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Very different testicular structures and spermatogenetic patterns have been found in fish of the teleost group. Two types of structures may be identified: (i) a tubular type with no lumen (in cyprinodonts); the cysts migrate from the blind end to the vas efferens during the process of spermatogenesis; (ii) a lobular type having a central lumen receiving the spermatozoa released from cysts which remain stationary along the lobule during spermatogenesis. Different spermatogenetic patterns are distinguished in salmonids and cyprinids. In the latter (carp. &Ivprinus ccarpio. and goldfish, Cciras.sius crur-atus). sonme germ cell types (e.g. type B spermatogonia and spermatozoa) are present throughout the year, allowing nearly continuous production of good-quality sperm. Studies of their endocrine patterns suggest that the GTH involved is controlled by external (mainly temperature but also photoperiod) and gonadal factors. The GTH stimulates androgen production and eventually controls spermatogenesis and spernmiation. In salmonids, the two major events of the testicular cycle, spermatogenesis and spermiation, are temporally separated by a stage of spcmsatozoal "maturation" during which the spermatozoa undergo physiological changes. Sperm quality declines during the period of spe'rmiation. The initiation of a new spermatogenetic cycle seems possible only when the spermatozoa have been eliminated from the testis. either by the normal process of spermiation or by intratesticular resorption. This also illustrates the spatias independence of spermatogenesis and spermiation. ' The endocrine pattern of spermatogenesis in saimonids is similar to that in carp but seems different as regards spermiation. This spermiation process includes two steps, initiation and amplification, which require different GTH levels and steroid balance. Among the steroids, I 1-ketotestosterone is the major androgen found in the plasma, but its superiority over other androgens in regard to spernsiation remains to be demonstrated. Environmental factors (photoperiodic changes and temperature) may act directly on the central nervous system to control gonadotropin secretion. Temperature may also directly influence the gonad, somatic or germ cells, and steroid metabolism which acts either locally on the gonad or more centrally. The regulation of spermatogenesis in fish appears to be more subtle than previously believed. Major unknowns are whether there is a second GTH and, if so, its site of action; which steroids are directly involved in the control of sperinatogeriesis in the lobules, and which are the target cells; and which factors regulate testicular size and which pituitary GTH secretion. Finally, the poor yield of spermiation is intriguing and requires further study, considering its practical implication in fish-farming.

Lipids are the predominant source of energy for fish and are stored in fat depots in different parts of the body regions. This review focuses on visceral, subcutaneous and intramuscular adipose tissues that interfere with carcass and fillet yields and with flesh quality. The morphological, cellular and biochemical characteristics of these tissues are discussed as well as the different mechanisms involved in the regulation of their lipid metabolism. Particular emphasis is given to the modulation of these characteristics and mechanisms by different extrinsic (food composition, water parameters) and intrinsic (selective breeding, life cycle status) factors. This review focuses on recent studies that take into account the present challenges of fin-fish aquaculture, which are principally (1) the replacement of fish oil and meal by vegetable oil and meal due to the need for sustainability and the limited availability of fish to prepare food pellets, and (2) selective breeding programs to improve fish growth and flesh quality. These studies apply various modern technologies to different fish species, including the development of cell culture systems and transcriptomic and proteomic techniques. This review highlights that fish adipose tissues differ in their localization and their morphological characteristics and that they show a large plasticity in their responses to variations of both extrinsic and intrinsic factors. These different responses reinforce the idea of their differential participation in fish lipid homeostasis

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