Gut clearance and pigment destruction in a herbivorous copepod, Acartia tonsa, and the determination of in situ grazing rates

Gut clearance rates of starving and continuously feeding Acartiatonsa were estimated. During the initial 30 min the rates weresimilar (0.045 and 0.048 min^–1, respectively; 14°C)but thereafter starving animals expelled the remains of theirgut contents at half the rate (0.019 min^–1) of fed ones(0.048). Pigment destruction was estimated by (i) incubationexperiments over 3–4 days, (ii) silica to pigment ratioin algae and faeces and (iii) by gut filling experiments. Theincubations showed that 8% of the ingested pigments were destroyedto nonfluorescent residues during gut passage. The silica topigment ratio method gave an average of 11 % (1 –24) destructionand gut-filling experiments showed no systematic differencebetween ingestion measured as gut filling rate (fluorescence)and particle reduction. ¹Present address: Kristineberg Marine Biological Station, S-45034 Fiskebäckskil, Sweden     

Feeding by the planktonic calanoid copepod Acartia clausi Giesbrecht on natural suspended particulate matter of varying quantity and quality

The defaecation rate of females of the calanoid copepod Acartia clausi Giesbrecht as a measure of the ingestion rate was measured in relation to quantity and quality of natural suspended particulate matter. In the April–September experiments, phytoplankton was the dominant component of suspended particulate matter and the ambient chlorophyll a concentration was high, ranging from l.11 to 23.91 μg·l⁻¹. The defaecation rate became saturated below the ambient particle concentration and the weight-specific defaecation rate was rectilinearly fitted against particle concentration. In terms of chlorophyll a, the critical concentration for saturation of the weight-specific defaecation rate ranged from 0.616 to 1.710 μg·l⁻¹. In the October–November experiments, the ambient chlorophyll a concentration ranged from 0.27 to 0.53 μg·l⁻¹ and non-living particles became the dominant component. No critical concentration was observed and the slope of the line describing the relationship between particle concentration and weight-specific defaecation rate was greatly reduced in comparison with that obtained in the April–September experiments. The reduction of the slope may be due to the dominance of non-living particles.     

Variability of Acartia clausi in the Black Sea

There are two morphotypes of A. clausi in the Black Sea. The diversity in the spine ornamentation of the last prosomal and urosomal segments generally has been decreasing in both morphotypes during the period of this study (1976–1990). Seasonal data showed increased variability in April.     

Relationship between functional response and gut transit time in the calanoid copepod Acartia clausi: role of food quantity and quality

The relationship between ingestion rate and gut transit timeof the calanoid copepod Acartia clausi was examined in laboratoryexperiments with five different diets: (i) living cells of thediatom Thalassiosira weissflogii, (ii) detrital cells of thesame diatom, (iii) 50:50 mix of the two previous diets on aprotein basis, (iv) dinoflagellate cells of Prorocentrum micansand (v) Prorocentrum minimum. Ingestion followed a Holling type2 response for diets 1 and 4 and a linear one for diets 2, 3and 5. Gut transit time varied with food abundance only whenthe copepods were fed with the living diatom. The gut evacuationrate increased with the concentration of T. weissflogii withvalues of 0.010, 0.020, 0.032, 0.042 min^–1, correspondingto gut transit time of 97, 50, 31 and 24 min, measured at 50,110, 130 and 275 µg protein L^–1, respectively. Copepodsfed with dinoflagellates, mixed and pure detrital diets exhibitedlonger and similar gut transit times ranging from 85 to 166min, depending on diet. The coupling between ingestion rateand gut transit time measurements is discussed in the contextof copepod feeding strategies.     

Nitrogen excretion by the calanoid copepod Acartia tonsa: results of mesocosm experiments

Rates of excretion of NH₄⁺ urea and dissolved primary amines(DPA) by Acartia tonsa were measured over two 24-h periods inestuarine mesocosms. Animals (     

A simulation of the distribution of Acartia clausi during Oregon Upwelling, August 1973

The distribution of the estuarine copepod Acartia clausi incoastal waters off Oregon during an upwelling period in August1973 is simulated. A time dependent, two dimensional (x, z,t) model relates maximum offshore extent of the copepod's fourlife stages (egg, nauplius, copepodite, and adult) to intensityof the wind stress driving the upwelling circulation, stagedevelopment time, and mortality. Realistic solutions are obtainedby using actual intermittent wind forcing recorded by an anemometerat Newport. Offshore transport is overestimated when the circulationmodel is driven by theoretical continuous winds, suggestingzooplankton may be washed out of coastal upwelling zones (e.g.off Northwest Africa) which undergo periods of prolonged upwelling. With an accurate model of offshore transport and stage developmenttime, the mismatch between predicted and observed distributionsmay be used to estimate field mortality of the various stages.     

