Distribution and diel vertical migration of the euphausiid Thysanoessa macrura in Gerlache Strait,Antarctica

Thysanoessa macrura was found throughout Gerlache Strait, Antarctica, during four surveys carried out from 30 October to 23 November 1989, with the highest abundance being 332 individuals m^–2 (0–290 m). Reproduction had begun just before the surveys took place, as indicated by the presence of females with attached spermatophores and of larvae. Thirteen-month old females were reproductive. Larvae in 9 depth strata between 0–290 m were dominated by calyptopis stages, and developed from calyptopis 1 to furcilia 1 during November. Larval abundance was not correlated to chlorophyll a concentration, which showed a consistent east-west gradient in Gerlache Strait with highest concentrations (>30 mg chlorophyll a m^–3) in bays of the Antarctic Peninsula. Survival of larvae appeared to not be affected by phytoplankton abundance. Older T. macrura showed strong diel vertical migration between the surface at night and depths to 120 m during mid-day. Larvae were consistently found in the chlorophyll a-rich upper 50 m during night (90%) and day (81%), while adults and juveniles were found in the upper 50m at night (83%), but only 16% remained there during the day.     

Diel vertical migrations of invertebrate predators (Leptodora kindtii ,Thermocyclops taihokuensis, andMesocyclops sp .) in a shallow, eutrophic lake

We studied vertical distribution patterns of three invertebrate predators – Leptodora kindtii, Mesocyclopssp., and Thermocyclops taihokuensis– in a shallow eutrophic lake, Lake Suwa , Japan. From June to October in 2000 and 2001, we collected samples in the lake center in order to examine the vertical distribution patterns and the densities of the predators in the water column during the day (0900) and at night (2330). We also examined phototactic behavior of Leptodora in the laboratory. The three invertebrate predators showed clear migration patterns. Leptodora and Thermocyclops displayed a typical migration, avoiding the surface and maintaining a high abundance in deeper water during the day, and being distributed uniformly during the night. Mesocyclops, on the other hand, showed no clear vertical distribution pattern in the water column. However, Mesocyclops showed higher densities in the water column during the night than during the day. It suggests that they stayed just above the bottom during the day and migrated upward during the night. Leptodora also showed such a density difference between day and night. In the laboratory, Leptodora showed strong negative phototactic behavior. The observed density changes between day and night in Leptodora and Mesocyclops suggests the possible underestimation of their population density by usual sampling methods, and thus the impact of predation on populations of prey zooplankton species may also be underestimated in shallow water bodies.     

Evidence for up-estuary transport of puffer Takifugu larvae (Tetraodontidae) in Ariake Bay, Japan

The present study determined whether puffer Takifugu rubripes and T. xanthopterus larvae use selective tidal stream transport (STST) for migration into the nursery area. The influence of the tidal cycle on the vertical distribution of Thkifugu larvae was investigated during a 24 h sampling period at one location off Shimabara Peninsula in Ariake Bay. Samples were collected in three depth layers, from near the sea floor to near the surface (5, 20 and 30 m depth). The change in vertical distribution in relation to tidal phase was not observed. This data did not support STST hypothesis. Diel vertical migration was observed irrespective of tidal phase, where larvae migrated to the middle layer during the night, and sank to the bottom layer during the day, however, larvae hardly emerged into the surface layer during the study period. In Ariake Bay, the residual current leads to a layered vertically stratified structure, in which surface water flows towards the mouth and the middle-bottom water flows toward inner part of the Bay. It is suggested that Takifugu larvae use not STST but residual currents for transport into the nursery ground, namely, undergoing nocturnal diel vertical migration in the water column between the middle layer and the bottom layer where the net flow is northward.     

Vertical distribution of two substages of stage Calyptopis I of Euphausia superba Dana,in South Orkneys Area

Summary Samples of Euphausia superba larvae at the Calyptopis I stage, taken in the upper 200 m near South Orkney Islands, have been analyzed discriminating early and advanced substages on the basis of the presence or absence of pleon segmentation, evidenced through the actual cuticle. A significant difference in pleon length between these forms is demonstrated, suggesting growth between them. Results show no difference between these in relation to their vertical distribution; supporting the view that developmental ascent has been already completed in the area. In both substages, a significant difference between night and day catches existed, but no evidence of diel migration was found in their vertical distributions.     

