Oncaea Spp Research Articles

Aggregate-colonizing copepods in a glacial fjord: Population dynamics, vertical distribution and allometric scaling of growth and mortality rates of Microsetella norvegica and Oncaea spp.

Most global analysis on the role of copepods in food web efficiency or carbon sequestration ignore the harpacticoid or poecilostomatoid copepods that are under-estimated in traditional zooplankton surveys and under-studied with respect to their ecology and behavior. Nevertheless, when small-mesh-size nets are used these groups appear to dominate zooplankton abundance and sometimes even biomass from Arctic to tropical seas. We studied the seasonal succession of abundance, body size, vertical distribution, reproduction, growth and mortality of two aggregate-colonizing copepods, Microsetella norvegica and Oncaea spp. in a glacial fjord, to investigate the allometric scaling of their vital rates and the correlation between their reproduction, mortality and vertical distribution and environmental variables. Although both species are known to feed on marine snow, they differed in population dynamics, vertical distribution and environmental tolerance. Also, in contrast to most sac-spawning copepods, both M. norvegica and Oncaea spp. had a high specific mortality of eggs and early naupliar stages, and the allometric scaling of their egg size and growth differed from calanoid and cyclopoid copepods. Our results suggest that these non-calanoid copepods do not necessarily share the same habitat or respond similarly to the environment, and that our understanding of the allometric scaling of copepods is incomplete if we do not consider these copepod groups. M. norvegica and Oncaea spp. form by virtue of their high abundance an important part of oceanic food webs, and should be included if we are to understand the future of the ocean ecosystems.

The missing piece of the upper mesopelagic carbon budget? Biomass, vertical distribution and feeding of aggregate-associated copepods at the PAP site

Although zooplankton are recognized as important consumers of marine snow, our knowledge of the contribution of the small (<1 mm) zooplankton to the degradation of sinking particles, and therefore on the efficiency of the biological pump, is limited. To estimate the marine snow consumption by small aggregate-feeding copepods, we measured the daily changes in biomass and vertical distribution of the harpacticoid Microsetella norvegica and the poecilostomatoid Oncaea spp. over a period of 10 days and combined these with estimates of their feeding and respiration rates. We estimated copepod feeding rates in multiple ways, using: gut chlorophyll content, maximum ingestion rates from functional responses on two types of aggregates, carbon demand based on the respiration measurements, and egg production rates. Microsetella norvegica and Oncaea spp. biomass levels varied > 2-fold between sampling dates for geographically-close sampling stations. Microsetella norvegica resided mainly in the surface layer, at or immediately below the fluorescence peak, while Oncaea spp. was typically deeper. Microsetella norvegica fed actively on Trichodesmium filaments, with a maximum ingestion rate of ca. 0.11 µg C ind.−1 d−1, while their ingestion of detritus aggregates was low. Feeding on Trichodesmium was reflected in their gut chl-a content, which was high for M. norvegica at all but one sampling time. In contrast, Oncaea spp. had significantly lower gut chl-a content and may have been feeding on other types of aggregates. Respiration of both copepods was variable and as much as 0.08 µL ind.−1 h−1 for M. norvegica and 0.04 µL ind.−1 h−1 for Oncaea spp. Based on individual biomass, vertical distribution, and carbon demand, aggregate-associated copepods could degrade up to 79 ± 33 mg C m−2 d−1, a value similar to the surplus in the epipelagic carbon budget of the area. This large degradation rate demonstrates that zooplankton < 1 mm can have a large influence on the vertical flux and that the factors controlling their abundances and feeding rates should be high in the priority list of future zooplankton studies.

