Structure of and energy reserves in the liver of wild and cultured yellowtail flounder , Limanda ferruginea

Knowledge of the structure and energy reserves, in the liver of commercially important fish species, is important in understanding metabolic processes and in assessing the impact of potential environmental physical and chemical stressors in both wild and cultured stocks. The present study investigated the microscopic morphology and histochemistry (total and neutral lipids, glycogen) of liver tissue of wild (3) and cultured (1) sexually immature female and male yellowtail flounder (Limanda ferruginea Storer), sampled in late April 2001. Hepatosomatic indices [HSI: (liver weight/body weight)liver weight)·100] of cultured fish were significantly higher than those of wild fish. Females in the cultured group had significantly lower HSIs than males. The liver of both wild and cultured L. ferruginea was interspersed with pancreatic tissue. The main components of the liver tissue were irregular cords of hepatocytes arranged in tubules which surrounded vascular sinusoids. The hepatocytes contained an abundance of lipid, much of which appeared to be neutral lipids, in both sexes of the cultured fish. Total and neutral lipid droplets were larger, and the area occupied by these droplets was significantly greater in both cultured females and males compared to the wild fish, suggesting lipidosis in the cultured fish. In the cultured fish these differences were sex-dependent, the females having significantly more total and neutral lipids in hepatocytes than the males. This suggests a potentially greater storage capacity in females and/or a higher lipid metabolism in males. There were no statistically significant differences in glycogen content between the cultured and wild fish, or between the sexes in both sampling groups. Introduction Yellowtail flounder (Limanda ferruginea, formerly Pleuronectes ferrugineus) is a benthic species of the family Pleuronectidae, found along the North American Atlantic coast, mainly at depths around 60 m at temperatures of 3–5 C (Langton 1979; Pitt 1983). Before the fishing moratorium established in 1992, this groundfish species was important commercially on the Grand Banks of Newfoundland, and its decline, along with the decline of other fish stocks, is a concern. It is well known that the marine ecosystem is increasingly exposed to a wide variety of physical and chemical stressors, such as those produced by oil and gas exploration and exploitation, which may harm fisheries through direct and indirect destruction of fish stocks. The level of contaminants, such as polycyclic aromatic compounds, in sediments correlates with increased prevalence of hepatic lesions in fish, particularly in species of Pleuronectidae (Myers et al. 1998, and references therein). L. ferruginea could be an excellent candidate in monitoring programs to protect the quality of the stocks (Collier et al. 1998) and an important species for the aquaculture industry (Brown et al. 1995). Hatchery conditions (i.e. diet, temperature, population density, interspecific competition) may also affect several physiological processes such as growth and reproduction in fish (Blaxter 1975; Herbinger and Friars 1992; Whalen 1999). Age and size of wild yellowtail flounder, at sexual maturity, vary depending on the geographic region, year class, and sex. Males (<26 cm) and females (32– 40 cm) from southern areas (off New England) mature at ages 2–4 years, while males and females from northern areas (Grand Banks of Newfoundland) mature at ages 4.4–5.5 years (25–30 cm) and 6.1–6.8 years (34 cm), respectively (Royce et al. 1959; Morgan and Walsh 1997). Sexually mature females and males develop a stock of vitellogenic oocytes and spermatozoa, respectively, during the winter months. The peak spawning period is in mid-May for fish off New EngMarine Biology (2003) 143: 257–265 DOI 10.1007/s00227-003-1083-y G.E. Fåhraeus-Van Ree Æ D.R. Spurrell Communicated by J.P. Grassle, New Brunswick G.E. Fåhraeus-Van Ree (&) Æ D.R. Spurrell Department of Biology, Memorial University of Newfoundland, St. John’s, Newfoundland, A1B 3X9, Canada E-mail: gvanree@mun.ca Fax: +1-709-7373018 land (4.9–12.3 C) and during the latter half of June for fish on the Grand Banks of Newfoundland (2–4 C) (Royce et al. 1959; Pitt 1970; Howell 1983; Clearwater 1996; Manning and Crim 1998). Cultured yellowtail flounder sexually mature earlier and grow faster than wild fish, with males maturing at age 22 months (mean total length of 17.3–20.1 cm) and females at age 22– 34 months (mean total length of 18.3–24.3 cm). Both sexes start to spawn in June (A.J. Manning, unpublished data). The liver of fish plays an important role in intermediary metabolism, storage of reserve compounds such as lipids (triacylglycerol) and glycogen, the biotransformation and detoxification of lipophilic organic compounds (e.g. xenobiotics), and reproduction (vitellogenesis). Therefore, monitoring the effects of chemical and physical stressors in fish has been focussed on the liver. Many histological, histochemical, and enzyme cytochemical and biochemical parameters have been used to diagnose stress in the liver of marine teleosts, flatfish species in particular (e.g. Köhler 1990; Carr et al. 1991; Köhler and Pluta 1995; Köhler et al. 1992, 1998; Husøy et al. 1996; Vethaak and Wester 1996; Collier et al. 1998; Myers et al. 1998), notably, differences in the quantities of stored lipids and glycogen in the hepatocytes. Metabolism of these energy-rich storage products is variable, especially depending on water temperature, reproductive, and nutritional status, as well as age and state of health of the fish (Welsch and Storch 1973, and references therein). Early signs of liver injury may be accompanied by lipid accumulation and/ or excessive storage or depletion of glycogen in the hepatocytes (Köhler et al. 1992; Hinton 1993). The histomorphology of liver of flatfish such as winter flounder (Pseudopleuronectes americanus), European flounder (Platichthys flesus), English sole (Pleuronectes vetulus), and dab (Limanda limanda) resembles that of other teleosts (Hampton et al. 1985, 1988; Morrison 1987; Stehr et al. 1988; Köhler 1989; Vethaak and Wester 1996, and references therein). The basic morphology and physiology of the liver of yellowtail flounder have not been studied to date, and sex differences in the studied species, and the potential differences between wild and cultured fish have also received little attention. The main objectives of the present study were to: (1) examine the liver morphology of yellowtail flounder and (2) investigate potential differences in lipid and glycogen storage in female and male hepatocytes in wild and cultured sexually immature yellowtail flounder. Materials and methods