Machiko Nishino

Seasonal variations of Microcystis populations in sediments of Lake Biwa, Japan

Seasonal variations of colony numbers of Microcystis aeruginosa(Kütz.) Kütz. and M. wesenbergii(Komárek) Komárek in N. V. Kondrat. in sediments of Lake Biwa were investigated over a period of 1 year. At two stations located in the shallow South Basin of Lake Biwa (ca. 4 m water depth), the colony number of Microcystisfluctuated seasonally. The number had a tendency to gradually decrease from winter to early summer, while it increased through mid-summer and autumn. Since the Microcystispopulation in sediment was rather small, intensive growth and accumulation in the water column should be important for the formation of Microcystisblooms in Lake Biwa. Microcystiscolonies in the sediment samples after June were observed to be floating in a counting chamber under a microscope. The observation suggests that the recruitment of Microcystis colonies into the water column mostly occurs in early summer. The number of Microcystiscolonies in the deep North Basin of Lake Biwa (70 – 90 m water depth) was larger than in the South Basin. Because the seasonal variation of colony numbers was not observed in the North Basin, and Microcystiscells do not have gas vesicles, these colonies will not return into the water column. The colonies isolated from the sediment of the North Basin were able to grow in cultured conditions, in the same way as those from the sediment of the South Basin. Therefore, Microcystiscolonies may survive for a long time under stable conditions of low temperature (ca. 8 °C) and darkness, in the sediment of the deep North Basin, accumulating gradually each year.

Vertical distributions of sulfate-reducing bacteria and methane-producing archaea quantified by oligonucleotide probe hybridization in the profundal sediment of a mesotrophic lake

Abstract Vertical distributions of sulfate-reducing bacteria and methane-producing archaea were investigated in the profundal sediment of a freshwater lake using membrane-immobilized small subunit rRNA hybridization with group- and genus-specific oligonucleotide probes. The annual average of the relative abundance of small subunit rRNA hybridized with all probes for sulfate-reducing bacteria to total small subunit rRNA was 2.3% at 0-2 cm and increased with depth up to 22.9% at 8-14 cm where sulfate concentration was less than 10 nmol ml(-1) in interstitial water, suggesting that these bacteria may survive on alternative metabolisms. The signal of probe Dsv687 (the family Desulfovibrionaceae and some Geobacteraceae) was the main factor in this increase. The relative abundance of methane-producing archaea to total small subunit rRNA was highest (7.8%) at 8-14 cm, dominated by the order Methanosarcinales. The metabolic rates measured in the sediments demonstrated that the peaks of sulfate reduction and methane production were separated vertically, and were not linked to their small subunit rRNA distributions. Our data indicate that sulfate-reducing bacteria can coexist with methane-producing archaea from 0 to 20 cm in the freshwater lake sediment.

New records and distributions of two North American branchiobdellidan species (Annelida: Clitellata) from introduced signal crayfish, Pacifastacus leniusculus, in Japan

This is the first report of two North American branchiobdellidans, Sathodrilus attenuatus Holt, 1981, and Xironogiton victoriensis Gelder and Hall, 1990, on the signal crayfish, Pacifastacus leniusculus (Dana, 1852) introduced into Japan from the Columbia River system, northwestern North America. Signal crayfish from 12 localities in eastern and northern Hokkaido, Japan, were examined and each supported S. attenuatus. In addition, an individual of this species was found on preserved material from Ishikawa Prefecture, central Honshu. All of these branchiobdellidans reported in Hokkaido most probably came from the original population of signal crayfish introduced into Lake Mashu, Hokkaido, Japan, in 1930. It is suggested that the use of non-pathogenic branchiobdellidans, when present, provides an easy method for tracing the spread of crayfishes around Japan and could also be applied in other countries and continents. Specimens of X. victoriensis were only found on crayfish in a stream at Akashina in Nagano Prefecture, central Honshu, Japan. Although the signal crayfish appears to be displacing the endemic Japanese crayfish, C. japonicus, no native branchiobdellidans were found on any of the introduced signal crayfish examined.

