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Animals affect element cycling in ecosystems by consumption and excretion. Amphidromous shrimps frequently dominate low-mid altitude streams, where downstream connectivity to oceans is sustained. Although shrimps’ direct influence on benthic communities has been studied, little is known about their influences on nutrient cycling. Here, we hypothesized that the dominance of shrimps alters nutrient mineralization by benthic macroinvertebrates in streams due to the difference in the quality and quantity of excretion between shrimps and aquatic insects. We tested this hypothesis through a field manipulative experiment, excretion measurements of animals, and field surveys. In the field manipulative experiment, the presence of shrimps slightly decreased the biomass of aquatic insects but tripled total benthic macroinvertebrate biomass directly through their own biomass. The mass-specific NH₄⁺ excretion rate by shrimps was similar to aquatic insects, and the areal NH₄⁺ excretion by benthic macroinvertebrates was increased by 2.5 times in the presence of shrimps. In contrast, shrimps excreted significantly less soluble reactive phosphorus (SRP) than aquatic insects, and the presence of shrimps did not affect areal SRP excretion by benthic macroinvertebrates. The field survey showed a positive correlation of NO₃⁻ concentration with the shrimp density, inferring the excess NH₄⁺ was nitrified. Although the nutrient concentration of stream water is frequently attributed to watershed conditions, the results of this study indicate that downstream connectivity to oceans may also influence nutrient dynamics of the stream through the density of amphidromous shrimps.

Hatakejima Experimental Field is located in Tanabe Bay, Wakayama Prefecture, Japan, which is composed of Hatakejima Island and Komarujima Islet, connected to the former in low tide. Hatakejima Island was purchased by Kyoto University and was designated as the “Hatakejima Experimental Field” in 1968. The year 2019 marks the 50th year of the long-term surveys that have been formally conducted on the experimental field since 1969 (i.e., the Century of Research Project). We conducted a field survey to record the macrobenthic fauna of the experimental field in 2019. A total of 168 species of 11 phyla were recorded in this survey. In each phylum, the number of species is listed as follows in descending order: Mollusca (78 spp.), Arthropoda (27 spp.), Echinodermata (23 spp.), Annelida (21 spp.), Cnidaria (7 spp.), Porifera (3 spp.), Nemertea (3 spp.), Platyhelminthes (2 spp.), Chordata (2 spp.), Bryozoa (1 sp.), and Hemichordata (1 sp.). We also recorded and discussed the influence of recent environmental changes around the Hatakejima Experimental Field. Tropical sea urchin species disappeared in the winter of 2017–2018 following the large meander of the Kuroshio Current, which led to decreasing water temperatures. The population of the seagrass Zostera japonica drastically decreased on the western sandy shore of the island in 2019, most likely because of two big typhoons in September 2018. We must conduct continuous observations to aid the recovery of seagrass-associated communities and protect the experimental field to keep high biodiversity of macrobenthic fauna in the future.

A blue jellyfish body fragment of Rhopilema asamushi Uchida, that is identifiable by mitochondrial gene analysis (COI gene sequences: 644 bps), is washed ashore at Hatakejima Island in Tanabe Bay, Wakayama Prefecture, Japan. This occurrence is the first record in the Pacific side of southern Japan.

At the head of Tanabe Bay, on the Pacific coast of Japan, dense populations of Proboscidactyla ornata medusae were observed at 24℃, 34 psu from July 2001 to September 2002 (maximum 472.4 individuals m−3). In the laboratory, 288 immature medusae of P. ornata collected from Tanabe Bay in May–June 2006 were cultured individually and fed with asphyxiated Artemia nauplii under 12 experimental conditions (four different temperatures (17℃, 20℃, 23℃ and 26℃) combined with three different salinities (28 psu, 31 psu and 34 psu)). For 30 days, 94% of the medusae survived at all conditions. The maximum number of clones produced was 0.32 clones medusa−1 day−1 at 20℃, 34 psu: the number of medusae increased 10.5 times in 30 days. The maturation percentage at 23℃ (54.2–58.3%) was higher than at 17℃ (12.5–25.0%) and was positively correlated with the number of nauplii ingested. Temperature-specificity of energy usage was observed: 20℃ for asexual budding and 23℃ for sexual reproduction. The direct relationship between budding and temperature is demonstrated among hydromedusae for the first time in this study. Additionally, the influence of salinity on budding suggests a hypothesis on an advantage of budding at offshore and high saline waters.

We fully described two Koellikerina species from shallow waters in Japan with illustrations and photographs. Koellikerina constricta (Menon, 1932), bearing characteristic bell constriction, expands its distribution to Kuchinoerabu Island, Kagoshima Prefecture (northern limit). Koellikerina bouilloni n. sp. is added to eight known species of the genus Koellikerina on the basis of an absence of bell constriction, a colorless body, and the presence of adaxial ocelli, perradial gonads, and a peduncle by examining a specimen from Tanabe Bay, Wakayama Prefecture and 18 specimens formerly collected from Papua New Guinea. Although bell constriction has been criticized as a taxonomic character, we conclude that it is reliable among Koellikerina because all the K. bouilloni specimens at variable developmental stages lack bell constriction and are clearly distinguishable from K. constricta.

Sixty-nine Chiropsalmus quadrigatus medusae were collected from Okinawa Island, Japan, in June-August 2000. The bell height ranged from 2.5 to 97.6 mm. The numbers of fine rings on polished statoliths were counted, and the coefficient of variation for the within-individual counts for the four statoliths was 3.3± 1.9% (mean::tSD, n=17). The slope of the linear regression of the number of rings against collecting date in 61 medusae was near 1.0, suggesting that the statolith rings are daily increments. The relationship between bell height and number of rings fitted a logistic growth curve. And, the relationship between statolith length and number of rings fitted the Gompertz growth curve. A check ring was present at a position of 5-10 rings from the center of each statolith. The backcalculated dates of check ring formation dated mainly from early to mid June, suggesting that the polyp of C. quadrigatus finished the metamorphosis to medusa and the medusa was liberated from a substratum during this period.