Diel patterns of emergence of crinoids (Echinodermata) from within a reef at Lizard Island,Great Barrier Reef,Australia

Periods of emergence of nine species (88 individuals) of crinoids (Comanthus parvicirrus, Clarkcomanthus albinotus, Comaster multifidus, Oxycomanthus comanthipinna, Oligometra serripinna, Comanthus gisleni, Comanthus wahlbergi, Comatula purpurea, and Oxycomanthus exilis) were monitored at Lizard Island, Queensland, over seven days in March 1983. One species (O. serripinna) was fully exposed and the others partially exposed. Two patterns of emergence were species-specific: emergent both day and night, and emergent only at night. Intraspecific exposure patterns were generally synchronous in six species and asynchronous in three species. The number of species visible was relatively constant during all dives, while at least twice as many individuals were visible at night compared to day. Degree of emergence was determined as the number and length of arms extended into the water column. Except for species fully exposed, degree of crinoid emergence was generally minimal at midday and maximal at twilight and night. Increases in both number of crinoids visible, and the amount of an individual's emergence, corresponded to decreasing light intensities, even during daytime. It is suggested that the patterns of exposure are a response to increased prey abundance at twilight and night, and also a means of avoiding diurnal predators. Gonads on the longest arms of some partially exposed crinoids were not extended into the water, thus protecting the gonads from predators.     

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     

Parasitic flies (Diptera: Phoridae) influence foraging rhythms and caste division of labor in the leaf-cutter ant,Atta cephalotes (Hymenoptera: Formicidae)

Summary Three lines of evidence, including interspecific comparisons, temporal division of foraging between size castes, and experimental manipulations, suggest that the diurnal parasitoid Neodohrniphora curvinervis (Diptera: Phoridae) influences both the caste sizes and numbers of leaf-cutter ants (Atta cephalotes) that leave their underground nests to collect leaves. At Parque Nacional Corcovado in Costa Rica, A. cephalotes was attacked by Neodohrniphora during the daytime, and foraged less during the day than at night; a closely related ant at the same site, A. colombica, had no phorid parasites and foraged exclusively during the day. Most daytime foragers of A. cephalotes were smaller than the lower size threshold for attack by Neodohrniphora, while nocturnal foragers, active when parasitoids were absent, were both larger than this threshold and within the energetically optimal size range for foraging. When I supplied artificial lighting to allow phorids to hunt at A. cephalotes colonies past dusk, ants foraged less than when light was provided but flies were removed. The influence of Neodohrniphora on the foraging activity of A. cephalotes may explain why investigations focusing on abiotic factors have largely failed to discover what drives this ant's daily foraging cycles, and suggests that forager sizes are influenced not only by energetic efficiency, but also by the threat of parasitism.     

Transmission of Pseudoterranova decipiens (Nematoda: Ascaridoidea) via benthic macrofauna to sympatric flatfishes (Hippoglossoides platessoides,Pleuronectes ferrugineus,P. americanus) on Sable Island Bank,Canada

Larval ascaridoid nematodes were surveyed in three flatfish species, Hippoglossoides platessoides, Pleuronectes ferrugineus, and P. americanus, from Sable Island Bank in February and June 1989. Pseudoterranova decipiens (sealworm) abundances differed significantly with both host species, the heaviest levels being found in H. platessoides, and host length, infections generally increasing with length, but not between February and June samples. Moderate numbers of larvae were found in P. ferrugineus while P. americanus were seldom infected. Two third-stage P. decipiens larvae (L₃), 3.2 and 7.0 mm in length, were recovered from the cephalothoraxes of two Mysis mixta in H. platessoides stomachs while five L₃'s (2.8 to 8.7 mm in length) were found among food (polychaetes, cumaceans, tanaids, amphipods) in the stomach of a P. ferrugineus. Levels of P. decipiens infection appear to be directly related to consumption of suprabenthic prey, which commonly occur in the diet of H. platessoides but are exploited less frequently by P. ferrugineus and rarely by P. americanus.     

