Individual Locomotor Activity Research Articles

Cooperation by harvester ant queens brings direct energetic benefits during colony founding

Cooperative behavior by nonrelatives is an evolutionary puzzle, because costs of cooperation are often strong while selective factors favoring cooperative groups remain unclear. In some populations of the seed harvester ant Pogonomyrmex californicus, unrelated queens form groups at colony founding (pleometrosis), whereas in other populations, colonies are initiated by single queens (haplometrosis). We tested the hypothesis that energetic savings of grouping contribute a benefit that may enhance queen success during colony founding and early growth for pleometrotic colonies. We measured metabolic rates and locomotor activity of individuals or pairs of freshly collected, newly mated P. californicus queens from pleometrotic and haplometrotic populations. Population source did not significantly affect metabolic rates, but paired queens from both populations had mass-specific metabolic rates only 86% of those of individual queens. The metabolic effect of grouping was not explained by differences in locomotion. To test whether this degree of energetic saving could be biologically significant, we assessed metabolic rate, body mass, and egg production for pleometrotic pairs at four time points of colony founding and compared the energetic content of stored lipid to energy use up to the point of worker emergence. Metabolic rates dropped over time for queens, as did body mass, and gas exchange switched from continuous to discontinuous, suggestive of metabolic suppression. Total joules required for queens to reach the worker emergence stage were 1.5 times the energy content of stored lipids, consistent with the need for foraging for these queens, and supporting the hypothesis that energetic savings of grouping can be beneficial by extending energetic stores and reducing the need for risky foraging.

Artificial Neural Network (ANN)-Based Pattern Recognition Approach Illustrates a Biphasic Behavioral Effect of Ethanol in Zebrafish: A High-Throughput Method for Animal Locomotor Analysis.

Variations in stress responses between individuals are linked to factors ranging from stress coping styles to the sensitivity of neurotransmitter systems. Many anxiolytic compounds can increase stressor engagement through the modulation of neurotransmitter systems and are used to investigate stress response mechanisms. The effect of such modulation may vary in time depending on concentration or environment, but those effects are hard to dissect because of the slow transition. We investigated the temporal effect of ethanol and found that ethanol-treated individual zebrafish larvae showed altered behavior that is different between drug concentrations and decreases with time. We used an artificial neural network approach with a time-dependent method for analyzing long (90 min) experiments on zebrafish larvae and found that individuals from the 0.5% group begin to show locomotor activity corresponding to the control group starting from the 60th minute. The locomotor activity of individuals from the 2% group after the 80th minute is classified as the activity of individuals from the 1.5% group. Our method shows three clusters of different concentrations in comparison with two clusters, which were obtained with the usage of a statistical approach for analyzing just the speed of fish movements. In addition, we show that such changes are not explained by basic behavior statistics such as speed and are caused by shifts in locomotion patterns.

Exploratory Study on Individual Locomotor Activity in Local Dual-Purpose and Commercial Breeder Pullets.

Improving animal welfare is a prerequisite for the societal acceptance of poultry production. Support for improvements requires practical tools to quantify animal welfare and identify predispositions at the individual level, where possible. In this study, the activities and behavior of dual-purpose chickens (N = 245) and commercial breeders (N = 224) were analyzed. The general locomotor activity (GLA) data were collected using an RFID system over five days with 9-to-14-week-old animals. The results show that the animals of comparable age and stocking density differed from each other in their activity (p ≤ 0.001) according to breed, but no sex differences were observed (p = 0.159). No correlations were found between GLA and plumage condition (p > 0.05). The individual variations within the breeds are presented and discussed on an animal-by-animal level, providing new insights into the individual behavioral variability of chickens. The RFID systems can reliably generate GLA data that help to understand the potential interplay between behavior and animal welfare. The technology is also suitable for creating individual (personality) profiles that can be used for breeding. With a better understanding of the role of activity, husbandry and management practices can be adapted to improve animal welfare.

