Rhincalanus Nasutus Research Articles

Spatiotemporal Distribution Characteristics of Copepods in the Water Masses of the Northeastern East China Sea

To understand the effects of variable water masses in the northeastern East China Sea (Korea South Sea), planktonic copepods were seasonally sampled. Out of a total of 106 copepod species, 85 were oceanic warm-water species, and the number of species varied in summer, autumn, spring, and winter. The study area was divided into two or three regions according to the degree of influence of the water masses. Canthocalanus pauper, Clausocalanus furcatus, Oithona plumifera, Oncaea venella, Oncaea venusta, and Paracalanus aculeatus showed a positive correlation with water temperature and salinity and were indicator species of warm currents. Calanus sinicus, known as an indicator species of the Yellow Sea Bottom Cold Water, showed a high abundance and occurrence ratio in the western sea of the study area from spring to autumn. Moreover, Acartia pacifica indicated the extension of coastal waters to offshore areas. Several oceanic warm-water species (A. danae, Centropages gracilis, Labidocera acuta, Rhincalanus nasutus, and Temoropia mayumbaensis) were considered indicator species of the Taiwan Warm Current. Our results suggest that the spatiotemporal distribution patterns of indicator species are partly explained by different water masses.

Checklist of copepods from Gulf of Nicoya, Coronado Bay and Golfo Dulce, Pacific coast of Costa Rica, with comments on their distribution.

A list of 54 copepod species (Crustacea) in 23 families is presented for the Pacific coast of Costa Rica. Identifications are from zooplankton samples of the Victor Hensen Expedition during December 1993 and February 1994. Samples were taken with a Bongo net (0.60 m net opening, 2.50 m net length) with 200 microns mesh size. Oblique hauls were done from the surface to the ground at a towing speed of aprox. 1 knot. 37 species (68.5%) were found in the Gulf of Nicoya, 36 in Golfo Dulce (66.6%) and 17 (31.4%) species were common to both gulfs, while only twelve species (22.2%) were found in Coronado Bay. Four species (7.4%) were distributed along the coast and were common to the three regions: Paracalanus parvus, Euchaeta sp., Oithona plumifera and O. similis. Eleven species of calanoids found normally in the Costa Rica Dome show the influence of typical oceanic waters principally at the mouth of Gulf of Nicoya. Differences were observed in the composition and presence of the copepod species when the inner and outer (upper and lower) parts of both gulfs were compared. Gulf of Nicoya was dominated in its upper part by typical neritic estuarine species like Acartia lilljenborgii, Paracalanus parvus and, Hemyciclops thalassius as well as species of Pseudodiaptomus. On the other hand a more oceanic composition of copepods was observed in the lower part of the gulf. Both small species, like Oncaea venusta, as well as larger species, such as Pleuromamma robusta, Eucalanus attenuatus, E. elongatus and Rhincalanus nasutus, were typical of these waters. Oithona plumifera and O. similis were found in the lower part too; and both species are typical from oceanic water. Coronado Bay was characterized by the presence of typical oceanic species like Neocalanus gracilis, Euchaeta longicornis, Eucalanus attenuatus and Haloptilus ornatus with more transitional species like Clausocalanus pergens and C. furcatus near the coast. In the Golfo Dulce differences in copepod composition were also observed, but the separation of the species was not so evident. Outer stations were represented by oceanic species like Paracalanus aculeatus, Pleuromamma gracilis, Lucicutia ovalis, Candacia catula, Euchaeta wolfendeni and Oncaea mediterranea, while the inner station, located at the upper part of the Gulf, was more characterized by a mixed copepod group, with both neritic species like Pseudodiaptomus wrigthi, Acartia danae, A. clausi, Canthocalanus pauper as well as oceanic species like Scolicithricella marginata, Saphirina nicromaculata or Oncaea conifera. Two species of Coryceaus, C. flaccus and C. speciosus, were identified in the outer stations of Golfo Dulce, while C. brehmi was found in inner stations of Gulf of Nicoya. The majority of copepods found are typical of the east Pacific. This paper constitutes an additional work about the copepods in the Gulf of Nicoya and the first report of copepod species for Coronado Bay and Golfo Dulce.

