Biomass and production of Argyrodiaptomus furcatus,a tropical calanoid copepod in Broa Reservoir,southern Brazil

The biomass and the production of Argyrodiaptomus furcatus (Sars), the most abundant copepod in Broa Reservoir (São Carlos, São Paulo State), were estimated, determining in the laboratory the development time and the quantity of organic carbon and establishing the relationship between these two parameters. The daily production was calculated from P = B(1- e^gt) and the annual production was obtained by integrating daily production against time. The maximum production of Argyrodiaptomus furcatus in the reservoir depends on the region considered and on the period of the year. The maximum production was 45.15 mg C m^–3d^–1 in March, 1976 at station II, region of macrophytes and 6.74 mg C m^–3d^–1 at station IV, near the dam. The mean production for the year is 6.26 mg C m^–3d^–1 at station II and 1.43 mg C m^–3d^–1 at station IV.     

Spatial and temporal distribution of copepod communities in the Adriatic Sea

The distribution of planktonic copepods in the Adriatic Seahas been analyzed on the basis of 132 samples collected at 35fixed stations during 4 seasonal cruises. A total of 127 specieshave been determined and 3 characteristic copepod communitieshave been identified. The distribution of copepod species hasbeen found to reflect the dual physiognomy of the Adriatic.The shallow northern section is characterized by high densityvalues, low species diversity, and the dominance of speciesbelonging to the estuarine and coastal communities. The relativelydeep waters of the southern section are characterized by lowdensity values, high species diversity, and the presence ofmany species belonging to the upper, middle, and lower zonesof the oceanic community. Temporal variations in the horizontalspread of these 3 communities are discussed in relation to seasonalcycles in abundance, vertical migration patterns, and the currentregime in the Adriatic.     

Spatial and temporal variations in nematode assemblages in tropical seagrass sediments

The changes in the community dynamics of infaunal nematodes associated with tropical Australian intertidal seagrasses at 4 estuarine sites were investigated through 3 seasons (autumn, winter and spring). Nematode densities were highest in winter in all but one of the sites, ranging from 1971 to 3084 inds./10 cm², with one site showing a highest density of 3411 inds./10 cm² in spring. Multiple regression revealed significant correlations between nematode density peaks and seasonal changes in temperature, salinity and surface seagrass cover. Non-metric Multi-Dimensional Scaling revealed that the communities were characterised by relatively low within-site spatial variability but relatively high temporal variability through the three seasons. This temporal variability was largely due to significant increases in abundance of epistrate-feeding species in winter and spring. An investigation of the dominant epistrate-feeding nematodes revealed that predominantly infaunal species were responsible for overall winter and spring density increases. This study provides further evidence of the role played by temperature in regulating tropical, intertidal meiobenthic communities but also indirectly provides evidence of micro-scale seagrass canopy effects (micro-algal supply and availability) that may further enhance the impacts of these larger-scale seasonal environmental changes on the infaunal nematode community.     

Climatic regime shifts and decadal-scale variability in calanoid copepod populations off southern California

Decadal‐scale climatic regimes and the shifts between them have important impacts on marine ecosystems. Climatic regime shifts have been observed or hypothesized in the North Pacific basin in 1976–77 and 1989. This paper examines long‐term (1951–99) trends in calanoid copepod populations off southern California, and the evidence for responses to regime shifts. Most of the species of calanoid copepod that were analysed underwent one or more step changes during the 49 years covered by the study. All but one of these changes occurred in five periods: the late 1950s, late 1960s, mid‐1970s, early 1980s and around 1990. The late 1960s changes are considered to be artifacts of an increase in sampling depth. Strong El Niño conditions affected California waters during the late 1950s and early 1980s. The step changes of the mid‐1970s and late 1980s to early 1990s may have been responses to regime shifts or other climatic events. 28% of the species and subspecies responded to the 1976–77 event, all increasing in abundance. Another 28% of the copepod categories underwent step changes around 1990, most decreasing. Evidence for regime shifts in the hydrographic variables that were examined is mixed. The 10‐m temperature increased in the mid‐1970s. Abrupt changes in variables around 1990 were short‐lived. However, the population responses around 1990 and to the El Niños of the late 1950s and early 1980s indicate that some species of calanoid copepods may respond on longer time scales to environmental conditions that persist only a few years.     

