Patterns of life history among cyclopoid copepods of central Europe

• 1 Life history characters (body size of adults, egg diameter, egg sac length and breadth) of nineteen species of central European cyclopoid copepods were measured and sexual size dimorphism (adult female length x adult male length^?1), relative egg size (egg weight X body weight^?1), weight of adult females and of eggs, egg sac shape (egg sac length x egg sac breadth^?1), and reproductive effort (clutch weight produced per female weight per day) were calculated to detect trends in life history strategies.
• 2 Typical planktonic species exhibited the lowest reproductive effort. Among planktonic species, the value for egg sac shape increased with clutch size.
• 3 Large species and small species exhibited different trends in life history characters. Large species had larger clutches, larger eggs, and a greater sex size dimorphism than small species. However, small species had a greater relative egg size.
• 4 Large species live in cold water and reproduce during the spring bloom of phytoplankton where the production of large clutches with relatively small eggs is advantageous. Reserves are unnecessary for juveniles because food is abundant. Small species generally are most abundant during the warm season, when conditions are less predictable, and relatively large eggs, possibly provided with reserves, are advantageous.

     

Influence of spatial heterogeneity on predation by the flatworm Mesostoma ehrenbergii (Focke) on calanoid and cyclopoid copepods

Through laboratory experiments, we analysed the influence ofspatial heterogeneity on predation by Mesostoma ehrenbergiion the calanoid Boeckella gracilis and the cyclopoid Acanthocyclopsrobustus in four horizontal and two vertical spatial arrangements.This spatial heterogeneity simulated that of Juncaceae stems,a major macrophyte in the natural environment of these zooplankton.Our results indicated that M. ehrenbergii preyed differentlyon these copepod species. The rate of predation of M. ehrenbergiion A. robustus females reached saturation in all the treatmentswith and without horizontal spatial heterogeneity, but lowerpredation rates were observed in the medium heterogeneity treatment.Predation rates of Mesostoma on B. gracilis increased with theincrease in prey abundance in the treatment without heterogeneity,while predation rates reached saturation in the treatments withhorizontal spatial heterogeneity. Mesostoma ehrenbergii consumedmales and females of B. gracilis in each of the arrangementstested. In natural habitats, interaction between extent of macrophytedevelopment and intensity of predation by M. ehrenbergii oncopepods species may be expected. We suggest that the structuralcomplexity given by macrophytes in Patagonian fishless habitatsprovide a bottleneck for M. ehrenbergii predation. This paper was presented at Plankton Symposium III, held atFiguera da Foz, Portugal between 17 and 20 March 2005, underthe auspices of the University of Coimbra and the Universityof Aveiro, and coordinated by Mário Jorge Pereira andUlisses M. Azeiteiro.     

The lipid biochemistry of calanoid copepods

Calanus species, particularly those in high latitudes, can accumulate large oil reserves consisting predominantly of wax esters. These wax esters consist predominantly of 16:0, 20:1 (n–9) and 22:1 (n–11) fatty alcohols, mainly formed de novo by the animals from non-lipid dietary precursors, esterified with various fatty acids that are often polyunsaturated fatty acids and largely of dietary, phytoplanktonic origin. Wax ester formation is maximal in copepodite stages IV and V. The lipids are elaborated not primarily for buoyancy regulation but as a source of metabolic energy during overwintering, particularly for reproduction. Large quantities of wax esters are utilised for gonadal development when stage V copepodites mature to females. Development of stage V copepodites to males is not accompanied by wax ester utilisation but males consume large amounts of these lipids in physical activity during reproduction. The role of wax esters in the life history of calanoids is illustrated with particular reference to a comparison of Calanus finmarchicus and Metridia longa in Balsfjord, northern Norway.     

Origin and evolution of the parasitic cyclopoid copepods

Six of the 10 recognised families of the order Cyclopoida are parasitic, with 4 of them occurring on marine invertebrates and the remaining 2 on freshwater gastropods and fishes, respectively. A cladistic analysis of the 10 families indicates that evolution of parasitism occurred twice in the history of the cyclopoids. The first attempt was made by the marine epibenthic ancestors seeking food and shelter in sessile tunicates — the ascidians. This event led to the evolution of 2 ascidicolous families: Archinotodelphyidae and Notodelphyidae. The descendant of this lineage had also invaded the mantle cavity of marine bivalve molluscs, eventually leading to the evolution of the Mantridae. The second attempt for the parasitic mode of life was launched by the ancestor which was the sister group of the ancestral cyclopoids — the most successful family of freshwater copepods. This ancestral stock, while living in the coastal zone, split into 2 groups: one group stayed behind in the ocean and colonised again the ascidians; the other groups invaded freshwater and evolved into the fish-parasitising Lernaeidae and the gastropod-parasitising Ozmanidae.     

