Effect of stocking density on growth and survival of Clarias batrachus (Linn.) larvae and fry during hatchery rearing

Experiments were conducted to determine optimum stocking density for Clarias batrachus larvae and fry during hatchery rearing. The increase in stocking density decreased the total weight, specific growth rate (SGR) and percent weight gain of Clarias larvae during a 13‐day experiment. Survival rate was highest at a stocking density of 1000 m^?2 and lowest at 5000 m^?2. Stocking density did not influence the total biomass production of larvae. Clarias batrachus fry performance was studied during a 28‐day hatchery rearing experiment whereby fry stocked at a density of 100 m^?2 attained the highest total body weight (P < 0.05). The survival rate greatly declined to 59–61% by a density increase to 300 m^?2 and above. Stocking density influenced growth and survival of C. batrachus larvae and fry during hatchery rearing. The best performance was obtained when larvae were stocked at 2000 m^?2; survival was highest with C. batrachus fry stocked at 200 m^?2.     

Magnetic field direction differentially impacts the growth of different cell types

Magnetic resonance imaging (MRI) machines have horizontal or upright static magnetic field (SMF) of 0.1–3 T (Tesla) at sites of patients and operators, but the biological effects of these SMFs still remain elusive. We examined 12 different cell lines, including 5 human solid tumor cell lines, 2 human leukemia cell lines and 4 human non-cancer cell lines, as well as the Chinese hamster ovary cell line. Permanent magnets were used to provide 0.2–1 T SMFs with different magnetic field directions. We found that an upward magnetic field of 0.2–1 T could effectively reduce the cell numbers of all human solid tumor cell lines we tested, but a downward magnetic field mostly had no statistically significant effect. However, the leukemia cells in suspension, which do not have shape-induced anisotropy, were inhibited by both upward and downward magnetic fields. In contrast, the cell numbers of most non-cancer cells were not affected by magnetic fields of all directions. Moreover, the upward magnetic field inhibited GIST-T1 tumor growth in nude mice by 19.3% (p < 0.05) while the downward magnetic field did not produce significant effect. In conclusion, although still lack of mechanistical insights, our results show that different magnetic field directions produce divergent effects on cancer cell numbers as well as tumor growth in mice. This not only verified the safety of SMF exposure related to current MRI machines but also revealed the possible antitumor potential of magnetic field with an upward direction.     

Use of a magnetic compass for Y-axis orientation in premetamorphic newts (Triturus boscai)

Experiments were carried out to investigate whether premetamorphic larvae of Boscas newt (Triturus boscai) are capable of using the geomagnetic field for Y-axis orientation (i.e., orientation toward and away from shore). Larvae were trained outdoor in two different training configurations, using one training tank aligned along the magnetic north–south axis, with shore facing north, and another training tank positioned with its length along the east–west axis, with shore located west. After training, premetamorphic newts were tested in an outdoor circular arena surrounded by a pair of orthogonally aligned cube-surface coils used to alter the alignment of the Earths magnetic field. Each newt was tested only once, in one of four magnetic field alignments: ambient magnetic field (i.e., magnetic north at North), and three altered fields (magnetic north rotated to East, West, South). Distributions of magnetic bearings from tested larvae indicated that they oriented bimodally along the magnetic direction of the trained Y-axis. These findings demonstrate that T. boscai larvae are sensitive to the geomagnetic field and can use it for orienting along a learned Y-axis. This study is the first to provide evidence of Y-axis orientation, accomplished by a magnetic compass, in larval urodeles.     

Blockade of sensory neuron action potentials by a static magnetic field in the 10 mT range

To characterize the inhibitory effect of a static magnetic field, action potentials (AP) were elicited by intracellular application of 1 ms depolarizing current pulses of constant amplitude to the somata of adult mouse dorsal root ganglion neurons in monolayer dissociated cell culture. During the control period, <5% of stimuli failed to elicit AP. During exposure to an ?11 mT static magnetic field at the cell position produced by an array of four permanent center-charged neodymium magnets of alternating polarity (MAG-4A), 66% of stimuli failed to elicit AP. The number of failures was maximal after about 200-250 s in the field and returned gradually to baseline over 400–600 s. A direct or indirect effect on the conformation of AP generating sodium channels could account for these results because (I) failure was preceded often by reduction of maximal rate of rise, an indirect measure of sodium current; (2) recovery was significantly prolonged in more than one-half of neurons that were not stimulated during exposure to the MAG-4A field; and (3) resting membrane potential, input resistance, and chronaxie were unaffected by the field. The effect was diminished or prevented by moving the MAG-4A array along the X or Z axis away from the neuron under study and by increasing the distance between magnets in the XY plane. Reduction of AP firing during exposure to the ?0.1 mT field produced by a MAG-4A array of micromagnets was about the same as that produced by a MAG-4A array of the large magnets above. The ?28 mT field produced at cell position by two magnets of alternating polarity and the ?88 mT field produced by a single magnet had no significant effect on AP firing. These findings suggest that field strength alone cannot account for AP blockade. © 1995 Wiley-Liss, Inc.     