Diel vertical migration by males,females, copepodids and nauplii in a limnetic population of diaptomus (Copepoda)

Diel vertical migration (DVM) by the sexes and life history stages of the copepod Diaptomus novamexicanus (Herrick) was investigated during two summers in a subalpine lake (Castle Lake, California, USA). Migratory behavior was quantified repeatedly by 1300 and 21–2200 hr sampling in the epilimnion. Population density and vertical distribution were estimated routinely at 1300 hr and at 2200 hr on selected dates. Males had greater night to day biomass differentials and ratios than females. The sexes showed comparable DVM participation (% of total biomass migrating) and amplitudes of migration (distance of ascent). Copepodids displayed higher participation, night/day ratios and amplitudes than younger copepods, but adult participation differed only from that of nauplii. Active participation in high amplitude ascents thus increased with age and was greatest in adult males. Analysis suggests that such relative behaviors probably occur in most limnetic Diaptomus, and that the magnitude of ascent into the epilimnion is associated with daytime vertical distribution.     

Ovigerity,selective predation,and variable diel vertical migration in Euchaeta elongata (Copepoda: Calanoida)

Summary We present a statistical analysis of a previously published (Yen, 1983) but heretofore unanalyzed data set on the vertical distributions and diel vertical migration (DVM) of adult females of the marine planktonic copepod Euchaeta elongata in Dabob Bay, Washington, USA. Non-ovigerous females were strongly migratory on all four dates sampled, residing between 75–175 m during the day and at shallower depths during the night, commonly entering the upper 50 m of the water column. In contrast, ovigerous females were non-migratory or weakly migratory, largely remaining between 100–175 m both day and night, and entering the upper 50 m of the water column only rarely. Thus non-ovigerous females always migrated much more strongly, as measured by both amplitude of migration and the proportion of animals migrating, than did ovigerous females. These results led us to hypothesize that differential susceptibility to visually orienting predators was the cause of these differences in DVM behavior in female E. elongata, and we subsequently undertook an experimental study of the feeding selectivity of the copepod's natural predator, Pacific herring (Clupea harengus pallasi). Pacific herring exhibited a highly significant preference for ovigerous over nonovigerous adult female E. elongata. The demographic consequences of variable DVM in adult female E. elongata were investigated by way of life table analyses. Results indicated that under conditions of thermal stratification of the water column there is a distinct demographic disadvantage (reduced rate of realized population growth) incurred by non-migratory or weakly migratory ovigerous females due to delayed egg development at cooler subsurface temperatures. We conclude that ovigerous female E. elongata remain at depth both day and night to avoid visually orienting predators, and that such behavior must afford the copepod a demographic advantage of no less than a 26% reduction in adult mortality to offset the demographic cost of delayed egg development.     

Phytoplankton vertical distributions and composition in Baltic Sea cyanobacterial blooms

We studied the vertical structure of the phytoplankton community in two toxic cyanobacterial blooms in the offshore Baltic Sea. In 1994, vertically separated potentially toxic, diazotrophic and mixotrophic species (belonging to Cyanophyceae, Dinophyceae and Prymnesiophyceae) dominated. In 1997, picocyanobacteria, mainly in colonies, made up 40–50% of the total phytoplankton carbon biomass in the top 20 m both day and night. Colony-forming species of picocyanobacteria seem to be occasionally important and hitherto underestimated in the Baltic Sea.We found species-specific depth distribution patterns. Nodularia spumigena and Anabaena spp. were observed mainly above 10 m depth, while Aphanizomenon sp. was mostly found deeper, especially at night. Dinophysis norvegica was only abundant near the seasonal pycnocline and showed very limited diurnal migration. Other flagellates, including small Cryptophyceae and 10 identified Chrysochromulina species, occurred down to 40 m depth. Their vertical migration may help to retrieve nutrients from below the summer pycnocline.We conclude that considerable differences in dominating functional groups may occur between years/bloom stages, and that the vertical distribution pattern of many species is recurring at similar environmental conditions, suggesting species-specific niche-separation.     

