Spatial variations in fish assemblages in soft-substrata estuarine tidepools（n = 55, 0.6-6.4 m² were investigated on tidal flats 0-4 km from the mouth of the Tama River estuary, central Honshu, Japan in early June 2003. A total of 1,838 individuals, representing 2 families and 11 species, were collected during the study period. All fishes collected were less than 50 mm SL, being mostly gobiid juveniles and adults. Acanthogobius flavimanus was the most abundant species, comprising 52.2% of the total individual number, followed by Pseudogobius masago（24.6%）, Gymnogobius macrognathos（12.7%）, G. breunigii（7.0%）, Mugil cephalus cephalus（1.0%）, Favonigobius gymnauchen（0.9%）, Mugilogobius abei（0.7%）and Eutaeniichthys gilli（0.5%）. Of these, six benthic gobies except for G. breunigii and M. cephalus cephalus occurred at different densities in the lower, middle and upper estuarine areas. The canonical correspondence analysis using densities of abundant species in each tidepool revealed that spatial variations in the fish assemblage structures were largely associated with environmental variables, including mud shrimp-burrow density, median grain size, salinity, height above low tide level, water temperature, pool size and water depth.
This study estimated the area of eelgrass beds in Matsukawa-ura Lagoon and investigated spatiotemporal changes of the area using satellite images obtained after the 2011 off the Pacific coast of Tohoku Earthquake. Furthermore, we examined the spatial distribution of mud contents in the lagoon after the earthquake. In April 2012, the eelgrass beds were observed in small area（0.013 km²）of the northern part of the lagoon. Almost all eelgrass was physically removed by devastating tsunami waves. In September 2014, the bed area had increased sharply to approximately 0.39 km². The spatial distribution also expanded from the northern to the central lagoon. In November 2015, the rate of increase was only approximately 10% compared with the previous year. No remarkable change was found in the spatial distribution. The mud content was 0-79%, the eelgrass bed was observed in the area where the mud contents were less than 30%. The eelgrass bed area in 2015 was roughly twice that in 2010. Eelgrass beds had expanded to the upper edge of the waterway after the earthquake. The subtidal zone, which is suitable for eelgrass growth, expanded because of ground subsidence.
Planktonic copepods can detect potential preys and predators through mechanoreception. Sensing a certain level of deformation rate of ambient water, they escape from the source of stimulus. Quantification of the deformation rates that evoke the escape behavior of copepods may thus help understand their living strategies. The term “zooplankton” generally refers to assemblages of individual zooplankters, and “zooplankton” has been usually studied by ignoring inter-individual differences. We here observed and quantified individually the behaviors of female Oithona davisae under spatially changing deformation rates produced with a suction flow system. Female O. davisae typically escaped after being drawn to areas with deformation rates
ranging 0.1-1.9（0.54 ± 0.45）s-1. To escape, they jumped towards lower-deformation conditions
with higher speed and longer distance than without stimulus, showing that they can detect not only the strength but also the directional information of flow fields. Moreover, significant interindividual differences in the behavior were observed, indicating that copepods are a group of organisms with different individual characteristics. Our results also suggest that female O. davisae prefers to stay in a quiescent environment where local deformation rate is smaller than 0.1 s-1. Because female O. davisae ambushes prey by detecting weak hydromechanical signals, staying in environments with lower deformation rates may be beneficial to detect prey.
The amounts of leaf litterfall and leaf litter removal by the feeding of a numerically abundant sesarmid crab Parasesarma bidens were examined in a subtropical mangrove forest, dominated by the red mangrove Rhizophora stylosa, on Iriomote Island, southern Japan, in 2016. Field observations and stable isotope analyses showed that P. bidens consumed and assimilated leaf litter rather than other primary food sources such as macroalgae and microphytobenthos. Measurements of the leaf litterfall amount were conducted using litter traps for four weeks in May and June. The weekly and estimated annual litterfall amounts were 3.5 g/0.25 m² and 730g/m² （dry weight), respectively. Leaf litter consumption by P. bidens, determined using a field cage experiment in August, indicated that a single crab consumed 1.5 g/week（dry weight), the estimated annual consumption under natural crab density（8 individuals/m²）being 626 g/m² （86% of total litterfall amount). We conclude that leaf litter feeding by P. bidens significantly contributes to the organic matter cycling within the mangrove ecosystem.