Producing juvenile Artemia as prey for Octopus vulgaris paralarvae with different microalgal species of controlled biochemical composition

The major bottleneck of Octopus vulgaris culture is the rearing of its paralarval life stage, being the obtainment of adequate live prey to feed paralarvae one of the key issues for the success of the culture of this valuable species. Artemia has been widely used as a single prey or in combination with crustacean zoeae as food items for paralarvae, but few works have reported the biochemical composition of these prey. The gross biochemical composition and fatty acid profile of Artemia juveniles enriched with four marine microalgae of controlled biochemical composition was assessed, as well as the fatty acid composition of newly hatched O. vulgaris paralarvae, in order to estimate which prey would be more suitable to meet the nutritional requirements of O. vulgaris paralarvae. Microalgae were cultured semi-continuously with a daily renewal rate of 30% of the volume of the cultures in nutrient saturated conditions, in order to achieve biomass of constant and optimal biochemical composition. Artemia juveniles of two different sizes (1.5-2.0 mm and 3.0−3.5 mm), appropriate to feed O. vulgaris paralarvae, were obtained by growing Artemia nauplii with Tetraselmis suecica for 2 and 4 days and then enriched for 26 h with four marine microalgae: T. suecica, Isochrysis galbana, Isochrysis aff. galbana (T-ISO) and Rhodomonas lens. The protein content of R. lens (62% of dry weight) was considerably higher than that of the remaining microalgae (42–44%) (P < 0.001), whereas lipid and carbohydrate were significantly higher in both T-ISO and I. galbana (P < 0.05). Small juvenile Artemia (3-day old, 1.5-2.0 mm) contained nearly 51% protein (of dry weight) regardless the enrichment diet used, with the exception of individuals enriched with I. galbana (AISO) (41%) (P < 0.01). In these juveniles, lipid percentages were higher when enriched with T-ISO (group AT-ISO) or R. lens (ARHO), both with circa 16% (P < 0.05); whereas carbohydrate was higher in juveniles from groups AISO or AT-ISO (11%) (P < 0.05). Large juvenile Artemia (5-day old, 3.0-3.5 mm) had higher protein percentages than small juveniles with values ranging between 64 and 68% for all treatments, whereas the lipid fraction among groups increased in the order: ARHO (10%) < ATET = AT-ISO (16%) < AISO (18%) (P < 0.05). The lowest percentage of carbohydrate was found in group ARHO (6%) (P < 0.01). Maximum protein/energy ratio was observed in 5-day old juveniles from group ARHO (P/E ratio = 31). The highest percentage (% total fatty acids) of eicosapentaenoic acid (20:5n−3) in small juvenile Artemia was found in individuals from groups AISO or ARHO (circa 9%), whereas in 5-day old juveniles the highest value was found in group AISO (14.6%) (P < 0.05). Regarding docosahexaenoic acid (22:6n−3), small juveniles from groups AT-ISO or AISO had higher values (1.9 and 1.5%, respectively) than juveniles from group ARHO (1.0%) (P < 0.05), whereas in 5-day old Artemia maximum percentage of 22:6n−3 was found in group AT-ISO (3.9%) (P < 0.05). None of the Artemia juveniles enriched with T. suecica contained 22:6n−3. The fatty acid composition of O. vulgaris paralarvae revealed a much higher percentage of 22:6n−3 (18.3%) than the values found in the enriched Artemia juveniles, suggesting a deficit of this fatty acid in Artemia. Even though the highest sum of 20:5n−3 and 22:6n−3 was found in Artemia juveniles enriched with I. galbana, if the general biochemical composition of Artemia juveniles is taken into consideration, the enrichment with R. lens would provide the best composition to meet the possible nutritional requirements of O. vulgaris paralarvae.