Copepod photobehavior in a simulated natural light environment and its relation to nocturnal vertical migration

To investigate the roles of light in initiating, controlling and directing nocturnal vertical migration, photoresponses of the adult, female copepod Acartia tonsa Dana were measured under simulated natural underwater light conditions using a video system. Copepods were adapted to a range of background light levels and tested with the following stimuli: absolute quantal intensity, absolute change in quantal intensity and relative (%) change in quantal intensity. The stimulus initiating vertical movements was relative change in quantal intensity, while responsiveness was controlled by the level of light adaptation. A. tonsa swam upward in response to decreases. Response with minimal stimulation occurred at an adaptation intensity close to that in the copepod's natural habitat at the time of the migratory ascent (near the bottom of the Newport River estuary, North Carolina, near sunset). Analysis of the angles of upward movement showed that light is not a directional cue. Relative increases in intensity resulted in sinking, with minimal stimulation required at an adaptation intensity close to that in the field when the migratory descent occurs near sunrise. These results offer a reasonable explanation of how light cues may shape nocturnal vertical migratory patterns.     

Circadian rhythm of swimming activity in juvenile pink salmon (Oncorhynchus gorbuscha)

The circadian rhythm of swimming activity and the role of the daily illumination cycle in the synchronization of this rhythm were studied in individual juvenile pink salmon. Sixty eight percent of all fish examined (n=38) were day-active when exposed to a 12 h L:12 h D cycle; the remaining fish were nocturnally active. One half of the fish tested under laboratory conditions of continuous, constant light intensity (LL) and constant temperature showed unambiguously endogenous activity rhythms with circadian periods for up to 10 d. The remaining fish were arrhythmic. Mean period length of the free-running activity rhythms for diurnal fish in LL shortened with constant light intensity increasing from 6 to 600 lx, as predicted by the circadian rule. In contrast, mean free-running period for nocturnal fish did not vary significantly with similarly increasing constant light intensity. Mean swimming speed (activity level) of both diurnal and nocturnal fish increased significantly with increasing light intensity. This is suggestive of a positive photokinetic response. When subjected to a phase-delayed LD cycle, the fish resynchronized their daily rhythms of activity with this new LD cycle after only one transient cycle in most instances. Hence, the timing of the daily activity rhythms appeared to occur through the direct masking action of the illumination cycle on activity, rather than through entrainment of an endogenous circadian system.     

Diurnal and nocturnal feeding rate in Kentish plovers <Emphasis Type="Italic">Charadrius alexandrinus</Emphasis> on an intertidal flat as recorded by telescopic video systems

To elucidate the effects of light, site, temperature, time after emersion, and wind speed on foraging attempt rate (FAR) (attempts/unit time) and feeding success (FS) (captures/attempts) in the obligate visual foraging shorebird, Kentish plovers Charadrius alexandrinus, field observations were performed at a sandflat in Tokyo Bay, using a visible-light telescope camcorder during the day and a thermal infrared telescope camcorder at night. The re-analysis capability and frame-step replay of highly magnified video-images can contribute to accurate measurement of feeding behaviour over conventional focal observation techniques. Kentish plovers increased both FAR and FS in areas of high prey (polychaetes and crabs) density, resulting in a synergistic increase in feeding rate (captures/unit time). In areas of high prey abundance, FAR was higher at lower wind speed. FS increased with increasing time after emersion. Increasing temperatures induced a positive effect on FAR but a negative effect on FS. The effect of light on FAR was not observed; however, time-to-defecation occurrence was 2.2-fold shorter at night than during the day, indicating that feeding rate and FS are higher at night. These results are attributable to an increase in availability of cues due to higher nocturnal activity in polychaetes. Since available foraging time (emersion time) at night was 1.7-fold longer than during the day in the present study period, Kentish plovers could capture 3.7-fold (2.2 × 1.7) more prey at night than during the day. Results from this study indicate that nocturnal feeding in overwintering plovers is not a compensation but a major foraging activity to meet their energy requirements. An erratum to this article can be found at     

Diurnal and nocturnal feeding rate in Kentish plovers <Emphasis Type="Italic">Charadrius alexandrinus</Emphasis> on an intertidal flat as recorded by telescopic video systems

