The three Hediste species H. japonica, H. diadroma and H. atoka (Polychaeta: Nereididae) live in burrows in estuarine intertidal flats in eastern Asia and have been regarded as surface deposit feeders. Their filter-feeding behavior was investigated in laboratory experiments where the worms were placed in a U-shaped glass tube. The behavior is basically the same among the three species and the same as that previously observed in H. diversicolor inhabiting Atlantic coasts and consists of the following four steps: (1) Moving up to the end of the tube; (2) retreating slowly down, with the anterior body expanded laterally and moving from side to side in semicircles, making a transparent, funnel-shaped mucous net; (3) orientation of the head at 2.5–6.0 cm below the end of the glass tube, and pumping water through the net by means of active undulating movements of the body for 1–5 min; (4) moving forward and swallowing the mucous net together with the trapped particles. This behavior is accelerated by adding organic particles to the seawater. Such facultative filter feeding is judged as a unique generic characteristic of Hediste among the Nereididae and its ecological significance is discussed in relation to their estuarine habitats.
The diatom Eucampia zodiacus is a harmful species that indirectly causes bleaching damage to nori (Pyropia) cultivation through the competitive utilization of nutrients during its bloom. In the present study, to develop an understanding of the population ecology of E. zodiacus and identify countermeasures to reduce the negative impacts to aquacultured nori, we investigated the cell density, cell size reduction and restoration of E. zodiacus at two sampling stations in northern Harima-Nada, eastern Seto Inland Sea, from April 2006 to March 2009. Vegetative cells of E. zodiacus were detected almost all-year round in the water column. Cell densities were generally higher at the northwest station (Stn. Aioi), where initial appearances of E. zodiacus blooming are often detected in Harima-Nada, than at the northeast site (Stn. Futami). The restoration of cell size occurred once every autumn with great regularity, and the cells with maximum size gradually decreased and reached the minimum size one year later at both sampling stations, suggesting that synchronous changes in cell size occurred throughout the whole area of Harima-Nada. Eucampia zodiacus is considered to adapt during the stratified period by reducing its cell size. In the future, predicting the occurrence timing of nori bleaching by E. zodiacus will probably be improved by using the data obtained at Stn. Aioi in addition to that at Stn. Futami.
Since Park et al. (2006) succeeded in cultivating the toxic dinoflagellate Dinophysis acuminata and maintaining them by feeding them the ciliate Myrionecta rubra grown with a cryptophyte Teleaulax sp., the present study is the fifth report of propagation of a Dinophysis species (Dinophysis tripos) under laboratory conditions and describes the maintenance of clonal strains kept at relatively high abundance (>2,000 cells mL-1) for a relatively long period (>6 months) when fed on M. rubra with the addition of Teleaulax amphioxeia. Dinophysis tripos grew at a growth rate of 0.54 divisions day-1, reaching a maximum concentration of ca. 2,200 cells mL-1 during the first 14-day growth experiment. At the end of the incubations, the total amounts of pectenotoxin-2 and dinophysistoxin-1 in the D. tripos culture reached up to 2,780±216 (mean±SD) and 1.8±3.2 ng mL-1, respectively, therefore, we confirmed active toxin production. However okadaic acid (OA) was not detected from this culture before or after the incubation. These data clearly indicate that D. tripos needs to feed on their ciliate prey to actively produce the toxins.
Dependency of carbon cell quota (C) on cell volume (V) was studied for the Prymnesiophyte Isochrysis galbana. Changes in the cell volume and carbon quota during cell divisions were examined at 25°C in a continuous culture with an enriched f/2 medium under a 12/12 h light/dark cycle at an irradiance of 45, 150, and 365 μmol m-2 sec-1. Carbon quota estimates followed the relation log C=b log V+log a, where b is the slope and log a is the intercept of the relationship. Significant relationships between carbon quota and cell volume during cell divisions were obtained at the three irradiances tested. The value of the slope and the intercept ranged from 0.59 to 0.98 and from 0.22 to -0.64, respectively, however, they were not significantly different between the three irradiances. The relationship between carbon quota to cell volume during cell divisions also corresponded well to previously published carbon quota to cell volume relationship values within the size range of nanophytoplankton ranging from 2 to 20 μm, even though the irradiances used in the experiments differed from those studied previously. Due to the close similarity in cell size and division manner between the majority of nanophytoplankton communities and I. galbana, the results of the present study can be applied to natural assemblages of nanophytoplankton to estimate their carbon biomass in the euphotic layer regardless of sampling time on a given day.
Predation pressure of Noctiluca scintillans on diatoms and thecate dinoflagellates was investigated off the western coast of Kyushu, Japan by a comparison between the taxonomic composition of prey in the food vacuoles of N. scintillans and that in the ambient water. The ratios of total diatoms and centric diatoms in the food vacuoles were higher than those in the ambient water, while thecate dinoflagellates and pennate daitoms showed a reverse trend. These results indicate that predation pressure of N. scintillans on total diatoms is higher than on thecate dinoflagellates, and that the pressure on centric diatoms is stronger than that on pennate ones. Among the centric diatoms, Chaetoceros and Bacteriastrum were dominant in the food vacuoles and their respective ratios were higher than in the ambient water. Their chain-forming colonies with long setae appear to be easily trapped by N. scintillans tentacles and this is probably the cause of the high predation pressure of N. scintillans on centric diatoms.
Nine specimens of the planktonic chaetognath Eukrohnia kitoui, which was originally described from Tokyo Bay 32 years ago, were collected from an underwater canyon in Tosa Bay, Japan. This is the second record of the species. We compared E. kitoui and the closely allied E. calliops based on individual data of the two species reported in the previous and the present studies. Since allometric growth was observed in morphometric characters, specimens smaller than 10 mm in body length were omitted in this comparison. The results revealed that the ratios of tail segment and eye lengths to body length and the numbers of hooks and posterior teeth on the head, which were described as distinguishable features between the species in the previous study, were significantly different between the species. However, the values of these characters overlapped between the species except for the eye/body ratio, indicating that the eye size is the most critical feature distinguishing them. The present specimens showed that the relative position of the transverse musculature to the ventral ganglion, which was another diagnostic character of E. kitoui in the previous study, varied greatly according to growth. Considering the characteristic sampling sites (underwater canyons) and a long absence of occurrence records, E. kitoui is probably a rare species restricted to waters in or near coastal underwater canyons, as observed in E. calliops.
Relationships between the sediment hardness and the upper distribution limit of the burrowing amphipod Haustorioides japonicus were examined on 18 sandy shores in Niigata and Toyama prefectures, on the Japan Sea coast. Sediment hardness was measured by three methods (using a vane tester, a digital force gauge, or a cone penetrometer) and compared with the estimates from a previous laboratory experiment. Results showed that the vane tester measurements of the hardness of the sandy sediment corresponding to the upper distribution limit on the shores represented a similar value to the measurement obtained in the laboratory. Some similarities between the measurement mechanisms of the vane tester and the burrowing behavior of the amphipods may explain the results. Consideration of the mechanism of hardness measurement will improve understanding of the relationships between sediment hardness and the distribution of burrowing animals on sandy shores.