Population interaction: A synchronizer for the persistent rhythmicity of Chaoborus larvae

Fifty chaoborid larvae (Diptera: Chaoboridae) were maintained singly in 200-ml. experimental chambers with lake water and sediments. The larvae, entrained in a reversed light-dark cycle-L D 12: 12 (21–09), were monitored at two-hour intervals to observe whether or not the larvae were in the planktonic state. Results indicated that under constant conditions individual larvae varied in the time of movement out of the sediments and subsequent time spent in the planktonic state. A significant difference was observed when the individual activity patterns (i.e. the number of times an individual was observed to be planktonic during the 12-hour observations period) were summated and compared with the population data previously published by LaRow (1968). However, when larvae whose individual activity patterns were recorded, were pooled in a single experimental chamber the activity pattern then observed was identical to that previously reported. It was concluded, that the persistent, diurnal rhythm observed in Chaoborus is most likely a population rhythm and the interaction of the individuals within the population aids in the synchronization of the rhythm.The observation of larvae maintained singly in experimental chambers also enabled one to determine the actual percentage of the benthic population that leave the sediments and become planktonic each night. During the summer months at least 82.8 percent of the fourth instar larvae leave the sediments and enter the water column.     

Innate preference for magnetic compass direction in the Alpine newt, <Emphasis Type="Italic">Triturus alpestris</Emphasis> (Salamandridae,Urodela)?

Magnetic compass orientation was first discovered for migrating/homing birds in which all individuals of a population or species prefer a predictable magnetic direction during a particular migratory situation. If all other sensory cues are absent, the Earth’s magnetic field may serve as a reference for other orientation mechanisms. It will be demonstrated that alpine newts (Triturus alpestris, Salamandridae) spontaneously align according to the natural or the deviated magnetic field lines of the Earth. They are able to do this in the dark and by apparently seeking to maintain a specific angle with respect to the magnetic field vector. When the horizontal component of the magnetic vector was eliminated, animals became disoriented, and orientation became random. We infer that the animals observed had learned to prefer a particular magnetic direction following environmental/geographical cues. Alternatively, the magnetic directional alignments are innate as, e.g. in migrating birds, but these may be modified/altered according to season, age, hormonal status, and environmental factors such as “landmarks”, light-, sound-, or olfactory cues. Numerous observations of the aligning showed that the preference for a certain magnetic compass direction/axis was not only individual but also specific for the population-subgroups tested. Specimens roughly preferred magnetic directions close to east or west. However, the larvae were able to learn to align to obviously attractive hiding spots (tubes) that were provided in a direction that deviated with respect to the first magnetic preference. The new conditioned alignments were, again, referred to magnetically by the animals and remained stable, even if the hiding tubes were absent. Animals preferred that direction until, eventually, a new directional cue became attractive.     

Chronic static magnetic field exposure alters microvessel enlargement resulting from surgical intervention.

Magnetic field therapy has recently become a widely used complementary/alternative medicine for the treatment of vascular, as well as other musculoskeletal pathologies, including soft tissue injuries. Recent studies in our laboratory and others have suggested that acute static magnetic field (SMF) exposure can have a modulatory influence on the microvasculature, acting to normalize vascular function; however, the effect of chronic SMF exposure has not been investigated. This study aimed to measure, for the first time, the adaptive microvascular response to a chronic 7-day continuous magnetic field exposure. Murine dorsal skinfold chambers were applied on day 0, and neodymium static magnets (or size and weight-matched shams) were affixed to the chambers at day 0, where they remained until day 7. Separate analysis of arteriolar and venular diameters revealed that chronic SMF application significantly abrogated the luminal diameter expansion observed in sham-treated networks. Magnet-treated venular diameters were significantly reduced at day 4 and day 7 (34.3 and 54.4%, respectively) compared with sham-treated vessels. Arteriolar diameters were also significantly reduced by magnet treatment at day 7 (50%), but not significantly at day 4 (31.6%), although the same trend was evident. Venular functional length density was also significantly reduced (60%) by chronic field application. These results suggest that chronic SMF exposure can alter the adaptive microvascular remodeling response to mechanical injury, thus supporting the further study of chronic application of SMFs for the treatment of vascular pathologies involving the dysregulation of microvascular structure.     

