Metamorphosis in summer flounder: manipulation of thyroid status to synchronize settling behavior, growth, and development

Abstract

In the aquaculture of summer flounder ( Paralichthys dentatus), the inherent variation in growth and settling behavior during metamorphosis may lead to cannibalism and necessitate increased labor due to grading. Our goal was to use thyroid hormone manipulation to synchronize settling behavior and produce a uniformly sized cohort of juvenile summer flounder. Premetamorphic flounder were treated with exogenous thiourea (TU, 30 ppt, an inhibitor of thyroxine (T4) synthesis). Following removal of TU, metamorphosing flounder were exposed to either exogenous T4 (T4-Na salt, 100 ppb, dissolved in DMSO) or DMSO only. A control (no TU, then DMSO only, when appropriate) was used for comparison. Settling behavior in the control was prolonged, with the Peak Settlement Interval (defined as the interval beginning on the day the first 20% settled until the day 80% had settled) requiring 9 days, and a settling rate of 4.1±0.2 fish/day. Settling behavior was synchronized by thyroid hormone treatment. The Peak Settlement Interval was reduced (5 days) and settling rate increased (8.5±0.6 fish/day) by TU treatment. Treatment with TU followed by T4 further reduced the Peak Settlement Interval (3 days) and increased settling rate (11.2±0.6 fish/day). Development rate (DR) and growth rate (GR) were reduced by TU treatment, but following TU with T4 treatment restored DR and GR to control levels. However, TU treatment eliminated the larger, potentially cannibalistic juveniles found in control tanks. Both treatments reduced variance in GR, whereas only TU-T4 reduced variance in DR. Percent survival was unaffected by treatment. In aquaculture, the effective synchronization of settling behavior by thyroid hormone manipulations should reduce the labor costs associated with grading. We recommend a treatment regime that could aid in the production of more uniform cohorts of juvenile summer flounder.