Ultrastructural and histological study of degenerative changes leading to black gills in grass shrimp exposed to a dithiocarbamate biocide

Abstract

Pathologic “milky” and brown-black gills of grass shrimp, Palaemonetes pugio, exposed to low levels (5 to 60 ppb) of a 50% potassium-dimethyldithiocarbamate biocide (Busan-85) for 14 days were studied using light and electron microscopy. In shrimp exposed to 5 ppb Busan-85, a gradation of degenerative changes could be ascertained both within the gills of individual shrimp and among the gills of different shrimp. Primary degenerative changes, first evident in the apical epithelium of the lamellar plates, included: swollen mitochondria, dilated rough endoplasmic reticulum, the appearance of a dense grainy material within the subcuticular spaces and of paracrystalline elements within the epithelial cytoplasm, the formation of autophagosomes, and a loss of membrane continuity. Granular hemocytes migrated into the subcuticular spaces and appeared to phagocytose the grainy material, while other hemocytes phagocytosed and encapsulated the pathological epithelium. Later, the lamellar tips became congested with numerous interdigitating hemocytes which formed a “plug.” A new epithelium, derived partly from mitosis of basal lamellar tissues, developed subjacent to the plug and contributed to the formation of an abnormal and sometimes perforated cuticle during premolt. At ecdysis, the hemocyte plugs in the apical region of the lamella were sloughed resulting in markedly truncated and swollen lamellae. This process of pathological lamellar reduction appears to be an important defense mechanism in response to branchial trauma caused by exposure to environmental contaminants.