The effect of reproductive condition on egg production rates in the planktonic copepod Centropages typicus

Centropages typicus females oscillated between a dark, ripestage of oogenesis and a light, unripe stage of primary oocytes.The number of eggs released after 24 h under different experimentalconditions was strongly dependent on the reproductive statusof females at the time of incubation.     

Naupliar growth versus egg production in the calanoid copepod Centropages typicus

Growth rates of the copepod Centropages typicus were assessedduring May 1998 in the Alboran Sea. The goal of the work wasto compare field growth rates of juvenile (artificial cohortmethod) and adult copepods (female egg production rates) underan extensive range of natural conditions. The results showedthat adult and juvenile growth rates were similar in some stations,while in others no such relationship was apparent. In contrastto the common belief that juvenile growth may be saturated undernatural conditions whereas adults may be food-limited, in mostof our study, adult growth rates were greater than juveniles.We discuss these results in the light of food availability atthe surveyed stations. In summary, the assumption that copepodgrowth rates estimated through egg production rates of adultsare equivalent to juvenile growth is not always valid.     

Is fecundity modified by crowding in the copepod Centropages typicus?

Experiments were conducted to test the effect of overcrowdingon egg production rates in the copepod Centropages typicus.Fertility was not significantly reduced by the presence of otherconspecifics when the volume of water was the same for femalesincubated alone or in the presence of other females and males.There was no evidence of chemical cues affecting egg productionwhen females were kept in large (1 female in 100 ml sea water[SW] or small (1 female in 25 ml SW) volumes of water, withor without partitions, and in the presence or absence of foodor males. Also, spawning rate was not reduced when females wereincubated in flow-through experimental chambers which alloweddiffusion of pheromones between females incubated alone or inhigh densities (100 ind. in 50 ml SW). Only when females wereplaced at unnaturally high densities (1 female ml^–1) wasegg production significantly reduced, probably due to physicaldisturbance and increased number of collisions rather than chemicalcommunication among individuals. The results show that chemicalcues do not affect spawning behaviour when population densitiesare close to field conditions. By contrast, overcrowding inducedby swarming or aggregation may potentially limit reproductivesuccess due to reduced feeding, and production and cannibalismof eggs.     

Seasonal fluctuations in fecundity and hatching success in the planktonic copepod Centropages typicus

A mismatch of {small tilde}2–3 months was observed betweenreproductive rates and population abundances in the planktoniccopepod Centropages typicus, with the highest production atrelatively low female abundance and low production at high abundance,during the course of a 2 year study in 1989 and 1990 in neriticwaters of the Gulf of Naples. During this period, egg mortalitywas at times severe, with values as high as 59% in February1989. The seasonal trend in percentage hatching success didnot match seasonal fluctuations in breeding intensity, and wasnot correlated with variations in environmental variables suchas temperature and chlorophyll a. The results of experimentsusing the fluorescent dye Hoechst 33342 showed that unhatchedeggs had been fertilized and that in most cases developmenthad proceeded to an advanced stage before death of the embryo.The causes of high egg mortality are uncertain, but here wegive evidence, for the first time, that egg mortality may notonly be due to infertility caused by failure to remate.     

Development, growth and egg production of the two copepod species Centropages hamatus and Centropages typicus in the laboratory

Laboratory experiments were set up in order to determine andcompare developmental rates, growth rates, generation timesand egg production rates for the two calanoid copepod speciesCentropages typicus and Centropages hamatus. The nauplii showeda higher developmental rate than the copepodites for both specieswith quite different individual stage durations, which gaveno indication of isochronal development. For C typicus equiproportionaldevelopment was found. The growth rates were exponential andhighest for the largest species C typicus, and for both speciesthe juvenile growth rates were very similar to the egg productionrates of the adults.     

Laboratory study of the spawning rate of the calanoid copepod Centropages typicus: effect of fluctuating food concentration

The spawning rate of laboratory-reared Centropages typicus fedHymenomonas elongala increases with food concentration, up toa value of {small tilde}2800 µg C (16 500 cells) ml^–1.An alternation of a low food (1000 cells ml^–1) and highfood concentration (16 500 cells ml^–1) is not favourableto egg release when its periodicity is 1 or 2 days, whereasit may be of advantage if it is longer (3–6 days). Inthe latter case, Centropages typicus will benefit best fromthe rich food diet if this coincides with (or just follows)the last moult.     