Feeding habits and dietary overlap during the larval development of two sandperches (Pisces: Pinguipedidae)

Two species of sandperch (Pinguipedidae: Perciformes), Prolatilus jugularis and Pinguipes chilensis, inhabit the coastal waters of the South Pacific. Both species have pelagic larvae with similar morphology, but their diet preferences are unknown. Diet composition, feeding success, trophic niche breadth and dietary overlap were described during larval stages for both species. In the austral spring, larval P. jugularis (3.83-10.80 mm standard length [SL]) and P. chilensis (3.49-7.71 mm SL) during their first month of life had a high feeding incidence ( > 70%) and fed mostly on copepod nauplii ( > 80% IRI), Rhincalanus nasutus metanauplii and Paracalanus indicus copepodites. The number of prey ingested was low (mean: 4-5 prey per gut) and independent of larval size; total prey volume and maximum prey width increased as larvae grew. Mouth opening and ingested prey were greater in larval P. jugularis than in P. chilensis, leading to significant differences in prey composition among larval species, in terms of prey number and volume. Pearre’s trophic niche breadth was narrow for both species (0.159±0.07 for P. jugularis; 0.156±0.03 for P. chilensis) and independent of larval size. Dietary overlap was high inter- and intra-species in larvae with a mouth gape < 900 μm. These results suggest the relative importance of both larval species as primary consumers of the pelagic web in nearshore environments of rocky temperate areas.

Vertical pelagic habitat of euphausiid species assemblages in the Gulf of California

We describe the seasonal changes in the horizontal and vertical distribution and abundance of euphausiid species associated with seven physical and 61 biological variables in the Gulf of California (24–31°N). Euphausiid community structure was explored in the epipelagic habitat (<200m) in January, July, and October 2007 and in epipelagic to bathypelagic habitats (<1400m depth) in May 2015. Twelve euphausiid species comprising two distinct regional assemblages were identified. Nyctiphanes simplex and Nematoscelis difficilis were the most abundant species (>90%) in all cruises carried out in the 26–31°N region and Euphausia distinguenda in the 24–26°N region (mostly in October >90%). We confirmed that Euphausia gibboides and Nematobrachion flexipes inhabit the mesopelagic habitat, adapted to <1ml O2 l–1 environmental condition. Although Euphausia lamelligera and N. simplex populations were concentrated in well-oxygenated water (>3ml O2 l–1) near the surface (<50m), they were also detected in low densities in the hypoxic mesopelagic habitat (250–800m), but only at oxygen levels less than 0.09ml O2 l–1. Stylocheiron affine and Stylocheiron carinatum were numerous well below the seasonal thermocline (100–350m). Nematoscelis difficilis and N. simplex extended into the hypoxic mesopelagic habitat, but at low densities. Multivariate analyses showed six seasonal and regional krill assemblages, which are characterized by different species (Similarity percentage analysis). Canonical Correspondence Analysis show that, of 68 variables, three abiotic factors (temperature, dissolved oxygen, and OMZ depth) and eight biotic factors (accessory pigments; zeaxanthin and 19'-hexanoyloxyfucoxanthin, and the abundance of six copepod species; Candacia pectinata, Canthocalanus pauper, Centropages furcatus, Rhincalanus nasutus, Scolecithrix danae, and Temora discaudata), were the most influential variables associated with the vertical distribution and abundance of euphausiids. Krill‒copepod (predator–prey) relationships may be important determinants of daily and seasonal vertical distribution patterns. We conclude that the euphausiid vertical habitat reaches down to 900m depth (with historical records of six species collected between 1000 and 2280m) but with lower diversity and abundance in the mesopelagic and bathypelagic habitats than in the epipelagic habitat.

Ecological characteristics of eucalanoid copepods of the eastern tropical North Pacific Ocean: Adaptations for life within a low oxygen system

