Onset of nutrient consumption during early life stage digestive system development of two tuna species (Thunnus orientalis and Thunnus albacares).

To specify the timing of exogenous nutrient consumption in the larvae of two commercially important tuna species, the Pacific bluefin tuna (PBF) Thunnus orientalis and the yellowfin tuna (YFT) Thunnus albacares, the gene expressions of peptide transporter 1 (PEPT1) were examined. The mRNA expressions of PEPT1 first occurred at 2 days post hatching (dph) in PBF larvae and 3 dph for the YFT, and PEPT1 was found to only be expressed in the intestinal tract. The histological changes of the digestive tract of the YFT larvae were observed and compared to PBF larvae from a previous study. The intestines were developed at the hatching day for both species. It was found that the developmental timing of internal organs differed between the species, with the YFT showing an approximately one-day delay. The major organs such as liver, pancreas and gall bladder that excrete digestive enzymes appeared at 1 dph for PBF and 2 dph for YFT. The development of external morphological features was similar to organ development timings, with mouth-opening and first feeding starting at 2 dph for PBF, and 3 dph for YFT. Growth during the first month is rapid and variable for both species, ranging from 1.06 to 1.56 mm/d. Our findings provide new information about the early onset of feeding and larval development for the two species which would contribute to future aquaculture.