Species identification of the rotifer Brachionus plicatilis sp. complex used in Japanese hatcheries and the implications for application

Precise identification of rotifer strains is beneficial for sustainable management and productivity improvement in hatcheries. By combining DNA barcoding, taxonomy, and morphometric analysis, this study verified the identities of 15 rotifer strains commonly utilized in Japanese public hatcheries or research institutes and discussed their significance in the application. The tested strains were categorized into large (L) type, Brachionus plicatilis sensu stricto (Amami, Shizuoka, and Notojima strains) and B. plicatilis “Nevada” (Obama, Shimane, and Tokushima-Kashima (large) strains), small (S) type, Brachionus koreanus (Kagoshima-Suiken, Mie-Owase, Yaeyama, Chlorella industry, Kindai, and Tokushima-Kashima (small) strains), and super-small (SS) type, Brachionus rotundiformis (Okayama, Oita, and Thai strains). Except for lorica size, anterior spine shape is an effective characteristic that can be used for morphological distinguishment. L-types have obtuse anterior spines, whereas S-types, including SS, have sharply pointed spines. L-types prefer relatively low temperatures and salinity, making them suitable for rearing cold-water larval species and allowing for storage and transport at low temperatures (e.g., 4 °C). Conversely, small rotifers, particularly SS-types, are adapted to high temperatures and salinity, exhibiting superior energy content, nutritional enrichment efficiency, and resilience to stressors, and can be cultivated at high/ultrahigh density. Species-specific biological preferences provide essential insights for management strategies via environmental manipulation.