Diel vertical migrations ofMeganyctiphanes norvegica in the Kattegat: Comparison of net catches and measurements with Acoustic Doppler Current Profilers

Diel vertical migration of a stable and well-defined population of Nordic krill,Meganyctiphanes norvegica (Crustacea, Euphausiacea) was investigated during eight days in August 1989, in the Läsö-Deep, East of the Danish island Läsö. Net catches with a multi-net (MOCNESS) and measurements with a moored and a shipboard Acoustic Doppler Current Profiler (ADCP) were compared. Backscattered energy as a measure for biomass gave good correlations to the dry weight ofM. norvegica and smaller zooplankton from net catches. Diel migratory patterns matched well, as determined, parallel with both methods. Migratory vertical velocity was determined with ADCP at 2–3 cm sec^?1. The potential for the use of ADCPs for biological investigation is discussed. Vertical migration was dependent on environmental parameters. The krill did not cross a temperature barrier of 14°C, although rich food sources were situated beyond it. Differences in salinity did not play a role. Currents were involved in plankton distribution. Light was an important Zeitgeber (synchronizer) and determined the density of the krill aggregations. Feeding behaviour did not interfere with the light-induced migratory pattern of Nordic krill at the Läsö-Deep.     

Acoustic observations of zooplankton in lakes using a Doppler current profiler

1. In this paper we test the usefulness of acoustic backscatter measurements from a 614 kHz Acoustic Doppler Current Profiler (ADCP) for the qualitative and quantitative characterisation of zooplankton distributions in lakes. ADCP‐based backscatter estimates were compared with frequent net hauls obtained during a calibration experiment in which the acoustic backscatter was strongly dominated by vertical migrating Chaoborus flavicans larvae. 2. The correlation between backscatter estimates and the C. flavicans concentration was very good. Vertical swimming speed of larvae, measured directly by the ADCP, was up to a maximum of 5 mm s^?1 and agreed very well with the observed vertical movement of the backscatter contour lines. Although the strong backscatter from C. flavicans overwhelmed the signal from the remaining zooplankton, a good correlation between backscatter strength and the total remaining zooplankton concentration, dominated by Cyclops spp., was found for the depth and time intervals where no C. flavicans were present. 3. In addition to the calibration experiment, longer‐term ADCP measurements from different lakes revealed a strong temporal correlation between the onset of the up‐ and downward migration of zooplankton and the local sunset and sunrise. 4. We conclude that ADCPs can be used to monitor plankton distributions both temporally and spatially. It also seems possible to estimate plankton densities after appropriate calibration.     

Swimming and feeding behaviour of the planktonic copepod Clausocalanus furcatus

The objective of this study was to quantify motion and feeding behaviour of the small calanoid Clausocalanus furcatus, which is a common and abundant species in oligotrophic environments. Adult females were video recorded at 60 fields s^-1 using video equipment which allowed us to follow each free-swimming individual continuously in 3 1 vessels. At 20C under dark conditions, C.furcatus moved continuously along convoluted small loops at a mean speed of 10 mm s^-1 (corresponding to 10 body lengths s^-1). This motion was occasionally interrupted by sudden somersaulting performed at very high speed (up to 17 mm s^-1). The copepods only occasionally sank. There was no evidence that C.furcatus created feeding currents. Dinoflagellate cells offered as food appeared to be perceived by direct encounter at high speed, being instantaneously captured and ingested or rejected. The capture rates were correlated with the frequency of somersaulting. By moving fast and by associating a high turning rate with a high frequency of crossing the previous tracks, C.furcatus searched 22-26% of the explored volumes. The motion and feeding behaviour of C.furcatus show that the foraging tactic of this species is to explore small volumes of water rapidly. This strategy appears, so far, to be unique among small planktonic copepods.      