Cell clarification and size separation using continuous countercurrent magnetophoresis

Nonmagnetic microparticles (e.g., cells, polymer beads) immersed in a magnetic fluid (ferrofluid) under a nonuniform magnetic field experience a magnetophoretic force in the direction of decreasing magnetic field strength. This phenomenon was exploited in the development of a continuous magnetophoretic countercurrent separation for the removal and concentration of micron-sized particles from aqueous suspensions, and in particular as a viable approach for cell clarification of raw fermentation broth. A magnetic fluid is added to the cell suspension, the mixture is introduced to the magnetic separator, which consists of an open flow tube passing between pairs of magnets that move in a direction counter to the flow of the suspension. The cells are pushed ahead of the magnet pairs owing to the magnetophoretic forces acting on them, collected in a tube upstream of the feed injection point, and removed as a concentrated suspension for further treatment.     

Movement of dissolved oxygen in a constant magnetic field

Some regularities of the movement of liquid-solved oxygen in the constant magnetic field have been studied. Redistribution and specific dynamics of the changes of pO2 in a vessel with a physiological solution under the effect of the magnetic field are shown. The data obtained make us to believe that when interpreting the results of biological experiments applying magnets movement of oxygen in the magnetic field should be taken into account as well as possible change of metabolic processes in this case.     

Dosimetry in magnetic fields with dedicated MR-compatible ionization chambers

MR-integrated radiotherapy requires suitable dosimetry detectors to be used in magnetic fields. This study investigates the feasibility of using dedicated MR-compatible ionization chambers at MR-integrated radiotherapy devices. MR-compatible ionization chambers (Exradin A19MR, A1SLMR, A26MR, A28MR) were precisely modeled and their relative response in a 6MV treatment beam in the presence of a magnetic field was simulated using EGSnrc. Monte Carlo simulations were carried out with the magnetic field in three orientations: the magnetic field aligned perpendicular to the chamber and beam axis (transverse orientation), the magnetic field parallel to the chamber as well as parallel to the beam axis. Monte Carlo simulation results were validated with measurements using an electromagnet with magnetic field strength upto 1.1 T with the chambers in transverse orientation. The measurements and simulation results were in good agreement, except for the A26MR ionization chamber in transverse orientation. The maximum increase in response of the ionization chambers observed was 8.6% for the transverse orientation. No appreciable change in chamber response due to the magnetic field was observed for the magnetic field parallel to the ionization chamber and parallel to the photon beam.Polarity and recombination correction factor were experimentally investigated in the transverse orientation. The polarity effect and recombination effect were not altered by a magnetic field.This study further investigates the response of the ionization chambers as a function of the chambers’ rotation around their longitudinal axis. A variation in response was observed when the chamber was not rotationally symmetric, which was independent of the magnetic field.     

Migratory Orientation of Blackcaps (Sylvia atricapilla): Population-specific Shifts of Direction during the Autumn

The population-specific orientation of two groups of blackcaps (Sylvia atricapilla), one from southwest Germany, the other from easternmost Austria, was studied outdoors in Emlen funnels. We investigated whether a seasonal shift in the migratory direction — as expected for the Austrian birds from ringing recoveries — occurs under experimental conditions and in a seasonally constant magnetic field. The West German birds, for which no shift was expected, oriented southwest during the entire season. The Austrian birds oriented southeast in October and southsouthwest in November. The clockwise shift by about 60° occurred within a 10-day period. The results indicate that in this species seasonal changes of migratory direction are probably based on an endogenous program, occur without the birds actually migrating and are independent of changes in the magnetic field. Our results provide further evidence that directional shifts in Sylvia warblers may be controlled by a different mechanism than in pied flycatchers (Ficedula hypoleuca).     

