33 research outputs found

    Diel migration and feeding patterns of the chaetognath, Sagitta friderici, off the west coast of South Africa

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    The vertical distribution patterns of adult and juvenile Sagitta friderici were investigated over a 48 h period, using samples collected at different depth strata with an RMT 1 × 6 net. Juvenile chaetognaths were generally found at depths of less than 50 m throughout the day and night, and exhibited limited diel migration patterns. Adults, on the other hand, migrated more extensively and were generally found below 50 m during the day and scattered throughout the water column or near the surface at night. Using the mean depths at each sampling time, the migration patterns of adult chaetognaths closely followed that of the large calanoid copepods. Juvenile chaetognaths, on the other hand, showed no significant correlation with any group of copepods. The greatest proportion of chaetognaths with food in their guts were not found at the depths of maximum copepod abundance, suggesting that the chaetognaths had not been feeding at the depths where they were caught. There was little evidence of a diel feeding rhythm, with no significant difference between the number of prey per chaetognath during the day or night. The majority of food items ingested were copepods (85.7%), while cannibalism on smaller chaetognaths occurred in 2.4% of the samples. The size of copepods consumed by adult chaetognaths was estimated from the width of the copepod mandibular blade, and was found to range from 0.16–1.96 mm prosome length. Although there was a large range of prey sizes which any given predator could consume, there was a clear trend of increasing prey size with increasing predator size. The energetic equivalent of ingesting a single copepod represented 47.4% of the body carbon of the juveniles, but only 16.7% for the adults. Consumption of two or more copepods increased this to 30.3% body carbon of the adults, but cannibalism on smaller chaetognaths represented an average of 46.8% of adult body carbon. Assuming equal handling costs, this implies that cannibalism is energetically much more advantageous for the adults than consuming copepods. Digestion times of copepod prey items were estimated from an exponential equation relating digestion time to temperature using values from the literature for six different chaetognath species. This yielded an ingestion rate of 0.5 copepods chaetognath–1 d–1, which resulted in 1.0–5.3% of the copepod standing stocks or 2.4–14.0% of the copepod production being consumed per day by the Sagitta friderici population

    Growth rates of copepods in the southern Benguela upwelling system: The interplay between body size and food

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    Copepod growth rates were estimated from shipboard measurements of egg production of adult female Calanus agulhensis, Calanoides carinatus, Nannocalanus minor, and Centropages brachiatus and molting rates of juvenile stages (N6-C5) of C. agulhensis. Data were obtained during austral spring and summer of 1989-1995 in the southern Benguela upwelling system. While maximum growth rates showed less than a threefold decline over the body-size range examined (525-2,763-mu m total length), probably owing to allometric considerations, mean growth rate decreased by one order of magnitude, suggesting limitation of growth rate by an environmental factor Most of this decline in mean growth rate was attributable to food limitation of large copepods. Frequency distributions of growth rate under low food densities were severely skewed toward slow growth rate for large copepods, whereas they were more symmetric for smaller copepods. In contrast, at high food concentrations, the frequency distributions had a high degree of symmetry for all copepods. These frequency distributions were interpreted in terms of a probabilistic model describing the encounter rate of copepods with suitably sized food particles. The effect of food limitation on growth rate was evaluated by regressing the coefficient of variation of growth rate against body size. A strong positive relationship was found (r(2) = 0.93, P < 0.001), indicating that small copepods were always sowing well, whereas the growth rate of large copepods was more variable. It is suggested that this difference is a consequence of the ability of small copepods to consume small particles, which are present at a relatively constant background density

    Book Reviews

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    Book Review 1Book Title: Zoological Catalogue of Australia Vol. 33 EchinodermataBook Authors: F.W.E Rowe &amp; J. Gates CSIRO 1995Book Review 2Book Title: A Natural History of Inhaca Island, MozambiqueBook Author: Edited by Margaret Kalk Published by Witwatersrand University Press, 1995. 3rd Edition. 365 pagesBook Review 3Book Title: Mechanics and Physiology of Animal SwimmingBook Authors: Editors: Linda Maddock, Quentin Bone &amp; Jeremy M.v. RaynerCambridge University Press 1994. 250 pages.Book Review 4Book Title: Chironomids: From Genes to EcosystemsBook Author: Edited by Peter CranstonCSIRO, Australia 1995. 482 pages.Book Review 5Book Title: Ecology and Morphology of CopepodsBook Authors: Edited by F.D. Ferrari and B.P. BradleyKluwer Academic Publishers, Dordrecht, The Netherlands, 1994. 530 pages. Reprinted from Hydrobiologia, Vols 292/293 (1994).Book Review 6Book Title: Restoration of Endangered Species. Conceptual Issues, Planning and ImplementationBook Authors: Edited by M.L . Bowles &amp; C. J. WhelanCambridge University Press, 1994. 394 pages

    gbpA as a Novel qPCR Target for the Species-Specific Detection of Vibrio cholerae O1, O139, Non-O1/Non-O139 in Environmental, Stool, and Historical Continuous Plankton Recorder Samples

