A subterranean species of pseudocrangonyctid amphipod, Pseudocrangonyx gudariensis Tomikawa & Sato, sp. n., is described from the spring-fed stream Gudari-numa in Hakkoda Mountains, Aomori Prefecture, northern Japan. Pseudocrangonyx gudariensis is morphologically similar to P. coreanus Uéno, 1966 and P. febras Sidorov, 2009 based on its relatively small body size, small number of articles of rami of pleopods, and urosomite 1 without basal setae. However, P. gudariensis is distinguished from those species based on the following characteristics: from P. coreanus, antenna 2 of female without calceoli, palmar margins of gnathopods 1 and 2 with distally notched robust setae, inner margin of inner ramus of uropod 2 with 4 robust setae, and basal part of inner ramus of uropod 2 without slender seta; and from P. febras, carpus of gnathopod 2 without serrate robust setae on posterodistal corners, peduncle of pleopods 1 and 2 with setae, and longer article 2 of uropod 3. Phylogenetic analyses using nuclear 28S rRNA and histone H3, and mitochondrial cytochrome c oxidase subunit I and 16S rRNA markers showed that P. gudariensis is placed among known Pseudocrangonyx Akatsuka and Komai, 1922 species. However, its exact phylogenetic position within the genus could not be determined. The polyphyly of the Japanese Pseudocrangonyx species indicates that multiple colonization events of Pseudocrangonyx ancestors to the Japanese Archipelago could have occurred. The reliability of the past Pseudocrangonyx records from Japan is briefly discussed.

Sixty species of freshwater Cladocera (Crustacea, Branchiopoda) are recorded from Lake Tonle Sap and its adjacent waters, Cambodia. Four species, Ceriodaphnia laticaudata, Bunops cf. tuberculatus, Leydigia australis, and Chydorus cf. dentifer are newly recorded from Southeast Asia, and occurrence of Macrothrix pholpunthini is secondary confirmed in the world. The present cladoceran fauna is characterized by having rich epiphytic and benthic forms. It can be associated with the shallow and vegetation-rich habitats of the study sites.

This is the first report of two North American branchiobdellidans, Sathodrilus attenuatus Holt, 1981, and

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Effects of environmental variables on the distribution of benthic macroinvertebrates inhabiting sediments were studied at 25 sites along the shoreline of Lake Takkobu in the Kushiro wetland of northern Japan in summer 2003. During the last decade, the lake's status has undergone a drastic shift from clear water dominated by submerged macrophytes to turbid water dominated by phytoplankton. The canonical correspondence analysis showed that four environmental variables explained the significant variation in the macroinvertebrate species composition: submerged plant biomass, bottom sediment organic matter content (OMC), distance from the mouth of the Takkobu River, and bottom-layer pH. Five species of Chironomidae [Chironomus sp. (except plumosus group), Psectrocladius sp., Corynoneura sp., Parachironomus sp. arcuatus group, and Zavreliella sp.] occurred in sites with relatively lower pH and a high submerged plant biomass, whereas three species of Tubificidae (Tubifex tubifex, Aulodrilus limnobius and Aulodrilus sp.) and two of Chironomidae (Nanocladius sp. and Monodiamesa sp.) occurred in sites with high pH and little vegetation. The three Tubificidae species also preferred organic-rich sediments. Irrespective of aquatic vegetation, Sphaerium sp. (Bivalvia) and Monodiamesa sp. (Chironomidae) occurred in low-OMC sites, whereas Tanypus sp. (Chironomidae) preferred high-OMC sites. The number of macroinvertebrate taxa showed the highest correlation with the number of submerged plants, suggesting that macroinvertebrate species richness was related mostly to submerged plant species diversity in this lake. The quantity and species richness of submerged plants and OMC are thus important determinants of the community structure of macroinvertebrates inhabiting sediments in Lake Takkobu.