Morphological and Genetic Identification of Spontaneously Spawned Larvae of Captive Bluefin Tuna in the Adriatic Sea

The spawning behavior of captive Bluefin Tuna (Thunnus thynnus) was observed in cages at an Adriatic facility in summer 2011. Approximately 20,000 eggs were collected from cages and transferred to a land-based nursery, with an estimated fertilization success rate of 80%. Eggs were spherical with a diameter of 1,035.06 +/- 20.16 m and were pelagic with a single oil globule (227.63 +/- 8.07 m). Newly hatched larvae (3.08 +/- 0.14 mm total length) followed scombrid larval development: melanophores were scattered over the body, head, notochord, and yolk, except finfold; eye pigmentation was observed 1.5 days posthatch, pectoral fins started to appear, and two-thirds of the yolk sac were absorbed; the mouth developed at 2.5 days posthatch, and larvae began feeding upon enriched rotifers. After mortalities occurred in the nursery (5 days posthatch), DNA was extracted from a sample of larvae. An 890 base pair fragment of the mitochondrial partial control region was amplified and sequenced to genetically confirm fish species identity. Based on multiple sequence alignment, larval sequences showed no ambiguities to the T. thynnus mitochondrial control region, thereby confirming spontaneous spawning in captivity. Although such an event has previously been reported in the Mediterranean, this is the first report supported by both morphological and genetic data.