MOVEMENT ACTIVITY OF ROUND GOBY (NEOGOBIUS MELANOSTOMUS PALLAS, 1814) UNDER THE INFLUENCE OF GEOMAGNETIC FIELDS

Introduction. Behavioral traits of fish are an important manifestation of their response to external stimuli. Locomotor activity reflects the influence of specific factors on fish behavior and the role they play in their lives. Among the least studied factors are fluctuations in the geomagnetic field and their impact on the locomotor activity of the round goby Neogobius melanostomus Pallas, 1814. Aim. The aim of this study was to investigate the influence of fluctuations in the geomagnetic field on the locomotor activity of the commercially important speciesof benthic ichthyofauna, the round goby Neogobius melanostomus Pallas, 1814, in the Black Sea. Methods. The ichthyological material was collected in the coastal waters of the Odesa Bay, ranging from Cape Northern Odesa to Cape Velikyi Fontan, during a scientific research survey conducted by the I.I. Mechnikov Odesa National University (ONU). Laboratory investigations were conducted in the aquarium room of the Aquatic Biology and General Ecology Department of the I.I. Mechnikov Odesa National University. Hourly changes in fish locomotor activity were recorded and analyzed as a result of the study. During the specified observation period, periods of increased geomagnetic activity were identified and compared to periods when this activity was within normal range. An original methodology called “Computer Vision Method” was employed for processing the visual data and tracking of the laboratory animals. Results. The maximum values of geomagnetic activity during the observation period, classified as a first intensity magnetic storm of class G1, were observed on February 3, 2022, from 6 to 12 hours, and on February 4 and 10, 2022, from 15 to 21 hours. When comparing the levels of fish locomotor activity during these time intervals with their activity during the time intervals on days with minimal geomagnetic activity (February 8, 9, and 15, 2022), a decrease in locomotor activity of individuals during magnetic storms of 27% to 42% can be observed. Conclusion. The fluctuations in locomotor activity of the round goby during increased geomagnetic activity up to the level of magnetic storms indicate the ability of this species to perceive magnetic fields and respond to changes in its properties. Specifically, when the Kp-index values corresponded to a first intensity geomagnetic storm (G1), a decrease in fish locomotor activity was observed compared to the activity during the same time intervals on days with minimal geomagnetic activity.

Effects of coumaphos on locomotor activities of different honeybee (Apis mellifera L.) subspecies and ecotypes

The effects of acute sublethal doses of coumaphos, an acaricide used against Varroa destructor infestation in beekeeping, on the locomotor activities of four native honeybee subspecies of Türkiye including two ecotypes (Carniolan honeybee -A. m. carnica, Syrian honeybee -A. m. syriaca, Caucasian honeybee- A. m. caucasica, and Muğla and Yığılca ecotypes of Anatolian honeybee A. m. anatoliaca) were investigated using an individual locomotor activity monitoring system. Analysis of locomotor activity data in the first 12-h, last 12-h, and 24-h time periods showed that bees from caucasica and carnica subspecies were not affected by coumaphos at all three acute doses (1, 2, and 5 μg coumaphos in 10 μl sucrose syrup for each bee). In contrast, bees from A. m. syriaca subspecies showed significantly elevated locomotor activity levels at 2 and 5 μg coumaphos doses within the first 12 h. Bees from both Muğla and Yığılca ecotypes of anatoliaca subspecies also showed elevated locomotor activity levels at 5 μg coumaphos dose but the magnitude of increase was lower in these ecotypes compared to that seen in syriaca subspecies in the first 12-h period. In general, increasing doses of coumaphos resulted in increased locomotor activity (locomotor activity), with differences in sensitivity across honeybee populations. Possible mechanisms underlying this variance and suggestions for further studies are discussed.

Seasonal Changes in Locomotor Activity Patterns of Wild Social Natal Mole-Rats (Cryptomys hottentotus natalensis)

Differences in individual locomotor activity patterns may be linked to a number of ecological factors, such as changes in ambient temperature or photoperiod. Observations on subterranean mammals suggest that they exhibit diel rhythms despite the lack of visual cues in their underground burrows, but it is unknown how seasonality and individual characteristics affect their activity. In this study we use RFID technology to monitor daily activity patterns of wild, social Natal mole-rats (Cryptomys hottentotus natalensis) during the summer and winter to investigate how their activity varies with season and whether their activity depends on individual characteristics such as body mass, sex and reproductive status. We found that in winter, individuals were more active during the time with the highest soil temperatures, whereas in summer, they showed a bimodal activity pattern during early morning and late afternoon coinciding with cooler soil temperatures. Individual characteristics, including reproductive status, did not affect general activity indicating that reproductive and non-reproductive individuals contribute equally to cooperative behaviors. We suggest that the activity patterns may be a behavioral adaptation to avoid extreme burrow temperatures and a mechanism to maintain a stable core body temperature. We highlight the advantages of RFID technology to study wild small mammal movements.