Use of cyclopoid copepods for mosquito control

The New Orleans Mosquito Control Board mass produces Mesocyclops longisetus and Macrocyclops albidus for introduction to mosquito breeding sites as a routine part of control operations. Mesocyclops longisetus is used in tires that collect rainwater; M. albidus is used in temporary pools. Field trials in a Spartina marsh, rice fields, and residential roadside ditches in Louisiana suggest that M. longisetus and M. albidus could be of use to control larvae of Anopheles spp. and Culex quinquefasciatus. Mesocyclops longisetus has proved to be effective for Aedes aegypti control in cisterns, 55-gallon drums, and other domestic containers in Honduras.     

Variation in torpidity of diapause in freshwater cyclopoid copepods

By successively filtering off animals emerging from mud cores taken to the laboratory, the degree of torpidity within the same species was found to vary considerably both time and space. Variation in time from the start of dormancy follows a pattern of diapause development similar to that of insect diapause. In some localities dormancy is deep and prolonged, while in others it is less pronounced and lasts for a shorter period. The depth of torpidity during winter seems to be correlated to an induction early in the year. The intensity of diapause varies along the slope of the localities and down into the bottom mud. In some cases high temperatures seem to induce a deeper torpidity.     

Emergence and dynamics of cyclopoid copepods in an unpredictable environment

SUMMARY. 1. We studied species composition, abundance and population dynamics of cyclopoid copepods emerging from dormancy from the sediments of a temporary pond in South Carolina in 1985, 1988 and 1989. During a drought in 1988–89, the maximum hydroperiod was 19 days; in 1985 and late 1989-early 1990, the hydroperiods were 57 and 118 days. We also report on species present in 1984 and 1987, two years that had longer hydroperiods, and on abundance of cyclopoids in 1984. 2. Within a day after standing water appeared, fourth-instar copepodids of Diacydops haueri and D. crassicaudis brachycercus became active. These two species appeared every time the pond filled in winter, spring, or autumn. Other species, including Acanthocyclops vernalis, were usually not collected until weeks or months after the pond filled. Because the times and durations of fillings were extremely variable, species composition differed among years, with the most species (eleven) appearing in 1984, the year with the longest hydroperiod. 3. The abundances of emerging Diacydops were much lower in 1988 and 1989 (range of means from seven fillings in spring and autumn: 675–7382 animals m^?2) than 1985 (range of means from three fillings in winter: 26,037–107,418 animals m^?2). Low abundances of emerging animals could have been caused by poor survival of dormant animals, poor production during preceding seasons, or incomplete emergence of the dormant populations. 4. Substrate samples from the dry pond were collected in spring, summer, and autumn 1988 and winter 1989 to measure emergence of the cyclopoids in laboratory incubations. Population densities of emerging animals were much lower in 1988 and 1989 (range of means from seven experiments: 0–120 animals m^?2 over the first 3 days) than in similar experiments in 1984 (Taylor & Mahoney, 1990, means from two experiments: 3630 and 6890 animals m^?2). 5. Despite the low abundance of animals emerging from dormancy in late 1989, the cyclopoid populations in 1990 reached similar densities of copepodids (10^4?-10⁵ animals m^?2) to those reached in 1984 and 1985. These results suggest that short generation time and high reproductive capacity permit rapid recovery from population reductions.     

Perceptive performance and feeding behavior of calanoid copepods

The goal of this study was to determine variables associatedwith calanoid feeding behavior, and thus, to improve our understandingof the basics of calanoid feeding rates. These variables includedperiods and frequency of appendage motion, rates of cell clearance,distance at which a copepod first reacts to a cell which iseventually captured, and rate of water flow through the areacovered by the motions of a copepod's feeding appendages. Theeffects of these variables on feeding rates were determinedfor copepodids and adult females of the calanoid copepod Eucalanuspileatus at phytoplankton concentrations covering the rangeencountered by this species on the south-eastern shelf of theUSA. Our results indicate that the distance at which E.pileatusperceives phytoplankton cells increases {small tilde}2-foldas food concentrations decrease from 1.0 to 0.1 mm³ l^–1.These results lead us to hypothesize that this is due to increasedsensitivity of chemosensors on the copepods' feeding appendages.This 2-fold increase in perceptive distance amounts to a near4-fold increase in perceived volume which is close to the 6-foldincrease in volume swept clear (VSC) from 1.0 to 0.1 mm³ l^–1of Thalassiosira weissflogii. We assume that the increases inVSC by planktonic copepods, when food levels are below satiation,are largely a function of the sensory performance of the individualcopepod.     