Effects of moonlight on the vertical migration patterns of demersal zooplankton

The diel vertical migration patterns of demersal zooplankton, those organisms which habit bottom substrates but periodically emerge to swim freely in the water column, water determined throughout the lunar cycle. Demersal zooplankton were quantitatively sampled on a subtidal sand flat in the Gulf of California every 2 h for 24-h periods at new, full, first, and last-quarter moons, both as they emerged into the water column and as they returned to the benthos. Demersal zooplankton rarely migrated during daylight. Three general patterns of migration were observed. (1) Polychaetes and cumaceans emerged from the benthos at dusk, regardless of the phase of the moon. Polychaetes returned to the benthos throughout the night while cumaceans returned near dawn. (2) Species of amphipods and isopods exhibited significant avoidance of moonlight, delaying emergence until moonset or returning to the benthos at moonrise. (3) Species of copepods, mysids, shrimp, Branchiostoma (cephalochordate), and tanaids emerged into the water column throughout the night. The timing of migration was highly variable and did not correlate with the presence or absence of moonlight. Large zooplankton migrated less frequently into the water column during moonlit periods than small forms, suggesting that nocturnal predation by visually oriented planktivorous fish may be an important selective pressure.Demersal zooplankton emerged into artificially darkened emergence traps in significantly higher numbers during daylight and during full and quarter moons than into undarkened control traps, demonstrating that absence of light is a major cue stimulating migration. Reentry traps resting on the bottom captured higher densities of demersal zooplankton than either emergence traps or reentry traps suspended off the bottom. Thus, many demersal zooplankton remain near the bottom, rarely swimming far into the water column. Some trap avoidance was observed and current methods for collecting demersal zooplankton are evaluated. Since most demersal zooplankton remained in the water column only a short time, dispersal, particularly over short distances, may be a major advantage of migratory behavior. Migration facilitates rapid recolonization of disturbed or defaunated sites, disrupts and mixes bottom sediments, and results in daily variation in the microdistribution, patchiness, and species composition of the benthic fauna.     

Changes in circadian parameters of humbug damselfish,Dascyllus aruanus according to lunar phase shifts in Micronesia

The objectives of this study were to test the nighttime effects of the lunar phase on circadian rhythm in the humbug damselfish, Dascyllus aruanus. We measured moonlight intensities at eight different phases across the lunar cycle. At each lunar phase, the circadian rhythm was evaluated by measuring the clock genes cryptochrome 1 and period 2. In addition, we measured arylalkylamine N-acetyltransferase 2 (AANAT2), melatonin and melatonin receptor 1 (MT-R1). The moonlight intensity was highest at full moon and lowest during the waning crescent. Clock gene expression was highest during the full moon compared to the other phases. By contrast, the plasma concentrations of AANAT2 and melatonin and the MT-R1 mRNA expression were highest during the full moon phase. Our results suggest that moonlight affects circadian rhythm patterns in the humbug damselfish. There is a need to investigate potential other physiological effects of lunar phase shifts.     

Advection of euphausiids in a Norwegian fjord system subject to altered freshwater input by hydro-electric power production

Due to hydro-electric power production, the freshwater inputto the Sandsfjord system has increased during winter, whilethe freshwater input to Jøsenfjorden has been removed.In studies conducted during autumn and winter, both Meganyctiphanesnorvegica and Thysanoessa raschii were most common in Jøsenfjorden,and T.raschii was almost wanting in the Sandsfjord system. Theeuphausiids generally swam to the surface layer at night, butavoided low salinity water in the uppermost metres of the Sandsfjordsystem. Still, in most locations 50% of M.norvegica was foundin the upper 10 m. Their vertical distribution overlapped withfreshwater-driven currents, and estuarine circulation was apparentlyprominent in governing the horizontal distribution within theSandsfjord system. At the entrance to the fjord, M. norvegicawas distributed deeper than within the fjord, reducing the importanceof transport with estuarine circulation relative to currentsdriven by wind at the coast. Estimates of advective rates ofM.norvegica suggest that exchange with coastal waters was moreimportant for the standing stock than local growth within thefjords. In Jtfsenfjorden, maxima of T.raschii were always foundin a zone of shallowing bottom topography (bottom depth 130m), where this species possibly maintained its horizontal positionby means of vertical migrations.     

