ruffe, Gymnocephalus cernuus (L.) under different thermal conditions. Acta Ichthyol. Piscat. 34 (1): 51-72. Background. Ruffe, Gymnocephalus cernuus is a small fish inhabiting vast areas of Europe, Asia, and also North America. Its substantial geographical dispersal range in different climatic zones, in waters of different parameters indicates that it is a particularly plastic species. Recent implementation of new fish-processing machinery has increased dramatically the economical importance of ruffe, which is now treated as a gourmet food. In the wake of the increased interest in this fish we decided to study aspects of its developmental biology. Methods. Fertilised eggs and the developing embryos of ruffe were incubated under five different temperature regimes. They were monitored in two planes, horizontal and vertical, using a light microscope fitted with a digital camera and connected to a computer with a monitor and a VCR. The data collected were processed using MultiScan software. Results. Perivitelline space in ruffe constitutes as much 65% of the egg volume. This facilitates movements of an embryo and enhances processes of gas exchange through mixing of perivitelline fluid. It finally contributes to the distribution of the hatching enzyme on the inner surface of the egg shell. Similarly as in the other percid fish, the lack of integration between the "lipid raft" with the embryonic disc, causes the fall of the disc on the lateral side of the vitelline sphere. This lateral position persists throughout all sequential stages of the embryogenesis. The diversified thermal regimes of the development affect the timing of the embryogenesis, as well as the condition, weight, and the size of newly-hatched ruffe larvae. The highest rate of specimens hatched in good condition-marked by the highest weight and body length-was observed at the optimal temperature range of 16-18°C. Newly-hatched larva is small