Three types of haemocyte -hyaline cells, small granule and large granule haemocytes -have been identified in the circulation of blue crabs, Callinectes sapidus Rathbun, that were collected and processed in North Carolina, USA in February 1991. Differentiation of haemocytes begins in the haematopoietic tissue and continues in the circulation. Hyaline cells are distinguished morphologically by their relatively small size, high nucleocytoplasmic ratio and sparse cytoplasmic granule content. Small and large granule haemocytes are distinguished, respectively, by a preponderance of small and large electron dense granules in their cytoplasm. Furthermore, a light and electron microscopic examination of the clotting process performed in July 1991 revealed that hyaline cells initiated coagulation when exposed to seawater, whereas granular haemocytes did not undergo marked changes when so treated. These distinct functional and morphologic features suggest that the hyaline cell and granular haemocytes represent distinct cell lines. It remains unclear whether small granule haemocytes differentiate into large granule haemocytes.

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