Three‐dimensional fate mapping of larval tissues through metamorphosis in the glass sponge Oopsacas minuta

The tissue of glass sponges (Class Hexactinellida) is unique among metazoans in being largely syncytial, a state that arises during early embryogenesis when blastomeres fuse. In addition, hexactinellids are one of only two poriferan groups that already have clearly formed flagellated chambers as larvae. The fate of the larval chambers and of other tissues during metamorphosis is unknown. One species of hexactinellid, Oopsacas minuta, is found in submarine caves in the Mediterranean and is reproductive year round, which facilitates developmental studies; however, describing metamorphosis has been a challenge because the syncytial nature of the tissue makes it difficult to trace the fates using conventional cell tracking markers. We used three‐dimensional models to map the fate of larval tissues of O. minuta through metamorphosis and provide the first detailed account of larval tissue reorganization at metamorphosis of a glass sponge larva. Larvae settle on their anterior swimming pole or on one side. The multiciliated cells that formed a belt around the larva are discarded during the first stage of metamorphosis. We found that larval flagellated chambers are retained throughout metamorphosis and become the kernels of the first pumping chambers of the juvenile sponge. As larvae of O. minuta settle, larval chambers are enlarged by syncytial tissues containing yolk inclusions. Lipid inclusions at the basal attachment site gradually became smaller during the six weeks of our study. In O. minuta, the flagellated chambers that differentiate in the larva become the post‐metamorphic flagellated chambers, which corroborate the view that internalization of these chambers during embryogenesis is a process that resembles gastrulation processes in other animals.     

Aversive responses of captive sandbar sharks Carcharhinus plumbeus to strong magnetic fields

This experimental study focused on the possible deterrent effect of permanent magnets on adult sandbar sharks Carcharhinus plumbeus. Results showed that the presence of a magnetic field significantly reduced the number of approaches of conditioned C. plumbeus towards a target indicating that adult C. plumbeus can be deterred by strong magnetic fields. These data, therefore, confirm that the use of magnetic devices to reduce shark by‐catch is a promising avenue.     

Germination of soybean seeds exposed to the static/alternating magnetic field and algal extract

In the present study, the effect of the static and alternating magnetic field applied individually and in combination with an algal extract on the germination of soybean seeds (Glycine max (L.) Merrill) and chlorophyll content was examined. The exposure time of seeds to the static magnetic field was 3, 6, and 12 min, whereas to the alternating magnetic field was 1, 2.5, and 5 min. The static magnetic field was obtained by means of a permanent magnets system while the alternating magnetic field by means of magnetic coils. Algal extract was produced from a freshwater macroalga—Cladophora glomerata using ultrasound homogenizer. In the germination tests, 10% extract was applied to the paper substrate before sowing. This is the first study that compares the germination of soybean seeds exposed to the static and alternating magnetic field. The best effect on the germination and chlorophyll content in seedlings had synergistic action of the static magnetic field on seeds for 3 min applied together with the extract and alternating magnetic field used for 2.5 min. It is not possible to clearly state which magnetic field better stimulated the germination of seeds, but the chlorophyll content in seedlings was much higher for alternating magnetic field.     

Treefall gaps versus forest understory as environments for a defoliating moth on a tropical forest shrub

Summary The moth Zunacetha annulata (Dioptidae) is a specialist on the understory shrub Hybanthus prunifolius (Violaceae) in the forest of Barro Colorado Island (BCI), Panama. The larvae, which are capable of defoliating entire shrubs, concentrate their attack upon the small minority of H. prunifolius individuals that grow in treefall gaps. Field experiments demonstrated that larval growth rates were 37% higher, and weights at pupation 25% higher, on shrubs in gaps than on shrubs in the understory. In a common environment in the laboratory, growth rates of larvae were 23% higher on foliage taken from shrubs in gaps than on foliage from shrubs in the understory.However, larvae grown in a temperature regime simulating that of gaps did not grow faster than larvae in an understory regime, when the two groups were reared in growth chambers on foliage taken from the same shrubs. In the field, predation appeared higher in gaps: experimental groups of larvae survived at rates of 65% per day in gaps and 78% per day in the understory. Quality of foliage, and not direct effects of the environment, appears to be responsible for the observed pattern of defoliation by this moth.     