Diel vertical migration of the copepod <Emphasis Type="Italic">Thermocyclops inversus</Emphasis> (Kiefer, 1936) in a tropical reservoir: the role of oxygen and the spatial overlap with <Emphasis Type="Italic">Chaoborus</Emphasis>

The effect of water transparency, dissolved oxygen concentration and the invertebrate predator Chaoborus brasiliensis on the day–night vertical distribution of the copepod cyclopoid Thermocyclops inversus was investigated in a shallow tropical reservoir, Nado Reservoir, Belo Horizonte, Brazil. Diel cycles were carried out over a period of 12 consecutive months, between October 1999 and September 2000. The different developmental stages of T. inversus exhibited diel vertical migration (DVM) and displayed a clear ontogenetic trend, with the amplitude of DVM increasing with the age of the organism, and ranging from 0.4 m to 0.8 m for nauplii, 0.4 m to 1.2 m for copepodite, and 1.1 m to 2.1 m for adults. We observed that seasonal changes in dissolved oxygen and C. brasiliensis directly influenced the vertical distribution of the copepod population in this reservoir. Furthermore, it was showed that the diurnal vertical migration is an important predator avoidance behavior since it diminished the spatial overlap between prey and its potential predator. This finding supports the hypothesis that the vertical migration is a defense mechanism against predation. Thus, T. inversus is able to remain in the anoxic layers during day light hours, and at night they move upwards avoiding hypolimnetic waters to escape from predation by Chaoborus.     

Evaluation of genes involved in Norway lobster (<Emphasis Type="Italic">Nephrops norvegicus</Emphasis>) female sexual maturation using transcriptomic analysis

In marine ecosystems, the most significant migration observed in terms of biomass distribution is the one connected with the vertical movements in the water column. In the present study, the vertical profiles of the mesopelagic shrimps Gennadas elegans, Eusergestes arcticus, Sergia robusta, and the epipelagic Parasergestes vigilax in the Balearic Sea (western Mediterranean), during the stratified (summer) and non-stratified (autumn) hydrographic conditions, were investigated through their ontogeny, from the larval to adult stages. The mesopelagic adults were observed to move down to the deeper layers during the night more than during the daylight hours. Most larvae aggregated within the limits of the upper water column. The P. vigilax larvae were collected only during the stratified period. The first two larval stages vertical distribution indicates that the mesopelagic crustacean spawning could occur at greater depths. During the non-stratified period, the larvae of the mesopelagic species tended to remain at about 500 m depth at night, rising towards the upper layers at sunrise. Vertical patterns are discussed, as strategies associated with predator–prey trade-offs. To our knowledge, the present study is the first such attempt to jointly analyze the vertical migrations of the developmental stages of the pelagic shrimps in the Mediterranean Sea.     

Day‐Night Vertical Distribution and Feeding Patterns of Fourth Instar ofChaoborus Larvae in a Neotropical Reservoir (Socuy Reservoir,Venezuela)

The day‐night vertical distribution, diel feeding activity and diet of fourth instar of Chaoborus larvae were analyzed in lacustrine zone of a neotropical reservoir which shows seasonally contrasting hypolimnetic oxygen conditions. Larvae stayed in sediment and water bottom during day and ascended to surface during night. Results indicate that feeding activity is limited mainly to the plankton population. Phytoplankton, rotifers or remains of Chaoborus larvae were not found in crops. With the exception of ostracods, all crustacean prey available in the zooplankton occurred in the guts. Ceriodaphnia cornuta and Moina micrura were the most frequent food items (about 75% of occurrence frequency) and were positively selected. The remainder crustacean zooplankton taxa were negatively selected by larvae. The most intense feeding activity in larvae occurred near midnight and sunrise, in dates when the hypolimnion was anoxic. When oxygen was available on the bottom, a higher and not changing diel feeding activity was detected. Our results indicate that vertical migration may promote a spatial separation between larvae and zooplankton, and feeding activity of larvae occurred only when both overlapped. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Vertical distributions and relations of euphausiid populations off Elephant Island,March 1984

