Contribution of gut content to the nutritional value of Brachionus plicatilis used as prey in larviculture

With the aim of assessing the potential value of gut content in the rotifer Brachionus plicatilis during the standard procedures of enrichment and posterior residence in the larval tanks, we have examined by image analysis the changes in gut volume during the filling and subsequent evacuation process when feed is no more available. The gut filling pattern has been examined at different microalgal concentrations of Nannochloropsis gaditana ranging between 0.4 and 20.8 × 106 cell ml− 1. The rotifer gut was completely filled in 120 min and the gut volume became significantly higher in rotifers fed at the highest microalgae concentrations tested. The gut volume accounted for up to 15% of the body volume. When harvested with a non-submerged filter and resuspended in clean seawater, the rotifers evacuated the gut quickly, losing 60% of its content in the first 5 min. Contrarily, when the rotifers were rinsed carefully using a submerged filter while maintaining the water volume, the gut was evacuated progressively and needed 1 h to lose 60% of the content. Moreover, we have examined the changes in dry mass and energy. After 2 h of gut evacuation the rotifers lost 20% of the initial dry weight, and 38% after 24 h of starvation. In terms of energy the rotifers lost 43% of the caloric content after 24 h of starvation. The findings of this study confirm the importance of performing appropriate feeding protocols to supply living prey of high nutritional quality in larviculture.

► Filling pattern and maximum gut volume depends on microalgae concentration ► Rotifers needed 2 h to fill completely the gut a 20 °C of temperature. ► Gut content may reach up to 15% of body volume. ► Rotifers evacuate 2/3 of gut content in few minutes during the filtering and cleaning. ► Loss of gut content is delayed when the cleaning is done with submerged filters.