Life cycles and seasonal vertical distributions of copepods in the Iceland Sea

The sub-Arctic Iceland Sea is an important feeding area for the Icelandic capelin stock with copepods as a major food item. Information on the life history of copepods in the area is limited, and therefore the major aim of the present paper is to describe the population development and ontogenetic vertical migrations of dominant copepods in the area. Depth stratified samples were collected on six cruises covering all seasons of the year from 2006 to 2008. Calanus hyperboreus dominated the biomass (~ 45% of copepods), with C. finmarchicus ranking second (~ 28%) and Metridia longa third (~ 17%). The copepods differed in their life history strategies. C. hyperboreus is unique in that it has the deepest winter distribution (~ 800–1000 m), reproduces at depth in February–March in the absence of phytoplankton food, and spends relatively short time in the surface layers during summer. C. finmarchicus also hibernates at depth, however much shallower (~ 200-600 m), and the reproduction at the surface in May–June is timed to the productive period of phytoplankton. A 2- to 3-year life cycle is proposed for C. hyperboreus and a 1 year for C. finmarchicus. Pseudocalanus spp. appear to have a similar life history as C. finmarchicus, while occupying intermediate depths (~ 400–1000 m) during the overwintering period. The life history of the omnivorous species M. longa appears relatively decoupled from the phytoplankton spring bloom. The same applies to the omnivorous species Oithona spp. and Oncaea spp. The information presented adds to the comparative description and understanding of copepods in sub-Arctic areas in general.

Comparative productivity and biomass yields of the Guinea Current LME

Plankton production patterns in the Guinea Current Large Marine Ecosystem (GCLME) were investigated using data obtained from Continuous Plankton Recorder (CPR) tows carried out in the GLCME, from December 1995 to November 1999. Four CPR routes were established between Cote d'Ivoire and Cameroon as part of the pilot phase of the GCLME project, funded by the Global Environment Facility. Phytoplankton and zooplankton yields were greatest during the major upwelling compared to the other identifiable hydrographic seasons. The phytoplankton was dominated by diatoms whereas smaller zooplankton (<2mm) occurred in greatest abundance. Analyses of ten years sea surface temperature (SST) data from 1990 to 2000 revealed two thermal regimes; a warmer phase followed by a colder phase that commenced in 1997. Annual phytoplankton and zooplankton yields in 1997 were relatively higher than the other years of the CPR sampling. Analyses of zooplankton community structure, in terms of species occurrence and relative abundances, was found to be similar during the minor and major upwelling. The zooplankton taxa contributing to dissimilarity in community structure were Chaetognath (<2mm), Oithona spp., Oncaea spp, copepod nauplii, echinoderm larvae, Para-Pseudocalanus, Temora spp., Eucalanus spp., and Clausocalanus spp. Spatial and seasonal variation in phytoplankton communities did not follow any pattern. It was observed from a multiple regression analysis that phytoplankton distribution alone accounted for a significant proportion of the variation in zooplankton distribution in all hydrographic seasons except the major upwelling, during which period sea surface temperature contributed over 60% of the observed variation. This investigation using CPR is the first of its kind in the GCLME and thus provides a baseline information to guide future studies.

Inter-annual comparison of diet and daily ration of larval jack mackerel Trachurus japonicus in the southern East China Sea

We describe and compare the feeding habits of Trachurus japonicus larvae from 2.4 to ,8-mm body length (BL) sampled during February to March of 4 consecutive years (2002–2005) in the primary spawning ground along the shelf-break region of the southern East China Sea. Dietary overlap based on Schoener’s index revealed high similarities (58–81%) among the 4 years. In larvae 5–8-mm BL, the diet was strongly dominated by calanoid copepodites and Oncaea spp. Larvae ,5-mm BL fed occasionally on copepod nauplii and gastropod larvae, as well as the above two main prey items. Feeding was mainly restricted to 0700–2100 h and feeding incidence was 78–90 and 100% in larvae ,5 and 5–8-mm BL, respectively, without inter-annual variation. The mean number of prey items per gut was positively correlated with larval size, and in each size class it did not significantly differ among the 4 years. Based on information on the feeding period (14 h), the mean number of prey items, and gut evacuation time (2 h), we estimated that larvae ,3-mm BL would consume 4–8 prey items day, and this increased greatly with larval growth, and larvae 7–8-mm BL consume 56–60 prey items larvae day. We estimated that 0.5% of the available prey items were consumed by larvae day in 2005 when both prey and larval abundance data were available, implying that the larvae have only limited effects on prey abundance.