The calanoid copepods of the Nile system

At least 11 or 12 calanoid taxa occur in the Nile system. One species is typical of the river and of Lake Chad (Th. galebi); two are restricted to Lake Victoria (Th. galeboides, T. stuhlmanni), but both might be only subspecies to more widespread species. One is restricted to Lakes Edward and George (T. worthingtoni) but is a little known species. No calanoids are on record from Lakes Albert and Kyoga, while the species reported from Lake Turkana (T. banforanus) is out of range, and almost certainly represents an erroneous record. Among the remaining species, four are East-African, ranging from the southern tip of the continent to the Ethiopian plateau and the Nile valley (P. schultzei, Th. mixtus, T. kraepelini, T. cf orientalis), while two are Sahelian species that span Africa from east to west (M. mauretanicus, T. processifer et ssp.).Two new synonyms are introduced. One new subspecies (T. processifier friedae) is described from the Ethiopian plateau. It is suggested that Tropodiaptomus orientalis (Brady, 1886), the type species of its genus, should not be considered a nomen dubium, but should be redescribed on its type female(s) and on topotypical males from Sri Lanka.     

Mating frequency and interspecific matings in some freshwater cyclopoid copepods

Laboratory investigations with Cyclops vicinus and Metacyclops minutus documented that mating frequency decreased significantly after the initial mating, i.e. re-mating is relatively rare in cyclopoid copepods. Re-mating is unnecessary because females are able to fertilize multiple clutches of viable eggs from one insemination. Similar sized Cyclops vicinus and Cyclops furcifer interbreed frequently. Interbreeding does not occur when size differences and taxonomic differences are as great as between C. vicinus and M. leuckarti. In C. vicinus and M. leuckarti, the duration of the last mating phase (spermatophore transfer until release of the female) and consequently the duration of the entire mating process, is different. I conclude that the low re-mating frequency in cyclopoid copepods probably evolved to accelerate clutch production in unpredictable environments and to reduce predation risk. The occurrence of interspecific mating discourages the co-existence of similarly sized, related species. Mating behaviour may have a considerable influence on zooplankton community structure.     

Feeding and digestive activity of cyclopoid copepods in active diapause

Three different methods were used to compare the feeding patterns of active cyclopoids and cyclopoids in active diapause from different habitats. First, we offered ¹⁴C- labeled algal detritus to stage III, stage IV (two different physiological groups), and adult females of Acanthocyclops vernalis from temporary water-bodies of the Volga Delta. Second, we compared the functional responses of active and actively diapausing cyclopoids preying on copepod nauplii. Stage III, stage IV (various physiological groups), stage V, adult males and females of Acanthocyclops viridis, Cyclops kolensis, C. abyssorum sevani from the Volga Delta, Lake Sevan (Armenia), and Mozhaisk Reservoir (Moscow District) were tested. Finally, we examined the activity of digestive enzymes (proteases) of CIV of C. kolensis from Mozhaisk Reservoir at different temperatures and temperature acclimation regimes. All three approaches used have led to similar conclusions. Actively diapausing cyclopoids were capable of feeding on both plant and animal prey; however, their feeding rates and digestive enzyme activity were 2–8 times lower than those of active cyclopoids of similar size. At the initial stage of active diapause the feeding rates were comparable with those of active cyclopoids then declined to a low level during approximately two weeks. Transfer of cyclopoids undergoing winter diapause from ambient temperature (4 °C) to warmer water (20 °C) resulted in dramatic increase of proteases activity only if the acclimation period was relatively long (two weeks).     

Body size and sexual size dimorphism in calanoid copepods

Patterns of sexual size dimorphism and body size in calanoid copepods are examined. We hypothesize that favorable conditions for development will result in large body size and high sexual size dimorphism among populations of a given species and that differences in this allometric relationship among species is governed by the male's role in insemination. We confirm that there is a greater advantage to large female size, normally the larger sex, when compared to males, hence leading to selection for developmental patterns favoring high size dimorphism. Individuals from populations of four centropagid copepod species were measured; other sizes were obtained from published sources. In the four species we examined, the relationships between prosome length and both clutch size and the ability to produce multiple clutches with one insemination were determined. Results show a trend toward hyperallometry in all centropagid species examined: sexual size dimorphism increases with increasing size. Large females produce larger clutches and more additional clutches on one insemination. That hyperallometry is not observed in diaptomid copepods may result from the greater role the male plays in reproduction. Males are needed for each clutch produced, hence the selective pressure to be larger is greater than that in the centropagidae.     

