Gaetice depressus, a varunid crab common on intertidal boulder shores, is a potential key organism for monitoring organic matter flow through the food web. In order to elucidate its biogeochemical role, the diet source and trophic position of this crab on the boulder shore of an island off the Izu Peninsula, Japan, were estimated using three approaches: foregut content examination, stable isotope signature (δ13C and δ15N) and pigment biomarkers. The results suggest that G. depressus utilizes green macroalgae (Ulva sp., Ulvaria sp.) as its main diet source together with red macroalgae (Gloiopeltis complanata, Gl. furcata). This crab also utilizes periphytic and planktonic microautotrophs (occasionally tissues of heterotrophs) when macroalgae prove insufficient due to seasonal change. Therefore, G. depressus can be considered to be an omnivore since it consumes both autotrophs and heterotrophs, although it obtains organic matter mostly from autotrophs.
The seasonal dynamics and the net growth rates of the planktonic freshwater cyanobacterium Cylindrospermopsis raciborskii were monitored in a shallow artificial pond in northern Taiwan from February 2010 to January 2011. C. raciborskii was present in the pond throughout the study, and its abundance was positively correlated with water temperature. The net growth rates of C. raciborskii displayed no clear seasonal pattern, whereas the highest value (0.184 d−1) was observed in early July when the water temperature sharply increased to the annual highest level. The rapid growth of C. raciborskii led to bloom formation, which persisted over the summer months (July and August). Dilution and washout of trichomes by rainfall significantly affected the dynamics of C. raciborskii, including leading to the termination of a bloom.
Climate change has the potential for intensification of typhoons, which will cause stronger effects on aquatic ecosystems in the future. The effect of typhoon Mawar (T0511), passing Manazuru Port located in the western part of Sagami Bay, Japan, was investigated from August to September 2005. Immediately after the passage of Mawar, photosynthetically available radiation showed high values, salinity decreased dramatically and nutrient concentrations (NO2+NO3, PO4 and Si(OH)4) increased. Skeletonema spp. and Leptocylindrus spp. were dominant after the passage of Mawar, and their succession was linked to the variability of the N/P ratio. Primary production was highest at 349 mg C m−3 day−1 three days after Mawar, and high assimilation numbers lasted for nine days. The integrated primary production during the nine days after Mawar was 2.1×103 mg C m−3, which accounted for 7.2–9.1% of the annual primary production in the upper waters of Sagami Bay. The study confirms that enhanced primary production induced by episodic typhoon events in temperate coastal regions are significant, and should be considered in annual primary production estimates.
Bioturbators affect other benthic animals by two processes: sediment reworking and changes in chemical properties. Crab exclusion experiments were conducted on an intertidal mud flat in the Tama estuary to examine the effects of the burrowing crab Macrophthalmus japonicus (Ocypodoidea) on the densities of sympatric macro-infauna in 2010 and 2011; and on sediment parameters in 2011. In 2010, approximately 1 month after M. japonicus was excluded, the densities of Corbicula japonica and Hediste sp. or spp. were significantly greater than those of the control, with M. japonicus present. Concerning sediment parameters: chlorophyll-a content, water content and total nitrogen content at the surface significantly increased and subsurface oxidation-reduction potential significantly decreased in the exclusion treatment compared with control conditions. These results appear to indicate that sediment reworking by M. japonicus was the main process affecting the sedimentary environment and density of C. japonica and Hediste sp. or spp. However, the magnitude of the effects of M. japonicus apparently varies from year to year because there were no significant differences in the densities of any macro-infauna species in 2011.
To determine the process of cellulose degradation in mangrove estuaries of the South-Western Islands, Japan, the cellulase activity of sediments was examined at a total of 9 sites on Okinawa Main Island (the River Gesashi and Manko), Ishigaki Island (Nagura Gulf and River Miyara), and Iriomote Island (rivers Urauchi, Mare, Hinai, Shira and Maira). Zymographic analysis was used to record cellulase activity in all sediment samples and in the meiobenthos. A 27-kDa cellulase activity band was expressed by several animal groups examined in the meiobenthos, including Gammaridea, Turbellaria, Ostracoda, Oligochaeta, Nematoda, and Polychaeta. However, the mangrove whelk, Terebralia palustris, which is a widely distributed member of the macrobenthos in these areas except Okinawa Main Island, expressed cellulase activities at 29, 31, 49, and 56 kDa. Interestingly, 31-kDa cellulase activity levels were recorded both in sediments and in the feces of T. palustris. The results of this study indicate that various benthic animals contribute to cellulose degradation in mangrove sediments of estuaries in the Southwestern Japanese Islands.
We studied the effects of Heterosigma akashiwo, a harmful algal species, in the diet of juvenile Manila clams (Ruditapes philippinarum), because it often blooms in coastal waters where aquaculture of the clams is carried out. Weight of soft tissue of clams increased when they were fed H. akashiwo added at 8,000 cells mL−1, with weight growth about 8% higher than that of clams fed only Chaetoceros neogracile. In addition, a significant increase in the body-mass index and glycogen content was observed when they were fed H. akashiwo compared with C. neogracile. Analysis of the chemical composition of H. akashiwo, C. neogracile, Pavlova lutheri and Nannochloropsis sp. showed a higher total sugar and acidic sugar content in H. akashiwo than in the other species, whereas the protein content was almost the same for all species except Nannochloropsis sp. These observations suggest the possibility that the sugar content of phytoplankton is an important factor affecting the growth of juvenile clams.
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