Vertical migration and diel activity patterns of hydromedusae: studies in a large tank

Vertical distributions and swimming behaviors of 7 species ofhydromedusae (Aequorea victoria, Bougainvillia principis, Gonionemusvertens, Mitrocoma cellularia, Phialidium gregarium, Polyorchispenicillatus, and Stomotoca atra) were observed over 24-h periodsin a 2 m high, 1500 1 transparent tank. In this tank, most speciesperformed well-marked diel vertical migrations that were mediatedby swimming. Manipulation of the light regime showed the didswimming behaviors to be light-dependent rather than intrinsic,even in species that do not possess recognized photoreceptors.Correlations between vertical migration and spawning times forseveral species of medusae suggest that in cases when verticalmigration reduces the distances between individuals (e.g., inmass movements to the surface) just prior to spawning, enhancedfertilization success may result from such movements. ¹Present address: Friday Harbor Laboratories, Friday Harbor,WA 98250, USA.     

Photoresponses of the copepod Mesocyclops edax

The predatory copepod Mesocyclops edax is an important componentof many zooplankton communities where it typically makes extensivedid vertical migrations. To describe the effect of light onadults we measured their photoresponses in the laboratory. Theresponse spectrum is characterized by a wide plateau of greatestsensitivity from about 480 – 580 nm. These animals areadapted to perceive light during the day since their regionof maximum sensitivity overlaps the spectral region of highestquantal intensity underwater (575 – 700 nm). The thresholdintensity for positive phototaxis by dark adapted animals wasabout 5 x 10^–1 Wm^–2 at 540 nm, and they were positivelyphototactic up to an intensity of 5 x 10^–1 Wm^–2.Above this intensity phototaxis is no longer observed. Light-adaptedanimals were less sensitive than dark-adapted, but their generalpattern of response to light intensity did not differ. Thereis no rhythm in phototaxis. Their photoresponses may providea mechanism for controlling vertical migration so as to minimizeexposure to planktivorous fish. ¹Contribution No. 1375-AEL from UM-CEES, Appalachian EnvironmentalLaboratory.     

Phototaxis, gravitaxis and vertical migrations in the marine dinoflagellate Prorocentrum micans

Abstract The phototactic orientation of the marine dinoflagellate Prorocentrum micans was studied at three different ages and at several light intensities. High irradiances caused the cells to show negative phototaxis and low irradiances caused positive phototaxis. The precision of negative phototaxis reached a maximum in the early afternoon, while the precision of positive phototaxis was found to peak in the morning and at night. The cells also showed a pronounced negative gravitactic orientation, which had a maximum in precision in the early afternoon. The degree of gravitaxis was found to be constant over time when the cells were confined to a closed cuvette for up to 9 h. As a consequence of the orientation strategies, populations of Prorocentrum micans showed daily vertical migrations in a 3-m Plexiglas column. They accumulated in the top layers in the afternoon and were almost randomly distributed during the rest of the day.     

OBSERVATIONS ON THE SWIMMING AND BUOYANCY OF SOME THECOSOMATOUS PTEROPOD GASTROPODS

Cymbulia peroni produces forward propulsion only on the upstrokeof its ‘wingbeat’ the downstroke opposing slightnegative buoyancy. The species' torpedo-shaped pseudoconchaprovides stability as well as buoyancy. Corolla ovata producespropulsive power on the downstroke of its ’wings‘and shows periods of active upward swimming alternating withpassive downward gliding/parachuting. Both cymbuliid speciesare gelatinous slow movers, close to neutral buoyancy, and areincapable of effective diurnal vertical migration. Two cavoliniidspecies were also studied; Cilo pyramida and Cuvierina columnella.Both were dense and sank quickly if they ceased active swimming.Both had postural responses to slow sinking, but vigorous activitywas required to hover in the water column. Clio could not climbrapidly enough to carry out effective diurnal vertical migration,but was capable of fast horizontal swimming during which thetriangular shell appeared to act as a hydrofoil. The smallerCuvierina could not swim quickly in a horizontal direction,but was capable of swimming vertically at rates sufficient tomaintain station with moving isolumes and may therefore carryout significant diurnal vertical migrations. (Received 28 September 1989; accepted 12 December 1989)     

Diel vertical migration of Cryptomonas and Chromatium in the deep chlorophyll maximum of a eutrophic lake