Sexual dimorphism in calanoid copepods: morphology and function

Mate location and recognition are essentially asymmetrical processes in the reproductive biology of calanoid copepods with the active partner (the male) locating and catching the largely passive partner (the female). This behavioural asymmetry has led to the evolution of sexual dimorphism in copepods, playing many pivotal roles during the various successive phases of copulatory and post-copulatory behaviour. Sexually dimorphic appendages and structures are engaged in (1) mate recognition by the male; (2) capture of the female by the male; (3) transfer and attachment of a spermatophore to the female by the male; (4) removal of discharged spermatophore(s) by the female; and (5) fertilization and release of the eggs by the female. In many male calanoids, the antennulary chemosensory system is enhanced at the final moult and this enhancement appears to be strongly linked to their mate-locating role, i.e. detection of sex pheromones released by the female. It can be extreme in calanoids inhabiting oceanic waters, taking the form of a doubling in the number of aesthetascs on almost every segment, and is less expressed in forms residing in turbulent, neritic waters. Mate recognition is a process where chemoreception and mechanoreception presumably work in conjunction. The less elaborate male chemosensory system in the Centropagoidea is counterbalanced by females playing a more active role in generating hydromechanical cues. This is reflected in females in the shape of the posterior prosomal margin, the complexity of urosomal morphology and the size of the caudal setae. Visual mate recognition may be important in the Pontellidae, which typically show sexual dimorphism in eye design. The most distinctive sexual dimorphism is the atrophy of the mouthparts of non-feeding males, illustrating how copepod detection systems can be shifted to a new modality at the final moult. In the next phase, the male captures the female using the geniculate antennule and/or other appendages. Three types of antennulary geniculations are recognized, and their detailed morphology suggests that they have originated independently. Grasping efficiency can be enhanced by the development of supplemental hinges. The scanty data on capture mechanisms in males lacking geniculate antennules are reviewed. It is suggested that the loss of the antennulary geniculation in many non-centropagoidean calanoids has evolved in response to increasing predator pressure imposed on pairs in amplexus. Spermatophore transfer and placement are generally accomplished by the modified leg 5 of the male. In some males, leg 5 consists of both a chelate grasping leg and a spermatophore-transferring leg, whereas in others, only the latter is developed. Tufts of fine setules/spinules and/or sclerotized elements on the terminal portion of the leg are involved in the transfer and attachment of the spermatophore. The configuration of gonopores, copulatory pores and their connecting ducts in the female genital double-somite is diversified in the early calanoid offshoots such as Arietellidae and Metridinidae, whereas in more derived groups, it is constant and invariable, with paired gonopores and copulatory pores located beneath a single genital operculum. The absence of seminal receptacles in most Centropagoidea limits the female's ability to store sufficient sperm for multiple egg batches, suggesting that repeated mating is necessary for sustained egg production. Discharged spermatophores are usually removed by the female leg 5 and/or specialized elements on other legs. In Tortanus (Atortus) Ohtsuka, which has rudimentary fifth legs in the female and complex coupling devices in the male, a spermatophore supposedly remains on the female urosome, since eggs appear to be released from a ventral opening of the spermatophore. The type of sexual dimorphism is closely related to habitat and biology. Some hyperbenthic families never show multiplication of aesthetascs on the male antennule, whereas families of the open pelagic realm such as the Aetideidae always have non-feeding males exhibiting secondary multiplication of antennulary aesthetascs. The various aspects and diversity of calanoid sexual dimorphism are herein considered in an evolutionary context.     

