Beneficial effects of carpet shell clam (Ruditapes decussatus) depuration during short periods of conditioning in shellfish hatchery: Role of the temperature and phytoplankton on reduction and diversity of vibrios

Abstract

Broodstock conditioning in hatcheries is the step previous to spawning and its optimization may be a key to the success of larval cultures. Cleaning and brushing of the broodstock and the utilization of antibiotics to reduce their vibrios load are used regularly as routine prophylactic measures prior to spawning induction. The development of protocols to reduce these bacteria using cheap and harmless techniques is of utmost importance for commercial bivalve production in hatcheries. With this aim, we have evaluated initially different conditionings (A–D) during short periods (a total of four weeks): first two weeks under gradient temperature (increasing + 0.3 °C day− 1 from 14.5 °C to 20 °C), without (A) or with phytoplankton (B), and constant temperature (20 °C) without (C) or with phytoplankton (D). Afterwards, all conditionings were kept at 20 °C and fed for two more weeks. Furthermore, broodstock optimal feeding time was re-evaluated during a second trial series. In all conditionings, bacterial loads were determined in terms of marine heterotrophic bacteria (MHB) and presumptive vibrios (PV). Broodstock under the optimal short period of conditioning (conditioning C) obtained the best gonadal development and a significant reduction in PV load at a lower expense. A total of 61 PV were isolated from all conditionings and identified by sequencing the 16S rRNA gene. Splendidus clade was dominant in the samples coming from natural beds. Diversity of vibrios changed throughout the conditionings in the hatchery favoured by exogenous factors whose effect was mainly observed at the end of the trials: Splendidus clade was also dominant in the broodstock conditioned at gradient temperature and Mediterranei and Harveyi clades were prevalent at constant temperature. Moreover, the percentage of transformation to D-larvae was estimated and the vertical transmission of vibrios from broodstock to eggs and D-larvae was suggested. Implementation of conditioning C reduces considerably the Vibrio load of clams without using antibiotics, and thus it represents a novel, cheap, environmental friendly and harmless methodology that can be easily transferred to commercial hatchery.