Haematology of the turbot, Psetta maxima (L.): ultrastructural, cytochemical and morphological properties of peripheral blood leucocytes

Optimum procedures for fish handling and sample processing for use when emp loying haematological parameters as health indicators in turbot, Psetta max ima (L.), have been established. We found thrombocytes to be the most abund ant blood cell, representing approximately 52% of circulating leucocytes (l ymphocytes, 40.8%; granulocytes, 5.6%; monocytes, 1.6%; total number of leu cocytes = 1.3 x 10(5) ml(-1); packed cell volume = 22.7%). The light- and e lectron-microscopical characteristics of these cell types are described, to gether with their cytochemical properties using Sudan Black B, Periodic Aci d Schiff, Non-specific Esterase, and Acid and Alkaline Phosphatase. Turbot thrombocytes showed a high degree of shape alterations when observed in liv e preparations using phase contrast microscopy, while ultrastructural obser vations following the in vitro uptake of carbon particles supported an acti ve process of phagocytosis by the thrombocytes, rather than passive entrapm ent. The lymphocytes of turbot are structurally similar to mammalian lympho cytes with the highest nuclear:cytoplasmic ratio of all the leucocytes obse rved. Small lymphocytes predominated, large lymphocytes forming less than 1 % of the total white blood cell population. The most frequent granulocyte t ype was a neutrophil-like cell with an eccentric nucleus, only rarely seen in segmented form. In vitro uptake of carbon particles by granulocytes was not observed under the conditions of the experiment, although turbot granul ocytes are capable of phagocytosis under different circumstances. These are discussed, along with other physiological and technical factors which can influence the blood parameter findings in fish.