Large populations of the phytoflagellates Cryptomonas rostratiformis,Cryptomonas phaseolus, Cryplomonas undulata and the phototrophicbacterium Chromatium cf. okenii have been observed in the oxic/anoxicboundary layer of the slightly eutrophic, dimictic lake Schlachtenseeduring the summer stratification. Vertical distribution of thesepopulations was studied with the help of a new close-intervalsampler and by in Situ fluorescence measurements with fine spatial(cm to dm) and temporal (h) resolution on 4 days in 1991, 1994and 1995. All populations lived close to the chemo dine butshowed a regular diel vertical migration with daytime ascentand night-time descent and a low migration amplitude. At leastfor one species— C. rostratiformis—the pattern ofmigration suggests that this behaviour has an endogenous origin.As a result of diel vertical migration, the population of C.rostratiformiswas spatially separated from the other dominating populationsduring daytime. Ultimate cause of diel vertical migration wasa better light supply of all populations. Another probable advantagewas the reduction in grazing pressure, as large percentagesof all populations moved into the anoxic, hydrogen sulphidecontaining water layers during night-time. ¹Present address Department of Lowland Rivers and Shallow Lakes,Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm260, D-12587 Berlin, FRG     

Seasonal and diet changes in ultrastructure in the dinoflagellate Ceratium hirundinella

The ultrastructure of the dinoflagellate Ceratium hirundinella(O.F.Mull.) Bergh. was studied in the plankton of a small productivelake throughout one season of its growth and during one 24-hperiod of that season. The results were related to measuredenvironmental variables. Samples for the seasonal study weretaken every 4 weeks and fixed immediately upon removal fromthe lake and samples for the diel study were fixed in situ atdepth. There was little difference in general ultra-structureof cells during the diel study or during the period of rapidgrowth but greater changes were apparent following excystmentand prior to encystment. Most changes were observed within thechloroplasts, pyrenoids and storage products. Changes in chloroplaststructure were shown to be related to the light conditions beingexperienced by the cells. Starch and lipid occurred in varyingproportions depending on the stage of the life cycle. Starchand associated pyrenoid clusters varied with irradiance levelover the 24-h period.     

Behavioral responses of Atlantic cod to sea temperature changes

Understanding responses of marine species to temperature variability is essential to predict impacts of future climate change in the oceans. Most ectotherms are expected to adjust their behavior to avoid extreme temperatures and minimize acute changes in body temperature. However, measuring such behavioral plasticity in the wild is challenging. Combining 4 years of telemetry‐derived behavioral data on juvenile and adult (30–80 cm) Atlantic cod (Gadus morhua), and in situ ocean temperature measurements, we found a significant effect of sea temperature on cod depth use and activity level in coastal Skagerrak. During summer, cod were found in deeper waters when sea surface temperature increased. Further, this effect of temperature was stronger on larger cod. Diel vertical migration, which consists in a nighttime rise to shallow feeding habitats, was stronger among smaller cod. As surface temperature increased beyond ~15°C, their vertical migration was limited to deeper waters. In addition to larger diel vertical migrations, smaller cod were more active and travelled larger distances compared to larger specimens. Cold temperatures during winter tended, however, to reduce the magnitude of diel vertical migrations, as well as the activity level and distance moved by those smaller individuals. Our findings suggest that future and ongoing rises in sea surface temperature may increasingly deprive cod in this region from shallow feeding areas during summer, which may be detrimental for local populations of the species.     

Effects of temporary food limitation on survival and development of brachyuran crab larvae

As a consequence of the combined effects of prey patchinessand diel or tidal vertical migrations in the water column, decapodcrustacean larvae may experience temporal or spatial variabilityin the availability of planktonic food. In a laboratory study,we evaluated effects of temporarily limited access to prey onthe larvae of three species of brachyuran crabs, Chasmagnathusgranulata, Cancer pagurus and Carcinus maenas. Stage-I zoeaewere fed ad libitum for 4 or 6 h per day (20 or 25% treatments;6 h tested in C. pagurus only), and rates of larval survivaland development were compared with those observed in continuouslyfed control groups (24 h, 100%). In C. granulata, we also testedif intraspecific variability in initial biomass of freshly hatchedlarvae originating from different broods has an influence onearly larval tolerance of food limitation. Moreover, we exposedembryos and larvae of this estuarine species to moderately decreasedsalinities to identify possible interactions of osmotic andnutritional stress. Finally, we evaluated in this species theeffect of food limitation on survival from hatching throughall larval instars to metamorphosis. In all three species, limitedaccess to prey had only weak or insignificant negative effectson survival through the Zoea-I stage. The strength of the effectsof temporary food limitation varied in C. granulata significantlyamong broods. However, no significant relationships were foundbetween initial larval biomass (C content) and either survivalor development duration. Strongly decreased survival to metamorphosiswas found when food limitation continued throughout larval development.Thus, early brachyuran crab larvae are well adapted to transitorylack of planktonic food. The capability of the Zoea-I stageof C. granulata to withstand nutritional stress also under conditionsof concomitant salinity stress allows them to exploit variousbrackish environments within estuarine gradients. However, continuedexposure to limited access to planktonic prey may exceed thenutritional flexibility of C. granulata larvae.     