Composition and community structure of zooplankton in the sea ice-covered western Weddell Sea in spring 2004—with emphasis on calanoid copepods

The mesozooplankton community, with special emphasis on calanoid copepods, was studied with respect to its species composition, abundance, vertical distribution and developmental structure during the “Ice Station POLarstern” (ISPOL) expedition to the ice-covered western Weddell Sea. Stratified zooplankton tows were carried out nine times between 1 December 2004 and 2 January 2005 with a multiple opening–closing net between 0 and 1000 m depth. Copepods were by far the most abundant taxon, contributing more than 94% of the total mesozooplankton. Numerical dominants were cyclopoid copepods, mostly Oncaea spp. A total of 66 calanoid copepod species were identified, but the calanoid copepod community was characterised by the dominance of only a few species. The most numerous species was Microcalanus pygmaeus, which comprised on average 70% of all calanoids. Calanoides acutus and Metridia gerlachei represented other abundant calanoid species contributing an average of 8% and 7%, respectively. All other species comprised less than 3%. The temporal changes in the abundance and population structure of M. pygmaeus and M. gerlachei were small while a shift in the stage frequency distribution of C. acutus was observed during the study: copepodite stage IV (C IV) dominated the C. acutus population with 48–50% during the first week of December, while C V comprised 48% in late December. C I and C II of C. acutus were absent in the samples, and males occurred only in very low numbers in greater depths. In M. gerlachei, C I was not found, whereas all developmental stages of M. pygmaeus occurred throughout the study. All three species showed migratory behaviour, and they occurred in upper water layers towards the end of the investigation. This vertical ascent was most pronounced in C. acutus and relatively weak in the other two species. In M. pygmaeus and M. gerlachei, copepodids were responsible for the upward migration in late December, while the vertical distribution of adults did not change. In C. acutus, all abundant developmental stages (C IV, C V and females) ascended to upper water layers. Almost exclusively (93%) medium- and semi-ripe females of C. acutus and M. gerlachei were found, and only 3–4% of the ovaries were ripe. The absence of C I and the low number of ripe females indicate that the main reproductive period had not started in C. acutus and M. gerlachei until the end of our study in early January. In contrast, the high portion of C I and C II of M. pygmaeus suggests that reproduction of this species had started in October–November and hence before the onset of the phytoplankton bloom in the water. The community structure did not differ between stations with one exception on 26 December, when the station was strongly influenced by the continental shelf.

Oxygen consumption in relation to motion of marine planktonic copepods

The goal of these studies was to quantify the energy consumption of abundant marine planktonic copepods with different motion behaviors but similar body weight by determining their oxygen consumption under conditions approaching those encountered in their natural environment. The motion behaviors of adult females of the 3 copepod taxa studied ranged from the continuously fast-moving Clausocalanus furcatus to the continuously slow-moving and feeding current-producing Paracalanus aculeatus to the occasionally moving Oncaea spp. At 20°C, the daily oxygen consump- tion per unit body weight decreased significantly with decreasing activity, i.e. C. furcatus > P. aculea- tus > Oncaea spp. Expressed as percent of body carbon metabolized per day, the average values were 23.4% for C. furcatus, 15.1% for P. aculeatus, and 10.5% for Oncaea spp. Oxygen consumption of P. aculeatus and Oncaea spp. was significantly higher at 23°C than at 20°C. It is concluded that not only body weight and temperature, but also the extent of motion will affect energy expenditures of planktonic copepods.

Food and feeding ecology of Brazilian sardine (Sardinella brasiliensis) larvae from the southeastern Brazilian Bight

Results from depth integrated and vertically stratified plankton samples collected in the southeastern Brazilian Bight were used to study the feeding behavior of Brazilian sardine (Sardinella brasiliensis) larvae. Sampling of the ichthyoplankton was carried out with 60 cm Bongo nets in the Bight during the spawning seasons of 1991/92 and 1992/93. The sampling of microzooplankton was carried out in the coastal region off Ubatuba, using the closing-type plankton net, in December 1995. The feeding study was based on a total of 901 captured larvae. Gut content analysis of the sardine larvae showed a diurnal pattern of food intake. Copepod nauplii dominated the diet of the preflexion and flexion larvae, but they were the second in abundance for the postflexion larvae which fed preferentially on copepodites and adults of Oncaea spp. Averaged feeding incidence of the 901 larvae was 37.6%, but it increased to 58.5% for day-caught larvae. Seventy percent of the food particles were found in the mid-gut and food eaten showed a natural increase in digestion from fore-gut to hind-gut. Vertical distribution of microzooplankton revealed that copepod nauplii were present in densities of 10-20 ind. L-1, mainly in the upper mixed layer (0-20 m depth), but higher densities of copepodite and adult of Oncaea, Oithona and Paracalamis were found within and beneath the thermocline. These results show that Brazilian sardine larvae can successfully adapt their diet, feeding on the most abundant food particles in the upper mixed layer of the survey area.

