In order to assess the relationship between the macrobenthic community and the sedimentary environment in Isahaya Bay, Kyushu, Japan, we conducted field surveys at 9 sampling stations in April 2014. Based on clustering analysis of abundance of the macrobenthic community, the 9 sampling stations were clearly divided into three groups(Groups A–C). The groups A–C were located at the outer(3 stations), center(2 stations), and inner(4 stations)parts of Isahaya Bay, respectively. Group A had low mud content(28.3±10.8%, mean±standard deviation)with low total organic carbon(TOC) (14.2±4.1 mg C g−1)and low acid volatile sulfur(AVS, 0.12±0.1 mg g−1). The dominant species in group A were a deposit- and filter-feeding amphipod, Photis longicaudata; a suspension-feeding bivalve, Veremolpa micra; and an omnivorous polychaete, Scoletoma longifolia. Group B had high mud content(74.6%)with high TOC(19.1 mg C g−1)but low AVS(0.20 mg g−1). The dominant species of this group was V. micra; a deposit-feeding bivalve, Theora fragilis; and a deposit- and filter-feeding amphipod, Ampelisca brevicornis. Group C had high mud content(87.8±4.8%), TOC(21.4±2.9 mg C g−1), and AVS(0.85±0.5 mg g−1)and was dominated by a deposit- and filter-feeding amphipod, Monocorophium sp.; T. fragilis; and a carnivorous polychaete, Glycinde sp. These results suggest that the distribution of macrobenthos corresponds with the sedimentary environment gradient in Isahaya Bay.
Lake Nakaumi is a brackish water body in the eastern part of Shimane Prefecture, Japan. The lake, which receives fresh water from Lake Shinji and several rivers, connects with the Japan Sea through a single narrow channel. Recently, aquaculture trials for the Manila clam, Ruditapes philippinarum, have been conducted in the lake. However, little is known regarding the ecological and physiological characteristics of the clam in this environment. In the present study, the reproductive cycle was investigated in clams sampled at a subtidal site from April 2008 to March 2011, using histological analysis and the change in condition index(CI). The water temperature, salinity and chlorophyll a + pheo-pigment content were monitored in lake water near the collection site of the clams. The CI of the clams increased from March to May and gradually decreased afterwards to September/October. The spawning period was determined to be August–October in 2008, Jun–August and October–November in 2009, and Jun–October in 2010. Thus, the time of initiation and annual number of the peak spawning periods varied yearly. While the water temperature(<5–>30℃)and chlorophyll a + pheo-pigment content(7–15 μg/l)were within the range for these parameters in the major clam habitats in the southern part of Japan, salinity(10–25 psu)was considerably lower than that in other areas. We presume that low and variable salinity conditions caused unstable spawning activity(delayed initiation and fluctuating annual peak of spawning period)of the clams in Lake Nakaumi.
The hermit crab Clibanarius infraspinatus generally inhabits the infratidal zone and rarely occupies the intertidal area. However, at the mouth of the Waka River, they are commonly found in the intertidal sand flats located in the shallow channels, where we conducted quantitative surveys at six stations to investigate their population structure between June 2013 and May 2014. Between June and September 2013 and in May 2014 we collected, on average, 213 crabs per month. However, between December and March we found no crabs, which suggests that they migrated seasonally. Ovigerous females were found only between June and September, and their numbers were maximum in August accounting for 36% of all female crabs that were collected in that month. Males were larger than females in terms of body size distribution in the population. The shield length of the smallest ovigerous female was 5.9 mm. Chela size of males became relatively larger than that of females with their age. The right chela was slightly larger than the left one in females. The sex ratio in the population was nearly 1:1. In contrast to the lower four sites, fewer crabs and no ovigerous females were collected at the upper two sites. This may be because the substratum of the upper sites was muddy, differing from a typical habitat for C. infraspinatus. This along with the relatively fewer ovigerous females collected even in a peak reproductive season suggests that the tidal flat surveyed in this study may not be physically suitable for C. infraspinatus.