The diel migrations and vertical distributions of zooplankton and micronekton in the Northwestern Mediterranean Sea. 2. Siphonophores, hydromedusae and pyrosomids

Vertical distributions and diel migrations in the 0–1000m water column at a position in the Northwestern MediterraneanSea are given in detail for the main species of gelatinous macrozoo-plankton:three siphonophores (Abylopsis tetragona, Chelophyes appendiculataand Lensia conoidea), one hydromedusa (Solmissus albescens)and one pyrosomid (Pyrosoma atlanticum). Extensive diel verticalmigration occurred in these five species, particularly in A.tetragona,S.albescens and P.atlanticum for which the bulk of the populationmigrated as a compact unit. Pyrosoma atlanticum underwent thelargest migration, with a mean amplitude of 515 m. The migrationpattern of L.conoidea was more complex. Its day-distributionwas bimodal and at night part of the population ascended towardsthe surface, while the other part appeared to undergo a weakreverse migration of     

Vertical distribution and diel migration of flagellated phytoplankton in a small humic lake

The vertical distributions and migrations are described of the most abundant flagellated phytoplankton species from the summer community of a small forest lake in southern Finland. The lake showed a steep and stable thermal stratification with a shallow oxygenated epilimnion. Horizontal variation of phytoplankton distribution within the lake was tested on two scales and found to be statistically significant only in the case of Mallomonas reginae. The vertical distribution of flagellated phytoplankton was assessed by reference to the distribution of a non-motile, neutrally buoyant species Ankyra judayi. Statistically significant, active vertical positioning was demonstrated for all the flagellates examined with the exception of Spiniferomonas bourrellyi. Diel vertical migrations were apparent for all species showing active positioning and the pattern of an evening descent and a morning ascent was ubiquitous. The extent and timing of diel migrations varied between species. The most extensive migrations were by Cryptomonas marssonii which crossed a temperature gradient of 14 °C and penetrated far into the anoxic hypolimnion. Several categories of competitive advantage can be gained by species undertaking such diel vertical migrations.     

The swimming and feeding behavior of Mesocyclops

The swimming and feeding behaviors of Mesocyclops are described from a review of the literature and personal observations. Mesocyclops exhibits considerable behavioral flexibility in response to environmental stimuli. Mesocyclops edax exhibits an increase in horizontal looping behavior at high prey densities, and performs a tight vertical looping behavior in response to the loss of captured prey. Ingestion rates by Mesocyclops are a complex function of prey density, morphology, and behavior in addition to prey size. Vertebrate predators induce a rapid escape response in Mesocyclops and may be responsible at least in part for their extensive diel vertical migrations. The complex behavioral patterns of Mesocyclops suggest that its distribution and abundance in nature will be distinctly nonrandom and influenced as much by its own behavioral responses as by other external physical factors such as water circulation patterns.     

Vertical migration for horizontal transport while avoiding predators: I. A tidal/diel model

Research into the vertical migration behavior of scallop (Placopectenmagellanicus) veligers has led us to examine whether these,and possibly other small zooplankters, may migrate in responseto a combination of tidal and diel stimuli. This paper usesHill's (1991) model to evaluate the horizontal transport effectsof such migrations. We demonstrate that most types of verticalmigration behavior reported in the literature (e.g. nocturnal,twilight, midnight sink) appear at different phases of the lunarcycle. Moreover, migrating in response to both of these cuesmay provide horizontal transport advantages if the zooplankteris very small (unable to migrate the full water column depth)and/or has difficulty determining its position in the watercolumn (especially if the behavior also holds it in regionsof increased shear). Such behavior need not interfere with otheradvantages of vertical migration, including avoiding predation,avoiding UV light, searching for patchy food, etc. Tidal/dielmigration may have distinct advantages for occupying new habitatsor coping with local changes associated with altered currentregimes. Because averaging the results of several days, samplingless frequently than every 2 h or sequential sampling of differentsites is likely to obscure the tidal portion of a tidal/dielmigration, such behavior could be common without being obviousto researchers. Aliasing of the lunar and solar cycles (a 14.8day period) may allow the detection of tidal period migrationsin long-term records with lower sampling frequencies.     