Evidence for hyporegulation in the calanoid copepod,Acartia tonsa

• 1.1. The euryaline calanoid copepod, Acartia tonsa, maintains haemolymph Na below that of the external medium in salinities above 34^o_oo (475 mM Na).
• 2.2. The measured transepithelial electrical potential. −9.97 ± 1.0 mV, indicates that Na is regulated out of electrochemical equilibrium.
• 3.3. Water osmotically lost in hyporegulation is replaced by Na-dependent absorption by the gut.
• 4.4. High osmotic water permeability is evidenced by the fact that with an increase in external salinity from 475 mM Na to 580 mM Na the copepod's drinking rate nearly doubles.
• 5.5. Sodium efflux measurements indicate that ionic permeability is much lower than other hyporegulating crustaceans.
• 6.6. The energetic advantage of hyporegulation in this species is considered.

     

Comparative effects of environmental conditions,in eutrophic polluted and oligotrophic non-polluted areas of the saronikos gulf (Greece), on the physiology of the copepod Acartia clausi

1. Digestive enzyme activity and the oxygen consumption rate of Acartia clausi from the polluted eutrophic Elefsis Bay and the non-polluted oligotrophic Vouliagmeni-Fleves area were compared.2. Out of 13 hydrolases whose activity was revealed, 10 had a higher activity in the Elefsis population than in the Vouliagmeni-Fleves one. Only amylase, alkaline phosphatase and esterase lipase had a higher activity in the Vouliagmeni-Fleves population.3. The high activity of amylase in Vouliagmeni-Fleves population seems to indicate that phytoplankton constitutes the most important part of A. clausi's food.4. Respiration rate was significantly lower in Elefsis population, irrespective of season.5. A significant seasonal variation was observed which was greater in the Vouliagmeni-Fleves population, with a 50% decrease in the respiration rate between spring and summer.6. The obtained results agree with previous literature data and show the physiological characteristics of A. clausi of Elefsis Bay, allowing it to thrive in polluted eutrophic waters.     

Morphological variability of geographically distinct populations of the estuarine copepod Acartia Tonsa

Variations in the number of spines on the left and right posterior dorsal and posterior margins of the prosome as well as the length of the prosome of Acartia tonsa from three estuaries, the upper western side of the Chesapeake Bay, Montauk Bay near the eastern end of Long Island Sound and the coast of Peru were determined. The length of the prosome and number of spines in each of the four locations were used as an indication of morphological similarity between the populations.     

DMSP-consuming bacteria associated with the calanoid copepod Acartia tonsa (Dana)

DMSP-consuming bacteria (DCB) were recovered from the body and fecal pellets of the copepod Acartia tonsa (Dana). The most probable number of DCB associated with starved A. tonsa was 9.2x10(2) cells copepod(-1). The abundance of DCB recovered from the copepod body increased to 1.6-2.8x10(4) after the copepod fed on DMSP-containing alga. DCB abundance associated with fecal pellets averaged 1.2x10(4) cells pellet(-1). In enrichment cultures, the DCB grew with a doubling time of 1.1-2.9 days, and consumed DMSP at a rate of 4.5-7.5 fmol cell(-1) day(-1). The apparent DMSP-to-DMS conversion efficiency was 25-41% for DCB from copepod body, and 99% for DCB from fecal pellets. Our study demonstrated that copepods and their fecal pellets may harbour dense populations of DCB, and that the copepod-bacteria coupling represents a novel mechanism for DMSP consumption in the water column.     

Turbulence decreases the hydrodynamic predator sensing ability of the calanoid copepod Acartia tonsa

The copepod Acartia tonsa is very sensitive to hydrodynamicsignals, including those made by approaching predators, andresponds with a vigorous escape jump. Whether the presence ofmoderate turbulence changes this ability to detect hydrodynamicsignals was investigated by comparing the response of copepodsto velocity gradients created by a siphon flow in turbulentand still water. Turbulence decreased the distance at whichA. tonsa initiated escapes from the siphon and increased thecapture rate, indicating decreased sensitivity to hydrodynamicsignals, but did not trigger unnecessary escape reactions thatmight produce fatigue.     