Locomotor behavior of Neocaridina palmata: a study with leachates from UV-weathered microplastics.

Weathering of plastics leads to the formation of increasingly smaller particles with the release of chemical compounds. The latter occurs with currently unknown environmental impacts. Leachate-induced effects of weathered microplastics (MPs) are therefore of increasing concern. To investigate the toxicity of the chemical mixtures from such plastics, we exposed the freshwater shrimp Neocaridina palmata to enriched leachates from unweathered and artificially weathered (UV-A/B light) MPs (≤1 mm) from recycled low-density polyethylene (LDPE-R) pellets and from a biodegradable, not fully bio-based starch blend (SB) foil. We analyzed the individual locomotor activity (moved distance and frozen events) on day 1, 3, 7 and 14 of exposure to five leachate concentrations equivalent to 0.40–15.6 g MPs L−1, representing the upper scale of MPs that have been found in the environment. The median moved distance did not change as a function of concentration, except for the unweathered SB treatment on day 14 that indicated hyperactivity with increasing concentrations. Significant impacts were solely detected for few concentrations and exposure days. Generally, no consistent trend was observed across the experiments. We further assessed the baseline toxicity of the samples in the Microtox assay and detected high bioluminescence inhibitions of the bacterium Aliivibrio fischeri. This study demonstrates that neither the recycled nor the biodegradable material are without impacts on test parameters and therefore cannot be seen as safe alternative for conventional plastics regarding the toxicity. However, the observed in vitro toxicity did not result in substantial effects on the behavior of shrimps. Overall, we assume that the two endpoints examined in the atyid shrimp N. palmata were not sensitive to chemicals leaching from plastics or that effects on the in vivo level affect other toxic endpoints which were not considered in this study.

Effect of constant and fluctuating temperature on the circadian foraging rhythm of the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae).

Understanding circadian foraging rhythms activity of the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae) foragers at different temperatures is an important step towards developing control measures in Integrated Pest Management (IPM) programs. In this study, the circadian foraging rhythm activities of S. invicta foragersat different temperature were investigated under laboratory and field conditions. Results indicated that the foraging activity increased after sunrise, and maximum foraging occurred at 14:00 (foraging rate was 69.22 ± 0.57 and 72.58 ± 1.15 foragers/min in the first and second year, respectively) in the tea fields of Guangzhou during autumn. Furthermore, foragers demonstrated circadian rhythms and exhibited a unimodal after 24 h. A significant correlation was found between foraging activity and temperature. S. invicta colonies were active at moderate soil temperatures (approximately 26.65 °C to 29.24 °C). The preferred temperature of the colonies was 26 °C, followed by 22 °C and 18 °C in the laboratory. The individual S. invicta activity was maximum at 17:00 (18.67 ± 1.66 times /10 min) and minimum at 5:00 (8.33 ± 2.51 times/10 min) at 26 °C. The fluctuating temperature had a significant impact on individual locomotor activity (r = 0.8979, P < 0.01) but did not alter the rhythm activity. Our results demonstrated that temperature might play an important role in circadian foraging rhythms activity of S. invicta. These results may have implications for the development of more effective fire ant management strategies.

Viral infection causes sex-specific changes in fruit fly social aggregation behaviour

Host behavioural changes following infection are common and could be important determinants of host behavioural competence to transmit pathogens. Identifying potential sources of variation in sickness behaviours is therefore central to our understanding of disease transmission. Here, we test how group social aggregation and individual locomotor activity vary between different genotypes of male and female fruit flies (Drosophila melanogaster) following septic infection with Drosophila C virus (DCV). We find genetic-based variation in both locomotor activity and social aggregation, but we did not detect an effect of DCV infection on fly activity or sleep patterns within the initial days following infection. However, DCV infection caused sex-specific effects on social aggregation, as male flies in most genetic backgrounds increased the distance to their nearest neighbour when infected. We discuss possible causes for these differences in the context of individual variation in immunity and their potential consequences for disease transmission.