To elucidate the effects of light, site, temperature, time after emersion, and wind speed on foraging attempt rate (FAR) (attempts/unit time) and feeding success (FS) (captures/attempts) in the obligate visual foraging shorebird, Kentish plovers Charadrius alexandrinus, field observations were performed at a sandflat in Tokyo Bay, using a visible-light telescope camcorder during the day and a thermal infrared telescope camcorder at night. The re-analysis capability and frame-step replay of highly magnified video-images can contribute to accurate measurement of feeding behaviour over conventional focal observation techniques. Kentish plovers increased both FAR and FS in areas of high prey (polychaetes and crabs) density, resulting in a synergistic increase in feeding rate (captures/unit time). In areas of high prey abundance, FAR was higher at lower wind speed. FS increased with increasing time after emersion. Increasing temperatures induced a positive effect on FAR but a negative effect on FS. The effect of light on FAR was not observed; however, time-to-defecation occurrence was 2.2-fold shorter at night than during the day, indicating that feeding rate and FS are higher at night. These results are attributable to an increase in availability of cues due to higher nocturnal activity in polychaetes. Since available foraging time (emersion time) at night was 1.7-fold longer than during the day in the present study period, Kentish plovers could capture 3.7-fold (2.2 × 1.7) more prey at night than during the day. Results from this study indicate that nocturnal feeding in overwintering plovers is not a compensation but a major foraging activity to meet their energy requirements.     

Settlement times of blue crab (Callinectes sapidus) megalopae during flood-tide transport

Settlement by blue crab (Callinectes sapidus Rathbun) megalopae on artificial settlement substrates was monitored relative to tidal currents throughout ten nights from July to September 1997 in which the phase relationship between tides and the light dark cycle differed. Most megalopae were in intermolt, and the total number settling to collectors sampled at hourly intervals was greater than totals on collectors immersed all night. Maximum settlement occurred at slack water before ebb tide (SBE), with a smaller peak at slack water before flood tide (SBF). These results support the hypothesis that during flood-tide transport (FTT) blue crab megalopae remain swimming during flood tide at night in response to water turbulence and settle in response to the decline in turbulence occurring near SBE. Settlement peaks near SBF can be explained by a behavioral response of megalopae to increasing salinity at the beginning of flood tide, which results in an ascent response lasting only a few minutes. Depth maintenance in the water column is not maintained at SBF because of low water turbulence. Since light inhibits swimming and upward movement into the water column, settlement, and, presumably, transport were reduced when SBE occurred near the times of sunrise and sunset. Collectively, these results suggest that the phase relationship between the tide and light: dark cycles affects FTT, the timing of settlement, and behaviors associated with habitat selection. Published online: 9 August 2002     

Light‐induced changes in rabbit pineal gland n‐acetyltransferase activity

N‐acetyltransferase (NAT) activity was determined in different ages of New Zealand White rabbit pineal gland using 2‐aminofluorene and p‐aminobenzoic acid as substrates and it was assayed by high pressure liquid chromatography. Rabbits of different ages were either sacrificed during the light phase, exposed to darkness or light for 1 min during the dark phase of the lighting cycle, returned to their cages in darkness for 30 min and then sacrificed. Pineal gland NAT activity in animal nocturnally exposed to 1 min of light was inhibited in animals 1 ‐day‐old of age. Nocturnal light exposure did not inhibit enzyme activity in 1‐day‐old rabbit, even though these animal displayed clear light : dark differences in pineal gland NAT activity. Nocturnal light exposure also did not inhibit night time levels of NAT activity in 1‐day‐old animals who had been bilaterally enucleated. The result suggested that this effect is retinally mediated. Pre‐treatment of 1‐day‐old and 60‐day‐old animals with the isoproterenol (beta‐noradrenoreceptor agonist drug), prevented the nocturnal light‐induced inhibition of NAT activity. The different sensitivity of 60‐day‐old and 1‐day‐old animals to different illuminances or durations of nocturnal light exposure, was that the duration or intensity of light exposure was enable to inhibit nocturnal NAT activity. The NAT activity was at least 3.2‐ to 4.6‐fold greater in 1‐day‐old rabbits compared to 60‐day‐old rabbits. Kinetic constants for arylamine NAT activity in pineal gland from rabbits were determined. K_m and F_max values for 2‐aminofluorene were 2.6‐fold higher for light exposure than for no light exposure rabbits. This is the first demonstration of the retina‐pineal gland pathway appears light‐induced changes in pineal glands of animals in 1‐day‐old of ages or older; but this pathway does not function in 60‐day‐old rabbits like manner in 1‐day‐old rabbits.     

