Integration and control: hormones

Fish, and other vertebrates, possess both a neural control system and a hormonal one dependent on the circulation. Fish hormones may be secreted by well-defined endocrine glands or by diffuse tissue and can have profound effects on the function of target tissues or organs which possess receptor molecules recognized by specific hormones. Some endocrine activities in fish are not relevant to mammals, and vice versa. Thus fish can have chromatophores controlled hormonally to change skin colour, the same hormones having different roles in mammals. Likewise, ‘prolactin’ in fish cannot regulate lactation. Individual endocrine glands, tissues and cells of fish are described in detail and the possible roles of their hormones are discussed. Fish endocrinology is becoming increasingly important with the realization of its potential applications in aquaculture and with the recognition of the consequences of pollutant endocrine disruption.