The ecology of the calanoid copepod Eurytemora affinis in salt marsh tide pools

We describe selected aspects of the ecology of the copepod Eurytemora affinis in tide pools of an inland salt marsh near L'Isle-Verte, Québec along the southern shore of the St. Lawrence estuary. E. affinis performed daily horizontal migrations moving from the centers of pools to the banks and into dense algae. Male E. affinis were mainly found in the center of the pools during twilight (21 : 00 hrs) and in dense algae in daylight (12: 00 hrs) whereas most females and copepodites were found near the banks at all three sampling periods. Although these daily movements among sites may have minimized predation by diurnally foraging sticklebacks (Pisces: Gasterosteidae), other explanations for the movements can not be excluded. We also quantified the effects of fish predation upon the population structure of E. affinis. Densities of all stages (nauplius, copepodite, adult) were significantly lower in pools with fish than in pools without fish. Female E. affinis were significantly smaller (mean length) in pools with fish than in pools without fish, indicating that the sticklebacks selectively ate larger females. Male-biased sex ratios were found in both types of pools, which excluded the possibility that biased ratios in this species are caused by selective predation upon the females.     

Spatial and temporal patterns in the occurrence of peritrich ciliates as epibionts on calanoid copepods in the Chesapeake Bay, USA

We investigated temporal and spatial patterns of distribution in two peritrich ciliates (i.e. Zoothamnium intermedium and Epistylis sp.) living as epibionts on calanoid copepods (i.e. Acartia tonsa and Eurytemora affinis) in Chesapeake Bay. Net tow samples collected along the main axis of the Bay were analyzed to estimate the occurrence of epibionts on copepods and to explore relationships among infestation prevalence, host abundance, and environmental variables. Zoothamnium intermedium and Epistylis sp. colonized populations of A. tonsa during spring and summer months, while only Z. intermedium colonized E. affinis during spring. Occurrence of epibionts on copepods showed high interannual variation, marked seasonality, and geographic heterogeneity. Extensive statistical analyses rejected simple scenarios of interactions between epibiosis, environmental variables, and host density, suggesting a more complex dynamics for the system. Analyses of epibiont colonies and zooids per host area (i.e. the sum of width and length of the body including antennae and swimming legs calculated assuming a cylindrical shape) were also performed. Overall, epibiont infestation prevalence (i.e. colonies/host area) and load (i.e. zooids/host area) were higher on copepodites than on adults for both host species, suggesting a preferential attachment to juveniles, or a higher predation pressure on adult stages. Infestation density and loads of both epibiont species were higher on the cephalothorax and abdomen of A. tonsa and E. affinis in comparison to the antennae and swimming legs, suggesting that ciliates can more easily colonize less active parts of the host.     

Diversity and structure of pelagic copepod populations in the frontal zone of the eastern Alboran sea

During the oceanographic campaign, ALMOFRONT I (April 24–May 26, 1991) a total of 50 vertical zooplankton hauls was carried out in the Alboran Sea using a triple net. Leg A (18 stations; 18 hauls) described a widely spaced grid in the region of the Atlantic current and some northern eddies, while Leg B (16 stations; 32 hauls, 6 hydrodynamically characterized sites sampled during a two day period) covered a more restricted area in the frontal zone. Ninety-one species of copepods were found in 100 samples collected by either 200 µm or 500 µm mesh size nets, including 32 genera and 26 families. Copepod abundance, structure indexes (species richness, evenness, Shannon species diversity index, standardized for unequal sample counts) and species abundance patterns (as rank-frequency diagrams) are presented and compared among the sites of leg-B. Copepod abundance was found to be the highest in the more productive sites of the Atlantic current. Structure indexes values and the changing shape of rank-frequency diagrams give a coherent view of the ecological succession stages of the copepod community. Juvenile stage (1) develops from the Atlantic divergence zone, left side of the jet. More mature stages (1 and 2) occur on the right side of the jet influenced by cross frontal flow. Both abundance and structure indexes decrease within an anticyclonic gyre south to the current. A situation more influenced by oligotrophic conditions was observed in a Mediterranean anticyclonic gyre north of the current. The importance of lateral displacement and meandering of the Atlantic current across the Alboran Sea to the community structure is discussed.     