Oogenesis in Centropages typicus (Copepoda, Calanoida)

We carried out a complete study of oogenesis in Centropages typicus using structural, ultrastructural and cytochemical data. The usual stages of oogenesis, i.e. germinative phase, premeiosis, primary and secondary vitellogenesis, were found. The latter two stages were the most typical. Primary vitellogenesis consisted of endogenous yolk accumulations; these substances, probably of lipoprotein or lipoglycoprotein nature, were produced at the granular endoplasmic reticulum level and then stocked in the reticulum cavities. During secondary vitellogenesis, endogenous yolk production continued, but we mainly observed the development of exogenous yolk accumulation (lipid droplets and protein globules) in the ooplasm. These accumulations resulted from the fusion of very numerous pinocytotic vesicles arising from the oolemma and containing substances probably brought to the oocytes by the hemolymph. The effect of various proteases on the vitellus globules caused a more or less marked digestion of their contents, tending to prove their protein nature. The end of vitellogenesis was marked by the appearance of vacuolar formations with dense lamellae which could correspond to cortical granules.     

Spatial distribution of the copepod Centropages typicus in Ligurian Sea (NW Mediterranean). Role of surface currents estimated by Topex-Poseidon altimetry

A particle-tracking model was used to simulate the dispersion and development of the planktonic copepod Centropages typicus during spring in Ligurian Sea. We show that mesoscale current structure, with a coastal jet and eddies, plays a key role in the transport and dispersion of C. typicus during its life cycle. Although, in the north, offshore Nice, cohorts can be advected southwestward out of Ligurian basin, more to the south others are retained in the central eddy and may give the start to the spring bloom of this species. However, input of individuals from the south through the Corsican Channel and along the west coast of Corsica may also be important in spring. This study shows that the ambit of C. typicus population is larger than the Ligurian Sea.     

Short communication. Food availability and diel feeding rhythms in the marine copepods Acartia grani and Centropages typicus

The effects of food availability on the daily feeding pattern of planktonic copepods was studied for two species: Acartia grani (cultured and wild specimens) and Centropages typicus (wild specimens). Both species showed clear diel feeding rhythms at high food concentrations, with significantly higher clearance rates during night hours. However, at limiting food conditions, while C.typicus maintained its daily feeding pattern, A.grani fed continuously, with similar day and night clearance rates. These results could be explained in relation to differences in predation risks and probability of experiencing starvation episodes in their respective habitats, as well as their capacity for dampening food fluctuations. On the other hand, the maintenance (although with less intensity) of the diel feeding rhythm in A.grani, when cultured in the laboratory for long periods of time (>10 generations) and in the absence of predators, suggests the importance of the endogenous control of copepod feeding patterns.      

Mediterranean marine copepods: basin-scale trends of the calanoid Centropages typicus

The Mediterranean Sea is located in a crossroad of mid-latitude and subtropical climatic modes that enhance contrasting environmental conditions over both latitudinal and longitudinal ranges. Here, we show that the large-scale environmental forcing is reflected in the basin scale trends of the adult population of the calanoid copepod Centropages typicus. The species is distributed over the whole Mediterranean basin, and maximal abundances were found in the north-western basin associated to oceanic fronts, and in the Adriatic Sea associated to shallow and semi enclosed waters. The peak of main abundances of C. typicus correlates with the latitudinal temperature gradient and the highest seasonal abundances occurred in spring within the 14–18°C temperature window. Such thermal cline may define the latitudinal geographic region where C. typicus seasonally dominates the >200 μm-sized spring copepod community in the Mediterranean Sea. The approach used here is generally applicable to investigate the large-scale spatial patterns of other planktonic organisms and to identify favourable environmental windows for population development.     

New view of population genetics of zooplankton: RAD‐seq analysis reveals population structure of the North Atlantic planktonic copepod Centropages typicus

Detection of population genetic structure of zooplankton at medium‐to‐small spatial scales in the absence of physical barriers has remained challenging and controversial. The large population sizes and high rates of gene flow characteristic of zooplankton have made resolution of geographical differentiation very difficult, especially when using few genetic markers and assuming equilibrium conditions. Next‐generation sequencing now allows simultaneous sampling of hundreds to thousands of genetic markers; new analytical approaches allow studies under nonequilibrium conditions and directional migration. Samples of the North Atlantic Ocean planktonic copepod, Centropages typicus, were analysed using restriction site‐associated DNA (RAD) sequencing on a PROTON platform. Although prior studies revealed no genetic differentiation of populations across the geographical range of the species, analysis of RAD tags showed significant structure across the North Atlantic Ocean. We also compared the likelihood for models of connectivity among NW Atlantic populations under various directional flow scenarios that replicate oceanographic conditions of the sampled domain. High‐density marker sampling with RAD sequencing markedly outperformed other technical and analytical approaches in detection of population genetic structure and characterization of connectivity of this high geneflow zooplankton species.     