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)     

Aerial insectivorous bat activity in relation to moonlight intensity

It is commonly assumed that aerial insectivorous bats in the tropics respond to moonlight intensity by decreasing their foraging activity during bright nights due either to an increase in predation risk, or to a reduction in insect availability.The effect of moonlight on bat activity can be measured both between nights and within a single night. However, few studies have simultaneously used both approaches, and most authors generally compare bat activity with lunar phases. Our main aim was to evaluate how moonlight influences aerial insectivorous bat activity at different time scales: between nights and within the same night. Activity of five bat species was measured using autonomous ultrasound recording stations and moonlight intensity percentages retrieved from the Moontool program nightly throughout a 53-day sampling period. Only one species (Myotis riparius) responded negatively to moonlight, while two species (Pteronotus parnellii and Saccopteryx leptura) increased their foraging activity in moonlight. For Cormura brevirostris and S. bilineata, moonlight intensity did not affect activity level. Bat activity was greater for all species at the beginning of the night, independent of the presence of the moon, indicating that foraging just after the sunset is adaptive. Thus, bat response to the effect of moonlight intensity is more apparent between nights than within a single night and may depend on species-specific traits, such as flight speed, flexibility in habitat use and body size.     

Behavioural differences in relation to pycnoclines during vertical migration of the euphausiids Meganyctiphanes norvegica (M. Sars) and Thysanoessa raschii (M. Sars)

An observation on differences in the vertical migration patternbetween Meganyctiphanes norvegica (M. Sars) and Thysanoessaraschii (M. Sars) caused by the presence of a thermocline isreported. Generally, M.norvegica did not migrate through a thermoclineat 50–60 m depth while T.raschii did. This observationis discussed in the context of how the combination of differencesin vertical migration and advection over sill depth may affectthe abundance of fjord stocks of these two euphausiid species.     

The orientation of the sandhopper <Emphasis Type="Italic">Talitrus saltator</Emphasis> during a partial solar eclipse

To acquire more information about the identification and use of the sun and other celestial cues in the sea–land orientation of the sandhopper Talitrus saltator, we carried out releases in a confined environment during a partial solar eclipse and at sunset. The sandhoppers were unable to identify the sun (86% covered) during the eclipse nor to use other celestial compass factors of orientation. This was probably due to the low level of light intensity (close to the minimum level for orientation recorded at sunset) and to the variations in intensity and pattern of skylight polarization.     

Lunar phobia in bats and its ecological correlates: A meta-analysis

Animals show several behavioral strategies to reduce predation risks. Presumably, moonlight avoidance is a strategy used by some nocturnal species to reduce the risk of predation. In bats, some research indicates that foraging activity is negatively correlated with moonlight intensity, a phenomenon better known as lunar phobia. However, the currently available evidence is contradictory because some bat species reduce their activity during nights with more moonlight while the opposite occurs in other species. We quantitatively evaluated the strength and direction of the relationship between moonlight intensity and bat activity using a meta-analysis. We also looked at some ecological correlates of lunar phobia in bats. Specifically, we examined foraging habitat and latitude as potential moderators of the size of the lunar phobia effect. Our results show that, regardless of the method used to evaluate bat activity, the overall relationship between moonlight intensity and bat activity is significant and negative (r = ?0.22). Species foraging on the surface of the water (piscivores and insectivores; r = ?0.83) and forest canopy species (i.e., big frugivores; r = ?0.30) are more affected by moonlight than those with different foraging habitats (understory, subcanopy, open air). Latitude was positively correlated with lunar phobia (r = 0.023). The stronger lunar phobia for bats foraging on the water surface and in the forest canopy may suggest that the risk of predation is greater where moonlight penetrates more easily. The significant effect of latitude as a moderator of lunar phobia suggests that there is a weak geographic pattern, with this phobia slightly more common in tropical bats than in temperate species.     

Short communication. Carbon:nitrogen ratios of Baltic Sea copepods-indication of mineral limitation?

The carbon (C) and nitrogen (N) content and the C:N ratio of two common calanoid copepods, Eurytemora affinis and Acartia bifilosa were measured during spring and summer at the SW coast of Finland, northern Baltic Sea. The C:N ratio of both copepod species was slow and stable (4-4.5), irrespective of sampling time, which implies N limitation at least during intermediate to high food concentrations in spring and early summer. In addition, experiments were conducted to reveal whether the diet of copepods affects their C and N content. Adding green algae Brachiomonas submarina in concentrations of 50-500 g C l^-1 to <100-m-filtered sea water did not significantly increase copepod C or N content in 3 day experiments. However, the C and N contents of E.affinis eggs were 2 times higher in high food concentration, which may increase survival of eggs and nauplii.      