On the move around the clock: correlates and determinants of cathemeral activity in wild redfronted lemurs (<Emphasis Type="Italic">Eulemur fulvus rufus</Emphasis>)

Whether animals are active at night or during the day has profound consequences for many aspects of their behavioral ecology. Because of ecological and physiological trade-offs, most animals, including primates, are either strictly nocturnal or diurnal. However, a few primate species exhibit cathemeral activity, i.e., their activity is irregularly distributed throughout the 24-h cycle. Details and determinants of this unusual activity pattern are poorly understood because long-term 24-h observations are not feasible in the field. We therefore used small data loggers to record the activity of cathemeral redfronted lemurs (Eulemur fulvus rufus) from several neighboring groups quantitatively and continuously over a complete annual cycle in order to evaluate various proposed proximate and ultimate determinants of cathemeral activity. Activity data were examined for variation as a function of ambient temperature, time of day, lunar phase, and season. We found that cathemeral activity occurred year-round and that, on average, 3.5 times more activity occurred during the day. Total and diurnal activity increased during the long days of the austral summer. Nocturnal activity increased during the longer nights of the cool dry season. Irrespective of season, lunar phase had a significant effect on the distribution of activity across the 24-h cycle, with most nocturnal activity recorded during parts of the night with greatest brightness. These data indicate that light availability is the primary proximate determinant for the patterning of cathemeral activity. Several lines of evidence suggest that cathemerality in lemurs has evolved from nocturnal ancestors and that it represents a transitory state on the way to the diurnal niche.Communicated by F. Trillmich     

Moon orientation in sandhoppers: effects of lighting treatments on the persistence of orientation ability

The moon orientation rhythm persists in sandhoppers removed from environmental entrainments relative to moon periodicity. In order to investigate the underlying mechanism, the crustacean amphipods Talorchestia capensis and Talitrus saltator were collected at new moon and exposed in the laboratory to an artificial light during the day, but to a variety of lighting treatments during the night: (1) continuous dark, (2) dim light in phase with the natural moon, (3) continuous dim light. The animals were tested under the moon 9–21 days later by using a new type of arena. The ability to orientate in a direction perpendicular to the shore persisted in sandhoppers under treatments 1 and 2, but not under treatment 3. A disturbance due to a phototactic tendency in Talorchestia was also observed in animals captured on the day of the test when exposed to unnatural lighting, but not when they were kept under natural light. The present findings show that the timing mechanism allowing compensation for changes in the moon’s position also persists in animals that have been long removed from entraining factors. The dependence of this orientation ability on nocturnal lighting and the disturbing effect of sudden changes in light intensity support the idea that the phasal lighting of the moon, and perhaps sunrise and sunset act as resetting factors for the moon’s orientation rhythm.     

Nocturnal feeding migrations of Nerita plicata,N. undata and N. textilis (Prosobranchia: Neritacea) on the rocky shores at Mkomani,Mombasa, Kenya

The vertical zonation of the three common rocky shore neritids at Mkomani, Mombasa, Kenya, Nerita plicata Linnaeus, N. undata Linnaeus, and N. textilis Dillwyn, as a function of feeding migrations and of size, was studied from 28 February to 24 March 1983. These snails perform feeding migrations at night starting at around mid-ebb tide and return to their resting positions with the flood tide. They remain in their resting positions throughout the day until the next nocturnal ebb tide. The direction of migration is sizerelated, with the larger snails of each species moving in the opposite vertical direction to the smaller ones, so that the populations as a whole exhibit no statistically significant net vertical displacement. The larger individuals of two of the species, N. plicata and N. undata, invariably move downwards to their feeding levels, while the smaller individuals move upwards; the larger individuals of N. textilis display a different pattern of migration, moving downwards on and around spring-tide days and upwards on and around neap tide days, while the smaller individuals move in the opposite directions. N. textilis rest above their feeding level around spring tides, and below that level around neap tides. It is demonstrated how these nocturnal migratory feeding rhythms are integrated into the spring-neap and seasonal cycles of the snails' daytime resting positions. The adaptive significance of these migrations is also discussed.     

