Endocrine Regulation of Compensatory Growth in Fish

Compensatory growth (CG) is a period of accelerated growth that occurs following the alleviation of growth-stunting conditions during which an organism can make up for lost growth opportunity and potentially catch-up in size with non-stunted cohorts. Fish show a particularly robust capacity for the response and have been the focus of numerous studies that demonstrate their ability to compensate for periods of fasting once food is made available again. Compensatory growth is characterized by an elevated growth rate resulting from enhanced feed intake, mitogen production and feed conversion efficiency. Because little is known about the underlying mechanisms that drive the response, this review describes the sequential endocrine adaptations that lead to CG; namely during the precedent catabolic phase (fasting) that taps endogenous energy reserves, and the following hyperanabolic phase (refeeding) when accelerated growth occurs. In order to elicit a CG response, endogenous energy reserves must first be moderately depleted, which alters endocrine profiles that enhance appetite and growth potential. During this catabolic phase, elevated ghrelin and growth hormone (GH) production increase appetite and protein-sparing lipolysis, while insulin-like growth factors (IGFs) are suppressed, primarily due to hepatic GH resistance. During refeeding, temporal hyperphagia provides an influx of energy and metabolic substrates that are then allocated to somatic growth by resumed IGF signaling. Under the right conditions, refeeding results in hyperanabolism and a steepened growth trajectory relative to constantly fed controls. The response wanes as energy reserves are re-accumulated and homeostasis is restored. We ascribe possible roles for select appetite and growth-regulatory hormones in the context of these catabolic and hyperanabolic phases of the CG response in teleosts, with emphasis on GH, IGFs, cortisol, somatostatin, neuropeptide Y, ghrelin and leptin

Gonadal differentiation and effects of temperature on sex determination in southern flounder (Paralichthys lethostigma) PII: S 0 0 4 4 -8 4 8 6 ( 0 2 ) 0 0 4 0 7 -6

Abstract Southern flounder (Paralichthys lethostigma) support valuable North American fisheries and show great promise for aquaculture. Because females grow faster and reach larger adult sizes than males, monosex culture of females is desirable for commercial operations. A detailed understanding of sexual development and its timing is critical to control sex and optimize culture. Structural and cellular sex-distinguishing markers were identified histologically, and then used to describe ovarian development in female and testicular development in male flounder. In presumptive ovaries of southern flounder, development of an ovarian cavity first occurs in fish ranging from 75 to 100 mm total length (TL). This is considerably delayed relative to that observed in the Japanese congener, Paralichthys olivaceus, where an ovarian cavity is seen in fish as small as 40 mm TL. The smallest southern flounder that possessed primary oocytes in the early perinucleolus stage was 115 mm TL. In presumptive testes, the formation of seminiferous tubules first occurs in fish of approximately 100 mm TL. Spermatogonia remained quiescent until most fish were over 100 mm TL. Overall, gonads from southern flounder greater than 120 mm TL commonly possess gonial cells undergoing meiosis, clearly differentiating sex. The effect of temperature on sex determination in southern flounder was addressed in a separate experiment. Juvenile southern flounder were grown at 18, 23, or 28jC for 245 days. High and low temperatures induced phenotypic sex reversal in juvenile southern flounder, producing a higher proportion of males (96% males at high temperature, P < 0.001, 78% males at low temperature, P < 0.01). Raising southern flounder at the midrange temperature held sex ratios close to 1:1. Sex ratios from these trials suggest that southern flounder possess a temperaturesensitive mechanism of sex determination similar to that shown for P. olivaceus, but possibly shifted towards warmer temperatures. These findings indicate that sex differentiation in southern flounder is distinguishable in most fish by 100 -120 mm TL and that sex determination is sensitive to temperature. This information is critical to the development of strategies to maximize the number of faster-growing females for commercial flounder culture.

Induced meiotic gynogenesis and sex differentiation in summer flounder (Paralichthys dentatus)

Meiogynogenesis and temperature manipulation were used to produce XX male summer flounder broodstock for future production of monosex (all female) populations. Meiogynogens were produced by fertilizing eggs with UV-irradiated (70 mJ/cm(2)) black sea bass sperm and applying 6-minute pressure shocks (58,600 kPa), two min post-fertilization. From 4 females, 132,000 eggs were produced, of which 95.6 +/- 1.8% were viable, 51.0 +/- 13.0% fertilized, and 15.9 +/- 8.3% hatched. Following metamorphosis. meiogynogens and controls were raised under a low temperature regime (12 degrees C gradually increased to 20 degrees C), 21, and 26 degrees C for up to 376 days post hatch (DPH). Female sex differentiation was greater in meiogynogens (62.5%) and control fingerlings (22.6%) raised under a low temperature regime compared to those raised at the higher rearing temperatures: 0% at 21 degrees C, and 0 and 3.9% at 26 degrees C in meiogynogens and controls, respectively. These results suggest that temperature, during the critical phase preceding gonadal development, influences sex differentiation in summer flounder. (C) 2009 Elsevier B.V. All rights reserved