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The red swamp crayfish Procambarus clarkii is an invasive alien species spreading worldwide. The sterile male release technique (SMRT) is among the methods used to contrast the growth of P. clarkii populations within invaded areas. In this study males underwent X-ray sterilisation with a dose of 40 Gy and their immunocompetence was analysed in comparison to untreated animals to ascertain whether radiation can affect welfare parameters other than reproductive organs. The present research investigated the immune function in P. clarkii males in term of (1) morphological haemocyte characterisation by transmission electron microscopy to identify the main phagocyting haemocyte after in vivo artificial non-self challenge with latex beads; (2) total and differential haemocyte counts; and (3) basal and total phenoloxidase activities as components of the humoral defence. Three types of circulating haemocytes were characterised via transmission electron microscopy: hyaline, semigranular and granular haemocytes. The ultrastructural features of haemocyte granules allowed the characterisation of a fourth type of haemocyte, the medium granule haemocyte. In vivo artificial non-self-challenge with latex beads identified the semigranular haemocytes as primarily involved in phagocyting activity. Circulating haemocytes of males irradiated with a dose of 40 Gy, after 20 days, showed a significantly lower diameter in the granules of hyaline and semigranular haemocytes, but no other evident ultrastructural alterations in comparison with un-irradiated animals were found. Irradiated males showed a significant decrease of about 80% of circulating haemocytes and an increase in frequency of semigranular and granular haemocytes. No significant differences in basal and total phenoloxidase activity were recorded and this could, in part, explain the good survival level of irradiated males despite the drastic decline of the haemocyte number. This study represents the basis to appraise whether SMRT affects important functions, such as those of the immune system, in addition to altering the gonad tissue. [ABSTRACT FROM AUTHOR]