Structure and swimming behavior of the larva of Halichondria melanadocia (Porifera: Demospongiae)

Larvae of the sponge Halichondria melanadocia are of the parenchymella type and, during swimming, can change shape rapidly from cigar-like to ovoid. Larvae collected in Hawaii displayed neither qualitative nor quantitative differences in behavior or structure from those collected in Florida. Floridian larvae were examined at 2, 28, 48, and 72 hr after release to assess anatomical changes correlated with duration of the free-swimming period. Although 2 hr larvae were significantly longer than 48 or 72 hr larvae, other differences were not observed. Positively phototactic throughout the free-swimming period, the larvae eventually begin to swim on or near the bottom of dishes, settle temporarily, but can resume swimming before permanent settlement is achieved. The larva is extensively flagellated and a band of long flagella separates the lateral and posterior regions. The epidermis is a tall, simple columnar epithelium composed of highly polarized, monoflagellated cells. The interior contains at least two distinct amoeboid cell types that intermesh with the basal ends of epidermal cells within a loosely defined cavity. No spicules are present. Choanocyte chambers, found within 3 of the 50 larvae that were serially sectioned, varied in size and complexity, but were not associated with canals. This report is the-first of such chambers in Halichondria larvae. Spicules and choanocyte chambers are somatic structures associated with adults, and their appearance in larvae is presumably a consequence of a heterochronic event, most likely acceleration. The evolutionary significance of the occurrence of these traits in Halichondria larvae awaits further developmental analysis and greater phylogenetic resolution.     

Magnetic compass orientation by larval Drosophila melanogaster

We report evidence for magnetic compass orientation by larval Drosophila melanogaster. Groups of larvae were exposed from the time of hatching to directional ultraviolet (365 nm) light emanating from one of four magnetic directions. Larvae were then tested individually on a circular agar plate under diffuse light in one of four magnetic field alignments. The larvae exhibited magnetic compass orientation in a direction opposite that of the light source in training. Evidence for a well-developed magnetic compass in a larval insect that moves over distances of at most a few tens of centimeters has important implications for understanding the adaptive significance of orientation mechanisms like the magnetic compass. Moreover, the development of an assay for studying magnetic compass orientation in larval D. melanogaster will make it possible to use a wide range of molecular genetic techniques to investigate the neurophysiological, biophysical, and molecular mechanisms underlying the magnetic compass.     

The geomagnetic environment in which sea turtle eggs incubate affects subsequent magnetic navigation behaviour of hatchlings

Loggerhead sea turtle hatchlings (Caretta caretta) use regional magnetic fields as open-ocean navigational markers during trans-oceanic migrations. Little is known, however, about the ontogeny of this behaviour. As a first step towards investigating whether the magnetic environment in which hatchlings develop affects subsequent magnetic orientation behaviour, eggs deposited by nesting female loggerheads were permitted to develop in situ either in the natural ambient magnetic field or in a magnetic field distorted by magnets placed around the nest. In orientation experiments, hatchlings that developed in the normal ambient field oriented approximately south when exposed to a field that exists near the northern coast of Portugal, a direction consistent with their migratory route in the northeastern Atlantic. By contrast, hatchlings that developed in a distorted magnetic field had orientation indistinguishable from random when tested in the same north Portugal field. No differences existed between the two groups in orientation assays involving responses to orbital movements of waves or sea-finding, neither of which involves magnetic field perception. These findings, to our knowledge, demonstrate for the first time that the magnetic environment present during early development can influence the magnetic orientation behaviour of a neonatal migratory animal.     

Intracellular magnetophoresis of amyloplasts and induction of root curvature

High-gradient magnetic fields (HGMFs) were used to induce intracellular magnetophoresis of amyloplasts. The HGMFs were generated by placing a small ferromagnetic wedge into a uniform magnetic field or at the gap edge between two permanent magnets. In the vicinity of the tip of the wedge the dynamic factor of the magnetic field, (H²/2), was about 10⁹ Oe² · cm^–1, which subjected the amyloplasts to a force comparable to that of gravity. When roots of 2-d-old seedlings of flax (Linum usitatissimum L.) were positioned vertically and exposed to an HGMF, curvature away from the wedge was transient and lasted approximately 1 h. Average curvature obtained after placing magnets, wedge and seedlings on a 1-rpm clinostat for 2 h was 33 ± 5 degrees. Roots of horizontally placed control seedlings without rotation curved about 47 ± 4 degrees. The time course of curvature and changes in growth rate were similar for gravicurvature and for root curvature induced by HGMFs. Microscopy showed displacement of amyloplasts in vitro and in vivo. Studies with Arabidopsis thaliana (L.) Heynh. showed that the wild type responded to HGMFs but the starchless mutant TC7 did not. The data indicate that a magnetic force can be used to study the gravisensing and response system of roots.Abbreviations HGMF high-gradient magnetic field - emu electromagnetic units - Oe Oersted We thank Dr. John Kiss, Miami University, Ohio for providing the Arabidopsis seeds. This work was supported by NASA grant NAGW-3656