Spatial distributions of copepod genera along the Atlantic Meridional Transect

The distribution of copepod taxa at a basin scale was analysed using three Atlantic transects (U.K. – Malvinas 1997, Malvinas – U.K. 1997, and South Africa – U.K. 1998). Integrated 200 m to surface zooplankton samples were taken daily, using WP2 nets (200-m mesh). The zooplankton were size-fractionated and sub-samples taken for carbon analysis. The remainder of the samples was preserved for taxonomic analysis of copepod genera. Multidimensional scaling (MDS) was used to identify zoogeographic regions from the copepod genera. Seven regions were identified: northern temperate, northern subtropical, equatorial, southern tropical, southern sub-tropical, southern temperate and Benguela upwelling. Analysis of similarity showed that most regions were significantly different from each other except: northern temperate and southern temperate, northern temperate and southern subtropical, and northern subtropical and southern subtropical. The genera significant in determining the regions were identified. These regions were compared to other schemes of biological and hydrographic areas. The MDS also showed that the copepod composition in the tropical and subtropical regions was less variable than the temperate and Benguela stations. Latitudinal trends in diversity and size were also investigated. Copepod genera showed a reduction in richness at higher latitudes. Copepod size did not show any substantial or consistent change with latitude along these transects, as demonstrated by both the numerical abundances in each size category, and the carbon biomass per individual. The proportion in each size fraction was quite uniform over the transect.     

The calanoid fauna (Crustacea,Copepoda) of the Cordillera Oriental of the Colombian Andes

Calanoid copepods belonging to four species and one subspecies from lakes and ponds mainly from the Cordillera Oriental (22) and Cordillera Central (1) of the Colombian Andes are recorded. Most of the water bodies studied are located in the páramo region, between 2996 and 4085 m altitude. The family Boeckellidae is recorded for the first time from Colombia. Taxonomic characteristics of Boeckella occidentalis Marsh, 1906, Boeckella gracilis (Daday,1902) and the diaptomid Prionodiaptomus colombiensis (Thiébaud, 1914) are discussed. A new diaptomid genus, Colombodiaptomus, with one species and one subspecies are established.     

Dominance,frequency and species richness patterns in occurrences of calanoid copepods in Moreton Bay,Queensland

Of 66 calanoid species recorded in Moreton Bay, the 11 which were most frequent (> 50%) were also amongst the most dominant. Greatest total number of species (56) and greatest species richness (per sample) was found in the oceanic-influenced eastern Bay regions, a decrease toward western Bay estuarine-influenced regions being correlated with a salinity (and temperature) gradient. Maximum species richness in all regions occurred in autumn, seasonal amplitude of change being greatest in the eastern Bay. These data are largely explained by increasing environmental stability in .eastern Bay waters, especially with autumnal oceanic intrusion.     

Diversity and distribution of freshwater copepods (Crustacea) in southeastern Mexico

Plankton and littoral samples were obtained from several ponds and lagoons of different environments in the Mexican state of Tabasco. These systems are located in the Grijalva-Villahermosa or the Usumacinta basins. Twenty-two copepod species were recorded, most of them cyclopoids with neotropical and Caribbean affinities. Physical and chemical data associated with the species, as well as morphometric parameters of the surveyed systems were measured and analyzed. A correlation was found between diversity and parameters related to the productivity and the shoreline development (D _L) of the systems; therefore the littoral heterogeneity could be used to predict the species number in shallow tropical systems. At a regional level, the Tabasco copepod fauna is similar to that known from the Yucatan Peninsula, but different from the faunal associations of central Mexico. Distributional patterns differed slightly between both basins and appear to be correlated with climatic variations. This first survey of the freshwater copepod fauna in the state of Tabasco shows it to be constituted mainly by neotropical forms.     

Life cycle strategies and seasonal variations in distribution and population structure of four dominant calanoid copepod species in the eastern Weddell Sea, Antarctica