Summary Distributional relationships are described for post-larval and larval Euphausia superba and Thysanoessa sp. (probably macrura) and post-larval Euphausia frigida collected in 0–70/80 m and 0–175/200 m depth ranges with a MOCNESS sampler north of Elephant Island (61°S, 55°W) during 17–23 March 1984. Larval E. superba (predominantly calyptopes stage 2 and 3) were rare shallower than 80 m at night. Day catches of post-larval E. suberba were small and night catches were primarily near the top of the thermocline above 50 m depth. Thysanoessa sp. occurred throughout the 0–200 m depth range and was abundant in the upper 80 m both night and day. E. frigida migrated to the upper 80 m at night from deeper day depths. Larval stages of E. superba and bost-larval stages of all three species demonstrated independent and variable vertical distribution patterns both night and day. Changes in E. superba abundance and distributional patterns could to a certain extent be associated with observed environmental changes. An increase in larval and decrease in post-larval E. superba abundances between 0–80 m was associated with an intrusion of cold water at depth. At night, vertically restricted concentrations of post-larval E. superba were associated with shallow mixed layer depths, and a significant vertical separation of developmental stages and size categories was observed only during periods of stratification in the upper 80 m. Fluctuations in the distribution and abundance of Thysanoessa sp. and distribution of E. frigida did not appear to be influenced by physical parameters within the upper 80 m. Within the 0–80 m depth range, the distributions of these two species differed from each other and from E. superba and showed large tow to tow variability that could not be related to physical parameters in the upper water column.     

Diel vertical migration of <Emphasis Type="Italic">Peridiniopsis niei</Emphasis>, Liu et al., a new species of dinoflagellates in an eutrophic bay of Three-Gorge Reservoir,China

In spring, a typical dinoflagellate (Peridiniopsis niei Liu et al.) constitutes most of the phytoplankton biomass in most of eutrophic bay of Three-Gorge Reservoir (TGR, China). There are few field observations on vertical migration of the members of the genus Peridiniopsis. We studied diel vertical migration of the new species (Peridiniopsis niei) at sampling stations A and B in an eutrophic bay (Xiangxi Bay) of TGR during both day and night in March 2007. The present study suggests that vertical migration by Peridiniopsis niei is similar in nature to the migration patterns observed for other dinoflagellates. Solar irradiance incident was an important factor regulating the ascent and descent of Peridiniopsis niei. The vertical descent of Peridiniopsis niei during nighttime was restricted when the thermocline (Temperature gradient >1°C m⁻¹) was present in the station B.     

Diel vertical migration of the loosejaw dragonfishes (Stomiiformes: Stomiidae: Malacosteinae): a new analysis for rare pelagic taxa

The diel vertical migration (DVM) of three genera of the stomiid subfamily Malacosteinae (Photostomias, Aristostomias and Malacosteus) was analysed from capture records of nearly 300 specimens in the Atlantic Ocean. To account for broad temporal and geographic scales encountered in this study, local time of capture was transformed to a corrected time representing position in a solar day. Species of Photostomias and Aristostomias undertake asynchronous DVMs characterized by a residence in the mesopelagic zone during the day and separate migrating and non‐migrating subpopulations at night. Species of Photostomias displayed an asynchronous DVM pattern characterized by a residence in the lower mesopelagic zone (>500 m) during the day and a segregated distribution at night. Specimens of Photostomias guernei captured at night in the mesopelagic were nearly identical in size to those captured in the epipelagic; however, day epipelagic specimens were stratified by size. In species of Aristostomias, few specimens were caught in the mesopelagic zone during the day and only small specimens were captured in the mesopelagic zone at night, indicating that sampling depth may not have been adequate to capture the bulk of mesopelagic daytime residents and the entire size range of the non‐migrating night‐time residents. In contrast, Malacosteus niger was distributed below 600 m, did not regularly migrate to the epipelagic zone and was stratified across the 700 m isobath. From these data, relationships between DVM patterns, morphology and foraging ecology are inferred and biases and applications of this method are discussed.     