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    The Vibrio cholerae N-acetyl glucosamine-binding protein A (GbpA) is a chitin-binding protein involved in V . cholerae attachment to environmental chitin surfaces and human intestinal cells. We previously investigated the distribution and genetic variations of gbpA in a large collection of V . cholerae strains and found that the gene is consistently present and highly conserved in this species. Primers and probe were designed from the gbpA sequence of V . cholerae and a new Taq-based qPCR protocol was developed for diagnostic detection and quantification of the bacterium in environmental and stool samples. In addition, the positions of primers targeting the gbpA gene region were selected to obtain a short amplified fragment of 206 bp and the protocol was optimized for the analysis of formalin-fixed samples, such as historical Continuous Plankton Recorder (CPR) samples. Overall, the method is sensitive (50 gene copies), highly specific for V . cholerae and failed to amplify strains of the closely-related species Vibrio mimicus . The sensitivity of the assay applied to environmental and stool samples spiked with V . cholerae ATCC 39315 was comparable to that of pure cultures and was of 10 2 genomic units/l for drinking and seawater samples, 10 1 genomic units/g for sediment and 10 2 genomic units/g for bivalve and stool samples. The method also performs well when tested on artificially formalin-fixed and degraded genomic samples and was able to amplify V . cholerae DNA in historical CPR samples, the earliest of which date back to August 1966. The detection of V . cholerae in CPR samples collected in cholera endemic areas such as the Benguela Current Large Marine Ecosystem (BCLME) is of particular significance and represents a proof of concept for the possible use of the CPR technology and the developed qPCR assay in cholera studies

    Unclear associations between small pelagic fish and jellyfish in several major marine ecosystems

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    During the last 20 years, a series of studies has suggested trends of increasing jellyfish (Cnidaria and Ctenophora) biomass in several major ecosystems worldwide. Some of these systems have been heavily fished, causing a decline among their historically dominant small pelagic fish stocks, or have experienced environmental shifts favouring jellyfish proliferation. Apparent reduction in fish abundance alongside increasing jellyfish abundance has led to hypotheses suggesting that jellyfish in these areas could be replacing small planktivorous fish through resource competition and/or through predation on early life stages of fish. In this study, we test these hypotheses using extended and published data of jellyfish, small pelagic fish and crustacean zooplankton biomass from four major ecosystems within the period of 1960 to 2014: the Southeastern Bering Sea, the Black Sea, the Northern California Current and the Northern Benguela. Except for a negative association between jellyfish and crustacean zooplankton in the Black Sea, we found no evidence of jellyfish biomass being related to the biomass of small pelagic fish nor to a common crustacean zooplankton resource. Calculations of the energy requirements of small pelagic fish and jellyfish stocks in the most recent years suggest that fish predation on crustacean zooplankton is 2–30 times higher than jellyfish predation, depending on ecosystem. However, compared with available historical data in the Southeastern Bering Sea and the Black Sea, it is evident that jellyfish have increased their share of the common resource, and that jellyfish can account for up to 30% of the combined fish-jellyfish energy consumption. We conclude that the best available time-series data do not suggest that jellyfish are outcompeting, or have replaced, small pelagic fish on a regional scale in any of the four investigated ecosystems. However, further clarification of the role of jellyfish requires higher-resolution spatial, temporal and taxonomic sampling of the pelagic community.publishedVersio

    Seasonal and event-scale variation in growth of Calanus agulhensis (Copepoda) in the Benguela upwelling system and implications for spawning of sardine Sardinops sagax

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    We investigated the growth rate and egg production of Calanus agulhensis, the dominant copepod on the western Agulhas Bank region of the southern Benguela upwelling system (South Africa), and assessed the implications for the spawning of sardine Sardinops sagax. Daily weight specific growth rate (SGR) of copepod developmental stages (N6 to female) was measured monthly between September and March 1993/94 and 1994/95. Seasonally, SGRs of small stages (N6 to C2) remained relatively constant. By contrast, SGRs of large stages (C3 to female) mirrored changes in chl a related to seasonal warming and wind patterns, with a moderate peak in September/October and a larger peak in January to March. Superimposed on this seasonal cycle were fluctuations in response to winds on the event scale. At the onset of upwelling, mean female SGR across the shelf was slow. During sustained upwelling, female SGR increased, with a peak associated with enhanced chl a in the upwelling front and slower rates inshore in newly upwelled water and offshore in oligotrophic water. Female SGR was fastest during prolonged quiescence when high chl a levels extended over most of the shelf. During downwelling, female SGRs decreased. again, with fastest rates inshore. Overall, SGRs were not related to temperature, but were related to chl a, with stronger relationships for larger stages. This suggested food limitation of larger stages, which may be a consequence of their preference for larger phytoplankton cells that only occur at high concentrations in restricted locations in time and space in the southern Benguela upwelling system. The spawning of sardine varied seasonally and coincided with the maximum chl a concentration and production of C. agulhensis eggs (daily egg production x female density), both of which are a good source of food for fish larvae. Thus the timing of sardine spawning maximizes the food available to their larvae
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