Survival, growth, ingestion rate and foraging behavior of larval green mandarin fish (Synchiropus splendidus) fed copepods only versus co-fed copepods with rotifers

The importance of copepods as the diet for rearing early larvae of the green mandarin fish Synchiropus splendidus has been demonstrated in our previous studies, however in those studies, copepods were always co-fed with rotifers in the attempt to compensate for low copepod feeding density due to substantially low copepod culture productivity as compared to rotifers. Hence to what extent the roles of rotifers played in the larval culture is unclear. In this study, survival and growth of newly hatched S. splendidus larvae were compared when they were fed copepods (Parvocalanus crassirostris) at different densities either alone or co-fed with ss-type rotifers (Brachionus rotundiformis) until 11 days post-hatching (DPH). It showed that co-feeding rotifers at all copepod densities tested (0.1, 0.5 and 1 copepod mL(-1)) did not produce clear beneficial effects, suggesting co-feeding rotifers is unnecessary for early larval rearing. A subsequent experiment compared larval ingestion rates on copepods and rotifers under diffident co-feeding conditions, the ingestion rates and electivity index calculated showed that copepods were always positively selected over the rotifers by all ages of larvae (4, 6, 8 and 10 DPH). Furthermore, analyzing larval feeding behavior sequence (i.e. targeting, attack, capture, rejection following capture, and ingestion) on the copepods and rotifers when the two prey were offered to larvae of different ages (6, 8, 10 and 12 DPH) at a same density (1 copepod +1 rotifer mL(-1)), it was clear larvae of all ages attacked copepods more frequently. Moreover, copepods were never observed being rejected after being captured by the larvae whereas rejection was common for rotifers. Interestingly, the ratios of attacking leading to successful capture were significantly higher on rotifers than on copepods by larvae of all ages. However, this was more than offset by substantial lower ratios of targeting leading to attacks and higher rejection ratios following capture (up to 60%) on rotifers, resulting in significantly lower rotifer ingestion rates compared to copepods by larvae of all ages. Our results provided insights into copepods' superior performance over rotifers for early larvae of S. splendidus, and the results suggest that co-feeding copepods with rotifers is not necessary for early larval rearing.