Escape reaction performance of myelinated and non-myelinated calanoid copepods

Calanoid copepods from seven families in three superfamilies were exposed to a controlled near-field hydrodynamic stimulus and their escape reactions were recorded using high-speed videographic techniques. Copepod species have two distinct mechanisms for increasing conduction speed of neural signals: larger diameter nerve axons and insulated axons, i.e., myelination. Myelinated axons have been found in certain species of the more recently-evolved calanoid superfamilies. Copepod representatives from these superfamilies were expected to have shorter response latencies than species from more ancestral superfamilies due to the increased conduction speed of nerve impulses in myelinated neurons. Using frame-by-frame playback and computerized motion analysis techniques, response latency, jump speed, and acceleration were measured. Kinetic performance of copepods was highly variable, with mean escape speeds ranging between 100-250 mm s^− 1 and accelerations of 9-230 m s^− 2. Minimum behavioral response latencies of 2 ms were recorded for both myelinated and non-myelinated calanoids. There was no significant difference between the response latencies of copepods from the myelinated and non-myelinated superfamilies. Furthermore, no relationships were found between copepod latency and size for either myelinated or non-myelinated species. Previous research may suggest that myelin may shorten the response latencies of certain calanoid species. However, our results show that non-myelinated copepods are also capable of responding rapidly, within as few as 2 ms, to hydrodynamic stimuli and produce similar kinetic performance to myelinated species. The main advantage of myelination over giant nerve axons is their more efficient transfer of nerve impulses resulting in a metabolic energy savings. Although this energetic reward would be important for copepods in food-limited environments, for coastal copepods, in food-rich habitats, either mechanism is a viable solution.     

Biological observations on small cyclopoid copepods in the Red Sea

The breeding activity, sex ratio and variation in body lengthof a number of cyclopoid copepod species from the upper 450mof the Red Sea were analyzed. In most species the body lengthof adult specimens was in the lower range or less than thosereported from other pelagic regions. In the central Red Seamost species had some individuals actively breeding, duringboth autumn and winter. To the north, breeding was observedin fewer species. Only two species, Paroithona sp. and Agetuslimbatus, however, seemed to find conditions for reproductionbetter in the north than in the central area. The sex ratioswere quite consistent for all species within the families Sapphirinidaeand Corycaeidae (high percentage of males) and Oithonidae (lowpercentage of males). Within the Oncaeidae, the percentage ofmales varied from a few percent to >60% of adults. Thereappears to be a relationship between sex ratio and size. Populationsof the very small Oncaea species, O.ivlevi, O.prendeli and O.zernovi,include far fewer males than those of their larger congeners.The available information on the biology of Oncaea is summarized.Peculiarities in their life histories and reproduction are discussed.     

The problem of induction and termination of diapause in cyclopoid copepods

The seasonal activity and diapause in Cyclopidae was investigated in natural water-bodies in the vicinity of Kiev and in experiments. It was established that in stenothermal species (cold-loving and warmth-loving) the temperature thresholds of passing into an active state are nearer to the temperature optimum of each species than the temperature thresholds of passing into resting state. But temperature seemingly is not the only factor leading to changes in the activity state of populations. It seems probable that in the appearance of a particular adaptation some definite irritating external factor, regularly repeating each year, and permitting adaptation to it, acquires the role of a switch-factor directing the physiological processes of the organism to move in a new way — entering the resting state or leaving it.     

Filtering and feeding rates of cyclopoid copepods feeding on phytoplankton

The algal biomass ingested by omnivorous cyclopoid copepods (Cyclops kolensis and C. vicinus) was measured by two methods in the hypertrophic Heiligensee in Berlin (West Germany). The clearance and ingestion rates inferred from measurements of natural populations of ¹⁴C labelled phytoplankton were compared with those obtained from chlorophyll a determinations using the presence/absence method (observed chlorophyll a content of natural lake phytoplankton with and without addition of cyclopoids). Both methods gave similar results. Nevertheless, the radio tracer method is preferred, mainly because the short feeding duration excludes high variations in both the food composition and food concentration that limit the presence/absence method.     

Mechanoreceptors in calanoid copepods: designed for high sensitivity

The mechanoreceptors of the first antennae of Pleuromamma xiphias, a mesopelagic calanoid copepod, are critical for the detection of potential threats. These receptors exceed the physiological performance of other crustacean mechanoreceptors in sensitivity to water velocities as well as in frequency response. A study of these receptors was initiated to elucidate structure–function relationships. Morphologically, the receptors resemble the arthropod scolopidial organs by the presence of a scolopale tube. However, the rigidity of the copepod receptors greatly exceeds those described for crustaceans and other arthropods. The scolopale tube completely encloses the distal dendrites and it is firmly anchored to the cuticle. Microtubules are organized in register and arise from microtubule subfibers associated with crescent-shaped rods which extend from the basal body region to the setal socket. The distal dendrites are filled with a large number of cross-linked microtubules. Termination of the distal dendrites inside the lumen of the setae is gradual with a firm anchoring to the cuticle. A likely mechanism for mechanotransduction would involve a linkage between individual microtubules and mechano-gated channels in the dendritic membrane. The rigidity probably contributes to the high frequency sensitivity, and termination of the dendrite inside the seta increases the overall sensitivity of these receptors.