Voluntary locomotor activity promotes myogenic growth potential in domestic pigs

Self-determined physical activity is an essential behavioural need and can vary considerably between individuals of a given species. Although locomotion is suggested as a prerequisite for adequate function of skeletal muscle, domestic pigs are usually reared under limited space allowance. The aim of our study was to investigate if a different voluntary locomotor activity leads to altered properties in the muscle structure, biochemistry and mRNA expression of selected genes involved in myogenesis and skeletal muscle metabolism. Based on a video tracking method, we assigned pigs to three categories according to their total distances walked over five observed time points: long distance, medium distance, and short distance. The microstructure and biochemistry parameters of the M. semitendinosus were unaffected by the distance categories. However, we found distance-dependent differences in the mRNA expression of the genes encoding growth (IGF2, EGF, MSTN) and transcription factors (MRF4, MYOD). In particular, the IGF2/MSTN ratio appears to be a sensitive indicator, at the molecular level, for the locomotor activity of individuals. Our results indicate that the myogenic growth potential of pigs under standard rearing conditions is triggered by their displayed voluntary locomotor activity, but the covered distances are insufficient to induce adaptive changes at the tissue level.

Rhodnius prolixus and R. robustus (Hemiptera: Reduviidae) nymphs show different locomotor patterns on an automated recording system.

BackgroundCircadian rhythms of triatomines, vectors of the etiological agent Trypanosoma cruzi responsible for Chagas disease, have been extensively studied in adults of the two most epidemiologically relevant vector species, Rhodnius prolixus and Triatoma infestans. However, little attention has been dedicated to the activity patterns in earlier developmental stages, even though triatomine nymphs are equally capable of transmitting T. cruzi to humans. Because circadian rhythms may differ even between closely related species, studies that focus on this behavioral trait can also be used to shed light on the taxonomy of controversial taxa, which becomes especially relevant regarding vector species.MethodsWe compared the daily locomotor activity patterns of second- and third-instar nymphs of Rhodnius prolixus and Rhodnius robustus in order to unveil possible behavioral differences between these cryptic species. Mitochondrial and nuclear markers were sequenced to confirm species identification.ResultsNymphs of both species had a bimodal pattern of locomotion and similar daily activity patterns, but R. prolixus is more active under light/dark cycles and depicts a more pronounced activity rhythm under constant darkness conditions.ConclusionsWe describe the implementation of an often-used automated method for the recording of individual locomotor activity to differentiate sibling species of Rhodnius with distinct epidemiological relevance. The higher levels of activity observed in the nymphs of R. prolixus could potentially contribute to increased vector capacity.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-016-1482-9) contains supplementary material, which is available to authorized users.

Group effect on individual locomotor activity of common toad (Bufo bufo L.) and brown frog (Rana temporaria L.) tadpoles

Different components of group effect on the locomotor activity of single common toad and brown frog tadpoles have been studied by estimating the length of their trajectories, the speed of movement, the number of crossings of experimental field zones, and the number and duration of stops. The results show that toad tadpoles are more active than frog tadpoles. Conspecific metabolites added to the water do not change the pattern of their locomotor activity. Under conditions of visual contact with other tadpoles, locomotor activity of a toad tadpole decreases, whereas that of a frog tadpole increases. Thus, tadpoles of the two amphibian species show species-specific behavioral responses to changes in experimental conditions.

Parametric and nonparametric entrainment of circadian locomotor rhythm in the Japanese honeybee Apis cerana japonica

The circadian rhythm of locomotor activity in the Japanese honeybee Apis cerana japonica was studied to determine the involvement of parametric and/or nonparametric entrainment. The rhythm was entrained to a skeleton photoperiod in which a 1-h first light pulse was imposed in the morning along with a second light pulse in the evening, as well as to a complete photoperiodic regime (LD 12:12). However, the timing of peak activity relative to the lights-off in the evening in the skeleton photoperiod was earlier than that in the complete photoperiod. A single daily light pulse in the evening entrained the rhythm, whereas a daily light pulse in the morning allowed free-running as in constant darkness. The free-running period (τ) of locomotor activity in constant light became longer as the light intensity increased. A Winfree's type I phase response curve of the locomotor activity rhythm was obtained using a single 1-h light pulse. The results suggest that both parametric and nonparametric entrainment are involved in the circadian rhythm of individual locomotor activity in this honeybee.