Vertical migrations in the tree crab Sesarma leptosoma (Decapoda,Grapsidae)

Sesarma leptosoma, somewhat similar to the Atlantic related species, Aratus pisonii, is an East-African mangrove crab which spends its entire life on the roots and branches of mangrove trees (mostly Rhizophora mucronata, Bruguiera gymnorhiza and Ceriops tagal). S. leptosoma never enters the water, nor does it ever venture onto the free mud surface at low tide. Part of the day and night it remains on the lower parts of the mangrove aerial roots, which are often encrusted with a wet algal mat of Bostrichia spp., searching for food and water. Twice a day, from ca. 06:00 to 08:00 hrs in the morning and 16:00 to 18:00 hrs in the afternoon, many of the crabs migrate as far as the leaves at the top of the tree on which they feed. However, they only spend a brief period among the leaves, from ca. 07:00 to 10:00 and 17:00 to 19:00 hrs, after which time they make their way back towards the roots again in two downward migrations. In the morning, the downward migration brings all the crabs back to the roots, but in the evening not all the crabs take part in the downwards migration and some of them pass the whole night in the tree tops. A comparison of the migration time patterns for two different periods of the year (June–July and November) shows that the number of crabs migrating along the tree trunk is modulated by the spring-neap tidal cycle, while the onset of daily migration seems to be controlled mostly by the light level and/or other climatological cues. The adaptive significance of this migratory behaviour is discussed. Observations reported in our study were made in Mida Creek, Dabaso, Kenya in 1991 and 1992.     

The contribution of seagrass beds (<Emphasis Type="Italic">Zostera noltii</Emphasis>) to the function of tidal flats as a juvenile habitat for dominant,mobile epibenthos in the Wadden Sea

Since the substantial loss of subtidal eelgrass (Zostera marina L.) in the 1930s, seagrass beds in the Wadden Sea are limited to the intertidal zone and dominated by Z. noltii Hornem. This study deals with the effect of vegetated tidal flats on quantities of mobile epifauna and proves empirically the function of seagrass canopies as a refuge for marine animals remaining in the intertidal zone at ebb tide. Drop-trap samples were taken in the Sylt-Rømø Bight, a shallow tidal basin in the northern Wadden Sea, on vegetated and unvegetated tidal flats during July and August 2002, and during the entire growth period of Z. noltii from May to September in 2003. The species composition in Z. noltii and bare sand flats showed minor differences since only two isopod species (Idotea baltica and I. chelipes) occurred on Z. noltii flats exclusively. Juvenile shore crabs (Carcinus maenas L.), brown shrimps (Crangon crangon L.) and common gobies (Pomatoschistus microps Krøyer) were also found abundantly on bare sand flats. However, the results showed significantly higher abundances and production of these dominant species on vegetated tidal flats. Additionally, the analyses of faunal size classes indicated higher percentages of small individuals in the seagrass bed during the entire sampling period. Despite drastic diurnal fluctuations of dissolved oxygen at low tide, faunal density in the residual water layer remaining in seagrass canopies at ebb tide was found to be consistently higher than that found in artificially created tide-pool units. Although species composition of mobile epifauna did not basically differ between vegetated and unvegetated tidal flats, Z. noltii beds are considered to contribute quantitatively to the function of tidal flats, as an extended juvenile habitat for some of the most important species of the Wadden Sea food web.Communicated by O. Kinne, Oldendorf/Luhe     

Factors influencing the association patterns of Hippolyte zostericola and Palaemonetes intermedius (Decapoda: Natantia) with seagrasses of the Indian River Lagoon, Florida

The ecological role and the association of Hippolyte zostericola (Smith, 1873) (Decapoda: Natantia) with different seagrass species in the Indian River Lagoon (Florida, USA) were investigated through field sampling and the analysis of diet, size–frequency distributions and laboratory experiments. Gut contents suggested that H. zostericola is a mesograzer which may be important in the transfer of primary production to higher trophic levels. Population size–frequency distributions showed a polymodal pattern variable according to the site. Preference experiments indicated that choice of seagrass species is not influenced by the available surface area of seagrass blades, nor by the presence of epiphytic food. Significant responses of the shrimp to the ambient light field, mediated by the seagrass canopy, were detected. A clear negative phototropism was observed for H. zostericola which may be an adaptive response to improve avoidance of visual predators. Received: 1 February 1998 / Accepted: 21 December 1998     

Representing the effects of diurnal variations in light on primary production on a seasonal time-scale

Rates of production by primary producers are strongly affected by light. Diurnal variations in irradiance produce characteristic diurnal patterns in primary production and respiration. In cases in which the processes of interest occur on a longer time-scale, it is not uncommon to ignore diurnal variations and use mean daily irradiance to force models of primary producers. This simplification reduces the computer processing time and input data requirements of models. However, this approach results in an error which may be significant if irradiance during part of the day is higher than that needed for growth at the maximum possible rate. Here, an alternative approach is presented and applied to a simulation of phytoplankton in a shallow lake. The model uses a semi-analytical calculation of mean daily growth rates that takes into account periods of supersaturation with respect to light to deliver results that conform closely to those of a model using hourly time-steps and irradiance forcing, but with a substantially smaller computational cost.     