The dominant Antarctic copepod species Calanoides acutus, Calanuspropinquus, Rhincalanus gigas and Metridia gerlachei were investigatedwith respect to their abundance, vertical distribution, developmentalstage composition, dry weight and lipid content. The specimenswere sampled during three expeditions to the eastern WeddellSea in summer (January/February 1985), late winter/early spring(October/November 1986) and autumn (April/May 1992) between0 and 1000 m depth to follow the seasonal development of thepopulations. Three species were most abundant in April, onlyC.propinquus reached highest concentrations in February. A seasonalmigration pattern was evident in all four species, but was mostpronounced in C.acutus. In October/November, they inhabiteddeeper water layers, their ascent started by mid-November andin mid-February the species concentrated in the upper 50 m,except for M.gerlachei (50–100 m). Their descent was observedin April/May. The stage composition changed dramatically withseason, the older developmental stages (CIII–CVI) dominatedthe populations in late winter/early spring, whereas youngerstages (CI and CII) prevailed during summer (C.acutus, C.propinquus)or autumn (R.gigas, M.gerlachei). Only C.acutus ceased feedingin autumn and diapaused at depth. Strong differences betweenseasons were also detected in dry weight and lipid levels, withminima in late winter/early spring and maxima in summer (C.acutus,R.gigas) or autumn (C.propinquus, M.gerlachei). Lipid reservesseem to be most important for the older stages of C.acutus andC.propinquus. Based on these seasonal data, different life cyclestrategies are suggested for the four species.     

Zooplankton abundance and transport in a tropical white-water river

Zooplankton abundance and transport were studied in the Apure River of western Venuzuela over a 15-month period. Much of the lower portion of the drainage basin, which is an extensive savanna of low relief, is subject to seasonal flooding. Although more than 50 zooplankton species were recorded during the study, nine rotifer species accounted for more than 90% of total density (mean, 138 individuals · L^-1). Copepods were represented primarily by Mesocyclops decipiens. Most cladocerans were planktonic, but cladocerans were not abundant. Crustaceans comprised 46% of annual mean zooplankton biomass (1.9 gC·L^-1) but only 2% of zooplankton numbers. The annual transport of zooplankton biomass from the watershed was 29 300 kgC. Zooplankton abundance showed a strong, inverse relationship to discharge. Secondary branches (caños) of the braided channel appeared to be the primary source areas, but populations of some species also reproduced in the main channel at low water. As the river began to rise, caños were flushed and thus abruptly ceased to serve as source areas. Zooplankton transport fell sharply and remained low until the river inundated the floodplain. Secondary production in the floodplain was exported to the river as long as a connection existed. After drainage ceased from the floodplain, transport fell to very low levels until caños again became suitable habitat. Seasonal fluctuations in river level regulate the development of source areas suitable for zooplankton growth and control the export of plankton from the source areas. Plankton in the running waters reproduce only at low water.     

Zooplankton Spatial and Temporal Distribution in a Tropical Oceanic Area off West Africa

Tropical instability vortices are believed to modify the trophic food web by affecting plankton production. Such instabilities have been evidenced in a tuna fishing area of the Atlantic (10°–20° W and 2°–5° N). However, the origin of the tuna abundance remains uncertain as no data on zooplankton communities on which tuna preys are supposed to fed upon are available. This study was focused on short term spatial and diel variations of mesozooplankton communities sampled at 2° N during and after the passage of an instability wave evidenced from satellite imagery, at 4° N and at 0°, i.e. in the Equatorial divergence. Samples were collected with an opening–closing multisampler and with a WP2 net (200 μm). Copepods prevailed (86–92% of the total zooplankton) with Clausocalanidae, Oncaeidae, Corycaeidae, Calanidae, Eucalanidae as dominant families. At 0° and at 2° N during the wave, large-sized organisms constituted over 50% of total dry weight (DW). These percentages decreased after the wave and at 4° N. Zooplankton DW and densities were highest at 0° and lowest at 4° N. At 2° N after the wave, we observed a strong increase in the abundance of zooplankton which surpassed that at 0°. This increase mainly resulted from an increase in small copepods and copepodites. Our results suggest that the high production linked to the upwelling of nutrients caused by the geostrophic divergence is partly advected northward by the instability wave. The advection did not reach 4° N which displayed the characteristics of a typical tropical structure.     

Some remarks on the latitudinal distribution of Cladocera on the Indian subcontinent

One hundred and thirty species of Cladocera have been recorded from the Indian subcontinent which extends from 6°N to 37°N latitude and covers an area of 4.5 million km². The equatorial region has few Daphnia species, all belonging to the sub-genus Ctenodaphnia while the more northern parts have more species of Daphni including Daphnia s. str. The limnetic Cladocera lacks the carnivorous Polyphemidae and Leptodoridae at lower latitudes (equatorial). The common limnetic species of the equatorial region are eurytopic and extend throughout the subcontinent.     

