HISTOLOGICAL, HISTOCHEMICAL AND BIOCHEMICAL CHARACTERIZATION OF LARVAL DIGESTIVE SYSTEM ONTOGENY IN TWO CHARACID SPECIES TO INFORM WEANING PROTOCOLS

The intensive culture of characid teleosts for ornamental trade is highly dependent on live Artemia nauplii through the larval stage. Live feeds exhibit disadvantages relative to prepared microparticulate diets (MDs), specifically pertaining to availability, labor, and cost. Larval Black TetraGymnocorymbus ternetzi and Neon Tetra Paracheirodon innesi exhibited poor survival and growth for MDs relative to Artemia. Following confirmation of live feed dependence at first feeding, digestive system ontogeny was characterized using histology, histochemistry, and digestive enzyme activities from the onset of exogenous feeding through the larval stage. Both species exhibited an agastric, altricial larval stage, as well as low digestive enzyme activity at the onset of exogenous feeding followed by abrupt increases in trypsin, lipase, and pepsin activity. In P. innesi, histological differentiation of the stomach, including gastric gland formation and production of neutral mucopolysaccharides, as well as the onset of pepsin activity, did not occur until 20 days post hatch (dph; Figure 1). For G. ternetzi, these developmental milestones were not reached until 22 dph. This shift from agastric to gastric digestive modes is indicative of a proliferation of digestive capacity and subsequent prey diversity.

Based on this information, experiments were conducted to evaluate different weaning times from Artemia to a MD. For each species, twenty replicate tanks were fed Artemia exclusively from hatch through the end of the trial, a MD exclusively, or were transitioned from Artemia to a MD at three different timepoints. For P. innesi fed until 32 dph, and weaning beginning at 12 dph and 17 dph, survival was similar to live Artemia (mean: 22.0 ± 1.7%), while weaning beginning at 22 dph resulted in lower survival (16.2 ± 1.3%); MD only resulted in the lowest survival (0.8 ± 0.3%). For G. ternetzi fed until 33 dph, weaning beginning at 13 dph exhibited higher survival (20.6 ± 1.8%) than live Artemia and weaning beginning at 18 dph and 23 dph (mean: 13.6 ± 1.5%), while the MD resulted in the lowest survival (0.8 ± 0.2%). For both species, weaning did not result in statistically higher growth than Artemia. These results indicate that weaning is possible prior to gastric differentiation, potentially resulting in the reduction of Artemia use in the larval culture of both characid species.