Euryhaline rotifer Proales similis as initial live food for rearing fish with small mouth

Abstract

The SS-type rotifer <i>Brachionus rotundiformis</i> is a common initial food for rearing fish larvae with a small mouth. However, there are commercially important fish species whose mouth sizes are too small to feed on SS-type rotifers. In 2004, we isolated a small (body length = 82.7 ± 10.9 μm; body width 40.5 ± 6.4 μm), flexible, and iloricate rotifer, <i>Proales similis</i> from an estuary in Okinawa, Japan. Under laboratory conditions (25 °C, 2–25 ppt) <i>P. similis</i> produced its first offspring on 2.5 to 2.8 days after hatching, and produced 4.3 to 7.8 offspring within 4.0 to 4.7 days life span. Batch cultured <i>P. similis</i> fed <i>Nannochloropsis oculata</i> suspension at 28.8 μg dry weight ml^{− 1} and cultured at 25 °C, 25 ppt filtered seawater, increased exponentially from 25 to 2400 ind ml^{− 1} after 11 days of culture with an overall intrinsic rate of natural increase (<i>r</i>) of 0.42 day^{− 1}. The growth rate of <i>P. similis</i> was not significantly different when fed fresh <i>N. oculata</i> and super fresh <i>Chlorella vulgaris</i>-V12®. Total lipid per wet weight of <i>P. similis</i> fed by <i>N. oculata</i> and <i>C. vulgaris</i> were 2.4 and 2.6%, respectively. The compositions of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and arachidonic acid (ARA) of <i>P. similis</i> fed <i>N. oculata</i> were 23.2, 0.0 and 5.3%, respectively, while these were 11.0, 17.5 and 0.5% respectively, when fed <i>C. vulgaris</i>. The use of <i>P. similis</i> to feed small mouth fish including seven-band grouper <i>Epinephelus septemfasciatus</i>, rusty angelfish <i>Centropyge ferrugata</i>, and humphead wrasse <i>Cheilinus undulatus</i> showed that it is an excellent starter food for these species because of their high selectivity index and improved survival. In addition, <i>P. similis</i> was ingested by Japanese eel <i>Anguilla japonica</i> larvae with a complicated digestive system. The use of <i>P. similis</i> as starter feed for small mouth fish larvae is highly recommended.