Historical biogeography and distribution of the freshwater calanoid copepods (Crustacea: Copepoda) of the Yucatan Peninsula, Mexico

Aim To determine and analyse the distribution of the freshwater calanoid copepod (Diaptomidae and Pseudodiaptomidae) fauna of the Yucatan Peninsula (YP) and its relation to the geological history of this Neotropical karstic plain. Location The Yucatan Peninsula, Mexico. Methods Plotting of geo‐referenced sites, analysis of local and regional geological history, analysis and comparison of regional and local records. Results The current composition and distribution of Diaptomidae and Pseudodiaptomidae in the YP mainly reflects recent, post‐Pliocene colonization events. This invasion did not reach America, but only parts of Central America (CA). The presence of diaptomids in the continent since the pre‐Cretaceous and the presumed post‐Cretaceous (Palaeocene–Oligocene) radiation of Diaptomidae in Middle America suggest earlier colonizations of the YP. The marine transgressions kept most of the YP submerged in different geological periods, thus eliminating any original primary freshwater colonizers, such as the diaptomids. The periods of marine regression probably represented opportunities for new waves of diaptomid colonization of the YP. The latest dispersal of diaptomids in the YP during the Holocene (8000 yr bp ) was probably an intermittent process because of the alternative dry and wet periods and interglacial transgressions. The presence of the Nearctic Leptodiaptomus and Arctodiaptomus in the YP and the current distribution of Mastigodiaptomus might represent remnants of earlier invasions of Diaptomidae in Middle America. The Neotropical Mastigodiaptomus probably originated in the Late Cretacic CA/proto‐Antilles complex. Forms derived from a M. albuquerquensis type ancestor radiated into the YP leaving relatively isolated populations of three species in the northern half of the peninsula. The distribution of the brackish water Pseudodiaptomus marshi well inside the coastline might have resulted from stranding and subsequent adaptation of this species during a marine regression in the Bacalar formation; this agrees with the vision of this taxon as being in process of invasion of freshwater environments. Main conclusions It is not probable that the South American (SA) diaptomid fauna originated from an invasion of upper Neotropical/Nearctic forms. The current distribution of freshwater calanoid copepods reflects relatively recent, post‐Pliocene biogeographical patterns, but probably older patterns are involved as well. The northern and eastern coasts of the Yucatan are the most recently colonized by diaptomids. Differing from other freshwater groups surveyed in the Yucatan that have marine relatives (i.e. fishes, amphipods, isopods, mysids, macrocrustaceans), there is no evidence of local vicariant events involving cave‐dwelling forms or marine relicts in the diaptomid fauna of the YP.     

Influence of short-term nutritional variations on digestive enzyme and fatty acid patterns of the calanoid copepod Temora longicornis

Temora longicornis, a dominant calanoid copepod species in the North Sea, is characterised by low lipid reserves and high biomass turnover rates. To survive and reproduce successfully, this species needs continuous food supply and thus requires a highly flexible digestive system to exploit various food sources. Information on the capacity of digestive enzymes is scarce and therefore the aim of our study was to investigate the enzymatic capability to respond to quickly changing nutritional conditions. We conducted two feeding experiments with female T. longicornis from the southern North Sea off Helgoland. In the first experiment in 2005, we tested how digestive enzyme activities and enzyme patterns as revealed by substrate SDS-PAGE (sodium dodecylsulfate polyacrylamide gel electrophoresis) responded to changes in food composition. Females were incubated for three days fed ad libitum with either the heterotrophic dinoflagellate Oxyrrhis marina or the diatom Thalassiosira weissflogii. At the beginning and at the end of the experiment, copepods were deep-frozen for analyses. The lipolytic enzyme activity did not change over the course of the experiment but the enzyme patterns did, indicating a distinct diet-induced response. In a second experiment in 2008, we therefore focused on the enzyme patterns, testing how fast changes occur and whether feeding on the same algal species leads to similar patterns. In this experiment, we kept the females for 4 days at surplus food while changing the algal food species daily. At day 1, copepods were offered O. marina. On day 2, females received the cryptophycean Rhodomonas baltica followed by T. weissflogii on day 3. On day 4 copepods were again fed with O. marina. Each day, copepods were frozen for analysis by means of substrate SDS-PAGE. This showed that within 24 h new digestive enzymes appeared on the electrophoresis gels while others disappeared with the introduction of a new food species, and that the patterns were similar on day 1 and 4, when females were fed with O. marina. In addition, we monitored the fatty acid compositions of the copepods, and this indicated that specific algal fatty acids were quickly incorporated. With such short time lags between substrate availability and enzyme response, T. longicornis can successfully exploit short-term food sources and is thus well adapted to changes in food availability, as they often occur in its natural environment due seasonal variations in phyto- and microzooplankton distribution.     

