Is spermatophore melanisation in captive shrimp (Litopenaeus vannamei) a result of an auto-immune response?

Male reproductive tract degenerative syndrome in shrimp is a condition associated with the melanisation and degradation of sperm housed in the spermatophores. While it has been suggested that this condition is associated with a number of external factors including temperature, diet and microbial infection, the cause of this condition is still unclear. This current study examines if the melanisation of the spermatophores may be a result of an auto-immune condition during which the spermatozoa are recognised as ‘foreign’ by the shrimp's immune system. Initial studies examined the morphology of spermatozoa extracted from normal and heavily melanised spermatophores. A larger percentage of abnormal sperm were found in the melanised compared with the normal spermatophores. There was no indication of any bacterial infection associated with the melanised spermatophores ruling this out as a potential trigger for the syndrome. Incubation of apparently healthy sperm from non-melanised spermatophores with blood cells (haemocytes) showed little interaction while incubation of abnormal sperm from animals with the syndrome with haemocytes resulted in their phagocytosis. Histological analysis of melanised spermatophores revealed the presence of abnormal sperm together with melanised and non-melanised amorphous debris but there was no indication of any haemocyte infiltration or general host reaction to the presence of these abnormal cells in vivo. Overall, although in vitro experiments indicated that damaged sperm together with attendant amorphous debris is apparently recognised as foreign leading to its engulfment by phagocytic haemocytes, such interaction was not observed in vivo. The possible reasons for this disparity are discussed.