Black Gill in Marine Decapod Crustaceans: A Review

Heavily melanized gills in crustaceans, often referred to as black gill, have been reported in both wild and cultured marine species. Tissue melanization is generally the result of a response of the crustacean innate immune system to the presence of an irritant or pathogen. While black gill can be caused by a variety of abiotic stressors and nutritional deficiencies, biotic agents are the cause of most reported black gill cases in crustaceans. In high density culture systems, fungi are identified as the most common causes of black gill outbreaks. In the wild, epidemic-scale outbreaks of black gill appear largely to be caused by ciliate rather than fungal infections. Black gill epidemics caused by ciliates have recently been reported in two commercially important fishery species including penaeid shrimp in the South Atlantic Bight USA (Western North Atlantic) and the Gulf of Mexico, and in pandalid shrimp in the Gulf of Maine, USA. Here we review the reports of the occurrence, causative agents, biology, ecology, and impacts of black gill on wild crustacean species of black gill with special focus on the pandalid shrimp species Pandalus borealis parasitized by the apostome ciliate Synophrya sp. and penaeid shrimp in the Western North Atlantic and Gulf of Mexico parasitized by a newly described apostome ciliate species Hyalophysa lynni. A review of the literature reveals large knowledge gaps with respect to black gill in both commercially exploited and other keystone crustacean species. Recommendations for future research include improved surveillance and identification of causative agents of black gill, improved understanding of their interactions with crustacean hosts including distribution, transmission, morbidity, and mortality, epidemiology, molecular biology, and relationship with climate.