Labral glands in Centropages typicus (Copepoda,Calanoida). I. Sites of synthesis

In Centropages typicus the labrum contains two symmetric gland clusters on the right and left sides. Each cluster comprises two principal elements: a vast unit (AU) located in the anterior, distal area and a dozen smaller units (LPU) in the lateral, posterior area. All these glandular units empty through several pores situated on the labrum. In addition, other secretory units (LDU) are observed in a more dorsal and posterior zone at the level of the perioesophageal nerve ring, and hence outside the labrum itself; these LDU probably secrete through the dorsal side of the stomodeum. All the glandular units (i.e., AU, LPU, and LDU) are organized in syncytia. They have the typical ultrastructural features of a secretory system (rough endoplasmic reticulum, numerous dictyosomes, and secretory granules, the latter amassed at one pole), and several stages of activity may be characterized. The biochemical composition of the products synthesized by rough endoplasmic reticulum and Golgi apparatus is probably very complex. Mucopolysaccharides and glycoproteins both appear to be contained in a single type of grain. The mucopolysaccharides would cause agglutination of food particles carried up to the stomodeum by water currents. The glycoproteins would consist of different enzymes functioning in a preliminary digestive phase occurring before the aliments enter the midgut.     

The copepod Centropages abdominalis as a carrier of the stalked ciliate Zoothamnium

Adult specimens of the calanoid copepod Centropages abdominalis collected from February through April 1986 in Shinhamako, a saline lake connected to Tokyo Bay, were heavily infested with the stalked ciliate Zoothamnium. The number of copepods and those infested varied during the infestation period; in February when the copepod population was large, the incidence of infestation was low, whereas in March when the copepod population was small, copepods infested with Zoothamnium were proportionally more abundant. Numerical variations of infested copepods may correspond to changes in the Zoothamnium population. No specimens of Acartia omorii, which coexists with C. abdominalis, were infested in 1986, although a small number of A. omorii were infested with peritrich ciliates in 1985. Some shrimp larvae were also covered with the same peritrichs as copepods. Nevertheless, as a whole, the relationship between the phoront and its carrier seems to be specific. The highest incidence of infested copepods was approximately 30% similar to that in 1985. The sex ratio of C. abdominalis changed on each sampling date but in general females were more numerous than males. The percentage of infested males was higher than that of infested females, suggesting that the former can be attached to more easily than the latter.     

Seasonal cycles in population abundances and egg production rates in the planktonic copepods Centropages typicus and Acartia clausi

Laboratory studies on the seasonal cycling of egg productionrates for Centropages typicus and Acartia clausi did not reflectseasonal changes in densities for these species at sea. Maximumegg production rates occurred from autumn to spring when populationabundances at sea were low. In late spring, an increment inpopulation numbers was followed by a drastic decline in eggdeposition which continued into summer. The percentage lossdue to mortality, calculated using egg data and total numberof copepodid stages CIII and CIV recorded at sea {small tilde}2weeks later, indicated greatest mortality (80–99%) fromeggs to copepodids during periods of high breeding intensity.These results suggest that high and low density phases in populationnumbers may be the outcome of variations in reproductive potentialand survival rates of eggs and immature stages rather than reproductivepotential per se.     

Cytochemical detection of phosphatase and arylsulphatase activities in the midgut of Centropages typicus (Copepod, Calanoid)

Ultrastructural study of the midgut of Calanoid Copepods revealed the presence of several cell types in all species. In a previous report we described and assigned a function to each of these cell types. In order to affirm the validity of those assignments we undertook an investigation of enzymatic activity especially of phosphatase and arylsulphatase. By cytochemical methods, alkaline phosphatase activity was detected in R-, R'-D- and B-cells, with labelling being observed on the apical plasmic membrane level in all four, and in B-cells on the pinocytotic vesicle membranes. Acid phosphatase and aryl-sulphatase activities were only detectable in B-cells; the most frequently labelled structures were located in the vacuolar system, dictyosomes and Golgi vesicles, although Golgi structures occasionally reacted to acid phosphatase. Nome of the dense bodies observed in B-cells reacted to arylsulphatase. Similarly they were unevenly labelled during acid phosphatase tests. Hence it may be assumed that dense bodies are not involved in hydrolases. It is possible that these enzymes originated from vesicles generated by the Golgi saccules surrounding and joined to the vacuoles, thus bypassing the lysosome I stage.     

The labral glands in Centropages typicus (Copepoda,Calanoida). II. Sites of secretory release

Two groups of external excretory pores associated with glandular units (AU and LPU) were observed on the labrum, one pair laterally and three pairs posteriorly. Each external pore leads to an underlying conical, flask-shaped epidermal chamber. The wide base of this chamber is perforated by an internal pore that delivers secretions from the excretory duct of a glandular unit. The chambers serve to protect the internal pores from turbulence in the outside environment. Expulsion of secretions from the chambers is probably brought about by contraction of labral striated muscles, which synchronizes opening of the AU and LPU pores. A complex funnel-shaped structure forms the internal end of the excretory duct between each chamber and the corresponding pole of accumulation for the secretory product of a glandular unit. This structure, composed of an epidermal syncytium lined by a sleeve of several aligned auxiliary cells, probably ensures a tight connection between the epidermal chamber and the syncytium. The dorsalmost glandular units (LDU) have no pores in the vicinity of their poles of accumulation. Instead they secrete through cuticular ducts delimited by aligned auxiliary cells. External pores for these canals have not yet been located. The secretions of lateral pores may be mucopolysaccharides that play an essential role in agglutination of food particles soon after capture, while the secretions of posterior pores may contain glycoproteins that mix with food only after ingestion into the buccal cavity and probably start the process of digestion.     