Vertical, lateral and longitudinal movement of zooplankton in a large river

1. The spatial distribution and movement patterns of zooplankton in large rivers are little known compared with those in lake environments. We conducted a series of studies in the Ohio River (U.S.A.) during the low flow period to assess diel vertical (DVM), longitudinal and lateral movement of crustacean zooplankton. 2. The dominant large zooplankter, the copepod Eurytemora affinis, showed a consistent vertical migration pattern of daytime ascent and night‐time descent during all sampling periods – the reverse of the most common migratory pattern of zooplankton in lakes. The cladoceran Bosmina migrated in a similar way in two of the three sampling periods. Surveys taken longitudinally in the river showed similar trends for both taxa. 3. During the lateral surveys, E. affinis was significantly more abundant in the shallow littoral zone during the night than in the daytime. The combination of vertical and lateral movement patterns along with the diel distribution of zooplanktivorous fish suggest that these movements are a predator‐avoidance mechanism. 4. Sampling programmes in large rivers should consider that larger zooplankton such as E. affinis may not be randomly distributed in the river channel and behaviours such as diel vertical migration may be just as evident in river habitats as in lakes.     

The Zooplankton community of Croker Passage,Antarctic Peninsula

Summary Zooplankton species composition, abundance and vertical distribution were investigated in the upper 1000 m of Croker Passage, Antarctic Peninsula during the austral fall (March–April, 1983). 106 species were identified, many being mesopelagic and reported previously from the Southern Ocean. The most numerous species (>1000/100 m³) were the copepodsMetridia gerlachei, Microcalanus pygmaeus, Oncaea antarctica andOncaea curvata. Oncaea curvata alone constituted half the zooplakton population. Zooplankton biomass was dominated by three copepod species,Metridia gerlachei, Calanoides aculus andEuchaeta antarctica,which comprised 74% of the biomass. Size analysis revealed most of the zooplankton numbers were in the >1 mm fraction. The biomass distribution was polymodal with major maxima in the >1 mm and the 4–4.9 mm size classe. The >1 mm peak, exclusive of protozoans, was primarily copepod nauplii and copepodites ofOncaea, Metridia andMicrocalanus. The 4–4.9 mm peak was mostlyCalanoides acutus andMetridia gerlachei.All of the principal species had broad vertical distributions both day and night. There was some suggestion of diel vertial migration byMetridia gerlachei andEuchaeta antarctica, with segments of their populations migrating into the upper 100 m and 200 m, respectively, at night. Most of the dominant and subdominant species were concentrated below 200 m,with only the subdominantOithona similis having its maximum in the epipelagic zone. The occurrence of zooplankton at winter depths appears to have been earlier in Croker Passage in 1983 than has been generally reported for waters south of the Polar Front.Total standing stock of net-caught zooplankton (>15 mm) in the upper 1000 m was estimated at 3.1 gDW/m², which is somewhat higher than values reported for the West Wind Drift and for open ocean areas of temperate to tropical latitudes.Euphausia superba (17–52 mm) dominated the pelagic biomass, exceeding zooplankton standing stock under a square meter of ocean by a factor of 15. This is in contrast to lower latitudes where zooplankton biomass is usually greater than macrozooplankton-micronekton.     

Summer depth selection in crustacean zooplankton in nutrient‐poor boreal lakes is affected by recent residential development

1. Strong vertical gradients in light, water temperature, oxygen, algal concentration and predator encounters during summer in stratified lakes may influence patterns of depth selection in crustacean zooplankton, especially Daphnia species. 2. To test how crustacean depth selection varies among lakes along a gradient of catchment disturbance by recent residential development and land use change, we calculated the weighted mean depth distribution of the biomass of crustaceans by day and night in eight nutrient‐poor boreal lakes. 3. Generally, the greatest biomass of crustaceans was located at the metalimnion or at the lower boundary of the euphotic zone during thermal stratification in July. The crustacean zooplankton avoided warm surface layers and tended to stay in colder deep waters by both day and night. They also remained at greater depths in lakes with a more extensive euphotic zone. 4. There was some evidence of upward nocturnal migrations of large Daphnia and copepods in some lakes, and one case of downward migration in a lake inhabited by chaoborid larvae. 5. Multivariate regression trees (MRT) were used to cluster crustaceans and Daphnia species in homogeneous groups based on lake natural and disturbance factors. For crustaceans, the depth of the euphotic zone, the sampling depth (epilimnion, metalimnion and hypolimnion), time (day or night) of sampling and the biomass of chlorophyll a were the main driving factors. For Daphnia species, the drainage area, the sampling depth, the cleared land surface area within the catchment and the concentration of total dissolved phosphorus were the main factors.     

