Effect of exogenous hormones on ovulation and gonadal steroid plasma levels in starry flounder, Platichthys stellatus

Han Kyu Lim

doi:10.1007/s10499-016-9971-6

Abstract

The present study was designed to examine the potential for inducing ovulation in starry flounder (Platichthys stellatus) using gonadotropin-releasing hormone analog (GnRHa) and human chorionic gonadotropin (hCG) to assess whether starry flounder are differentially responsive to GnRHa and hCG. Female starry flounder were injected or implanted with different doses of hCG or GnRHa pellets to examine their ovulation-inducing potential and effects on plasma levels of testosterone (T), 17β-estradiol (E2), and 17,20β-dihydroxy-4-pregnen-3-one (17,20βP). Blood samples were collected for up to 10 or 25 days post-injection or post-implantation in two separate experiments designed to mimic the early and middle stages of spawning, respectively. Fish treated with the GnRHa pellets (100 µg) showed a significant increase in the total number of stripped eggs relative to the controls. GnRHa administration had no effect on the floating rate or fertilization rate of ovulated eggs in the both experiments, whereas hCG injection affected both of these rates. Plasma T levels were not significantly different between the exogenous hormone-treated and control fish. In contrast, the plasma E2 level was elevated in those fish treated with GnRHa, regardless of injection or implantation, and was accompanied by increased numbers of stripped eggs in both experiments. Treatment with GnRHa resulted in higher 17,20βP levels compared to the controls, and there was a positive relationship between elevated plasma 17,20βP and an increase in ovulated eggs in response to GnRHa treatment. The implantation of starry flounder with GnRHa-containing pellets was effective at inducing sustained ovulation compared to hCG treatment.

Highlights

Starry flounder (Platichthys stellatus) is widely distributed throughout the northern Pacific Ocean, including coastal waters of Korea, Japan, Russia, Canada, and North America
The aim of this study was to determine the potential for inducing ovulation in starry flounder using gonadotropin-releasing hormone analog (GnRHa) and human chorionic gonadotropin to assess whether starry flounder are differentially responsive to GnRHa and hCG, based on the dose and delivery mode over short and long periods
The number of stripped eggs per 100 g body weight (BW) increased in fish injected with 100 lg of GnRHa compared with the other fish, the difference was not significant

Summary

Introduction

Starry flounder (Platichthys stellatus) is widely distributed throughout the northern Pacific Ocean, including coastal waters of Korea, Japan, Russia, Canada, and North America. There is growing interest in developing starry flounder aquaculture in Korea and China; current techniques are limited by unreliable egg production, similar to many other marine flatfish species. The mass yield of viable eggs is the first consideration in developing the commercial culture of this species. In cultured stocks of starry flounder, most females exhibit reproductive dysfunction, including a failure to ovulate and low-quality egg production. A variety of approaches have been used to address these problems, most of which involve artificial manipulation of the endocrine system. These treatments act at different levels in the hypothalamic–pituitary–gonadal axis, and they typically involve treatment with pituitary homogenates, purified gonadotropin preparations, and, more recently, synthetic agonists of gonadotropin-releasing hormone (GnRHa) (reviewed by Zohar and Mylonas 2001)

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