Tachykinin-related peptides modulate odor perception and locomotor activity in Drosophila

The invertebrate tachykinin-related peptides (TKRPs) constitute a conserved family, structurally related to the mammalian tachykinins, including members such as substance P and neurokinins A and B. Although their expression has been documented in the brains of insects and mammals, their neural functions remain largely unknown, particularly in behavior. Here, we have studied the role of TKRPs in Drosophila. We have analyzed the olfactory perception and the locomotor activity of individuals in which TKRPs are eliminated in the nervous system specifically, by using RNAi constructs to silence gene expression. The perception of specific odorants and concentrations is modified towards a loss of sensitivity, thus resulting in a significant change of the behavioral response towards indifference. In locomotion assays, the TKRP-deficient flies show hyperactivity. We conclude that these peptides are modulators of olfactory perception and locomotion activity in agreement with their abundant expression in the olfactory lobes and central complex. In these brain centers, TKRPs seem to enhance the regulatory inhibition of the neurons in which they are expressed.

Daily locomotor activity and melatonin rhythms in Senegal sole ( Solea senegalensis)

The daily locomotor and melatonin rhythms of the Senegal sole, a benthonic species of increasing interest in aquaculture, are still unknown, despite the fact that such knowledge is of prime importance for optimising its production. The aim of the present research was therefore to investigate the daily rhythms of locomotor activity and melatonin in the Senegal sole. For this purpose, the individual locomotor activity rhythms of fish were registered using a photocell. Plasma and ocular melatonin rhythms were studied in animals reared in circular tanks placed in earth under an LD 12:12 light regime and 16–18 °C temperature range (spring equinox). Blood and eye samples were taken every 3 h during a complete 24-h cycle. The impact of a light pulse in the middle of the dark period (MD) on plasma melatonin was also studied. Locomotor activity was mainly nocturnal, with 84.3% of the total activity occurring during darkness. The levels of plasma melatonin were higher at night (55 pg/ml) than during the day (2 pg/ml), while ocular melatonin levels appeared to be arrhythmic. Both weight and melatonin content were found to be significantly higher in the left eye in relation to the right eye. A light pulse in MD provoked a significant decrease in plasma melatonin levels. In summary, photoperiod is a key factor in synchronising locomotor activity and melatonin rhythms in the Senegal sole, whose nocturnal habits should be taken into account for their rearing by aquaculture.

Organization of the circadian system in the subterranean mole rat, Cryptomys hottentotus (Bathyergidae)

The mole rat, Cryptomys hottentotus (Bathyergidae) is a gregarious subterranean rodent, which shows no entrainment to ambient light–dark cycles. The locomotor activity of individuals or of a whole colony, which shows no circadian rhythmicity. Since the lack of both synchronization to light–dark cycle and an endogenous rhythm of locomotor activity could be related to the organization of the circadian system, we have investigated the neuropeptidergic features of the SCN and IGL, and have used pseudorabies viral tracing methods to identify the visual and circadian pathways in this species. The precise topographic distribution of certain neuropeptide populations in the SCN differs from typical rodent pattern of organization and may be correlated with the apparent absence of light entrainment of activity and lack of endogenous rhythmicity. The IGL is highly reduced in size. This structure can nevertheless be identified by the presence of NPY and CALB positive neurons, as well as by a dense network of SP fibers. Viral tracing using intraocular injection of the PRV-Becker and PRV-Bartha strains, leads to differential infection of neurons in circadian and visual structures. With the Bartha strain, infected neurons are principally observed in the SCN, whereas the Becker strain leads primarily to infection of the dLGN and shows an anatomical regression of visual structures. Transsynaptic retrograde infection of the retina contralateral to the injected eye reveals a morphologically homogeneous population, which resemble to retinohypothalamic ganglion cells described in other mammals.

Odor of specific alcohol solutions selectively alters locomotor activity of alcohol-drinking P rats

