Structural and ultrastructural aspects of the pericardial sac in some sea water teleosts

Generally, in fishes the parietal pericardium is considered to be a semi-rigid structure. It is widely known that the entire parietal pericardium in fish, in contrast to that of higher vertebrates, is fixed to the body wall which consists mainly of skeletal muscle. This structural arrangement could explain the low end-systolic intrapericardial pressure in fishes. The morphology of the parietal pericardium of 11 sea-water teleosts, either free or breeding, coming from various areas of the southern Italy was studied. Macroscopical evaluation, but even more, histological and ultrastructural studies, showed some differences among the hearts of the examined subjects, more pronounced in the European hake (Merluccius merluccius, L.), in which a true pericardial sac was demonstrated, well differentiated and distinguishable from the other structures, in contrast to the observations in other subjects. The pericardial connective tissue showed no continuity with that of the surrounding structures, mainly skeletal muscle. This difference could be retraced to the well-known pattern of life and, therefore, to other individual as well as evolutionary factors in comparison with other teleosteans examined.