The eastern tropical North Pacific Ocean (ETNP) is home to one of the largest and most severe mid-water oxygen minimum zones (OMZs). Members of the copepod family Eucalanidae are abundant in this region and display varied vertical distributions throughout the OMZ. This research assessed the diversity of ecological strategies used by these copepods to cope with the presence of the OMZ based on their biochemical, physiological, and behavioral characteristics. Five species of copepods (Eucalanus inermis, Pareucalanus attenuatus, Rhincalanus nasutus, Rhincalanus rostrifrons, and Subeucalanus subtenuis) were collected at the Costa Rica Dome (9°N, 90°W) and Tehuantepec Bowl (13°N, 105°W) during cruises in fall of 2007 and winter of 2008–2009. Adult females of all species were collected and analyzed for water, ash, carbon, nitrogen, hydrogen, phosphorus, protein and lipid content, lactate dehydrogenase (LDH) activity, and survivorship when exposed to prolonged low-oxygen conditions. Four distinct ecological strategies were observed for the five species based on genus. E. inermis, found throughout the OMZ and surface waters, had very high water content (94% of wet weight (WW)), leading to low organic content per unit WW. However, when corrected to ash-free dry weight (AFDW), protein and storage lipid contents of this species had intermediate (46 and 8% of AFDW, respectively) values compared to other species. E. inermis demonstrated high survivorship under low-oxygen conditions (>90%), and had detectable levels of LDH, indicating an ability to rely on anaerobic pathways. Rhincalanus spp., found primarily in the lower-oxygen subsurface waters, also had detectable LDH activity and high survivorship under low-oxygen conditions (>85%), but had much higher storage lipid levels (>37% of AFDW), very low protein levels (28–33% of AFDW), and low water content (87% of WW). S. subtenuis and P. attenuatus are both distributed primarily in surface waters, but showed distinct ecological strategies. S. subtenuis protein levels were very high (67% AFDW), storage lipid levels low (0%), and water content low (87% of WW). They also had very low survivorship under low-oxygen conditions (22%) and no detectable LDH activity. P. attenuatus also did not have detectable LDH activity or large lipid stores. P. attenuatus, however, had much lower protein content (30% AFDW) and higher water content (89% of WW), indicating a distinct ecological strategy. Comparisons of ETNP individuals with conspecifics or congeners from higher-oxygen environments suggest that low protein levels (indicative of lower overall activity) seen in some groups may be an adaptation for survival in lower-oxygen regions.

Energetics and carbon budgets of dominant calanoid copepods in the northern Benguela upwelling system

Respiration rates of 16 calanoid copepod species from the northern Benguela upwelling system were measured on board RRS Discovery in September/October 2010 to determine their energy requirements and assess their significance in the carbon cycle. Individual respiration rates were standardised to a mean copepod body mass and a temperature regime typical of the northern Benguela Current. These adjusted respiration rates revealed two different activity levels (active and resting) in copepodids C5 of Calanoides carinatus and females of Rhincalanus nasutus, which reduced their metabolism during dormancy by 82% and 62%, respectively. An allometric function (Imax) and an energy budget approach were performed to calculate ingestion rates. Imax generally overestimated the ingestion rates derived from the energy budget approach by >75%. We suggest that the energy budget approach is the more reliable approximation with a total calanoid copepod (mainly females) consumption of 78mg Cm−2d−1 in neritic regions and 21mg Cm−2d−1 in oceanic regions. The two primarily herbivorous copepods C. carinatus (neritic) and Nannocalanus minor (oceanic) contributed 83% and 5%, respectively, to total consumption by calanoid copepods. Locally, C. carinatus can remove up to 90% of the diatom biomass daily. In contrast, the maximum daily removal of dinoflagellate biomass by N. minor was 9%. These estimates imply that C. carinatus is an important primary consumers in the neritic province of the northern Benguela system, while N. minor has little grazing impact on phytoplankton populations further offshore. Data on energy requirements and total consumption rates of dominant calanoid copepods of this study are essential for the development of realistic carbon budgets and food-web models for the northern Benguela upwelling system.

Seasonality of the copepod assemblages associated with interplay waters off northeastern Taiwan

