Diel reproduction and vertical distributions of the marine cladocerans, Evadne tergestina and Penilia avirostris, in contrasting coastal environments

Evadne tergestina and Penilia avirostris were sampled duringseveral diel cycles in the well-mixed, turbid Inland Sea ofJapan and the more stratified, less turbid Gulf of Mexico shelf.Parthenogenetic female Evadne contained embryos with pigmentedeyes only at night, and apparently released neonates near dawnor in the morning. There was no significant diel vertical migrationin the Gulf of Mexico, and a ‘reverse’ migrationin the Inland Sea. Female Penilia could contain mature embryosat any time, but were most likely to do so at night. Peniliadid not migrate dielly, and in the Gulf of Mexico was concentratedin a nepheloid layer near the bottom.     

Feeding, growth and food conversion of the marine cladoceran Penilia avirostris

Ingestion and clearance rates, feeding behaviors and life historyvariables of the marine cladoceran Penilia avirostris were evaluatedover a range of food concentrations encountered in nature (0.01–3.0mm³ 1^–1 of Isochrysis galbana). Ingestion rates increasedand clearance rates decreased with increasing food concentration.No maximum feeding thresholds were observed over the range ofalgal concentrations offered. Weight-specific ingestion ratesdecreased with increasing body weight. Feeding behaviors suchas mandibular activity, abdominal claw rejections of cloggedfeeding structures and feeding appendage activity decreasedat a food level of 0.3 mm³ l^–1 of l. galbana. Peniliaavirostris had very poor survivorship at extremely low (0.01mm³ l^–1 and high (3.00 mm³ l^–1) food levels. Mortalitywas hardly affected at food levels of 0.03–1.0 mm³ l^–1Reproduction did not occur at food levels of     

Spatial and temporal variations of Penilia avirostris and Evadne tergestina (Crustacea,Branchiopoda) in a Tropical Bay,Brazil

We analysed monthly samples collected in Guanabara Bay, with a conical net of 200 m mesh during 1985. The bay was divided into three areas: an outer region (area A), influenced by oceanic waters; an inner region (area C), influenced by fluvial inflow; and a transition region (area B) with intermediate features. Penilia avirostris and Evadne tergestina were observed in the three areas, with greatest densities, however, in the outermost region, which had the highest salinities and lowest temperatures. Penilia avirostris was more abundant in summer (March), a period with the greatest relative densities of nanoplankton. Evadne tergestina was also abundant in summer, but its peak fell in November, a period with a relative increase in microphytoplankton density in the bay. The two species disappeared in winter: Penilia avirostris was absent from May to August, whereas Evadne tergestina disappeared in August and September.     

The trophic role of the marine cladoceran Penilia avirostris in the Gulf of Trieste

The herbivory of the marine cladoceran Peniliaavirostris was studied in the Gulf of Trieste(Northern Adriatic) from June 1993 to December1994 using the gut fluorescence method. P. avirostrisoccurred from June to December, butreached its greatest abundance in the summer months.A significant correlation between the gut pigmentcontent and chlorophyll a concentration in thesurface layer was established. Observations with anepifluorescence microscope revealed that the guts werefilled with fluorescing nanoplankton and picoplankton(cyanobacteria). Quantitative estimates indicated thatP. avirostris grazed less than 5% of theavailable chlorophyll a in more than half of allmeasurements, but removed most of the availablechlorophyll a in the surface layer during someperiods in September. It can therefore be concludedthat P. avirostris, together with planktonicprotists, plays an important role within the microbialloop.     

Increasing prevalence of the marine cladoceran Penilia avirostris (Dana, 1852) in the North Sea

Rising sea surface temperatures in the North Sea have had consequential effects on not only indigenous plankton species, but also on the possibility of successful colonisation of the area by invasive plankton species. Previous studies have noted the introduction and integration into the plankton community of various phytoplankton species, but establishment of zooplankton organisms in the North Sea is less well-documented. Examining continuous plankton recorder (CPR) survey data and zooplankton results from the Helgoland Roads study, the autumn of 1999 witnessed the occurrence of the marine cladoceran Penilia avirostris in large numbers in the North Sea. The rapid appearance of the species corresponded with exceptionally warm sea surface temperatures (SSTs). Since 1999, the species has become a regular feature of the autumnal zooplankton community of the North Sea. In 2002 and 2003, the species occurred in greater abundance than recorded before. It is suggested that increased autumn SSTs have proved favourable to P. avirostris, with warmer conditions contributing to the success of the species resting eggs and aiding colonisation.Communicated by H.-D. Franke     

Diel vertical migration of zooplankton in the Northeast Atlantic

Acoustic Doppler current profiler (ADCP) data collected duringAugust–September 1991 reveal the diel migration of zooplanktonin the northeast Atlantic (50–60     

Natural ingestion rates and grazing impact of the marine cladoceran Penilia avirostris Dana in Tolo Harbour, Hong Kong

The gut fluorescence method was used to study algal grazingby Penilia avirostris in a semiestuarine embayment. Grazingimpact estimated from ingestion rate and animal abundance revealedthat only {small tilde}1% of the algal biomass was grazed perday by the cladocerans.     

