Mass aggregations of Aurelia limbata have been reported along the Pacific coast of northern Japan, from spring to fall. The polyp stage is important for understanding the factors leading to mass occurrences of jellyfish, because polyps reproduce asexually and are responsible for the release of many ephyrae. Until the present report, the polyps of A. limbata had not been found in the wild and their ecology remained unknown. We found 18 polyps of A. limbata attached to two pieces of deep-sea debris, an aluminum beverage can and a plastic bottle, collected by bottom trawl at depths of 296 m and 392 m, respectively. Strobilation of the polyps was observed at 4°C without temperature change stimulation. This raises the possibility that strobilation occurs in low-temperature environments throughout the year. A large quantity of debris had sunk to the seafloor off the coast because of the tsunami tidal wave after the Great East Japan Earthquake, increasing the available substrate for A. limbata polyps. Additional ecological research on polyps and medusae in deep waters is necessary to predict future blooms of A. limbata.
Latitudinal distributions of pelagic chaetognaths from 3–34°N along the 137°E meridian in the Philippine Sea in winter from 1967 to 1995 were studied using the data of oceanographic observations by the Japan Meteorological Agency. Zooplankton was collected at almost every latitudinal degree in every winter (mid-January to early-February) by vertical hauls of a 0.33-mm mesh NORPAC net from 150-m depth. Twenty-six chaetognath species, including four mesopelagic species, were recorded. Flaccisagitta enflata was most abundant, comprising 25%, on average, of the chaetognath community, followed by Serratosagitta pacifica and Pterosagitta draco. The community structure was analyzed according to the following five areas along the survey line: Japan coast area (JC), Kuroshio Current area (KC), subtropical area (ST), North Pacific Equatorial Current area (NEC), and North Pacific Equatorial Counter Current area (NECC). The dominant species were S. pacifica and Mesosagitta minima in JC, Pseudosagitta lyra and Pt. draco in KC and ST, and F. enflata in NEC and NECC. The cluster analysis revealed that the community was generally more similar among the samples from the same area than those from different areas except for KC and ST, which were not separated into different cluster groups. This suggests that the chaetognath communities are location-specific. Multiple comparisons of the mean densities of 17 common species among these areas revealed six distribution types, i.e. JC type, JC-KC type, KC-ST type, NEC-NECC type, NECC types, and bimodal type. Possible mechanisms determining the distribution types, especially the bimodal distribution, are discussed in terms of environmental factors.
The scattering properties of phytoplankton is one of the main factors needed to model light propagation in the water column. Light scattering by phytoplankton depends on cell size and intracellular materials. In order to characterize the effects, we examined the scattering efficiency of dinoflagellates with large cell sizes and high intracellular carbon contents. Scattering properties of the dinoflagellates Prorocentrum micans and P. minimum were examined in semi-continuous cultures under two light-saturated conditions. Scattering coefficients of the cells at 676 nm (bph) were calculated as the difference between attenuation and absorption coefficients measured using a nine-wavelength absorption-attenuation meter. The bph(676) was normalized to the chlorophyll a (Chl a; b*ph) and cell concentrations (bcell). Lower b*ph(676) and higher bcell(676) were observed for the larger P. micans compared with the smaller P. minimum. The b*ph(676) increased with the ratio of cellular carbon to Chl a (C : Chl a). Both species indicated relatively high C : Chl a compared to other phytoplankton species. A reverse trend of b*ph(676) and bcell(676) between the species could reflect a negative relationship between the equivalent spherical diameter (d) and intracellular Chl a content (Chl ai). A dimensionless efficiency factor for scattering at 676 nm (Qb) was calculated experimentally from d, Chl ai, and b*ph(676). The Qb(676) of both species was two-fold higher than theoretical values based on the anomalous diffraction approximation. The experimentally high Qb(676) might reflect the high b*ph(676). The significant relationship between b*ph(676) and C : Chl a suggests that C : Chl a could be a proxy for scattering efficiency in relation to intracellular materials.
Sonication has been used in many studies that require observation of the cells of Microcystis aeruginosa. This study investigates the extent to which sonication (42 kHz, 150 W) destroys live and Lugol-fixed cells of M. aeruginosa, using two strains (NIES-1090 and NIES-1157). Live cells were easily destroyed by sonication regardless of growth stage. Sonication for 50 s caused up to 32.8% (NIES-1090) and 40.8% (NIES-1157) cell losses, and extended sonication (250 s) destroyed more cells. In contrast, sonication did not cause loss of cells that had been fixed with Lugol’s iodine solution. Interestingly, sonication often resulted in slightly higher counts of Lugol-fixed cells under a microscope. Sedimentation of Lugol-fixed cells is enhanced after sonication owing to the collapse of gas vacuoles and this appeared to facilitate reliable microscopic observations.