Cryptic ecological diversification of a planktonic estuarine copepod, Acartia tonsa

The recent discovery of cryptic species in marine holoplankton, organisms that 'drift' in oceanic currents throughout their life cycle, contrasts with their potential for long-distance passive dispersal and presumably high gene flow. These observations suggest that holoplankton species are adapting to surprisingly small-scale oceanographic features and imply either limited dispersal or strong selection gradients. Acartia tonsa is a widespread and numerically dominant estuarine copepod containing deep mitochondrial lineages within and among populations along the northwestern Atlantic coast. In this study, we intensively investigated A. tonsa populations in Chesapeake Bay with the goals of testing species status for the deep lineages and testing for their association with environmental features over space and time. Phylogenetic analyses of DNA sequences from mitochondrial cytochrome c oxidase I (mtCOI) and the nuclear ribosomal internal transcribed spacer (nITS) resolved two concordant monophyletic clades. Deep divergence between the two clades (13.7% uncorrected sequence divergence for mtCOI and 32.2% for nITS) and genealogical concordance within sympatric populations strongly suggest that the two clades represent reproductively isolated cryptic species. Based on restriction fragment length polymorphisms of mtCOI, representatives from the two clades were found consistently associated with contrasting salinity regimes (oligohaline vs. meso-polyhaline) with an overlap between 2 and 12 PSU in samples from 1995 to 2005. Finding these patterns in one of the best-known estuarine copepods reinforces the conclusion that marine biodiversity is underestimated, not only in terms of species numbers, but also with respect to niche partitioning and the potential importance of ecological divergence in marine holoplankton.     

Capture mechanisms used by the lobate ctenophore, Mnemiopsis leidyi, preying on the copepod Acartia tonsa

Although the lobate ctenophore Mnemiopsis leidyi is an influentialplanktonic predator, the mechanisms enabling it to capture itscharacteristically wide range of prey have not been systematicallyexamined. We recorded interactions between free-swimming M.leidyiand two stages (nauplii, adults) of the calanoid copepod Acartiatonsa in order to determine a mechanistic explanation of thisfeeding process. Prey encounter with Mnemiopsis involved twodifferent processes. The first depended on fluid motions createdby the nearly continuous beating of cilia lining the four auricles.These cilia created a low-velocity flow in which A.tonsa naupliiwere entrained (94% of naupliar encounters) and transportedpast the oral lobes onto the tentillae (oral tentacles). Thenauplii, although capable of rapid escape responses, generallyappeared to be insensitive to the current in which they werecarried. The second process relied upon the collision of swimmingprey with the inner surfaces of the oral lobes and was not obviouslyinfluenced by the auricular feeding currents. Adult A.tonsawere rarely entrained in the auricular flow, but, instead, propelledthemselves into contact with the oral lobes (97% of adult encounters).Both prey capture processes functioned simultaneously. The synergisticfunctioning of these processes probably explains the broad patternsof prey ingestion found by in situ studies of Mnemiopsis feeding.     

Feeding behaviour and migrations in a natural population of the copepod Acartia tonsa

This paper deals with the variations on feeding activities and diel migrations of Acartia tonsa Dana, the dominant copepod species in Berre lagoon (west Mediterranean French coasts).A 27 hour in situ study was carried out during June 1989, at a station located in the south west of the lagoon. Vertical profiles of salinity, temperature and dissolved oxygen taken each 12 hours showed a stratification of the water column in two distinct layers: (1) a superficial layer with higher temperature, moderate salinity, and high oxygen concentrations; (2) a colder, more saline and almost anoxic deep layer. In situ chlorophyll a measurements were made at –1, –3 and –5 m; concentrations were relatively homogeneous through the water column during the whole sampling period.Zooplankton samples were collected every 3 hours with a 200 µm mesh net, in three strata (0–2 m, 2–4 m, 4–6 m). A complete dominance of A. tonsa was observed in the zooplankton community. Our results point out clearly a nocturnal migration, with individuals concentrating in both superficial layers; thus an unimodal migratory pattern can be inferred. Gut flourescence measured following the Mackas & Bohrer technique (1976), showed higher values during night time, and values for females were the highest with wider day—night variations. Similar results were found in laboratory experiments with copepods fed with a culture of Dunaliella tertiolecta.Gut evacuation rate was measured in two laboratory experiments either mixing or separating males and females. Evacuation rate was 17.92 and 27.25 min for males and females respectively.Phytoplankton daily ration for A. tonsa calculated by gut flourescence and gut evacuation rate was particularly low, for it represents only 10% of the individual carbon weight.Moreover, grazing impact on phytoplankton is very restricted, it represents less than 1% of the daily phytoplankton stock.     

Do viruses affect fecundity and survival of the copepod Acartia tonsa Dana?