A predator’s body coloration enhances its foraging profitability by day and night

Few predators forage by both day and night. It remains unknown, however, how the costs and benefits of foraging or signaling are partitioned in animals that forage at all times. The orb-web spider Cyrtophora moluccensis is brightly colored and forages by day and night. We determined the benefits reaped when it forages by both day and night by estimating the biomass of prey caught in their webs. Additionally, we quantified whether the spider’s presence influences the number of prey caught by day and night and whether its colorful body is visible to diurnal and/or nocturnal insects using diurnal and nocturnal insect vision models. We found that approximately five times the biomass of prey was caught in C. moluccensis’ webs by night than by day. Hemipterans, hymenopterans, and dipterans were predominantly caught by day, while lepidopterans (moths) were predominately caught by night. Accordingly, we concluded that foraging by night is more profitable than foraging by day. We predicted that other benefits, for example, energetic advantages or enhanced fecundity, may promote its daytime activity. Foraging success was greater by day and night when the spider was present in the web than when the spider was absent. We also found that parts of the spider’s body were conspicuous to diurnal and nocturnal insects, possibly through different visual channels. The colorful body of C. moluccensis, accordingly, appears to influence its foraging success by attracting prey during both the day and night.     

Diel vertical migration and feeding patterns of Mysis mixta (Crustacea,Mysidacea) in the Baltic Sea

The extent of the nocturnal vertical migration of Mysis mixta Lilljeborg varied between early July and late October (of 1985 and 1986) in a coastal area of the Baltic Sea. Migration was more restricted in early July and late October. Seasonal changes in surface light levels and transparency were sufficient to explain the observed differences. Mysids avoided light levels above 10^-4 lux throughout the study period. Smaller juveniles migrated higher up than larger juveniles and adults. A two-layered distribution with part of the population close to the bottom was observed at night. Zooplankton were more abundant in water layers above the main concentration of mysids. M. mixta fed on phytoplankton, detritus, copepods, cladocerans, rotifers and tintinnids. Diel changes in gut fluorescence indicated a higher intake of phytoplankton at night, but levels were low compared to primarily herbivorous zooplankton. Comparisons of stomach contents of mysids caught at the bottom in the evening and in the water column at night showed a higher ingestion of zooplankton at night and of detritus during the day. Mysids caught at the bottom at night had an intermediate diet. Copepods and cladocerans constituted between 90 and 100% of ingested material by weight in all mysid groups.     

浙江天台山茶树光合日变化及光响应

自然条件下使用LCA－4型便携式光合测定系统研究了栽培天天台山主峰华顶山的茶树连体叶片的光合日变化及光响应。结果表明：在初夏晴天,上年越冬叶片（下称二年生叶）的光合速率,表观量子效率,羧化效率和饱和光强比展叶红16d的一年生叶高,CO2补偿点和光补偿点比一年生叶低,两者的净光合速率日进程曲线均为“双峰”型,午间胞间CO2浓度上升表明,此时净光合速率下降主要受非气孔限制因素的影响。一年生叶蒸腾速率高于二年生叶,它们日进程曲线为单峰河,午间最高,而气孔阻力进间最低。不同时段作光响应和CO2响应试验表明,上午茶树的表观量子效率,饱和光强和羧化效率较高,光补偿点和CO2补偿点较低。图3表4参26     

Influences of moonlight,ambient temperature,and food availability on the diurnal and nocturnal activity of owl monkeys (<Emphasis Type="Italic">Aotus azarai</Emphasis>)

The study of activity rhythms, their potential zeitgebers and masking factors among free-ranging primates has received relatively little attention in the past. Most primates are diurnal, a few of them nocturnal, and even fewer are cathemeral. Owl monkeys (Aotus azarai azarai) regularly show diurnal, as well as nocturnal, activity in the Argentinean and Paraguayan Chaco. The goal of this study was to examine how changes in activity patterns in owl monkeys of Formosa, Argentina are related to daily, monthly, and seasonal changes in temperature, light and food availability . During 1 year, I collected activity data from five groups followed continuously from dawn to dusk, dusk to dawn or uninterruptedly during 24 or 36 h for approximately 1,500 h. I kept hourly and daily records of temperature and light conditions, and I gathered monthly information on the density, distribution and abundance of food resources available to the monkeys. I found that the area of study is highly seasonal, and characterized by significant fluctuations in rainfall, temperature, photoperiod, and food availability. Owl monkeys had on average 5 h of activity during the day and 4 h during the night. The amount of diurnal activity remained fairly constant through the year despite seasonal changes in exogenous factors. Owl monkeys did not show changes in their activity patterns that could be attributed to changes in food availability. Nocturnal activity increased as the amount of moonlight increased, whereas diurnal activity decreased following a full-moon night. Ambient temperature was a good predictor of activity only when the moon was full. These results argue convincingly for an interaction between ambient temperature and moonlight in determining the observed activity pattern. It is then highly advisable that any evaluation of diurnal activity in cathemeral animals be analyzed controlling for the possible effects of moonlight during the previous night.Communicated by P. Kappeler     