Studies on the aquatic oligochaete fauna in Lake Biwa, central Japan. II. Records and taxonomic remarks of nine species

At least 41 taxa of aquatic oligochaetes including five new records have been recorded from the ancient Lake Biwa in Japan. The lake is characterized by eurybathyal occurrence of Branchiura sowerbyi. The posterior gill filaments of B. sowerbyi develop normally in lagoons adjacent to the lake, while in the lake they weakly develop or often disappear at littoral and sublittoral depths shallower than 30 m, and they almost completely disappear in the profundal zone. The profundal population of B. sowerbyi also differs from other populations in eastern Asia in having fewer and shorter hairs, and simple-pointed crotchets, or bifid crotchets with parallel teeth, in dorsal bundles of chaetae. This suggests that an intralacustrine speciation is progressing in the profundal population. An aberrant modification is found in the pectination of dorsal crotchets in Tubifex tubifex occurring at deep profundal depths of the lake. The ventral chaetae in segment II of Teneridrilus mastix are rarely modified in Lake Biwa. This requires cancellation of the modification of chaetae in II as a diagnostic characteristic of the genus Teneridrilus. Limnodrilus amblysetus, previously known only from China, is recorded from Lake Biwa. Descriptions and taxonomic remarks are provided for Haemonais waldvogeli (new to Japan), Pristina longiseta, Stephensoniana trivandrana and Ripistes parasita (new to Lake Biwa), and Ophidonais serpentina, based on Lake Biwa material.

Distributions of two ectosymbionts, branchiobdellidans (Annelida: Clitellata) and scutariellids (Platyhelminthes: "Turbellaria": Temnocephalida), on atyid shrimp (Arthropoda: Crustacea) in southeast China

Distribution of two ecologically similar but usually spatially separate ectosymbionts, branchiobdellidans (Annelida) and scutariellids (Platyhelminthes), on atyid shrimp (Neocaridina spp.) is reported from 18 localities in five Provinces of southeastern China. Prevalence was determined for the branchiobdellidan, Holtodrilus truncatus, found at seven locations, the scutariellid, Scutariella japonica, present at every site, and where cohabitation occurred. Both ectosymbionts showed a microhabitat predilection for the hosts branchial chambers and instances of cohabitation occurred at all seven locations where H. truncatus were collected, although not on every shrimp. On-site observations of live hosts supporting both ectosymbionts showed that neither H. truncatus nor S. japonica reacted aggressively or defensively towards the other when in close proximity. Instances of imported Chinese Neocaridina spp. into central Honshu Island, Japan, almost certainly came from areas in southeast China identified in this study. These imported populations are predicted to spread northwards into the area where endemic Japanese branchiobdellidans occur.

Disappearance of deep profundal zoobenthos in Lake Ikeda, southern Kyushu, Japan, with relation to recent environmental changes in the lake

A benthological survey in a deep caldera, Lake Ikeda, southern Kyushu, Japan, in 1998 revealed that no zoobenthos were found in the deep profundal, although two tubificid oligochaetes, Tubifex tubifex and Limnodrilus hoffmeisteri, and a chironomid, Procladius sp., were distributed in the upper profundal zone. This is the first record of oligochaete composition in the lake. Lake Ikeda had been typically oligotrophic until the 1940s, and zoobenthic assemblages were recorded throughout the profundal bottom in the 1920s and 1970s. Recent disappearance of the deep profundal zoobenthos could be caused by the stagnation of anoxic waters in the hypolimnion, in connection with eutrophication triggered by nutrient loading, as well as change in the thermal circulation system presumably caused by global warming.

Stomach contents of the landlocked dwarf ayu in Lake Biwa, Japan

Abstract The landlocked dwarf ayu (koayu) Plecoglossus altivelis in Lake Biwa was collected in June and July of 1995–1997, and its stomach contents were analyzed. On average, the standard length of koayu was 73 mm, its dry mass was 1.5 g, and the dry mass of the organisms in its stomach was 6.9 mg. Planktonic crustacea dominated the diet, and littoral benthic animals were minor prey. In particular, Daphnia galeata comprised a major fraction of the prey mass. Two copepods, Eodiaptomus japonicus and Mesocyclops dissimilis, were subdominant prey. About 80% of prey individuals had a dry mass of 2–10 μg, and individuals in this size range consisted mainly of D. galeata, with carapace lengths of 0.6 to 1.0 mm. Koayu occasionally ingested large organisms, such as D. galeata 1.5 mm in length, larval shrimps, aquatic insects, and even larval gobiid fishes. The stomachs of koayu were empty at night and were filled within 2 h after sunrise. The total prey mass then fluctuated until sunset, with a tendency toward reduction in the morning. The total prey mass and the size of ingested D. galeata tended to increase with predator size.

