The role of the green seaweed Ulva as a dietary supplement for full life-cycle grow-out of Tripneustes gratilla

This study investigated the use of five different feeding regimes, consisting of different combinations of fresh Ulva armoricana (FU) and dry formulated feeds containing 20% (w/w; 20U) or 0% (0U) dried Ulva, on Tripneustes gratilla somatic growth and gonad production (size, colour and texture) during full life-cycle grow-out in a flow-through tank system. The 32-week trial was divided into two phases, a somatic growth phase (20weeks) and a gonad enhancement phase (12weeks), with different combinations of feeds tested during each phase. The five feeding regimes included an FU only treatment for 32weeks; FU for 20weeks followed by the 20U diet for 12weeks (FU-20U); and, similarly, FU-0U, 20U-FU and 0U-FU treatment regimes for 20weeks and 12weeks, respectively. Comparable somatic growth was recorded during the first 20weeks of the trial from juvenile/small adult urchins fed the formulated feed containing Ulva (20U) and the fresh Ulva (FU) diet; whereas a formulated feed with the same nutritional properties, but without the addition of Ulva (0U), produced urchins that were significantly smaller. Gonad production during the first 20weeks of the trial was higher for urchins fed formulated feeds (0U and 20U); however, the reduced test size of urchins fed the 0U diet resulted in a significantly reduced gonad mass. Following the change in diets at 20weeks, both gonad size and colour were significantly affected. Formulated feeds promoted gonad growth of urchins previously fed FU, while administration of FU improved gonad colour of urchins previously fed the formulated diets. The FU only treatment produced gonads with the most desirable colour, but these gonads were smaller in size compared with urchins fed dry formulated feeds. These data suggest that the use of high protein formulated diets during the somatic growth phase of small adult urchins is not necessary, as comparable growth can be produced by feeding Ulva. Gonad production on a diet of Ulva alone, however, is significantly lower than that obtained using a formulated diet. It is therefore recommended that FU is used for the somatic growth phases of this species, while formulated feeds, especially the 20U diet, are used subsequently for gonad development and/or enhancement. The combination of sea urchin/Ulva systems will be very beneficial to the commercial production of Tripneustes gratilla, in terms of lowering farm operational costs, as fresh Ulva armoricana can produce better/comparable somatic growth to