Flux of decapod larvae and juveniles at a station in the lower Canal de Mira (Ria de Aveiro,Portugal) during one lunar month

Summary

Emigration and immigration of decapod larvae from estuaries depend on timing of larvae occurrence in the water column relative to the tidal, tidal amplitude and day cycles. The phase relation of these natural cycles varies with tidal regime and geographically, resulting in different time-patterns of hatching of first stage larvae and of presence of late stage larvae in the water column. Vertical migration behaviour according to phase of tide also controls transport inside estuaries. These mechanisms were investigated in a field study conducted on the northwest coast of Portugal where neap ebb tides occur during the night around the quarters of the moon. Flux of decapod larvae through one sampling station was measured during one lunar month at the Canal de Mira (Ria de Aveiro) in the spring of 1990. The sampling programme was comprised of a set of 25-h fixed station studies, separated by 25-h intervals during which no sampling took place. Plankton samples were collected with a pump every hour at three depths. Current velocity and direction at the standard depths, as well as height of the water column, were also measured every hour. Hourly instantaneous flux of larvae through a 1-m-wide vertical section of the Canal de Mira was calculated for the most abundant forms. A total number of 13 combinations of species and larval stages were analyzed, belonging to the families Atelecyclidae, Pirimelidae, Portunidae, Pilumnidae, Grapsidae, Palaemonidae, Crangonidae and Thalassinidae. Patterns of net larval flux along the lunar month could be grouped into three types. Type 1 includes first zoeas that were consistently exported to the sea. Type 2 comprises late zoeas, megalops and juveniles that were consistently imported into the estuary. First zoeas that were imported during some of the 25-h studies but were exported during the others were included in Type 3; in species of this type import periods appeared to alternate with export periods according to lunar phase. Flux of Type 1 larvae followed a semi-lunar pattern. Release activity of Type 1 zoeas took place during the night and started during neap tides around the quarters of the moon, but maximum releases occurred 3–4 h after high tide of average amplitude tides, 3–4 days after the quadratures. These observations agree with the hypothesis that hatching is timed to occur on ebb tides of the largest possible amplitude so that larvae are easily dispersed from areas with a high density of predator fishes. However, based on other observations on the Portuguese coast, it cannot be ruled out that hatching might depend on a minimum number of hours of darkness experienced by the females. Larvae included in Type 2 comprise forms that may have been retained inside the estuary for the entire larval phase, as well as one form that was imported from shelf waters. No semi-lunar pattern of import was detected in this last form. Fluctuations of net flux observed in Type 3 larvae, as well in other forms that were not included in any of the types, were more difficult to explain. These larvae were first zoeas of species belonging to different taxonomic, morphological and ecological groups and may show a diversity of adaptations to the way of life of the adults. Imports and exports of larvae depended not only on time-patterns of abundance, but also on time-patterns of larval vertical distribution. As a general rule, larval stages showed patterns of depth distribution that were consistent with vertical migration rhythmic behaviours synchronized with the tidal cycle. Though the effect was not always statistically significant, first-stage larvae were closer to the surface during ebb, especially during the night, enhancing seaward transport. On the contrary, later zoeal stages, megalops and juveniles were usually closer to the surface during flood, suggesting migration to the water column during this phase of the tide and landward transport.     

