Hello @Pedro I believe your shrimp may have Chitinolytic bacterial disease, Shell disease, Brown spot disease, Black spot disease, Burned spot disease, Rust disease. This is essentially a bacterial infection of Chitinolytic bacteria (Gram negative rods) including Vibrio spp. , Benekea spp., Pseudomonas spp., Aeromonas spp., Spirillum, spp. and Flavobacterium spp. The impact on the shrimp is visible as the exoskeleton becomes pitted, eroded, and melanized at the site of infection. Diagnosis: Erosion of chitin demarked by dark brown to black pigmentation demonstrates chitinolysis. Marks change colour from a rusty to a brown and finally a black colour. Ulcers form on these lesions, as the bacteria will destroyed muscle tissues under the shell. This opens the shrimp up to other secondary infections. The most common sites of infection is the gill, abdominal muscle belly, tail section and gastropods. Seriously infected will just lie on its side, only gastropods and gills in motion. http://shrimpkeepersforum.com/forum/topic/5052-shrimp-diseases-and-diagnosis/?page=1 From @jayc post.