This study investigated copepod assemblages in the regime around Turtle Island off northern Taiwan to trace South China Sea water (SCSW) flowing northward with the Kuroshio Current. Seasonal variations of copepod assemblages demonstrated a dynamic succession of changes in copepod populations; the average abundance for total copepods ranged from 102.58 ± 53.38 in December to 1669.89 ± 1866.17 in March (individuals m−3). A total of 87 copepod species representing 36 genera and 21 families were identified. Among all samples, Temora turbinata dominated the copepods by a relative abundance (RA) of 26.89 %, followed by Paracalanus parvus (RA: 22.34 %) and Corycaeus (Ditrichocorycaeus) affinis (RA: 12.77 %). Only the Acrocalanus gracilis species was recorded in all samples. Results of one-way ANOVA revealed that the number of copepod species, indices of richness, evenness, and Shannon–Wiener diversity differed significantly in five different cruises. The density of five copepod species (Gaetanus minor, Calanus sinicus, Eucalanus elongates, Rhincalanus nasutus, and Rhincalanus rostrifrons) exhibited a significant negative correlation with seawater temperature. In contrast, the density of Canthocalanus pauper and Undinula vulgaris was significantly positively correlated with seawater temperature. The cold-water indicator species, C. sinicus, recorded in samples of March and May indicated the effect of China Coast Water (CCW) on copepod communities in the study area. Furthermore, the presence of Calanoides philippinensis in May samples strongly indicated that the SCSW may reach the Turtle Island area. Consequently, C. philippinensis and C. sinicus can be used to trace SCSW and CCW, respectively, in the study area.

Distribution and life history of two planktonic copepods, Rhincalanus nasutus and Rhincalanus rostrifrons, in the northwestern Pacific Ocean

To clarify the life histories of two tropical–subtropical Eucalanidae species, Rhincalanus nasutus and Rhincalanus rostrifrons, we investigated the geographical and vertical distributions of their copepodid populations in the northwestern (NW) Pacific Ocean. Zooplankton samples were collected between 15°N and 49°N in the NW Pacific. Rhincalanus nasutus was widely distributed from 20°N to 49°N, whereas R. rostrifrons was restricted to more southerly latitudes (15–37°N). The occurrence of the two species in the surface layer was associated with seasonal spring ontogenetic vertical migration (OVM). Gonadal analysis indicated that, during spring, both species reproduce in the surface layers of the Kuroshio–Oyashio Transition Area (TA) and the slope water area, which is a region of the continental slope situated north of the Kuroshio axis. Our results suggest that both species might have 1 or 2 year life cycles. In comparison, dormant populations occupying the water below the surface layer mainly comprised adult females (50.2% for R. nasutus and 57.7% for R. rostrifrons), which implied that this is an advantageous strategy for this life state in response to irregular increases of primary production in tropical–subtropical oceans. Furthermore, dormancy in deep waters (500–1000m) by R. nasutus might indicate a strategy to avoid the relatively high predation risks incurred in shallower waters, due to its larger body size. In contrast, the smaller body size of R. rostrifrons facilitates dormancy in shallower waters (200–500m depth).

Development of Rhincalanus nasutus (Giesbrecht, 1888) (Copepoda, Rhincalanidae) from the Humboldt Current System

Morphological changes in the exoskeleton of naupliar stages 2-6 and copepodid stages I-V of Rhincalanus nasutus are described. On the distal segment of naupliar antennule, one lateral seta and two medial setae are added during each molt to nauplius 3-6. A naupliar arthrite is a thick, distinctive structure on the coxa of antenna, and the exopod of this limb is patterned from the large proximal segment of that ramus. The coxal gnathobase of the mandible appears at nauplius 4. Buds of maxillule and maxilla are added during the molt to nauplius 5, and buds of maxilliped and swimming legs 1-2 are added during the molt to nauplius 6. The somite bearing maxilla clearly articulates completely with the somite bearing the maxilliped on the last two naupliar stages. This unusual articulation does not appear during copepodid development, and the somite bearing swimming leg 2 is the anterior articulating somite during this later phase. One articulating somite is added during molts to copepodid II-V. Segmentation of the antennule is completed by copepodid IV. Segmentation of the exopod of antenna does not change during the copepodid phase, but one segment is added to the endopod of maxilla. Segmentation and setation of the maxilliped follow the usual calanoid pattern. Buds of swimming legs 3-5 are ventrolateral on their somite. Their presumptive exopod has two attenuations; their presumptive endopod bears an attenuation and a seta. Segmentation of the rami of swimming legs 1-4 follows the common pattern for copepods except for the exopod and endopod of swimming leg 1 which are only 2-segmented.