Rhythmic patterns of abundance in small sublittoral crustaceans: variety in the synchrony with day/night and tidal cycles

Temporal fluctuations of abundance (or emergence) in small benthic and planktonic crustaceans were studied in shallow subtidal waters (1.5 to 3.5 m in tide height). The abundances were more or less rhythmic, and showed wide diversity ranging from very clear nocturnal patterns to patterns in sychrony with the tidal cycle alone. These abundance patterns were classified into categories relating to the degree of synchrony with day/night and tidal cycles. Nocturnal patterns were especially strong in benthic crustaceans, which would be inactive during the daytime, being attached to algae and stones or disappearing into rock crevices, and actively swim in the water at night. Mysis larvae also showed a clear nocturnal pattern. Their lifestyle might be similar to that of many benthic animals. Other planktonic crustaceans drifting in the water showed weak nocturnal patterns. In some planktonic crustaceans (e.g., Calanoida), the ratio of abundance in the surface and bottom samples was reversed between day and night. Their pattern might be a manifestation of weak diel vertical movement between day and night. Furthermore, most patterns of zooplankton and benthos were modified in synchrony with tides to various degrees. Small crustaceans may respond to changes of hydrologic variables fluctuating with the tides, which may exogenously produce a weak tidal component in their emergence patterns. Received: 12 January 1998 / Accepted: 29 August 1998     

Influence of fuel load and weather on timing of nocturnal spring migratory departures in European robins, <Emphasis Type="Italic">Erithacus rubecula</Emphasis>

Time of departure and landing of nocturnal migrants are of great importance for understanding migratory strategy used by birds. It allows us to estimate flying time and hence the distance that migrants cover during a single night. In this paper, I studied the temporal schedule of nocturnal departures of European robins during spring migration. The study was done on the Courish Spit on the Baltic Sea in 1998–2003 by retrapping 51 ringed birds in high mist nets during nocturnal migratory departure. Take-offs of individual birds occurred between the first and tenth hour after sunset (median 176 min after sunset). Departure time was not related to fuel stores at arrival and departure, stopover duration and progress of the season. The results suggest that one reason for temporal variation in take-off time was differential response of European robins with high and low motivation to depart to such triggers as air pressure and its trend. If these parameters reach a certain minimum threshold shortly before sunset, robins with a high migratory motivation take off in the beginning of the night. When air pressure or its trend reaches a maximum, it may trigger to take off later during the night birds with lower initial motivation for departure, including those that have low refuelling efficiency. In regulation of timing of take-offs of robins, an important role is also played by their individual endogenous circadian rhythm of activity which is related to the environment in a complex way.     

Photosensitivity of the calanoid copepod Acartia tonsa

The light intensity and spectral sensitivities of the calanoid copepod Acartia tonsa Dana were determined by measuring phototactic responses. Adult females displayed only positive phototaxis. The dark-adapted copepod, which possesses a single naupliar eye, perceived light at intensities as low as 2.8x10¹¹ photons m^-2 s^-1. The action spectrum for positive phototaxis had no clear maxima but rather showed a broad range of greatest sensitivity from 453 to 620 nm. This sensitivity encompassed those wavelengths that are maximally available at the depth where the copepod is found during the day. This spectral overlap, coupled with the finding that the copepod requires light cues for nocturnal vertical miration, suggests that broad spectral sensitivity is an adaptive mechanism to maximize light intensity sensitivity during migration.     

Time stress,predation risk and diurnal–nocturnal foraging trade-offs in larval prey

Insect larvae increase in size with several orders of magnitude throughout development making them more conspicuous to visually hunting predators. This change in predation pressure is likely to impose selection on larval anti-predator behaviour and since the risk of detection is likely to decrease in darkness, the night may offer safer foraging opportunities to large individuals. However, forsaking day foraging reduces development rate and could be extra costly if prey are subjected to seasonal time stress. Here we test if size-dependent risk and time constraints on feeding affect the foraging–predation risk trade-off expressed by the use of the diurnal–nocturnal period. We exposed larvae of one seasonal and one non-seasonal butterfly to different levels of seasonal time stress and time for diurnal–nocturnal feeding by rearing them in two photoperiods. In both species, diurnal foraging ceased at large sizes while nocturnal foraging remained constant or increased, thus larvae showed ontogenetic shifts in behaviour. Short night lengths forced small individuals to take higher risks and forage more during daytime, postponing the shift to strict night foraging to later on in development. In the non-seasonal species, seasonal time stress had a small effect on development and the diurnal–nocturnal foraging mode. In contrast, in the seasonal species, time for pupation and the timing of the foraging shift were strongly affected. We argue that a large part of the observed variation in larval diurnal–nocturnal activity and resulting growth rates is explained by changes in the cost/benefit ratio of foraging mediated by size-dependent predation and time stress.     