Naturally occurring viruses are extremely abundant in aquaticsystems, and they infect bacteria, cyanobacteria, prokaryoticand eukaryotic phytoplankton, heterotrophic nanoflagellates,fish and mammals. Viral infections of single-celled organismshave been studied intensively in the past decade, but littleis known about the effects of viruses on aquatic metazoans,other than for some economically important species. Becausezooplankton assemblages are often dominated in number and biomassby copepods, we used them as model organisms to study the effectsof naturally occurring viruses on higher trophic levels. Weattempted to induce viral infection in laboratory-reared culturesof the estuarine copepod Acartia tonsa Dana by exposing themto elevated concentrations of natural viruses in seawater. Wefound no negative effects of such exposure on copepod fecundity,larval survival or adult survival.     

Comparison of the metabolism of Acartia clausi and A. tonsa: influence of temperature and salinity

In the Marseilles region (French Mediterranean coast), A. clausi is one of the most abundant copepod species of the Gulf of Fos while A. tonsa constitutes the almost exclusive copepod species of the Berre lagoon, a neighbouring semi-closed brackish area communicating with the gulf. As different ecophysiological capabilities to stand the various temperature, salinity and food conditions could explain why these two species do not coexist in the same environment, comparative experiments were performed on metabolism and feeding. The respiration and ammonia excretion of the two species were measured in different combinations of temperature (10, 15 and 20 degrees C) and salinity (15, 25 and 35 per thousand). For each temperature, at the salinity of 35 per thousand, respiration rates were less in A. clausi than in A. tonsa, the contrary being observed at the lowest salinity. At any temperature ammonia excretion was greater at the intermediate salinity in A. tonsa and least in A. clausi. In Acartia tonsa, Q(10) of respiration and excretion were minimum at the lowest salinity, while in A. clausi they were unaffected by salinity variation. The O:N atomic ratio (from respiration and ammonia excretion rates) was significantly more elevated in A. clausi (mean 21.2; range 13.6-28.7) than in A. tonsa (mean 11.3; range 4.2-25) suggesting a more proteinic oriented metabolism in the later. Feeding experiments where Dunaliella tertiolecta30 per thousand) or lagoon (<16 per thousand) salinity. The relationships between ingestion and food concentration in the two species were not significantly different. These different results are compared to other ecophysiological information concerning these Acartia species (survival tolerances, osmotic regulation, feeding behaviour) and are discussed in relation with the characteristics of their niches in the studied region.     

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.     

High-speed video analysis of the escape responses of the copepod Acartia tonsa to shadows

The copepod Acartia tonsa exhibits a vigorous escape jump in response to rapid decreases in light intensity, such as those produced by the shadow of an object passing above it. In the laboratory, decreases in light intensity were produced using a fiber optic lamp and an electronic shutter to abruptly either nearly eliminate visible light or reduce light intensity to a constant proportion of its original intensity. The escape responses of A. tonsa to these rapid decreases in visible light were recorded on high-speed video using infrared illumination. The speed, acceleration, and direction of movement of the escape response were quantified from videotape by using automated motion analysis techniques. A. tonsa typically responds to decreases in light intensity with an escape jump comprising an initial reorientation followed by multiple power strokes of the swimming legs. These escape jumps can result in maximum speeds of over 800 mm s(-1) and maximum accelerations of over 200 m s(-2). In A. tonsa, photically stimulated escape responses differ from hydrodynamically stimulated responses mainly in the longer latencies of photically stimulated responses and in the increased number of power strokes, even when the stimulus is near threshold; these factors result in longer escape jumps covering greater distances. The latency of responses of A. tonsa to this photic stimulus ranged from a minimum of about 30 ms to a maximum of more than 150 ms, compared to about 4 ms for hydrodynamically stimulated escape jumps. Average response latency decreased with increasing light intensity or increasing proportion of light eliminated. Little change was observed in the vigor of the escape response to rapid decreases in visible light over a wide range of adaptation intensities.     

Egg production rates of the neritic marine copepod Acartia tumida Willey in the Aleutian Archipelago

Acartia tumida, a neritic copepod of the northern North Pacific Ocean and Bering Sea, is an unusually large member of its genus, adult females measuring 2.0–2.4 mm in total length. In the summers of 1986 and 1987 we investigated egg production of A. tumida in nearshore habitats of several islands in the Aleutian Island chain. A. tumida was found within protected embayments, where it could reach adult densities as high as 1000m^–3. Highest egg production rates were measured at Amchitka Island (up to 86 eggs copepod^–1 d^–1 at 6°C), where the phytoplankton was dominated by chain-forming Thalassiosira spp. In situ egg production rates at Amchitka were more than twice as high as maximum rates measured with cultured T. weissflogii, a single cell diatom, or during blooms of chain-forming Chaetoceros spp. at Adak and Kiska Islands. Approximately 12–24 h was necessary for recent feeding to be reflected in egg production. At high food concentrations 75% of spawning occurred at night and in discrete clutches, a pattern not observed at a lower food concentration.