Relationships between fecal pellet production and feeding in the calanoid copepod Boeckella

The number, size (mm³ pellet ^–1) and total volume of fecalpellets produced (mm³ h^–1) by Boeckella titicacae increasedwith concentration of Chlorella and natural seston in threedifferent lakewaters from Bahia de Puno and Lago Grande of LakeTiticaca and nearby Laguna Arapa. The three lakewaters differedin the size and number of fecal pellets produced at seston concentrations>0.5 mg dry wt l^–1. However, the total volume of fecalpellets egested varied little. Large-sized food (>10 µg)resulted in larger pellets but smaller total fecal production.Food concentration had parallel effects on the production offeces and on feeding rates. Both fecal production and feedingincreased linearly with food concentration with saturation between5 and 7.5 x 10⁵ Chlorella ml^–1 (3.7–5.6 mg dry wtl^–1). Varied responses in fecal production when fed differentsize fractions of lake seston suggest Boeckella has a complexand flexible feeding behavior. Feeding preferences of B.titicacaeand B.occidentalis (Ivlev index) for two seston size fractionsare generally reflected by fecal production.     

Spatial and temporal variations in Caribbean zooplankton near Puerto Rico

Spatial and temporal patterns of abundance were examined inselected zooplankton categories sampled from oceanic surfacewaters (upper 100 m) south of Puerto Rico during 1980. In general,variations in abundances: (i) did not increase with spatialscales ranging from a few to –150 km; and (ii) were notcorrelated with geostrophic current patterns, These resultsimply a lack of zooplankton pattern characteristic of thesespatial scales, hence that corresponding physical processesdo not affect zooplankcton abundances in this region. Significantlyhigher abundances of zooplankton occurred during July in conjunctionwith peak chlorophyll a concentrations, the initial seasonaldecline in surface salinities and an eastward geostrophic flow.These observations are consistent with the hypothesis that upwellingat the edge of large low salinity lenses underlies increasedabundances of plankton. Because these variations in salinitiesresult from the annual pattern of rainfall in low latitude zonesof the Western Hemisphere, we conclude that the temporal patternof zooplankton abundance represents true seasonality and isassociated with large scale, regional processes occurring throughoutthe Eastern Caribbean.     

Shallow males, deep females: sex-biased differences in habitat distribution of the freshwater calanoid copepod Arctodiaptomus alpinus

In a small, circular high-altitude karst lake situated in the northeastern calcareous Alps of Austria the copepod Arctodiaptomus alpinus (Imhof) develops each spring from resting eggs after the lake basin fills in early spring. After mid-summer, late developmental stages and adults concentrate in a thin layer near the sediment throughout the lake basin. During daytime an average 86% and 71% of female and male copepods respectively resided near the bottom. Males exhibited stronger diel vertical migrations than females and part of the population concentrated in near-surface water at night, but the majority of copepods remained at the sediment during darkness. Macro-photographs and core samples revealed that the proportion of epibenthic male A. alpinus decreased constantly from >70% in 3 m depth to <20% below the 8-m isobath. The proportion of ovigerous females increased with depth, whereas non-ovigerous females were comparatively more abundant in shallow water. Concurrently the highest frequencies of copulating copepods were detected in areas of high male density. Above the 3-m isobath male A. alpinus formed dense swarms in scattered patches of the macrophyte Ranunculus eradicatus (>93% males; maximum abundance: 362 600 individuals m⁻²). Adults and larvae of the alpine newt Triturus alpestris are the top predators in this fishless alpine lake, and A. alpinus constitutes a major component of their diet. However, predator densities and different predation rates on male and female copepods were not sufficient to explain the observed horizontal distribution of A. alpinus . We argue that the optimization of successful sexual encounters in copepods is the ultimate driving force behind this segregation of both sexes of A. alpinus ('mate encounter hypothesis').