Magnetic compass mediates nocturnal homing by the alpine newt, Triturus alpestris

Experiments were carried out to investigate the use of magnetic compass cues in the nocturnal homing orientation of the alpine newt Triturus alpestris. Tests were carried out at a site 9 km to the east–northeast of the breeding pond. Newts were tested at night in an outdoor circular arena that provided an unimpeded view of celestial cues, in one of four symmetrical alignments of an earth-strength magnetic field. In tests carried out under partly cloudy skies newts exhibited homeward magnetic compass orientation. Because the moon was visible in some trials, but obscured by clouds in others, we investigated whether the presence of the moon contributed to the scatter in the distribution of magnetic bearings. When the moon was visible, the distribution of magnetic bearings was more scattered than when the moon was obscured by clouds, although in neither case was the distribution significant due, in part, to the small sample sizes. Moreover, when the moon was visible, newts oriented along a bimodal axis perpendicular to the moon azimuth, suggesting that the presence of the moon may have affected the newts behavior. To provide a more rigorous test of the role of magnetic compass cues when celestial cues were unavailable, nocturnal tests were carried out during the following migratory season under total overcast. In the absence of celestial compass cues, the distribution of magnetic bearings exhibited highly significant orientation in the homeward direction. These findings indicate that newts are able to orient in the homeward direction at night using the magnetic compass as the sole source of directional information. Moon light altered the newts behavior. However, this apparently resulted from the asymmetrical distribution of moon light in the testing arena, rather than the use of an alternative compass.     

Spring phenological responses of marine and freshwater plankton to changing temperature and light conditions

Shifts in the timing and magnitude of the spring plankton bloom in response to climate change have been observed across a wide range of aquatic systems. We used meta-analysis to investigate phenological responses of marine and freshwater plankton communities in mesocosms subjected to experimental manipulations of temperature and light intensity. Systems differed with respect to the dominant mesozooplankton (copepods in seawater and daphnids in freshwater). Higher water temperatures advanced the bloom timing of most functional plankton groups in both marine and freshwater systems. In contrast to timing, responses of bloom magnitudes were more variable among taxa and systems and were influenced by light intensity and trophic interactions. Increased light levels increased the magnitude of the spring peaks of most phytoplankton taxa and of total phytoplankton biomass. Intensified size-selective grazing of copepods in warming scenarios affected phytoplankton size structure and lowered intermediate (20–200?μm)-sized phytoplankton in marine systems. In contrast, plankton peak magnitudes in freshwater systems were unaffected by temperature, but decreased at lower light intensities, suggesting that filter feeding daphnids are sensitive to changes in algal carrying capacity as mediated by light supply. Our analysis confirms the general shift toward earlier blooms at increased temperature in both marine and freshwater systems and supports predictions that effects of climate change on plankton production will vary among sites, depending on resource limitation and species composition.     

Movement and activity patterns of the European lobster, Homarus gammarus, revealed by electromagnetic telemetry

European lobsters, Homarus gammarus (L.), were tracked on an artificial reef in Poole Bay on the south coast of England using an electromagnetic telemetry system which monitored movements between reef units and recorded body movements (pitching and rolling) detected with a tilt switch incorporated into the transmitting tag. Several environmental variables (water temperature, light, hydrostatic pressure, current velocity and direction) were recorded simultaneously by the telemetry system, which was self-contained on the seabed. Movements between units of the artificial reef (excursions outside shelter) were predominantly nocturnal, peaking 1.5 to 3 h after sunset and returning to low levels shortly before dawn. A marked decline in the number of inter-reef unit movements from late summer to winter was related to decreasing water temperature rather than to daytime light level, wave height or tidal range. Activity indicated by the tilt switch was also greater at night, but declined gradually from a peak early in the night to a minimum at around midday, on average, implying a degree of activity within reef units during daylight. As with movements between reef units, activity declined seasonally with decreasing water temperature; in addition, the diel pattern of activity disappeared in winter. Received: 9 February 1998 / Accepted: 24 July 1998