Evidence exists that the olfactory property of alcohol affects cerebral processes to influence the pattern of alcohol drinking in an experimental animal. In this study, the level of activity was quantified in the genetically selected alcohol-preferring P rats during exposure to the odor of 4 concentrations of alcohol. A Mini-Mitter ® transmitter was implanted intraperitoneally in each animal to record individual locomotor activity continuously, in terms of counts per unit time. In the first experiment, each rat was exposed between 1730 and 1930 h to the odor of 5%, 15%, 30%, or 60% alcohol placed in 2 petri dishes flanking both sides of each cage. The control condition was identical, except that the petri dishes were empty. In the second experiment, each rat was offered water and its individually preffered solution of alcohol in the drinking tubes during 1730 to 1930 h on alternate days of exposure to the odor of alcohol. During the odor condition alone, the 30% concentration of alcohol increased the activity counts significantly from the control level of 228.8 ± 23.9 to 303.5 ± 28.1 over the first 30 min of the 2-h test period. Plots of activity every 30 min for each alcohol solution, calculated as percent of control, also showed that the odor of 30% alcohol evoked significantly greater activity during the first and fourth intervals, in contrast to the exposure to 5%, 15%, and 60% alcohol, which failed to shift activity beyond the control level. When the P rats were exposed to 30% alcohol 24 h after 2 h of alcohol drinking, their profile of activity was concordant with that prior to alcohol drinking. These results demonstrate that the odor of a specific concentration of alcohol (i.e., 30%) selectively enhances the activity of the P rat. This could reflect an association of the sensory quality of alcohol with an anticipation of drinking. Finally, these experiments provide the first quantitative evidence that an olfactory cue associated with the unique odor of a specific solution of alcohol may determine the nature of the drinking response.

Naloxone reduces social locomotor activity in rats

Naloxone, a nonspecific opioid antagonist, has been found to decrease the activity and social behavior of rats tested in pairs but the effects on individual locomotor activity have been equivocal. In the present study, groups of male Long-Evans hooded rats recieved naloxone (1 or 4 mg/kg, IP) or vehicle alone (isotonic saline) 30 min prior to testing sessions. Individual locomotor activity was measured in two activity boxes (41-cm 3) equipped with two infrared photobeams using daily 30-min testing sessions for 5 consecutive days. Following a 1-week washout period (no testing), activity and social attraction (paired distance and contact) were examined in pairs of rats from each group using daily 15-min testing sessions for 4 consecutive days. Locomotor activity and its habituation were not significantly affected by naloxone in rats tested individually. However, both doses of naloxone significantly reduced paired locomotor activity compared to the control group. Measures of social attraction were not significantly affected by naloxone. The present findings suggest that naloxone does not produce nonspecific depressant effects on activity but rather may antagonize opioid release in situational contexts of high arousal (e.g., social activity) with consequent reduction of activity.

Oviposition Behavior and Locomotor Activity of Homoeosoma nebulellum (Lepidoptera: Pyralidae) Under Laboratory Conditions

Experiment to determine the oviposition behavior and locomotor activity of Homoeosoma nebulellum Denis and Schiffermüller were conducted under laboratory conditions with a photoperiod of 16:8 (L:D). When provided with oviposition sites dusted with sunflower pollen, females began to lay eggs on the day following mating, with a maximum fecundity during the first 2 d. During a single life span, the total fecundity per female ranged from 50 to 337 eggs (167.9 ± 72.2) ( |$\bar x$| ± SD). Most eggs were laid during the first 2 h of the scotophase. Locomotor activity of individuals of both sexes was recorded during the scotophase. Virgin females were active mainly at the beginning and end of this period. The activity of virgin males increased at the end of the scotophase, which coincides with female calling behavior. Mated females without oviposition sites showed a high and sustained level of activity.

RELATION BETWEEN DIEL VERTICAL MIGRATION AND LOCOMOTOR ACTIVITY OF A MARINE HYPERIIDEAN AMPHIPOD, THEMISTO JAPONICA (BOVALLIUS)

Specimens of a hyperiidean amphipod, Themistojaponica (Bovallius), were collected in the Sea of Japan off Wakasa Bay from different layers between 0 and 300 m depth in May 1982. Analysis of the specimens disclosed that: (1) juveniles were distributed in higher layers than adults during both day and night, and (2) both juveniles and adults were distributed in higher layers at night than in the daytime. Locomotor activity of individuals of T. japonica was observed in a laboratory aquarium. My observations indicated that: (1) any individual that had been swimming began sinking downward passively in the water as soon as it stopped its swimming motions, and (2) at night most of the individuals were swimming actively, separated from each other, while in the daytime they remained almost motionless near the bottom, in contact with each other, forming clusters. It is pointed out that the upward and downward migration, a phenomenon on the population level, and the rise and fall in the locomotor activity, a phenomenon on the individual level, are seen to occur simultaneously. This simultaneity is also reported in a fresh-water gammaridean amphipod, Anisogammarus annandalei. A suggested common mechanism which brings about the diel vertical migration of both of these two amphipod species is discussed.