Lipid composition of tropical and subtropical copepod species of the genus Rhincalanus (Copepoda: Eucalanidae): a novel fatty acid and alcohol signature

Rhincalanus nasutus, R. rostrifrons, and R. cornutus are primarily found in subtropi- cal and tropical waters and accumulate large lipid stores, an uncommon feature in low-latitude copepods. While R. nasutus fatty acid and alcohol profiles have been examined previously, little is known about lipids in R. rostrifrons or R. cornutus. Lipid profiles for wax ester, triacylglycerol, phospholipid, free fatty acid, sterol, and free fatty alcohol fractions were determined for these 3 species collected from the eastern tropical north Pacific, Gulf of California, and Gulf of Mexico. Storage lipids, primarily wax esters, were the dominant component (>88%) of total lipid in all 3 Rhincalanus species. R. nasutus, however, had distinctly different storage lipid fatty acid and alcohol profiles, with primarily 16:1(n-7)/18:1(n-9) fatty acids and 14:0/16:0 fatty alcohols, while R. cornutus and R. rostrifrons accumulated 14:0/16:0 fatty acids and 16:1/18:1 fatty alcohols. Species differences also were observed in sterol profiles, with R. rostrifrons having a cholesterol content of 75 to 76% versus 85 to 90% in R. nasutus and R. cornutus. The remainder of the sterol fraction was largely composed of 22-dehydrocholesterol in all species. In all 3 species, phospholipid fractions were dominated by 22:6(n-3), 16:0, 18:0, 20:5(n-3), and 18:1(n-9) fatty acids. The results of the present study suggest that genetic predisposition significantly governs lipid profiles, particularly storage lipids, within this genus. The storage lipid profiles observed in R. cornutus and R. rostrifrons were unique from their congeners and have not been reported previously for other copepod species.

Short-term variations in mesozooplankton, ichthyoplankton, and nutrients associated with semi-diurnal tides in a patagonian Gulf

The relationships between the distribution of different zooplankton and ichthyoplankton stages and physical and chemical variables were studied using samples and data (CTD profiles, ADCP and current meter measurements, nutrients, mesozooplankton, ichthyoplankton) obtained from different strata during two 24-h cycles at two oceanographic stations in a Chilean Patagonian gulf during the CIMAR 10-Fiordos cruise (November, 2004). A station located at the Chacao Channel was dominated by tidal mixing and small increments in surface stratification during high tides, leading to decreased nutrient availability. This agreed with short periods of increased phytoplankton abundance during slack waters at the end of flood currents. Increases in larval density for all zooplankton and ichthyoplankton taxa corresponded to the flooding phases of the tidal cycle. When the larval density data were fit to a sinusoidal model, the regression coefficients were high, suggesting that tides are important features that modulate short-term variations in plankton abundance. All larvae did not vary synchronously with the tidal phase; rather, time lags were observed among species. The abundances of older individuals of the copepodite Rhincalanus nasutus and all zoea stages of the squat lobster Munida gregaria increased during night flood tides, whereas younger stages increased during daytime flood tides. At a station located at the Queullin Pass, which was dominated by vertical stratification patterns, the variations in peak larval density were better fitted to the semi-diurnal sea level fluctuations. Other evidence indicated internal tides below the pycnocline, which could promote larval transport in deeper layers. In the overall picture that emerges from this study, planktonic organisms from different habitats and phylogenetic origins seem to respond to the local tidal regimes. In some cases, this response might be beneficial, transporting these individuals inshore to areas that are rich in food during the peak biological production season.

Integrated molecular and morphological biogeography of the calanoid copepod family Eucalanidae

