Ulva lactuca biofilter from a land-based integrated multi trophic aquaculture (IMTA) system as a sole food source for the tropical sea urchin Tripneustes gratilla elatensis

The performance of Tripneustes gratilla elatensis using IMTA-produced Ulva lactuca as the sole food source, in comparison to Gracilaria conferta and formulated diets, was evaluated in a long term (251-day) experiment in a land based facility in the tropical Gulf of Aqaba. T. gratilla elatensis fed IMTA-produced U. lactuca exhibited significantly (P < 0.01) better performance in terms of somatic growth, SGR, gonad yield, colour and survival, compared to sea urchins fed G. conferta and formulated diets. These differences can be attributed mainly to the high consumption rate and digestibility of the protein rich Ulva compared to the other diets. IMTA-produced Ulva as a sole feed was found to be a suitable diet for both somatic and gonadal growth. T. gratilla elatensis was shown as able to reach a commercial test size (~60 mm) in 5-6 months. T. gratilla elatensis grew (from spawning) in 400 days to 44.44 ± 2.01, 61.80 ± 3.69, and 73.28 ± 1.46 mm test diameter with average Specific Growth Rate (SGR) of 0.21 ± 0.02, 0.35 ± 0.05, and 0.42 ± 0.03% d−1 for formulated diet, G. conferta and U. lactuca diets, respectively. Gonad weights were 9.14 ± 2.3, 8.5 ± 2.7 g, and 24.6 ± 7.4 g. and Gonado-Somatic Index (GSI) reached 14.4 ± 2.01, 13.1 ± 5.0, and 13.7 ± 4.56 for formulated diet, G. conferta and U. lactuca diets, respectively, at the end of the experiment. No specific seasonality was observed with regard to gonad development, growth and GSI dynamics. Gonad growth and spawning events occurred throughout the year. T. gratilla elatensis fed IMTA-produced Ulva exhibited 90-100% commercially acceptable gonad colour, while the formulated diet (10% Ulva inclusion) showed poorer results of 30-60% acceptable colour and the G. conferta diet ca. 80%. In addition to enhancing sea urchin performance, U. lactuca as a biofilter in the IMTA considerably improved the system's sustainability and waste management.