Suspected Blue‐green Algal Poisoning in the Blue Shrimp

Blooms of a marine blue‐green alga, Spirulina subsalsa , were found to cause a particular disease syndrome, called hemocytic enteritis, in tank or raceway‐reared blue shrimp ( Penaeus stylirostris ). The disease was characterized by necrosis of the lining epithelium of the midgut and occasionally of the dorsal caecum and hindgut gland. Bacterial infections, due predominantly to Vibrio alginolyticus , were common in affected shrimp and were presumed to be a secondary condition resulting from necrosis of the gut epithelium. Intestinal lesions were induced in blue shrimp fed living cultures of S. subsalsa sp. as a diet supplement, but not in shrimp fed fresh cultures of other algal forms ( Oscillatoria sp. and Nitzchia sp.) indigenous to the shrimp culture tank and raceways. Blue shrimp reared in tanks in which S. subsalsa was the dominant algal form showed reduced growth rates, reduced biomass, and reduced survival compared to shrimp reared in tanks in which Enteromorpha sp. and various diatom species were dominant. Recent findings indicate that blooms of certain Oscillatoria sp. and possibly other blue‐green species may also produce the syndrome in tank or raceway‐reared blue shrimp.