Studies on the aquatic oligochaete fauna in Lake Biwa, central Japan. IV. Faunal characteristics in the attached lakes (naiko)

Surveys for macroinvertebrates occurring in 20 small attached lakes (naiko) adjacent to Lake Biwa, central Japan, conducted in 2001–2002 yielded 44 taxa of aquatic oligochaetes (Annelida, Clitellata). Descriptions and taxonomic remarks are provided for nine species: Aulophorus furcatus, A. tonkinensis, A. flabelliger, Pristina biserrata, P. proboscidea, Ilyodrilus templetoni, Allonais pectinata, Stylaria fossularis, and Dero dorsalis; the former six species of these are new to Japan. Stylaria fossularis is regarded as a distinct species, distinguished from the congener S. lacustris, on the basis of prostomium morphology and geographical distribution. No distinctive oligochaete fauna can be seen in the individual naiko, but as a collective body they maintain a rich fauna. This fauna was richer in naidines, while poorer in other groups of oligochaetes than those in Lake Biwa, and basically can be regarded as an extension of the fauna of littoral Lake Biwa. Epiphytic oligochaetes were more diverse and more abundant in the naiko than those in littoral Lake Biwa, probably because of higher temperatures, denser aquatic vegetation, and higher primary production. Summer oligochaete assemblages in the naiko were predominated by Stylaria fossularis and include several species characteristic of subtropical and tropical Asia, suggesting a close relationship to the fauna of the Indo-Malayan zoogeographical subregion.

Chromosomes of Phagocata kawakatsui and Bdellocephala annandalei from Lake Biwa-ko in Honshu, central Japan

Two lake-dwelling species of paludicolen triclads from Lake Biwa-ko (Honshû, Japan) were studied taxonomically and karyologically. (1) Phagocata kawakatsui Okugawa, 1956, is an epigean species usually inhabiting shallow springs and spring-fed streams in Central Japan. In Lake Biwa-ko, animals were obtained from several bottom stations of the littoral area in the southern part of the northern basin (3–70 m in depth). Chromosome numbers and karyotype: 2x=24 (2m+2sm+2sm+2m+2sm+2m+2sm+2m+2m+2sm+2m+2m). The first pair of metacentric chromosomes is very large in size. (2) Bdellocephala annandalei Ijima et Kaburaki, 1916, an endemic species, is distributed widely in the deep areas of the northern basin (30 to over 100 m in depth). Chromosome numbers and karyotype: 2x=28 (2m+2sm+2sm+2sm+2sm+2m+2m+2m+2m+2m+2m+2m+2m+2m) with the first pair of metacentric chromosomes very long.

Predator-prey relationship between the landlocked dwarf ayu and planktonic Crustacea in Lake Biwa, Japan

The predator-prey relationship was investigated between the landlocked dwarf ayu (koayu; Plecoglossus altivelis) and three planktonic crustaceans, Daphnia galeata, Eodiaptomus japonicus, and Mesocyclops dissimilis, in Lake Biwa. The abundance of each prey species and daphniid size composition in the lake was compared with those in the stomachs of koayu collected at the same time and place. D. galeata was the main item of diet of koayu in spite of their low density, and the larger individuals were selectively preyed upon by the predator. In the stepwise multiple regression of the mean number of each prey species in the stomachs on prey densities in the lake and predator mass, the density of the prey itself was always selected as the predictor variable. The density of E. japonicus was also selected as the regressor of daphniid number in the stomachs with a negative slope. Multiple regression was also performed each for two size-classes of D. galeata, and a negative effect of large daphniids was detected for the consumption of small ones. The present results and additional information enable estimation of the collective predation rate by the koayu population on the prey populations.