Lunar rhythms of vertical migrations coded in otolith microstructure of North Atlantic lanternfishes,genus Hygophum (Myctophidae)

Otoliths of five Hygophum species were examined by means of light and scanning electron microscopy. In otoliths of four species (H. benoiti, H. macrochir, H. reinhardtii and H. taaningi) a strong cyclic pattern of the incremental structure was observed. In the fifth species (H. hygomii) such a pattern did not exist. An analysis of archival data on mesopelagic collections suggested three types of Hygophum spp. migratory behavior in relation to the lunar cycle which corresponded with the otolith microstructure. In H. hygomii only limited influence of moon phase on the uppermost range of night-vertical migration toward the surface was observed. The abundant nighttime occurrence of this species moved from the 0 to 50 m into the 50 to 100 m depth strata at full moon. In H. benoiti, a great part of the population, mainly juveniles, showed a tendency toward cessation of the vertical migrations during the first and fourth quarters of the lunar cycle. H. macrochir and H. taaningi had the strongest correlation of behavior with the lunar cycle. Both species showed arrested vertical migrations at the new moon phase, staying at day depths during the night, i. e., below 400 m. Thus, sequences of clear growth increments in otoliths represented a fast-growth period associated with the night migration to the warm surface layers, while bands without easily distinguishable incremental structure were interpreted as a period of slow growth in deep, cold waters due to limitation of the upward migration range occurring approximately at new moon.     

Evaluating day–night changes in shallow Mediterranean rocky reef fish assemblages by visual census

Ecological information on coastal fish distribution patterns and habitat use can be greatly improved by nocturnal samplings and observations. To this purpose, the structure of a Mediterranean fish assemblage inhabiting the shallow rocky littoral of Linosa Island (Sicily Strait, Italy) was examined by using visual census to detect possible diel variations in species composition and abundance. Day–night fish distribution patterns were investigated by multivariate and univariate analyses. Overall, 42 fish taxa belonging to 19 families were recorded: 35 during the day and 24 during the night. Seventeen species were common to both diurnal and nocturnal assemblages. Within the diurnal assemblage, Chromis chromis was the most represented species (37.2%), followed by Thalassoma pavo (23.2%) and Sparisoma cretense (10.8%). Within the nocturnal assemblage, the most abundant taxon was Atherina spp. (33.9%), followed by Apogon imberbis (26.4%) and Boops boops (11.5%). Our results indicated wide variation in the abundance and species composition during the day and during the night. Multi Dimensional Scaling plot showed a clear-cut separation between the two assemblages and analysis of similarities found significant differences as well. SIMPER analysis revealed that ten species individually contributed by more than 2.5% to the dissimilarity between diurnal and nocturnal assemblages, T. pavo, C. chromis and S. cretense being the first three species in order of decreasing percentage. ANOVA performed on species richness and fish abundance detected significant differences between diurnal and nocturnal assemblages, the latter showing far lower average values for both variables.     

Modeling the effect of daily ration and feeding frequency on growth of redtail shrimp Penaeus penicillatus (Alock) at controlled temperatures

Obtaining optimal growth of redtail shrimp at an economic production cost is essential. An experiment was conducted to evaluate simultaneous effects of daily ration and feeding frequency on growth of redtail shrimp at controlled temperatures. Two series were operated contemporarily, one at 25°C and the other at 30°C. the three daily rations were 5, 15, and 25% of fresh body weight in association with three levels of daily feeding frequency: one, three, and five times. Each series lasted for 80 days. The growth rate and the combined effects were studied on day 40 and day 80 for each series.Response surface analyses showed that, at 25°C, two quadratic polynomial equations could describe the daily growth rates (G₁ and G₂) during growth periods 1 (day 1-day 40) and 2 (day 41-day 80). The optimal daily ration and feeding frequency in terms of maximum growth rate were found at 25°C to be 22.38% of fresh body weight with three feedings/day for growth period 1 and 22.68% of fresh body weight with three feedings/day for growth period 2.At 30°C, two dynamic models were derived for each growth period that included growth optimums of 21.94% and four feedings/day for the first growth period, and 21.27% and four feedings/day for the subsequent growth period.     