Diel vertical migration: modelling light-mediated mechanisms

Light is generally regarded as the most likely cue used by zooplanktonto regulate their vertical movements through the water column.However, the way in which light is used by zooplankton as acue is not well understood. In this paper we present a mathematicalmodel of diel vertical migration which produces vertical distributionsof zooplankton that vary in space and time. The model is usedto predict the patterns of vertical distribution which resultwhen animals are assumed to adopt one of three commonly proposedmechanisms for vertical swimming. First, we assume zooplanktontend to swim towards a preferred intensity of light. We thenassume zooplankton swim in response to either the rate of changein light intensity or the relative rate of change in light intensity.The model predicts that for all three mechanisms movement isfastest at sunset and sunrise and populations are primarilyinfluenced by eddy diffusion at night in the absence of a lightstimulus. Daytime patterns of vertical distribution differ betweenthe three mechanisms and the reasons for the predicted differencesare discussed. Swimming responses to properties of the lightfield are shown to be adequate for describing did vertical migrationwhere animals congregate in near surface waters during the eveningand reside at deeper depths during the day. However, the modelis unable to explain how some populations halt their ascentbefore reaching surface waters or how populations re-congregatein surface waters a few hours before sunrise, a phenomenon whichis sometimes observed in the field. The model results indicatethat other exogenous or endogenous factors besides light mayplay important roles in regulating vertical movement.     

Diel vertical migration arising in a habitat selection game

Predator and prey react to each other, adjusting their behavior to maximize their fitness and optimizing their food intake while keeping their predation risk as low as possible. In a pelagic environment, prey reduce their predation mortality by adopting a diel vertical migration (DVM) strategy, avoiding their predator during their peak performance by finding refuge in deep layers during daylight hours and feeding at the surface during the night. Due to the duality of the interaction between prey and predator, we used a game theory approach to investigate whether DVM can be a suitable strategy for the predator as well as the prey. We formulated three scenarios in plankton ecology in order to address this question. A novel finding is that mixed strategies emerge as optimal over a range of the parameter space, where part of the predator or prey population adopts a DVM while the rest adopt one or other “sit and wait” strategies.     

Diel vertical migration of Arctic zooplankton during the polar night

High-latitude environments show extreme seasonal variation in physical and biological variables. The classic paradigm of Arctic marine ecosystems holds that most biological processes slow down or cease during the polar night. One key process that is generally assumed to cease during winter is diel vertical migration (DVM) of zooplankton. DVM constitutes the largest synchronized movement of biomass on the planet, and is of paramount importance for marine ecosystem function and carbon cycling. Here we present acoustic data that demonstrate a synchronized DVM behaviour of zooplankton that continues throughout the Arctic winter, in both open and ice-covered waters. We argue that even during the polar night, DVM is regulated by diel variations in solar and lunar illumination, which are at intensities far below the threshold of human perception. We also demonstrate that winter DVM is stronger in open waters compared with ice-covered waters. This suggests that the biologically mediated vertical flux of carbon will increase if there is a continued retreat of the Arctic winter sea ice cover.     

Embryonic development time of Penilia avirostris Dana, 1852 in a tropical bay in Brazil.

Development time of embryos in the brood pouch of the cladoceran Penilia avirostris Dana, 1852, was estimated by collecting zooplankton daily for 15 days in surface water of Guanabara Bay, Brazil. Each day the maturity stage of embryos of 90 parthenogenic females was noted. Total development time (egg to birth) varied from 2 to 3 days, the immature phase (stages I to IV) being generally longer (2 days) than intermediate and mature phases (1 day, stages V to XII). Similar results were obtained from Bottrell's equation, which takes water temperature into account.     

