A series of oblique hauls with Bongo nets (0–1000 m) was made during the period of August 2002 through August 2004 in the Oyashio region, western subarctic Pacific, to investigate abundance, biomass and life cycle patterns of the three predominant euphausiids (Euphausia pacifica, Thysanoessa inspinata and T. longipes). While the three euphausiids occurred throughout the entire study period, E. pacifica was the most abundant (1,120 indiv.m−2, or 832 mg C m−2), followed by T. inspinata (163 indiv.m−2, or 144 mg C m−2) and T. longipes (73 indiv.m−2, or 75 mg C m−2). Judging from the occurrence of females with spermatophores and furcilia larvae, the spawning was considered to take place twice a year (April–May and August) for E. pacifica, year-round (peak season: March–May) for T. inspinata and in spring (March–May) for T. longipes. The population structure in terms of size (=total length) frequency distributions of the three euphausiids was characterized by the frequent co-occurrence of 2–3 cohorts in the same samples. The maximum size of males and females found were 21 mm and 24 mm, respectively, for E. pacifica, 18 mm and 23 mm, respectively, for T. inspinata, 27 mm and 31 mm, respectively, for T. longipes. Tracing the sequence of cohorts, the life spans of E. pacifica, T. inspinata and T. longipes were estimated to be 17–26 months, 17–19 months and 29–31 months, respectively. These results are compared with reports of the same species in other habitats in the light of regional variations.
The seasonality in abundance, population structure, and other biological aspects of the tropical shallow-water mysid Mesopodopsis orientalis were studied at an open sandy beach with fine sand on the southern coast of Penang Island, north-western Peninsular Malaysia, from December 2004 to August 2006. The monthly collections ranged from 0.7–534 indiv.m−2, with an overall mean of 49 indiv.m−2. The density, however, could reach as high as 8,656 indiv.m−2. There were no correlations between the abundance of mysids and the temperature, salinity, or tidal cycle, and the wide range in the variations of catches was considered to be mainly due to their heterogeneous distribution. Moreover, no distinct pattern was found in abundance between day and night and again between shady and sunny areas. Males accounted for 33.3–56.5% (mean; 47.3%) of the monthly samples; females predominated over males in the entire shore population. Reproduction of M. orientalis in Penang waters is continuous throughout the year and seasonality in reproductive traits is non-existent or, if present, considerably reduced. The number of eggs/embryos carried by females exhibited a positive correlation with female size, and the maximum brood size was 24. The egg diameter ranged from 0.30–0.42 mm, with a mean of 0.37 mm, and was independent of female size. Thus, the clutch size of the Penang population of M. orientalis was more fecund than that observed in the estuarine counterpart, Mesopodopsis tenuipes, but the former was smaller in size than the latter during all stages in its life cycle.
We investigated seasonal variations in abundance and cell volume of picoplankton (heterotrophic bacteria and phycoerythrin-rich cyanobacteria) along with environmental conditions at a coastal site in an oyster-farming area in northern Japan. Samples were collected once or twice a month from July 2002 to July 2004 and analyzed using epifluorescence microscope methods. Abundances of bacteria and cyanobacteria both increased from summer (June–August) to autumn (September–November) and decreased from winter (December–February) to spring (March–May). The range of seasonal abundances of bacteria was within one order of magnitude, but that of cyanobacteria extended over almost three orders of magnitude. Bacterial and cyanobacterial abundances were both positively correlated with temperature. However, the abundance of cyanobacteria decreased at high temperatures when salinity was below 31 psu. Cell volumes of bacteria and cyanobacteria varied inversely with seasonal patterns of abundance. Cell volumes of cyanobacteria were negatively correlated with temperature, whereas those of bacteria showed no significant correlation with temperature. Mean water-column carbon biomass ranged from 23.4 to 174.3 μg-C L−1 for bacteria and from 0.1 to 18.4 μg-C L−1 for cyanobacteria. Maximum values of mean water-column biomass of bacteria reached 179% of the estimated biomass of phytoplankton carbon, whereas that of cyanobacteria was 58% of picophytoplankton biomass and 22% of phytoplankton biomass. Bacterial biomass levels similar to those of phytoplankton and the high contribution of cyanobacteria to picophytoplankton in summer imply that the picoplankton assemblage plays an essential role in planktonic food webs in the bivalve-farming area.
Shell utilization patterns of the intertidal hermit crab Pagurus filholi were compared between two distant local populations from Oura Bay, Shimoda, central Japan, and Hakodate Bay, Hakodate, northern Japan, in conjunction with the shell resource availability and shell utilization patterns of sympatric hermit crabs. Gastropod species of shells dominantly utilized by P. filholi differed between Oura Bay and Hakodate Bay, and shell utilization patterns did not correspond to the availability of shells in both study localities. Between P. filholi and other sympatric hermit crabs, there were some clear differences in shell species utilized in both localities, and values of niche overlap indices in shell and habitat utilizations between P. filholi and other crabs were higher in Hakodate Bay than in Oura Bay. The rich supply of shell resources in Hakodate Bay compared to Oura Bay is considered to be a factor responsible for the difference in the degree of niche overlapping between the two localities.
The diet of the Japanese mitten crab Eriocheir japonica was investigated by examining the stomach contents of middle- and large-sized crabs (carapace width >10 mm). Crabs were collected in the freshwater area (growth area), tidal river, and seacoast area (reproductive area) of the Saigo River, Fukuoka Prefecture, Japan. Analysis of the diet of E. japonica was carried out using the frequency of occurrence and percentage point methods. The stomach contents were classified into 4 large and 27 small categories. It was evident that E. japonica was an omnivore and a deposit feeder, mainly feeding on the detritus derived from vascular plants growing along the river channel. Filamentous algae Cladophora sp., aquatic insects (e.g., Chironomidae), sediments such as mud or sand, and artificial products such as plastic or yarn were often found, suggesting that E. japonica mainly feeds on detritus lying on the substrate without sorting well. Stomach contents of adult crabs in the tidal area exhibited higher frequency of animal materials (i.e., fish, gastropods, and crabs) than those of young and adult crabs in the freshwater area, suggesting that adult crabs in the tidal area eat more animal materials. Such an omnivorous feeding habit is similar to other related brachyuran crabs (grapsoidae) and may be one of the important factors which enables E. japonica's wide distribution along the river and seacoast.
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