Japanese medaka (Oryzias latipes) is known to deposit fertilized eggs on aquatic substrates in shallow waters. In order to examine the influence of water depth on embryonic development of fertilized eggs of an orange-red variety and wild medaka, we observed fertilized eggs for 18 days and compared the hatching rate and the hatching time at different water depths of 5, 30, and 65 cm. The hatching rate was lower at 65 cm depth than at 5 cm depth and the hatching time was longer at 65 cm depth than at 5 cm depth with both the orangered variety and the wild medaka. These results indicate that deep water is not suitable for the embryonic development of medaka and suggest that preservation of natural conditions in shallow waters of rivers and ponds is important for conservation of wild Japanese medaka.

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Labroides dimidiatus is a well-known species of cleaner fish, while there are not many observation records on cleaning behavior of Plotosus japonicus. In this study, cleaning behavior of P. japonicus and L. dimidiatus was observed in a large aquarium tank (approximately 1000 m³ in capacity) at the Enoshima Aquarium to examine the relationship between their cleaning behavior and client fishes’ requesting behavior. P. japonicus was observed to perform cleaning behavior on 32 client species, some of which (17 species) displayed requesting behavior to P. japonicus. A total of 53 client species were recorded for L. dimidiatus, many of which (26 species) displayed requesting behavior to L. dimidiatus. We confirmed that in addition to L. dimidiatus, P. japonicus and some client species also developed their relationship through symbiotic coactions.

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In this study, we radio-tracked four masked palm civets from summer to autumn and estimated their home range size in a rural landscape of the Shonai region, Yamagata Prefecture, northeastern Japan. By field survey, we obtained location data for three male civets (19, 213, and 166 points) and one female (61 points). The home range size was 27 to 303 ha and 79 to 329 ha based on the 100% maximum convex polygon (MCP100) and the 95% fixed kernel method (FK95), respectively. These home range sizes may fall between the home range sizes of masked palm civets inhabiting forests and peri-urban landscapes.

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