Solar and lunar rhythms in the occurrence of Astronesthes niger (Pisces) at the sea surface

Abstract

The mesopelagic fish Astronesthes niger concentrates in the uppermost centimeters of the sea when both sun and moon are below the horizon. Probability of positive catches increases towards the times of lower culmination of sun and moon and perhaps is highest when both culminations are about synchronous. Positive catches were made only when the moon was near its first or third quarter, but this might be somewhat influenced by the material which was not evenly distributed in relation to moonphases. It is assumed that A. niger needs moonlight for vertical migration, but avoids the surface during illumination.     

Diel and lunar spawning periodicity of the hawkfish Paracirrhites forsteri (Cirrhitidae) on the reefs of Kuchierabu-jima Island, southern Japan

We observed spawning behavior of the hawkfish Paracirrhites forsteri on reefs of southern Japan. Spawning generally occurred after sunset, prior to the full and new moon with semi-lunar spawning peak periodicity. No egg predation was observed in spawning after sunset, and high tide often occurred at dusk in the lunar phases. Thus, diel timing and lunar synchronicity may increase larval survival. We found that the mating activity tended to start earlier in the day during early mating season than during mid-to-late mating season. The advantages of the earlier start of mating activity were also examined in relation to adult biology contexts.     

Temperature selection in Brook Charr: lab experiments,field studies,and matching the Fry curve

Zooplankton diel vertical migration (DVM) is an ecologically important process, affecting nutrient transport and trophic interactions. Available measurements of zooplankton displacement velocity during the DVM in the field are rare; therefore, it is not known which factors are key in driving this velocity. We measured the velocity of the migrating layer at sunset (upward bulk velocity) and sunrise (downwards velocity) in summer 2015 and 2016 in a lake using the backscatter strength (VBS) from an acoustic Doppler current profiler. We collected time series of temperature, relative change in light intensity chlorophyll-a concentration and zooplankton concentration. Our data show that upward velocities increased during the summer and were not enhanced by food, light intensity or by VBS, which is a proxy for zooplankton concentration and size. Upward velocities were strongly correlated with the water temperature in the migrating layer, suggesting that temperature could be a key factor controlling swimming activity. Downward velocities were constant, likely because Daphnia passively sink at sunrise, as suggested by our model of Daphnia sinking rate. Zooplankton migrations mediate trophic interactions and web food structure in pelagic ecosystems. An understanding of the potential environmental determinants of this behaviour is therefore essential to our knowledge of ecosystem functioning.

     

Light-Sensitive Vertical Migration of the Japanese Eel Anguilla japonica Revealed by Real-Time Tracking and Its Utilization for Geolocation

Short-time tracking (one to eight days) of the Japanese eel (Anguilla japonica) using ultrasonic transmitter was performed in the tropical-subtropical area adjacent to the spawning area and temperate area off the Japanese Archipelago. Of 16 eels (11 wild and five farmed) used, 10 wild eels displayed clear diel vertical migration (DVM) from the beginning, while the other five farmed eels tracked for 19 to 66 hours did not. During daytime, a significantly positive correlation between migration depth and light intensity recorded on the vessel was observed in the 10 wild eels, indicating that the eels were sensitive to sunlight even at the middle to lower mesopelagic zone (500 to 800 m). During nighttime, the eel migration depth was observed to be associated with the phase, rising and setting of the moon, indicating that the eels were sensitive to moonlight at the upper mesopelagic zone (<300 m). Two of 10 wild eels were in the yellow stage but shared similar DVM with the silver stage eels. Swimbladders of three silver stage eels were punctured before releasing, but very little effect on DVM was observed. The eels very punctually initiated descent upon nautical dawn and ascent upon sunset, enabling us to determine local times for sunrise and sunset, and hence this behavior may be used for geolocating eels. In fact, estimated positions of eels based on the depth trajectory data were comparable or even better than those obtained by light-based archival tag in other fish species.     