The behavior of heterotypic resting schools of juvenile grunts (Pomadasyidae)

Resting schools of juvenile grunts (less than 12 cm length), composed primarily of Haemulon flavolineatum and H. plumieri, were studied from 1972 to 1976 on a series of patch reefs surrounded by seagrass beds on the northeast coast of St. Croix, U.S. Virgin Islands. Juvenile grunts form large inactive multispecies schools in reef areas by day. Repeated censuses over several years reveal that certain parts of the reef, particularly those with large formations of the corals Porites porites and Acropora palmata, are traditional schooling areas. When the fishes reach a size of about 15 cm, the H. plumieri form homotypic schools which roam the patch reefs but are no longer associated with particular formations of coral. In contrast, larger H. flavolineatum are uncommon on the patch reefs and they may move offshore to deeper water. The schools of juvenile grunts migrate some distance away (often 100 to 300 m) into seagrass-covered areas to feed on invertebrates at dusk each day. The routes which are followed into the seagrass beds are precise and persist over years. At dawn the grunts return to the reef on the same routes. Mass marking and release experiments show that juvenile grunts will move over long distances (approximately 3 km) to reach a home reef.     

Diel and seasonal changes in the distribution of fish on a southeast Brazil sandy beach

The effect of diel and seasonal changes in the distribution of fishes on a subtropical sandy beach on the southeastern coast of Brazil were studied. Seine netting was carried out on both seasonal and diel scales between July 1998 and June 1999. A total of 46 fish species was recorded, six being numerically dominant: Anchoa tricolor, Gerres aprion, Harengula clupeola, Atherinella brasiliensis, Mugil liza and Diapterus rhombeus. Seasonal changes in abundance of dominant species were detected. Species dominant in winter were Anchoa tricolor, H. clupeola and Atherinella brasiliensis; in spring, Anchoa tricolor and G. aprion; in summer G. aprion and D. rhombeus; and in autumn M. liza and H. clupeola. Overall, diel patterns did not reveal any significant trends; however, if we consider each season separately, an increase in A. tricolor abundance was recorded during the day in winter/spring, being replaced at night by H. clupeola in winter, and by G. aprion in spring. Increases in number of individuals and biomass at sunset, and decreases during the night were recorded. The winter/spring inshore/offshore movements at diel scale performed by the three most abundant species demonstrate that diel fluctuation acts more at a species-specific level than at a structural one; in summer there was no evidence of diel movements due to a heavy influx of G. aprion and D. rhombeus, which use the area throughout day and night, increasing overall abundance. Seasonal movements seems to be related to ontogenetic change in species, while diel movements in the fish assemblage seem to be more related to physiological requirements, such feeding activity of each particular species, than to physico-chemical conditions.Communicated by O. Kinne, Oldendorf/Luhe     

Food selection by laboratory-reared larvae of the scaled sardine Harengula pensacolae (Pisces,Clupeidae) and the bay anchovy Anchoa mitchilli (Pisces,Engraulidae)

Food selection by laboratory-reared larvae of scaled sardines Harengula pensacolae, and bay anchovies Anchoa mitchilli, was compared. Natural plankton was fed to the larvae during the 22 days following hatching. Food levels in the rearing tanks were maintained at an average of 1,600 to 1,800 potential food organisms per liter. Larvae of both species selected as food copepod nauplii, copepodites, and copepods; initial feeding was on organisms of 50 to 75 body width. Larvae of H. pensacolae averaged 4.2 mm in total length at hatching and those of A. mitchilli about 2.0 mm. H. pensacolae larvae grew about 1.0 mm per day and A. mitchilli 0.70 mm per day. The mean number of food organisms in each digestive tract was greater in H. pensacolae than in A. mitchilli, and the difference in number increased as the larvae grew. Average size of food organisms eaten increased for both species with growth, because of selection by the larvae; the average size of copepodites and copepods in digestive tracts increased at a faste rate in H. pensacolae than A. mitchilli. A. mitchilli longer than 8 mm did not eat copepod nauplii.Contribution No. 170, Bureau of Commercial Fisheries Tropical Atlantic Biological laboratory, Miami, Florida 33149, USA.     

Growth and moulting of Neomysis integer (Crustacea: Mysidacea)

The growth and moulting of Neomysis integer (Leach) was investigated in the field and the laboratory. In the Ythan estuary, Aberdeenshire, Scotland, monthly samples taken from November 1976 to October 1978 revealed that the summer generation juveniles and mature individuals grew at a rate of 4 to 5 mm and 1 to 2 mm monthly, respectively. The winter generation had a growth rate of 3 to 4 mm monthly for juveniles and about 1 mm for mature individuals; during the winter there was a period of 3 mo when growth was almost completely stopped. Mysids reared in the laboratory on Artemia sp. nauplii had an average daily growth rate of 0.06 mm at 9°C and 0.09 mm at 16°C. The growth factors of N. integer ranged from 3 to 17% for mature and immature individuals, respectively. Intermoult periods ranged from 3 to 7 d in immature mysids to 12 to 18 d in mature mysids. Average laboratory growth curves calculated from information on growth factors and intermoult periods indicate that at 9°C (winter generation) it takes N. integer 277 d to grow to be a 15 mm mature individual, whereas at 16°C (summer generation) it takes 188 d. N. integer moults 24 times as it grows from a juvenile to a mature individual.     

