Mysids from a tropical estuary (Cochin backwater) in India were studied based on samples collected over a period of one year. Four species belonging to three genera) were represented: Mesopodopsis orientalis, Mesopodopsis zeylanica, Rhopalophthalmus indicus and Kochimysis pillaii, among which Mesopodopsis zeylanica was the most dominant species, recorded throughout the year with its peak abundance observed during the monsoon period (June to September). The wide range of fluctuation in their population density was considered to be mainly due to their heterogeneous distribution. Reproduction of M. zeylanica in Cochin backwater is continuous throughout the year. The number of embryos carried by a single female ranged from 7 to 12, and was correlated with female length (p<0.05), tending to increase with increasing the size of the female. Egg size varied between 0.38 and 0.43 mm, with no correlation with length of the female, and size differences were observed in the same brood. Males and females attain sexual maturity after reaching a total length of 5 mm and 4.5 mm respectively. The environmental parameters, salinity and chlorophyll a, have much influence on the population density of mysids.
Viral infection of the bloom-forming diatom Chaetoceros tenuissimus was discovered in 2008, and is now assumed to have a significant influence on the dynamics of C. tenuissimus populations in natural environments. Enumeration of C. tenuissimus and its viruses is essential when examining the host-virus relationship in situ; however, the diatom species is so small in size that its identification and counting by optical microscope is almost impossible. To resolve this problem, we have developed a TaqMan-based real-time polymerase chain reaction (PCR) method for detection and quantification of C. tenuissimus. We designed primers and a TaqMan probe to target the D1 region of its 28S ribosomal RNA (rRNA) gene; the established real-time PCR was specific at the species level by testing 41 microalgal strains including C. tenuissimus. Tris–EDTA buffer-based boiling method was shown to be efficient for extracting DNA from filter-trapped C. tenuissimus cells in this study. The detection range of the established TaqMan-based real-time PCR method for C. tenuissimus was 101 to 106 cells collected on a filter; the method was applicable for C. tenuissimus cells accompanied with natural microorganisms.
During our survey of the subtidal macrobiota on wave dissipating concrete blocks along the coast of Osaka Bay in 1997, we recorded a species of serpulid polychaete that was not immediately identifiable. In 2007, we re-examined the specimens and discovered that they were identical to Hydroides dianthus, native to the East Coast of North America. The density of the worm in Osaka Bay varied remarkably between sites and depths. A Steel-Dwass multiple comparison analysis was used to evaluate the differences among sites, depths and seasons. Spearman's rank correlation was used to examine the influence of physical factors of temperature, salinity and dissolved oxygen, and biological ones of the densities of coexisting organisms. The results suggest that salinity greater than 30 psu and biological interaction with macroalgae or mussel (Xenostrobus securis) are presently acting as limiting factors for the invasion of H. dianthus.
We examined the bias in the estimation of zooplankton biomass in net-samples in coral reef waters by measuring seston weight, which contains non-living matter (or detritus) and net-phytoplankton in addition to zooplankton. Net-samples were collected at a coral reef at Tioman Island, Malaysia, and divided them into two aliquots to be used for both measurements of seston weight and zooplankton biomass. Seston weight was on average 2.2 times higher than net-zooplankton biomass, and non-zooplankton content (detritus/phytoplankton) contributed on average 49.2% to the seston weight. Consequently, measurement of net-plankton seston weight as zooplankton biomass in coral reef waters is inadequate due to the highly variable contribution of detritus/phytoplankton content and involves the possibility of over-estimation of zooplankton biomass.
Onboard incubation experiments were conducted to examine the feeding rates and selectivity of the cyclopoid copepod Oithona similis on natural assemblages of microplankton during the spring bloom in the Oyashio region. O. similis cleared ciliates at higher rates (2.8–11.9 mL copepod−1 d−1) than dinoflagellates (1.4 mL copepod−1 d−1), with no detectable ingestion of diatoms. Clearance increased with ciliate size, reaching maximum rates at a size of ~35 μm equivalent spherical diameter. The total ingestion rates of protozooplankton (ciliates and dinoflagellates) by O. similis ranged between 24.8 and 41.5 ngC copepod−1 d−1, which exceeded by about 2–3 times the metabolic expenditure of the copepod. Applying the ingestion rates to abundance data of O. similis, we estimated that the copepod population removed each day 0.1–0.4% of the protozooplankton standing stock. These results indicate that protozooplankton, especially aloricate ciliates, are an important food resource available to O. similis even under bloom conditions in the Oyashio region, though the predation by the copepod did not have a significant impact on protozooplankton assemblages.
Stable isotope analyses are often used for the estimation of food web structures based on empirical fractionation (i.e., enrichment of carbon isotope signatures at 0–1‰ with one trophic level, and that of nitrogen at 3–4‰). We examined the effect of 80% ethanol fixation on both carbon and nitrogen stable isotope signatures of six species of macrobenthos samples. The effect of fixation was inconsistent (i.e. both enrichment and depletion occurred). The shift of nitrogen isotope signatures due to fixation ranged from −0.3 to +0.8‰, and the carbon signatures of fixed samples showed enrichment by −0.3 to +1.3‰ depending on the species. These changes would clearly affect estimation of trophic levels and energy flow in food webs. Ethanol fixation may not be a smart choice because of the inconsistency of the effect on isotope signatures. However, if the effect was corrected for each species, ethanol-fixed samples can be practically useful for food web analyses using stable isotopes.
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