Artificial light at night reverses monthly foraging pattern under simulated moonlight

Mounting evidence shows that artificial light at night (ALAN) alters biological processes across levels of organization, from cells to communities. Yet, the combined impacts of ALAN and natural sources of night-time illumination remain little explored. This is in part due the lack of accurate simulations of the complex changes moonlight intensity, timing and spectra throughout a single night and lunar cycles in laboratory experiments. We custom-built a novel system to simulate natural patterns of moonlight to test how different ALAN intensities affect predator–prey relationships over the full lunar cycle. Exposure to high intensity ALAN (10 and 50 lx) reversed the natural lunar-guided foraging pattern by the gastropod mesopredator Nucella lapillus on its prey Semibalanus balanoides. Foraging decreased during brighter moonlight in naturally lit conditions. When exposed to high intensity ALAN, foraging increased with brighter moonlight. Low intensity ALAN (0.1 and 0.5 lx) had no impact on foraging. Our results show that ALAN alters the foraging pattern guided by changes in moonlight brightness. ALAN impacts on ecosystems can depend on lunar light cycles. Accurate simulations of night-time light cycle will warrant more realistic insights into ALAN impacts and also facilitate advances in fundamental night-time ecology and chronobiology.     

Diel vertical migration and endogenous swimming rhythm in Asterope mariae (Baird) and Philomedes interpuncta (Baird) (Crustacea Ostracoda Cypridinidae)

Among the benthic ostracods Asterope mariae and Philomedes interpuncta, the adult males emerge at dusk from the benthos and swim straight up to the surface, where they soon concentrate in the neustonic realm. The migration starts when the irradiance is almost 1 W cm^-2 and the maximal density occurs at the surface when the light has reached 0.005 W cm^-2, before it is completely dark. The migration ends <1 h later and very few animals are observed in the water column later in the night. Upward migration does not result from a positive phototaxis to dim light, but rather from a strong negative geotaxis and an increase in kinesis or swimming activity which occur at around 1 W cm^-2. This activity is induced both by the end of the diurnal period of photoinhibition and by an endogenous circadian rhythm. This rhythm could be clearly observed in both species from the recordings made during up to 2 weeks in constant darkness. The length of the period was roughly 24 h (24 h 3 min in Asterope, 23 h 10 min i Philomedes). The timing of the activity phase in total darkness varied depending on the post-capture history of the experimental animals, especially on the time of onset of the experiment. The adaptive advantage of this rapid diel vertical migration may be a question of feeding, but this may also increase the nocturnal dispersal of the reproductive males, and thus promote the reproduction of the species.      

Factors affecting recruitment of glass eels into the Grey River, New Zealand

The arrival pattern of glass eels of the shortfin eel Anguilla australis and longfin eel Anguilla dieffenbachii was studied over a two successive migration seasons in the Grey River, South Island, New Zealand. Fishing was carried out on selected nights during September to November, for 3 h per night during 2000 and 2001. The number of shortfin glass eels exceeded longfins in both years; earliest glass eels (September) were larger than later glass eels (November), and longfins larger than shortfins, but there were significant differences in size between years for both species. Environmental variables affecting recruitment differed between years, but common variables for both years were sampling date, time after high tide (incoming tide preferred), time after sunset and moon phase; the influence of moon phase appeared to be expressed both through tides (spring tides preferred) and moonlight. During the year of higher discharge variability, both increasing discharge and increased turbidity were also important.     

The correlation of downwelling irradiance and staggered vertical igration patterns of zooplankton in Wilkinson Basin, Gulf of Maine

Field studies on the characteristics of light that influencevertical migrations in the mesopelagic realm are sparse, dueto the difficulty in simultaneously monitoring changes in speciesdistributions with changes in downwelling irradiance. Usingthe Johnson-Sea-Link submersible as a platform, in situ measurementsof the changes in downwelling irradiance at sunset were madesimultaneously with observations on changes in animal distributionpatterns in Wilkinson Basin, Gulf of Maine. The results indicatethat the vertical migrations for several species of large zooplanktonare staggered, with euphausiids (Meganyctiphanes norvegica)migrating first, cydippid ctenophores (Euplokamus) migratingnext, and two species of caridean shrimp (Dichelopandalus lepiocerusand Pasiphaea multidentata) migrating last Data collected ondaytime dives indicate that the daytime depth distribution isnot solely responsible for the migration order, and that differentspecies may be responding to different cues, or have differentthresholds for the same cue.