Two tidal-flat-dwelling ocypodoid crabs, Uca lactea and Ilyoplax pusilla show the similar habitat preferences with respect to intertidal height and substratum. Between populations of U. lactea co-occurring with I. pusilla and those without I. pusilla, their habitat conditions of intertidal height and substratum were compared. The habitat conditions were not significantly different between populations with I. pusilla and those without I. pusilla, suggesting the non-influence of I. pusilla on habitat use by U. lactea. Individual male U. lactea were found to be more aggressive against the neighboring conspecifics than against the neighboring I. pusilla. Less aggressiveness of U. lactea towards I. pusilla is attributable to the habitat use by U. lactea independent of the presence of I. pusilla.
The occurrence of the ghost crab Ocypode stimpsoni was surveyed at 32 beaches within southern Hokkaido, and Itanki Beach at Muroran was detected as the northernmost edge of its range. The burrow distribution of the ghost crab at Omori Beach, Hakodate, near the northern edge, was investigated from January to December 2014. Burrow opening was initially observed in late June near the upper area of the beach with vegetation. The number of burrows increased in the following months, with their distribution extending down the beach. During mid-summer with high temperature, severe dryness of the sand restricted the upper limit of the burrow digging area to immediately below the high-tide water mark with moist sand. The lower limit was determined by excavation of the sand by rough waves, concentrating the burrow area to a narrow band within the beach. In autumn, the dryness decreased and the burrow area extended upward, becoming wider. Burrow activity halted by the end of October. After October, the ghost crab overwintered in deep burrows within the upper area until late next June. These results suggest that at the northernmost edge of the distributional range, seasonal activity patterns are severely restricted by environmental factors such as extreme temperatures, dryness, and rough waves, which is coincident with the general rule of geographical range pattern of organisms. A possibility that the famous poet Takuboku Ishikawa played with the ghost crab at Omori Beach was discussed based on the findings.
To examine the effectiveness of different colored bottle traps for capturing an invasive crayfish Procambarus clarkii introduced from North America, we conducted two experiments (Experiments 1 and 2) in two ponds (the Onibasu and Raku-u-sho) in the Handayama Botanical Garden, Okayama, Japan. A set of traps consisted of three different colored bottles (black, white, and transparent). In Experiment 1, we compared the numbers of crayfish captured in different colored bottles; we set the three colored bottle traps with artificial bait at 10 stations in the Onibasu pond in the late afternoon and retrieved them early the following morning. In Experiment 2, we examined the difference in the numbers of crayfish captured during the day and at night; we set the same three colored bottle traps at 10 stations in the Raku-u-sho pond during the daytime and at nighttime and retrieved them in the late afternoon and early the following morning, respectively. In Experiment 1, there were multiple individuals in all of the traps, indicating that the plastic bottles were effective in capturing P. clarkii. The number of P. clarkii captured was significantly higher in the white and transparent traps than in the black traps. In Experiment 2, significantly more P. clarkii were captured at nighttime than during the day. The number of P. clarkii captured was not significantly different between the three colored bottles during the daytime. However, compared to the black traps, the white traps were significantly more effective in catching P. clarkii, but the effectiveness of the transparent traps was not significantly different from that of the black and white traps. Our results indicate that white or transparent plastic bottle traps were the most effective, especially at nighttime, for capturing invasive P. clarkii.
Estuaries, the main zones of transition between riverine terrestrial and marine ecosystems, are characterized by environmental fluctuations including changes in salinity, water temperature, and sediment biogeochemistry. In this review, I summarize a series of studies conducted in a freshwater-affected brackish lagoon (Idoura Lagoon, Sendai City, Northeastern Japan) in Sendai Bay. This lagoon system is considerably affected by irrigation water discharge from its catchment area during the rice-farming season (late April to early September). River-derived terrestrial materials are the main source of sediment organic matter in the lagoon. We found a distinct spatial change in the macrozoobenthic community structure along a salinity gradient from the freshwater inflow. Annual monitoring also revealed that the population dynamics of the polyhaline polychaete Pseudopolydora spp. were affected by seasonal changes in the salinity regime. Stable isotope ratios of the polychaetes Hediste spp. and the bivalve Laternula marilina also suggested that river-derived organic matter was a major component of the diets of these species during the irrigation period. Our results show that fluctuations in irrigation discharge have considerable effects on estuarine soft-bottom habitats, suggesting a significant linkage between terrestrial, riverine, and estuarine ecosystems under anthropogenically modified water-use conditions.