Prospecting the Photosynthetic Flatworm Symsagittifera roscoffensis as a Novel Fish-Feed

Symsagittifera roscoffensis is an intertidal Acoel flatworm that forms a symbiotic relationship with the alga Tetraselmis convolutae. Members of the genus Tetraselmis are known to have a high nutritional value and have been widely used to enrich intermediate prey for fish within the aquaculture industry; therefore, S. roscoffensis could be a good candidate as a trophic shortcut to deliver algal nutrition to fish. In this study, we investigated the likelihood of five ornamental tropical freshwater and six ornamental marine fishes to consume this worm, either as live feed or in freeze-dried form. We also tested the ability of S. roscoffensis to form a symbiotic relationship with alternative algal species, analysing the nutritional profile of S. roscoffensis when grown in different media. All the experimental fish consumed live worms to some degree, with the exception of one species (Meiacanthus grammistes); the response time to the worms ranged from 1.1–68.6 s for freshwater ornamental species to 1–24 s for marine ornamental species, and in most cases, this was comparable to or shorter than their response time to the reference diet Artemia. The fishes showed no negative effects after consuming the worms. We obtained similar results with freeze-dried worms in terms of the number of worms eaten, response time, and feeding time. Symsagittifera roscoffensis was able to form a symbiotic relationship with all the tested algal species of the genus Tetraselmis, but not with members of other genera. Worms grown in nutrient media (f/2 and f/4) had significantly higher contents of protein, pigments, and total and polyunsaturated fatty acids, including eicosapentaenoic acid (20:5n − 3) and α-linolenic acid (18:3n − 3), than those grown in seawater. These results show that S. roscoffensis was acceptable to many ornamental fish species, delivering key algal ingredients that are beneficial to fish health; hence, it is a promising alternative to conventional fish feeds for the ornamental pet trade.