Species range information forms the empirical data of pelagic biogeography. Early descriptions of canonical zooplankton distributions in the Pacific Ocean were based, in part, on distributional data from the planktonic copepod family Eucalanidae. A large-scale molecular survey of this group, covering Atlantic, Pacific, and Indian Oceans (1295 individuals), increased the total diversity from 24 to 39 evolutionarily significant units (ESUs). New biogeographies are presented here for 18 lineages within 10 described species in the genera Subeucalanus, Pareucalanus, and Rhincalanus. Integration of molecular and morphological data on diversity and distribution resulted in three primary outcomes: (1) the morphological species was confirmed to be valid, and the biogeographic distribution remains largely unchanged from prior reports, (2) the species was found to contain multiple ESUs, each of which has a more restricted distribution than the parent taxon, and (3) the species was found to contain multiple ESUs, whose biogeographic distributions remain unclear. Subeucalanus subtenuis, S. mucronatus, S. subcrassus, Pareucalanus attenuatus, P. langae, and P. parki are all valid genetic and morphological species, and prior distribution records from Fleminger (1973) and Lang (1965) are confirmed to be accurate. New records in the western Indian Ocean extend the biogeographic range of S. subtenuis, S. mucronatus, S. subcrassus, and P. langae. Subeucalanus pileatus, P. sewelli, and R. rostrifrons, all species with Indo-Pacific or circumglobal distributions, consist of genetically divergent, allopatric populations that subdivide the original biogeographic range. Subeucalanus crassus and Rhincalanus nasutus are species complexes containing 4-8 genetically divergent lineages, whose distributions are inadequately characterized. Although results suggest more restricted pelagic habitats for some eucalanid species, those habitats have been previously described for other zooplanktonic taxa and do not represent new pelagic distributional types.

Population dynamics and life strategies of Rhincalanus nasutus (Copepoda) at the onset of the spring bloom in the Gulf of Aqaba (Red Sea)

In the southern Gulf of Aqaba, the bulk of the population developed from stage CV to adult in the course of the 3-week study period. In contrast, immature CV stages dominated at the adjacent stations in the northern Gulf of Aqaba and in the northern Red Sea. Development was associated with the seasonal vertical migration from wintering mid-water layers and initiation of feeding starting as early as beginning of March in the southern Gulf of Aqaba. No upward migration was observed in the northern parts of the Gulf and in the northern Red Sea, where more than 90% of the females remained immature during our study. Lipids were dominated by wax esters in females and CV. The fatty acid and fatty alcohol compositions of females were very similar throughout the study region and period. Major fatty acids were 18:1(n29), 16:1(n27), 16:2(n24) and 20:5(n23). Our results support the previous reports of a seasonal dormancy of R. nasutus in the Gulf of Aqaba and suggest that the timing of vertical migration, feeding and maturation is closely coupled to the development of the spring bloom in oligotrophic subtropical waters.

Seasonal and inter-annual variation of mesozooplankton in the coastal upwelling zone off central-southern Chile

Zooplankton sampling at Station 18 off Concepción (36°30′S and 73°07′W), on an average frequency of 30 days (August 2002 to December 2005), allowed the assessment of seasonal and inter-annual variation in zooplankton biomass, its C and N content, and the community structure in relation to upwelling variability. Copepods contributed 79% of the total zooplankton community and were mostly represented by Paracalanus parvus, Oithona similis, Oithona nana, Calanus chilensis, and Rhincalanus nasutus. Other copepod species, euphausiids (mainly Euphausia mucronata), gelatinous zooplankton, and crustacean larvae comprised the rest of the community. Changes in the depth of the upper boundary of the oxygen minimum zone indicated the strongly seasonal upwelling pattern. The bulk of zooplankton biomass and total copepod abundance were both strongly and positively associated with a shallow (<20 m) oxygen minimum zone; these values increased in spring/summer, when upwelling prevailed. Gelatinous zooplankton showed positive abundance anomalies in the spring and winter, whereas euphausiids had no seasonal pattern and a positive anomaly in the fall. The C content and the C/N ratio of zooplankton biomass significantly increased during the spring when chlorophyll- a was high (>5 mg m −3). No major changes in zooplankton biomass and species were found from one year to the next. We concluded that upwelling is the key process modulating variability in zooplankton biomass and its community structure in this zone. The spring/summer increase in zooplankton may be largely the result of the aggregation of dominant copepods within the upwelling region; these may reproduce throughout the year, increasing their C content and C/N ratios given high diatom concentrations.

Effect of food composition on egg production and hatching success rate of two copepod species (Calanoides carinatus and Rhincalanus nasutus) in the Benguela upwelling system

We have analysed the daily egg production (EPR) and hatching success rates of the calanoid copepods Calanoides carinatus and Rhincalanus nasutus as a function of nano- and microplankton concentration and composition in the northern Benguela upwelling system off Namibia. Food concentration explained 55% (R. nasutus) to 62% (C. carinatus) of the EPR variability. We found no relation between the residuals of the food concentration–EPR regression and the percentage of the different taxonomic components of the nano- and microplankton. Nor was there a relation with the proportion of the diatom Skeletonema costatum that dominated the major blooms or with the number of nano- and microplankton species. We conclude that food quality differences could not be attributed to the relative composition of microplanktonic particles of the different groups (i.e. taxonomic composition).