Diel feeding behavior of neritic copepods during spring and fall blooms in Akkeshi Bay,eastern coast of Hokkaido,Japan

In situ diel feeding behavior of neritic copepods was investigated using the gut fluorescence method, during spring and fall bloom periods in Akkeshi Bay, on the eastern coast of Hokkaido, Japan. Acartia omorii and Paracalanus sp. were the dominant species during the fall, and Pseudocalanus spp. and A. longiremis during the spring. During both bloom periods, diel rhythms were always observed for the gut pigment contents of these dominant copepods, although there were interspecific differences in the pattern. The maximum gut pigment content was always observed during the night and the minimum during the day. For all species, except Paracalanus sp., the average gut pigment content during the night was significantly higher (p<0.05) than during daytime by factors of between 1.5 and 2.7. There were no significant differences between the gut evacuation rate constants determined during the day and the night, and initial gut pigment content had no effect on the value of gut evacuation rate constants. The instantaneous ingestion rates of individual copepods calculated from gut pigment and the mean value of gut evacuation rate constants followed the same diel rhythms as gut pigment contents. Copepod daily ingestion rates were higher than the daily requirements for respiration during both bloom periods. Estimated daily ration was 40 to 91% of body carbon during the fall bloom, and 17 to 28% during the spring bloom. The higher daily rations during fall were probably due to the difference in in situ temperature (ca. 14°C).     

Seagrass and algal beds as nursery habitats for tiger prawns (Penaeus semisulcatus and P. esculentus) in a tropical Australian estuary

We evaluated the importance of seagrass and algae to two species of tiger prawns (Penaeus semisulcatus and P. esculentus) by detailed sampling at four sites (two seagrass, two algae) in the Embley River estuary, and through sampling 26 sites in 7 adjacent estuaries at one time of year. Samples of tiger prawns were collected in the Embley River estuary with a small beam trawl at night every 2 wk from September to May for 2 yr (1990 to 1992). The two seagrass sites, which were 11 and 13 km from the river mouth, showed less seasonal variation in salinity than the two algal sites, which were 15 and 20 km from the river mouth. The algal beds at the two upstream sites almost disappeared during the wet season, but the biomass of seagrass did not change significantly between the wet and dry seasons. The grooved tiger prawn (P. semisulcatus), the main species at all sites, comprised 88% of the total tiger prawn catch over the two years. They were found at all sites during the pre-wet season, but after the onset of the wet season, they disappeared along with the algae, from the upstream sites. The brown tiger prawn (P. esculentus) was found almost exclusively (97% of the total catch) on the seagrass sites downstream. In the study of several estuaries, juvenile P. semisulcatus were caught at all 26 sites, and P. esculentus were caught in much smaller numbers, at 16 sites. Approximately equal numbers of P. semisulcatus were caught in seagrass and algal beds in the pre-wet season. Very few individuals >10 mm carapace length of either species, were caught. The results from this study highlight the importance of algal beds during the pre-wet season as nursery areas for one species of tiger prawn (P. semisulcatus).     

Diel burying by the tropical sea cucumber Holothuria scabra: effects of environmental stimuli, handling and ontogeny

Understanding concealment behaviour of marine animals is vital for population surveys and captive-release programmes. The commercially valuable sea cucumber Holothuria scabra Jaeger 1833 (Holothuroidea) can display a diel burying cycle, but is it widely predictable? Circadian burying of captive H. scabra juveniles, and both juveniles and adults in the wild, was examined in New Caledonia. Groups of ten cultured juveniles in mesh chambers in a tank were monitored for 24 h. Small juveniles (1–5 g) displayed an expected diel cycle of epibenthic foraging in the afternoon and night then burial in sediments in the morning. Burial was related significantly to both light and temperature in combination. Similar groups of juveniles were handled once or three times a day for 1 week then frequency of emergence during another week was compared to unhandled controls. Handling stress, whether occasional or frequent, significantly suppressed the frequency of their afternoon emergence from sediments for 4 days. In a coastal seagrass bed, burial and emergence of H. scabra were monitored during days of opposing tidal cycles in three seasons. Adults seldom buried during the day except in the cool season. At that site, most small hatchery-produced H. scabra juveniles were buried during most of the day, while larger juveniles showed little diurnal burying. This study underscores that the circadian behaviours of marine animals can exhibit substantial spatial variation, may be absent at certain sites or seasons, and can be mediated by a complexity of factors that vary over short timescales.     

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.