Two hundred years of zooplankton vertical migration research

Vertical migration is a geographically and taxonomically widespread behaviour among zooplankton that spans across diel and seasonal timescales. The shorter-term diel vertical migration (DVM) has a periodicity of up to 1 day and was first described by the French naturalist Georges Cuvier in 1817. In 1888, the German marine biologist Carl Chun described the longer-term seasonal vertical migration (SVM), which has a periodicity of ca. 1 year. The proximate control and adaptive significance of DVM have been extensively studied and are well understood. DVM is generally a behaviour controlled by ambient irradiance, which allows herbivorous zooplankton to feed in food-rich shallower waters during the night when light-dependent (visual) predation risk is minimal and take refuge in deeper, darker waters during daytime. However, DVMs of herbivorous zooplankton are followed by their predators, producing complex predator–prey patterns that may be traced across multiple trophic levels. In contrast to DVM, SVM research is relatively young and its causes and consequences are less well understood. During periods of seasonal environmental deterioration, SVM allows zooplankton to evacuate shallower waters seasonally and take refuge in deeper waters often in a state of dormancy. Both DVM and SVM play a significant role in the vertical transport of organic carbon to deeper waters (biological carbon sequestration), and hence in the buffering of global climate change. Although many animal migrations are expected to change under future climate scenarios, little is known about the potential implications of global climate change on zooplankton vertical migrations and its impact on the biological carbon sequestration process. Further, the combined influence of DVM and SVM in determining zooplankton fitness and maintenance of their horizontal (geographic) distributions is not well understood. The contrasting spatial (deep versus shallow) and temporal (diel versus seasonal) scales over which these two migrations occur lead to challenges in studying them at higher spatial, temporal and biological resolution and coverage. Extending the largely population-based vertical migration knowledge base to individual-based studies will be an important way forward. While tracking individual zooplankton in their natural habitats remains a major challenge, conducting trophic-scale, high-resolution, year-round studies that utilise emerging field sampling and observation techniques, molecular genetic tools and computational hardware and software will be the best solution to improve our understanding of zooplankton vertical migrations.     

The diel dynamics of vertical migrations in hortobiontic spiders (Aranei) in the clay semidesert of the northern Caspian region

The diel periodicity of vertical migrations of herbage-dwelling spiders has been studied in natural habitats of clay semidesert of the northern Caspain (Transvolga) region. Steppe habitats (microdepressions) and desert habitats (microelevations) are shown to have much in common: the abundance and the family composition of spider populations of both biotopes are similar; they slightly differ only during the hot summer season. The amplitude of diel fluctuations in spider abundance is relatively great, which is typical of open cenoses under semiarid conditions. In spring and autumn, the peaks of spider abundance in both types of habitats are at night, when air temperature is minimal. In summer, in addition to the nighttime abundance peaks, there are daytime peaks of similar size, due mostly to the increased activity of “southern” taxa, the Thomisidae in desert associations and the Thomisidae + Salticidae in steppe associations. The diel periodicity of vertical migrations in hortobiontic spiders is a complicated phenomenon determined by a number of factors. It is to some extent conditioned by environmental parameters and by vertical migrations of their potential prey, phytophagous insects. Thus, the ecological niches in spiders of different taxa are separated in time according to their morphological and behavioral adaptation to climatic conditions. This separation probably alleviates competition between populations of different taxa.     

A change in the pattern of vertical migration of Chaoborus flavicans after the introduction of trout

The pattern of diel vertical migration exhibited by Chaoborusflavicans in Lake Lenore, Washington has changed over the pastdecade. Formerly Chaoborus larvae and pupae were present inthe water column during the day. Since 1982, a strong patternof diel vertical migration has been evident in this population.Third and fourth instar larvae and pupae reside in the sedimentsduring the day and then ascend to the surface waters at night.The change in migratory activity of C. flavicans was coincidentwith the introduction of cutthroat trout (Salmo clarki henshawi)into Lake Lenore in 1979. Predation by trout on fourth instarlarvae and pupae of C. flavicans was probably responsible forthe observed changes in migratory behavior.     