Observations on egg production rates and seasonal changes in the internal morphology of Mediterranean populations of Acartia clausi and Centropages typicus

Egg production rates in wild populations of Acartia clausi and Centropages typicus, sampled biweekly in the Gulf of Naples from October 1985 to July 1987, showed marked seasonal fluctuations with maximum values in early spring that proceeded the annual maxima for adult female densities in summer. A positive correlation between chlorophyll a concentrations and egg production was evident only during the early spring phytoplankton bloom. A strong diminution in egg deposition occurred later in spring and continued throughout the summer notwithstanding high chlorophyll concentrations. In winter, when population abundances for adult females were lowest, egg production rates were always higher than in summer. Differences in egg production rates coincided with pronounced morphological changes between summer and winter populations of both species. The most striking of these changes consisted, in winter, in the presence of a dark brown fluid-like mass of granular material that seemed to freely bathe the gonads. The presence of this substance only during periods of elevated egg production suggests that it may enhance egg production rates when the adult population reaches minimum annual levels. Such a mechanism of self-regulation may operate to dampen the effects of environmental variability thereby contributing to maintain a conservative structure in coastal copepod communities.     

Seasonal Cycles of Egg Production of Two Planktonic Copepods, Centropages typicus and Temora stylifera, in the North-western Mediterranean Sea

Reproduction of the dominant copepods Centropages typicus andTemora stylifera was studied at a permanent station in the LigurianSea (north-western Mediterranean). Seasonal patterns of eggproduction, clutch size, egg size and female prosome lengthwere followed from January 1998 to December 1999. Female carboncontent and weight-specific egg production were compared inautumn 1998 and spring 1999. Reproductive patterns of C. typicusand T. stylifera were very similar, indicating that reproductionwas affected by the same environmental factors. Reproductiveactivity was highest in autumn in both species and years. Asecond peak of egg production was observed in early summer,which was less intense in 1999 after a bloom of salps. Egg productionrates reached maximal values of 33.5 and 33.3 eggs female^–1day^–1 and annual means of 10.8 and 11.7 eggs female^–1day^–1 in Centropages and Temora, respectively. Maximalweight-specific egg production was 0.21 day^–1 in bothspecies in November 1998, when female carbon contents were 6.7(C. typicus) and 12.0 µg (T. stylifera). No statisticalrelationship between egg production and food availability ortemperature was detected. Reproductive activity did not reflectthe seasonal abundance patterns, with C. typicus dominatingin spring and T. stylifera in autumn.     

Food limitation in a wild cyclopoid copepod population: direct and indirect life history responses

We studied the effects of food limitation on the population dynamics of the freshwater cyclopoid copepod Diacyclops thomasi in Oneida Lake, New York. In the field population, maximum juvenile abundance coincided seasonally with high phytoflagellate concentration. During the clear-water phase (a seasonal period of low algal density), D. thomasi disappeared from the water column, but fourth-instar copepodids (CIV) were found encysted in developmental arrest in the sediment. Laboratory assays of the effect of the density of two types of food on copepod life history parameters showed that temporal variation in the concentration of relatively small phytoflagellates significantly affected stage-specific development times. This food limitation was most pronounced during the clear-water phase, and supplementation of the diet with a laboratory-cultured phytoflagellate, Chlamydomonas, prevented food limitation. Although developmental arrest appears to be controlled primarily by photoperiod, availability of the larger, more mobile food, Euglena, also influenced the percentage of individuals entering developmental arrest in the laboratory. An investigation of the spatial and temporal emergence pattern in the field revealed that CIV copepodids started to emerge in late autumn and that emergence rates were significantly greater at deep-water locations (9–12 m water depth) compared with shallow-water locations (5–7 m). The clear-water phase in Oneida Lake is an annual event, probably produced by intense grazing by Daphnia pulicaria and Daphnia galeata. Food limitation is thus very likely a recurrent phenomenon for D. thomasi. This apparent seasonal competitive impact of Daphnia on Diacyclops affects both nauplii and immature copepodids. Diacyclops shows two types of responses to the food limitation: (1) the physiological response of slowed active development, and (2) the adaptive response of developmental arrest. Received: 3 November 1997 / Accepted: 1 March 1998