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Exposure to heavy metals can produce morphological and functional alterations in fish. It is well known that the marine organisms living in contaminated environments display damage attributable to the structures most directly exposed to the external environment such as the branchial epithelium, and thus involved in the interactions between it and the organism. The aim of the present work is to determine useful biomarkers in Liza aurata for identification of anomalies due to the toxic agents and factors implicated in the mechanisms controlling osmoregulation, detoxification, respiration and neurotransmission. The study of these biomarkers is an important ecophysiological factor and a tool for evaluating the risk of environmental variations in the coastal species exposed to anthropic pollution. Specimens of the golden grey mullet L. aurata (Perciformes, Mugilidae) were collected from the brackish swamps of Faro and Ganzirri (Messina, Italy), considered highly polluted environments; instead, the specimens collected from the Marinello (ME) and Fogliano (LT) swamps, considered a reference site, were used as the control group. In the branchial epithelium of specimens collected from highly polluted environments, in comparison with the control group, a decrease in immunoreactivity for serotonin was found, together with, the presence of a few apoptotic nuclei, revealed by TUNEL technique. The increased immunopositivity for nNOS is probably due to stress conditions. The local reaction to hypoxia involves a more intense production of vasoactive substances, like NO, by nitrergic innervation. In the specimens living in polluted environments, immunopositivity for metallothioneins and iNOS in the chloride cells is very interesting. The presence of both substances may be considered a further defensive mechanism also related to the significantly higher concentration of Pb found in gills and kidney. The results obtained show that, in stressed fish, the defensive processes increase to maintain the normal functions of the organs more exposed to the action of polluted substances.

A number of studies has been performed in recent years regarding the structure of the autonomic innervation of the gill and various organs of fishes, but only limited information exists on the role of the autonomic nervous system in air-breathing organs. The mechanisms for physiologically integrating gill and air-breathing organs are also not fully understood. The presence of neu-ropeptides and nNOS in gill autonomie nerves has been investigated in recent years by several Authors. Immunohistochemical localization of neuropeptides and nNOS in the autonomie nerves in the vasculatures and smooth muscle of air-breathing organs is reported for the first time. The function of NO in these tissues also remains to be elucidated especially, when the bulk of neurotransmission is attributable to cholinergic and adrenergic mechanisms. A challenge for future years is to understand the role of the autonomie nervous systems and the vasoactive substances in the in-series and in-parallel vascular connections between systemic gill and air-breathing organs.