The study showed that the clinical signs of black disease were characterized by the appearance of black spots. Mortality occurred after the appearance of the dark pigment. Scanning electron micrographs showed that bacteria were localized on the blackened parts; it is conceivable that bacteria are able to colonize the tissue surface of fairy shrimp. Lee, Yin, Ge & Sin (1997) showed that A. hydrophila invades the host cell by adhesion and that bacterial adhesion is the initial stage of the infection . In the present study, histopathology revealed that bacteria were found in scattered masses throughout the tissue and haemocytic encapsulation was present in the lesion. Encapsulation would occur in order to ingest and eliminate the invading particles. Furthermore, the dark pigment appears because of the action of phenoloxidase enzyme and melanization. Therefore, the black spots are a typical consequence of crustacean defence mechanisms to pathogenic bacteria. A haemocytic response to the bacterial infection was also found in the black parts of the infected shrimp, Litopenaeus vannamei (Boone) . Liu, Jiravanichpaisal, Cerenius, Lee, Soderhall & Soderhall (2007) reported that melanin formation plays a key role in the crayfish, Pacifastacus leniusculus (Dana), against Aeromonas
hydrophila infection. Similarly, dark pigment was also observed in the damaged organs of dead
kuruma prawn, Penaeus japonicus (Bate), infected with fungi (Duc, Wada, Kurata & Hatai 2010).
The histopathological results also demonstrated that generalized septicaemia was clearly observed in the body of affected fairy shrimp.