Distribution patterns of epipelagic copepods off Rio de Janeiro (SE Brazil) in summer 1991/1992 and winter 1992

The spatial distribution of copepod populations on the eastern continental shelf off Rio de Janeiro State, Brazil, was analysed in relation to the hydrographic regime in December–January (summer 1991/92) and July (winter 1992). Advection of cold and nutrient-rich South Atlantic Central Water (SACW) onto the shelf was a prevailing circulation mechanism in both seasonal cruises. The SACW occupied a larger proportion of bottom layers in summer as compared to winter, but only in the latter a well-defined upwelling event was recorded off Cape Frio. Two calanoid copepods were representative of shelf areas influenced by SACW intrusions: Calanoides carinatus, which occurred mainly during summer and Ctenocalanus vanus, recorded at high densities in both periods. Paracalanus quasimodo was the dominant copepod at inner- and mid-shelf stations both in summer and winter, followed by Temora spp. and Oncaea spp. These coastal species peaked at the upwelling site. A highly diversified and comparatively less abundant copepod assemblage, dominated by Clausocalanus furcatus and Oithona plumifera, was recorded in outer-shelf stations occupied by the oligotrophic Tropical Waters of the Brazil Current. We conclude that regional and seasonal contrasts in copepod species composition and abundance are controlled to a great extent by the advection and upwelling of the SACW onto the shelf.

Studies on chaetognaths off Ubatuba region, Brazil. II. Feeding habits

The diet of chaetognath species were studied by examining the gut contents of 9466 specimens collected off Ubatuba region, São Paulo State. The greatest proportion of chaetognaths (7119 individuals) showed their gut contents empty. Copepods, mollusc eggs, appendicularians, cladocerans and annelids were the most common food items in the gut contents of juveniles and mature stages. Cannibalism occurred in low frequency. In Summer the copepods Temora stylifera and Paracalanus spp were more abundant, whereas Oncaea spp and mollusc eggs were heavily preyed in Winter. There was a clear trend of increasing prey size with the developmental stage.

Studies on chaetognaths off Ubatuba region, Brazil. II. Feeding habits

The diet of chaetognath species were studied by examining the gut contents of 9466 specimens collected off Ubatuba region, Sao Paulo State. The greatest proportion of chaetognaths (7119 individuals) showed their gut contents empty. Copepods, mollusc eggs, appendicularians, cladocerans and annelids were the most common food items in the gut contents of juveniles and mature stages. Cannibalism occurred in low frequency. In Summer the copepods Temora stylifera and Paracalanus spp were more abundant, whereas Oncaea spp and mollusc eggs were heavily preyed in Winter. There was a clear trend of increasing prey size with the developmental stage.

Feeding of the carnivorous copepod Euchaeta antarcrica in Antarctic waters

Copepod Euchaeta antarctica stages V and V1 were caught during 24 h sampling programmes in h4arch 1989 at a coastal station in Gerlache S t r a~ t , Antarctic Peninsula, and in January 1990 at an oceanic s ta t~on in the north Weddell Sea. Gut content analyses showed that copepods made up 80 to 90 % of all food Items by number Metridia gerlachei, Calanoides acutus, Euchaeta spp , other large copepods, Oncaea spp , Olthona spp. and other small copepods were the main prey of E. antarctica (V, VI) Large copepods were probably more important as food (on a dry-weight b a s ~ s ) than small copepods. Mean number of prey per E. antarctica (V, VI) was 0.9 and 1.1 at the coastal and the oceanic station, respectively. Copepodids at stage V contamed more prey items (1 3) than adult females (0.5) at the coastal station In March A sample from South Georg~a showed that E antarctica feeds during winter E antarctica lnd~viduals were not eating continuously slnce, in all areas, over 30 O/O of them had empty guts. The parasitic dinoflagellate Blastodln~um sp. was found in the gut of 6 6 % of E. antarctica at the oceanic station. This infection d ~ d not affect the number of food items found in thelr guts.