Aspects of distribution, growth, and reproduction of the fluvial prawn, Macrobrachium shokitai, in a subtropical river on Iriomote Island, Japan

Abstract

This study investigates the distribution, growth, and reproduction of the fluvial, regional, and vulnerable species, Macrobrachium shokitai, on Iriomote Island in southwestern Japan. Prawns were captured at seven stations along a small, 4.0-km-long river, among ca. 40 rivers on the island. Animals were collected monthly during 2019 and abundance, carapace length (CL), sex, and the presence or absence of eggs attached to pleopods were documented. Prawns were primarily collected at stations >2.7 km above the river mouth at >270 m elevation. Stable, dense populations were observed above the waterfall. The sex ratio was significantly female-biased between January and August. Later in the year the ratio was 1 : 1. A bimodal CL distribution was recorded; with the emergence of larger individuals in some months. Lifespan appeared to be >1 year. Recruits, including several juvenile instars, were found between August and December. Ovigerous females were encountered from April to October but were more abundant from June to August. The phenology and duration of the breeding season were consistent with generic characteristics of the genus throughout subtropical and temperate East Asia.