Supplementation of a DHA commercial lipid emulsion in diets for tropical gar (Atractosteus tropicus) larvae: effects on growth, cannibalistic behavior, digestive enzyme activity, and aggressiveness-related genes.

This study evaluated growth indices, survival, digestive enzyme activities, and genes related to aggressiveness in tropical gar (Atractosteus tropicus) larvae fed with diets supplemented with a commercial product rich in lipids (CPRL) with a high docosahexaenoic acid (DHA) concentration. Experimental diets for A. tropicus larvae were supplemented with 20, 30, and 40 g/Kg of CPRL, compared to a control diet without CPRL supplementation. The four treatments were defined by the final percentage of DHA based on total fatty acids (FA) as follows: 3.2% (T3.2), 4.2% (T4.2), 5.3% (T5.3), and the control diet with 1.9% final DHA concentration (T1.9), respectively. All treatments were replicated three times in a recirculating system. Diets were tested in larvae from 3 to 13 days after hatching (DAH) (first stage) (200 larvae per replicate), fed four times per day with Artemia nauplii and experimental diet, and in a cohort of cannibalistic larvae from 14 to 24 DAH (second stage) (10 larvae per replicate) fed with T1.9 and T5.3. During both stages, water quality was monitored daily (27.36 ± 0.6 °C). In the first stage, no effects on survival, total length, or cannibalism were observed in larvae fed the commercial DHA lipid emulsion compared with T1.9 (p > 0.05). A difference in size (6.74 ± 4.82%) was recorded between cannibalistic and non-cannibalistic larvae (p < 0.05). In the second stage, the survival rate of the cannibalistic larvae in T5.3 was 35.00 ± 7.07%, compared to 30.00 ± 17.32% for T1.9 cannibalistic larvae (p > 0.05). Cannibalism was observed in 68.33 ± 7.63% of T5.3 larvae and 76.66 ± 5.77% of T1.9 larvae (p > 0.05). Differences in alkaline protease and trypsin activities were observed between the T1.9 and T5.3 treatments. The T5.3 cannibalistic larvae showed upregulation of avpi1, crh, gnrh1, htr1a, sstr1, and th and downregulation of hdc and tph1 aggressiveness-related genes compared to the cannibalistic larvae with T1.9 (p < 0.05). This may indicate a possible metabolic pathway related to cannibalistic behavior and with these developing strategies to prevent or reduce cannibalism during the larval stage and to enhance A. tropicus juvenile production.