The photobehaviour of Daphnia spp. as a model to explain diel vertical migration in zooplankton

Many pelagic animal species in the marine environment and in lakes migrate to deeper water layers before sunrise and return around sunset. The amplitude of these diel vertical migrations (DVM) varies from several hundreds of metres in the oceans to approx. 5–20 m in lakes. DVM can be studied from a proximate and an ultimate point of view. A proximate analysis is intended to reveal the underlying behavioural mechanism and the factors that cause the daily displacements. The ultimate analysis deals with the adaptive significance of DVM and the driving forces that were responsible for the selection of the traits essential to the behavioural mechanism. The freshwater cladoceran Daphnia is the best studied species and results can be used to model migration behaviour in general. Phototaxis in Daphnia spp., which is defined as a light-oriented swimming towards (positive phototaxis) or away (negative phototaxis) from a light source, is considered the most important mechanism basic to DVM. A distinction has been made between primary phototaxis which occurs when light intensity is constant, and secondary phototaxis which is caused by changes in light intensity. Both types of reaction are superimposed on normal swimming. This swimming of Daphnia spp. consists of alternating upwards and downwards displacements over small distances. An internal oscillator seems to be at the base of these alternations. Primary phototaxis is the result of a dominance of either the upwards or the downwards oscillator phase, and the direction depends on internal and external factors: for example, fish-mediated chemicals or kairomones induce a downwards drift. Adverse environmental factors may produce a persistent primary phototaxis. Rare clones of D. magna have been found that show also persistent positive or negative primary phototaxis and interbreeding of the two types produces intermediate progeny: thus a genetic component seems to be involved. Also secondary phototaxis is superimposed on normal swimming: a continuous increase in light intensity amplifies the downwards oscillator phase and decreases the upwards phase. A threshold must be succeeded which depends on the rate and the duration of the relative change in light intensity. The relation between both is given by the stimulus strength versus stimulus duration curve. An absolute threshold or rheobase exists, defined as the minimum rate of change causing a response if continued for an infinitely long time. DVM in a lake takes place during a period of 1-5-2 h when light changes are higher than the rheobase threshold. Accelerations in the rate of relative increase in light intensity strongly enhance downwards swimming in Daphnia spp. and this enhancement increases with increasing fish kairomone and food concentration. This phenomenon may represent a ‘decision-making mechanism’ to realize the adaptive goal of DVM: at high fish predator densities, thus high kairomone concentrations, and sufficiently high food concentrations, DVM is profitable but not so at low concentrations. Body axis orientation in Daphnia spp. is controlled with regard to light-dark boundaries or contrasts. Under water, contrasts are present at the boundaries of the illuminated circular window which results from the maximum angle of refraction at 48–9° with the normal (Snell's window). Contrasts are fixed by the compound eye and appropriate turning of the body axis orients the daphnid in an upwards or an obliquely downwards direction. A predisposition for a positively or negatively phototactic orientation seems to be the result of a disturbed balance of the two oscillators governing normal swimming. Some investigators have tried to study DVM at a laboratory scale during a 24 h cycle. To imitate nature, properties of a natural water column, such as a large temperature gradient, were compressed into a few cm. With appropriate light intensity changes, vertical distributions looking like DVM were obtained. The results can be explained by phototactic reactions and the artificial nature of the compressed environmental factors but do not compare with DVM in the field. A mechanistic model of DVM based on phototaxis is presented. Both, primary and secondary phototaxis is considered an extension of normal swimming. Using the light intensity changes of dawn and the differential enhancement of kairomones and food concentrations, amplitudes of DVM could be simulated comparable to those in a lake. The most important adaptive significance of DVM is avoidance of visual predators such as juvenile fish. However, in the absence of fish kairomones, small-scale DVMs are often present, which were probably evolved for UV-protection, and are realized by not enhanced phototaxis. In addition, the ‘decision-making mechanism’ was probably evolved as based on the enhanced phototactic reaction to accelerations in the rate of relative changes in light intensity and the presence of fish kairomones.     