Diel vertical migration and feeding of chaetognaths in coastal waters of the eastern Mediterranean

This study investigates the diel vertical distribution and the diet of the most important chaetognath species found in the 0–50 m surface layer of a coastal area in the eastern Mediterranean during a 24-hour period in July 2004. Among the recorded chaetognaths, Sagitta enflata was the most abundant species (41.6%), followed by S. minima (32.5%) and S. serratodentata (20.8%). Those three species exhibited a “twilight migration” pattern, with only small differences among them. Vertical separation was found between S. enflata and S. minima. Both species preyed mainly on cladocerans, although copepods were the most abundant group in the zooplankton assemblage. The chaetognath species followed partially the diel vertical migration of their prey. S. enflata showed high feeding intensity at different times in both day and night, while S. minima fed more intensively at midday (12:00) and at dusk (20:00), and S. serratodentata in the morning (08:00). It seems that in order to coexist in an area of low productivity the chaetognath species follow the basic ecological rules of space, time and food-type separation, in order to reduce the inter- and intra-specific competition. The high preference of S. minima and especially of S. enflata for the cladocerans made them probably the most important predators of cladocerans during summer.     

Diel vertical migration in zooplankton: rapid individual response to predators

While diel vertical migration in zooplankton has been shownrecently to be a predator avoidance behavior, the mechanismby which predators induce and maintain such behavior has beendebated. We report results of an in situ predator manipulationexperiment during which enclosed populations of the marine planktomccopepod Acaraa hudsonica rapidly changed their vertical distributionand diel migration behavior depending on presence or absenceof the planktivorous fish Casterosteus aculeatus These resultspoint unambiguously to phenotypic behavioral plasticity of individualplanktonic prey, not, as previously hypothesized, population-geneticlevel behavioral changes caused by selective fish predation,as the mechanism underlying changes in diel vertical migrationin this copepod.     

Diel vertical migration of a pelagic amphipod in the absence of fish predation

The pelagic amphipod, Hyalella montezuma, migrates vertically into the surface waters at twilight in Montezuma Well, Arizona, USA despite the absence of fish predators or thermal stratification. We suggest that a persistent, dense, neustonic algal assemblage may provide a food resource incentive for the twilight ascent.     

Diel vertical migration of major fish-species in Lake Victoria, East Africa

Understanding of migration patterns is essential in the interpretation of hydro-acoustic stock assessment data of partly demersal partly pelagic fish stocks. In this paper we provide this kind of information for some species that were common in the Mwanza Gulf of Lake Victoria in the 1980s, before and after the upsurge of introduced Nile perch (Lates niloticus). Detritivorous haplochromines and Nile tilapia (Oreochromis niloticus), both stay near the bottom during day and night. Feeding seems to occur predominantly during the day. The zooplanktivorous haplochromines and dagaa (Rastrineobola argentea) dwell near the bottom by day and migrate towards the surface during the night. They seem to follow their prey, zooplankton and lake-fly larvae. Piscivorous nembe (Schilbe intermedius) show similar migration patterns to zooplanktivorous fishes, but their behaviour cannot be unambiguously explained by pursuit of prey. Nile perch to some extend migrate into the column at night, though the majority remains near the bottom. Feeding takes place during day and night.     

Diel vertical migration of zooplankton following optimal food intake under predation

A model is developed to investigate the trade-offs between benefitsand costs involved in zooplanktonic diel vertical migration(DVM) strategies. The ‘venturous revenue’ (VR) isused as the criterion for optimal trade-offs. It is a functionof environmental factors and the age of zooplankter. Duringvertical migration, animals are assumed to check instantaneouslythe variations of environmental parameters and thereby selectthe optimal behavioral strategy to maximize the value of VR,i.e. taking up as much food as possible with a certain riskof mortality. The model is run on a diel time scale (24 h) infour possible scenarios during the animal’s life history.The results show that zooplankton can perform normal DVM balancingoptimal food intake against predation risk, with the profileof DVM largely modified by the age of zooplankter.     

Diel vertical migration in Mesocyclops edax: implications for predation rate estimates

The diel migration patterns of Mesocyclops edax and its preyin a small lake were followed in two studies separated by approximatelyone year. Gut contents of the predators were examined and selectivityindices calculated at each depth at 0100 h during 1980. Thethree principal zooplankton prey found in the guts of M. edaxwere Keratella, Kellicottia, and Bosmina. The predator and allthree major prey species exhibited unique and different dielvertical distribution and migration patterns. The complex natureof the spatio-temporal variation in prey density to which M.edax is exposed, demonstrates the dangers of using selectivityindices without knowledge of the distribution patterns of bothpredator and prey. An increase in vertebrate predation pressurefrom one year to the next is thought to be responsible for anincrease in the abundance of small zooplankton species, thedisappearance of two out of three of the large zooplankton species,and the onset of a pronounced nocturnal migration pattern inthe third large species. ¹Present address: Biology Department, Williams Hall #31, LehighUniversity, Bethlehem, PA 18015, USA