Seasonal and spatial variability of dominant copepods along a transect off Walvis Bay (23°S), Namibia

The study provides the first quantitative analysis of changes in zooplankton abundance and community structure in the Walvis Bay area, off Namibia, over a full seasonal cycle (February–December 2000). Zooplankton was collected monthly along a 90-nautical-mile transect. Calanoid copepods were numerically the most abundant mesozooplankton, Metridia lucens, Calanoides carinatus, Rhincalanus nasutus and Centropages brachiatus being the most common species, with each exhibiting a specific distribution pattern. Mean calanoid copepod abundance was lower during autumn, when upwelling was minimal, than during other times of the year. However, abundances were exceptionally low in September, when upwelling was most intense. Offshore transport is believed to be responsible for copepod losses from the area during that period. Abundances of C. carinatus and R. nasutus were high inshore between June and August, at the onset of the upwelling season, and peaked during October–December, towards the end of the upwelling season. These cool-water species were closely associated with upwelled water near the coast. The results concur with previous distribution patterns of zooplankton and their association with upwelling in Namibian waters. Findings suggest a possible decade-scale shift in the zooplankton community structure since the 1960s.

Differential dormancy of co-occurring copepods

Four species of planktonic calanoid copepods that co-occur in the California Current System ( Eucalanus californicus Johnson, Rhincalanus nasutus Giesbrecht, Calanus pacificus californicus Brodsky, and Metridia pacifica Brodsky) were investigated for evidence of seasonal dormancy in the San Diego Trough. Indices used to differentiate actively growing from dormant animals included developmental stage structure and vertical distribution; activity of aerobic metabolic enzymes (Citrate Synthase and the Electron Transfer System complex); investment in depot lipids (wax esters and triacylglycerols); in situ grazing activity from gut fluorescence; and egg production rates in simulated in situ conditions. None of the 4 species exhibited a canonical calanoid pattern of winter dormancy – i.e., synchronous developmental arrest as copepodid stage V, descent into deep waters, reduced metabolism, and lack of winter reproduction. Instead, Calanus pacificus californicus has a biphasic life history in this region, with an actively reproducing segment of the population in surface waters overlying a deep dormant segment in winter. Eucalanus californicus is dormant as both adult females and copepodid V’s, although winter females respond relatively rapidly to elevated food and temperature conditions; they begin feeding and producing eggs within 2–3 days. Rhincalanus nasutus appears to enter dormancy as adult females, although the evidence is equivocal. Metridia pacifica shows no evidence of dormancy, with sustained active feeding, diel vertical migration behavior, and elevated activity of metabolic enzymes in December as well as in June. The four species also differ markedly in water content, classes of storage lipids, and specific activity of Citrate Synthase. These results suggest that copepod dormancy traits and structural composition reflect diverse adaptations to regional environmental conditions rather than a uniform, canonical series of traits that remain invariant among taxa and fixed across a species’ range. Such interspecific and regional differences in life history traits need to be incorporated in models simulating Eastern Boundary Current pelagic ecosystem dynamics.

The relative importance of food and temperature to copepod egg production and somatic growth in the southern Benguela upwelling system