A metalimnetic oxygen minimum indirectly contributing to the low biomass of cladocerans in Lake Hiidenvesi – a diurnal study on the refuge effect

The diurnal vertical migrations of smelt (Osmerus eperlanus), larvae of phantom midge (Chaoborus flavicans) and cladoceran zooplankton in eutrophic Lake Hiidenvesi were studied in order to clarify the factors behind the low zooplankton biomass. In the study area, an oxygen minimum occurred in the metalimnion in the 10–15 m depth. No diurnal fluctuations in the position of the minimum were observed. Cladocerans inhabited the epilimnion throughout the study period and their vertical movements were restricted to above the thermocline and above the oxygen minimum. C. flavicansconducted a diurnal migration. During the day, the majority of the population inhabited the 12 – 15 m depth just in the oxygen minimum, while during darkness they were found in the uppermost 8 m. Smelts started ascending towards the water surface before sunset and reached the uppermost 3 m around 23:00. During daytime, the majority of smelts inhabited the depth of 7–9 m, where the water temperature was unfavourably high for them (18 °C). Smelts thus probably avoided the steep oxygen gradient in the metalimnion, whereas Chaoborusused the oxygen minimum as a refuge against predation. Those smelts that were found in the same water layers as Chaoborusused the larvae as their main prey. The metalimnetic oxygen minimum thus seemed to favour the coexistence of vertebrate and invertebrate predators, leading to a depression of cladoceran zooplankton.     

On the geographic distribution and migration of I- and II-group eel larvae as studied during the 1979 Sargasso Sea Expedition

During the 1979 Sargasso Sea Expedition, 423 larvae ofAnguilla anguilla and 5 larvae ofA. rostrata were caught on three Atlantic transects and two cruises in the Sargasso Sea. Results of the identification of the larvae by myomere counts, and limits of the occurrence of I- and II-group larvae are presented. Four standard fishing depths are compared. A range shallower than 25 m was found to be the optimal fishing depth by night for both larval length groups. The geographic distribution of length group I was observed in central and eastern North Atlantic. Available data indicate a migration of these larvae in a north easterly direction. Length measurements of the II-group larvae taken from catches on the European continental slope during the same expedition support this assumption.     

Hatching rhythms and dispersion of decapod crustacean larvae in a brackish coastal lagoon in Argentina

Mar Chiquita, a brackish coastal lagoon in central Argentina, is inhabited by dense populations of two intertidal grapsid crab species,Cyrtograpsus angulatus andChasmagnathus granulata. During a preliminary one-year study and a subsequent intensive sampling programme (November–December 1992), the physical properties and the occurrence of decapod crustacean larvae in the surface water of the lagoon were investigated. The lagoon is characterized by highly variable physical conditions, with oligohaline waters frequently predominating over extended periods. The adjacent coastal waters show a complex pattern of semidiurnal tides that often do not influence the lagoon, due to the existence of a sandbar across its entrance. Besides frequently occurring larvae (exclusively freshly hatched zoeae and a few megalopae) of the two dominating crab species, those of three other brachyurans (Plathyxanthus crenulatus, Uca uruguayensis, Pinnixa patagonica) and of one anomuran (the porcellanidPachycheles haigae) were also found occasionally. Caridean shrimp (Palaemonetes argentinus) larvae occurred in a moderate number of samples, with a maximum density of 800·m⁻³. The highest larval abundance was recorded inC. angulatus, with almost 8000°m⁻³. Significantly moreC. angulatus andC. granulata zoeae occurred at night than during daylight conditions, and more larvae (statistically significant only in the former species) during ebb (outflowing) than during flood (inflowing) tides. In consequence, most crab zoeae were observed during nocturnal ebb, the least with diurnal flood tides. Our data suggest that crab larvae do not develop in the lagoon, where the adult populations live, but exhibit an export strategy, probably based upon exogenously coordinated egg hatching rhythms. Zoeal development must take place in coastal marine waters, from where the megalopa eventually returns for settlement and metamorphosis in the lagoon. Significantly higher larval frequency ofC. granulata in low salinities (≤12‰) and at a particular sampling site may be related to local distribution patterns of the reproducing adult population. Unlike crab larvae, those of shrimp (P. argentinus) are retained inside the lagoon, where they develop from hatching through metamorphosis. They significantly prefer low salinity and occur at the lagoon surface more often at night. These patterns cannot be explained by larval release rhythms like those in brachyuran crabs, but may reflect diel vertical migrations to the bottom. It is concluded that osmotic stress as well as predation pressure exerted by visually directed predators (small species or life-cycle stages of estuarine fishes) may be the principal selection factors for the evolution of hatching and migration rhythms in decapod larvae, and that these are characteristics of export or retention mechanisms, respectively.