Physiology of Finfish and Shellfish

2.2.11. White Blood Cells (leucocytes)

Apart from erythrocytes with their respiratory pigment, fish blood contains several types of colourless or white cells, all of which are ovoid to spheroid in shape. The white blood cells varies between 20,000 to 1,50,000 per cubic millimeter of blood in different groups of fishes. The WBC are of 2 types called granulocytes and agranulocytes.

Granulocytes are sub divided into neutrophils, acidophils and basophils based on their staining reactions. Acidophils (eosinophils) and basophils are rare in fish blood. Of these, neutrophils and acidophils are phagocytes.

Agranulocytes are distinguished as lymphocytes and monocytes and are present in large numbers in blood.

Lymphocytes can vary in size (4.5µm to 12 µm in diameter) among species. The number of lymphocytes varies among species and can vary with season. Telostean lymphocytes appear to be produced by both thymus and kidney. Lymphocytes appear to differentiate into 2 populations, one concerned with the production of antibodies and other with cellular immunity.

Monocytes comprise a small proportion of the WBC population, unless foreign substances are present in the tissues or blood stream. Monocytes originate in the kidney, termed macrophages by some workers are important in defence, their role is to eat (phagocytize) anything they come across in the blood that might harm the fish such as bacteria, or parasite larvae.

WBC’s function in a variety of ways by riding away the foreign material (including invading pathogens) along with providing a mechanism for blood clotting. Measurements of change in total WBC number or in the percentage of various types can lead to a better understanding of the physiological or pathological state of the animal. Circulating WBC number can also vary through the year in some fish species. WBC counts generally show an inverse relationship with fish condition or health. Sick individuals would presumably make more WBC’s to synthesize antibodies, phagocytize bacteria, etc. (phagocytic activity – engulfing foreign cells).

The thrombocytes or platelets

These are small and spindle shaped cells and account for about ½ of all leucocytes in fish. These are involved in blood clotting.

Polymorphism in haemoglobin

Haemoglobin is the red colour respiratory pigments which is present in the RBC. Hb is present in majority of the fishes, but varies in its amount depending upon the total number of red cells in the blood. A few Antarctic ice fishes are without hb in their blood. These species are able to survive and live without hb because they are extremely sluggish in habit and have low oxygen requirement. They possess a large heart and blood volume as special adaptive features.