Short-term changes in the mesozooplanktonic community in theSeine ROFI (Region of Freshwater Influence)(eastern English Channel)

Discrete-depth, hourly mesozooplankton samples were collectedover a 92 h period in May 1992 at an anchor station within theSeine Region of Freshwater Influence (ROFI) (English Channel).The mesozooplankton community defined as a euryhaline marineassemblage was dominated by the calanoid copepods Acartia spp.,Temora longicornis and Centropages hamatus, the cladoceran Evadnenordmanni and the appendicularian Oikopleura dioica. The semi-diurnaltidal current was the dominant factor in determining the short-termtemporal changes in the community in terms of density and speciescomposition so that zooplankton patches displayed oscillatingmotion in relation to tidal advection. Although a few species(e.g. Pleurobrachia pileus) exhibited higher densities aroundlow tide, maximum densities were observed for most species (e.g.T.longicornis and E.nordmanni) around high tide, according tosalinity variations. Diurnal changes were only reported forcyclopoid copepods (i.e. Halicyclops sp. and Cyclopina sp.)which wer$$$ mainly endobenthic during the day and moved intothe water column at night. Besides temporal changes in depth-averageddensities, most species exhibited vertical migrations at dieland/or tidal periods. Tidal vertical migrations were reportedonly for a few taxa and could be the result of passive mechanisms(e.g. vertical mixing) rather than of active behaviour. Dielvertical migrations were observed in most of the abundant taxa.While this migration pattern did not appear to be an adaptationto predator avoidance within the Seine ROFI, it could regulatehorizontal transport of organisms and promote their retention.The consequences of the short-term mesozooplankton fluctuationsfor sampling designs are discussed.     

Food-web manipulations influence grazer control of phytoplankton growth rates in Lake Michigan

Stocking piscivorous salmonids in Lake Michigan produced dramaticalterations in food-web structure, including higher numbersof large-bodied zooplankton (especially Daphnia pulicaria),lower summer chlorophyll concentrations and increased watertransparency. Experimental determinations of epilimnetic phytoplanktongrowth rates and of zooplankton grazing rates indicate thatherbivorous zooplankton controlled algal dynamics during thesummer of 1983 because grazers occupied the surface waters throughoutthe day. In 1985, however, both large- and small-bodied Daphniamade approximately equal contributions to total grazer biomass,and all grazers displayed pronounced diel vertical migrations,visiting epilimnetic waters only at night. This prohibited zooplanktonfrom controlling algal dynamics because grazing losses did notexceed phytoplankton growth rates. The changes in zooplanktoncommunity composition and behavior observed in summer 1985 probablyresulted from increased predation by visually orienting planktivorousfish, especially bloater chub (Coregonus hoyi). Effects of food-webmanipulations on phytoplankton dynamics were evident only duringJuly and August. During spring and early summer copepods dominateLake Michigan's zooplankton community. Owing to their smallbody size, copepods are less susceptible to fish predation andexhibit much lower filtering rates than Daphnia. Variabilityin zooplanktivorous fish abundance probably has little effecton phytoplankton dynamics during spring and early summer.     

A free-drifting mimic of vertically migrating zooplankton

Many zooplankton species perform diel vertical migrations (DVM) which, in conjunction with vertical current shear, complicate the use of conventional fixed-depth drifters to account for advection. Here we illustrate the first use of an autonomous Vertically Migrating Drifter (VMD) to mimic DVM behavior. The vehicle resides within different subsurface layers at different times of day through either active hover cycles or passive drifts. It moves vertically between these layers at speeds comparable to those recorded for migratory populations. In this mode, it can be utilized as a tool to estimate the advection of migratory zooplankton in regions of high vertical current shear and can be employed as a Lagrangian tracer when attempting to sample the same population repeatedly over time.      

Effects of fish presence and simulated moonlight gradients on night-time horizontal movements of a predatory zooplankter, Chaoborus punctipennis

Some zooplankton, including Chaoborus punctipcnnis, have recentlybeen reported to undergo die1 horizontal migrations in additionto their widely known vertical migrations. In a series of laboratoryand field experiments, we tested the influences of gradientsin light intensity and fish presence on horizontal migrationin C.Punctipennis. In a small chamber, C.punctipennis showedno response to simulated moonlight gradients. They did, however,show significant movement away from fish held at one end ofthe chamber under uniform moonlight. A field experiment in alarge chamber indicated that the response to fish was inducedby chemical cues. When tested in the laboratory chamber withboth moonlight gradients and fish present, C.punctipennis becamepositively phototactic. This response increased the movementaway from fish when the fish were at the darker end of the gradient,or reduced it when the fish were at the brighter end. Ln combination,these experiments demonstrate that C.punctipennis show directedhorizontal movements in response to fish presence and moonlightgradients, suggesting a potential for these stimuli to influencehorizontal migrations observed in the field. Present address: Department of Biology, University of CaliforniaLos Angeles, CA 90095-1606, USA