The fecundity and somatic growth rates of Calanus agulhensis and Calanoides carinatus , the dominant large calanoid copepods in the southern Benguela upwelling system, as well as the fecundity of several other common copepods, were measured between September and March of 1993/94 and 1994/95. Mean egg production of most copepods was low at >30 eggs female -1 day -1 {Calanoides carinatus 23.7, Calanus agulhensis 19.0, Neocalanus tonsus 16.1 and Rhincalanus nasutus 26.1), whereas the mean fecundity of Centropages brachiatus was significantly greater (83.6 eggs female −1 day -1 ). This study also presents the first comprehensive field estimates of the fecundity of Nanno-calanus minor (mean: 26.1 eggs female −1 day −1 , range: 0.0–96.2 eggs female −1 day −1 ) and of somatic growth of N6 and all copepodite stages of Calanoides carinatus (decreasing from 0.58 day −1 for N6 to 0.04 day −1 for C5). Somatic growth rates of Calanus agulhensis also declined with age: from 0.57 day1 for N6 to 0.09 day1 for C5. Data on growth rates were used to assess the relative importance of food [as measured by total chlorophyll (Chi) a concentration], phytoplankton cell size (proportion of cells >10 µm) and temperature to the growth of copepods. Multiple regression results suggested that fecundity and somatic growth rates were positively related to both Chi a concentration and phytoplankton cell size, but not to temperature. Although it was not possible to separate the effects of Chi a concentration and phytoplankton cell size, data from previous laboratory experiments suggest that copepod growth is not limited by small cells per se, but by the low Chi a concentrations that are associated with these particles in the field. Despite growth not being directly related to temperature, a dome-shaped relationship was evident in some species, with slower growth rates at cool (<13°C) and warm (>18°C) temperatures. The shape of this relationship mirrors that of Chi a versus temperature, where poor Chi a concentrations are associated with cool and warm temperatures. It is concluded that the effect of food limitation on growth of copepods outweighs that of temperature in the southern Benguela region. Sources of variability in relationships between growth and Chi a concentration are discussed.

Fatty Acid Composition and Community Structure of Plankton from the San Lorenzo Channel, Gulf of California

The structure of the plankton community and fatty acid composition of nano-, micro- and zooplankton are described during four seasons of 1994 from the San Lorenzo Channel. During August, the warmest temperature in the surface water was observed and a thermocline developed between 20 and 30m. In the remaining months, a well-mixed layer occurred in the upper 30m. The chlorophyllacontent of the nanoplankton fraction (<38μm) was higher than the microplanktonic fraction (38–200μm) year round. Maximal chlorophyll values (1·5–3μl−1) occurred in January, which may be associated with organic matter, since phytoplankton was lower than at other seasons. The relative abundance of diatoms increased from January (57% of phytoplankton) to November (99%). The increment was mainly due toNitzschiaandChaetoceros. Dinoflagellates were always low (0·03–1·36 cellsml−1). Copepods (mainlyEucalanus) dominated the zooplankton in winter and fall, while in spring and summer, the abundance of doliolids was similar to the copepods (Nannocalanus minordominated).Four fatty acids (16:0, 16:1, 18:0, 18:1) were the most conspicuous in the plankton, representing usually between 40 and 80% of the total fatty acids throughout the water column. In winter, higher fatty acid content and higher relative amounts of 16:0 and 16:1 were observed than in the warm months. Stearic acid (18:0) peaked during fall. The major seasonal differences occurred in the nanoplankton, which had peaks of 20:5 during January, and 16:4 in April. A strong decrease in polyunsaturated fatty acids (PUFA) occurred during the warm months. The fatty acid composition of microplankton and larger zooplankton was similar in winter–spring. Individual copepods of selected species (Eucalanus sewelli,Rhincalanus nasutus,Centropages furcatusandLabidocera acuta) showed fatty acid profiles similar to the mixed zooplankton, with some differences in content of PUFA.

Characteristics of the habitat for larval development of three bathylagid fish species (Pisces: Salmoniformes)

Three bathylagid smelt species were identified: Bathylagus ochotensis Schmidt 1938, Bathylagus wesethi Bolin 1938 and Leuroglossus stilbius Gilbert 1890. A structure analysis of the copepod community was also carried out at the sampling stations and 55 species of copepods were identifíed. The copepod species Calanus pacificus Brodsky 1948, Acartia danae Giesbrecht 1889 and Labidocera trispinosa Esterly 1905 were dominant where B. ochotensis was found. The dominant copepod species where B. wesethi was found were C. pacificus, Corycaeus speciosus Dana 1848 and Paracalanus parvus Claus 1863. The copepod species C. pacificus, Pleuromamma abdominalis Lubbock 1888 and Rhincalanus nasutus Giesbrecht 1888 were found with L. stilbius. Both B. ochotensis and B. wesethi were found in the California Current, whereas L. stilbius occurred in moditied California Current waters. The biogeographic affinities of the copepod species were mainly tropical-equatorial, temperate and transitional, and a different habitat was determined for each bathylagid species based on these biogeographic affinities. It can, therefore, be inferred that distinct ecological conditions exist for each species in the study area.