1 NOVEMBER 1999
WATER QUALITY, CYANOBACTERIA (BLUE GREEN ALGAE) AND SHRIMP DISEASE
For those of you interested in information on cyanobacteria (blue-green
algae), my graduate research focused on correlations between water
quality in semi-intensive shrimp ponds, various species of cyanobacteria, and diseases (HE, vibrosis and IHHNV) in Belize. Don Lightner's laboratory conducted all the histopathological work on the shrimp samples and the University of Miami provided the electron micrographs of the blue-green algae.
A good deal of pond data was collected in this study to determine the
correlation between water quality parameters (temperature, salinity, pH, ammonia, nitrites, DO) and Oscillatoria lutea, Microcoleus lyngbyaceus, Schizothrix calcicola and Spirulina subsalsa. Shrimp were monitored and tested histopathologically for vibriosis, IHHNV and HE. The objective of the study was to induce HE in PL's by inoculating several PL rearing tanks with monocultures of the above mentioned blue-green algae. The algae cultures were started from filaments collected from the guts of the shrimp in the grow-out ponds during the months of Sept. Oct. Nov. and Dec.
Although HE was not induced, there was interesting information resulting from this study, and of course more questions concerning the cause and effects of blue-green algae and disease. However, what I did find was that for juvenile shrimp with cyanobacteria filaments comprising 50% or more of their total gut contents, had negative histopathological diagnosis for HE, but did have idiopathic lesions suggesting old resolving HE lesions or vibrosis. Growth rates for these pond-culture shrimp were extremely low and sporadic. It is possible that the juveniles developed lesions within the gastro-intestinal lining as a result of ingesting filaments. However, because they were exposed to the toxins at an early age, they may have developed an immunity
to the endotoxin and did not become infected with hemocytic enteritis. Low levels of IHHNV in the nerve cords were detected and as these were P. vannamei they also experienced runt deformity syndrome (RDS).
HE in shrimp appears to be caused by a lipopolysaccharide (LPS) endotoxin released in the gut from the digestive breakdown of ingested filamentous cyanobacteria. LPS molecules are constituents of the outer cell wall of some microorganisms, predominantly gram-negative bacteria and cyanobacteria (Lightner et al. 1988). This study and others done through the University of Arizona showed that shrimp continuously exposed to cyanobacteria appeared to be HE resistant, whereas their siblings reared in algae-free tanks indoors then moved to outdoors in algae-rich tanks soon developed severe HE.
Research by the US Fish and Wildlife Service found situations where a
single species of toxic alga dominates pond flora, toxins are released when competition for nutrients becomes intense, killing or inhibiting the growth of other algae. As the toxic alga uses up available nutrients, the species compete with itself continuously releasing toxins. Eventually, the water becomes toxic to zooplankton, insects, fish and sometimes even to animals that drink the water. (Herman and Meyer 1992).
I continued with this line of research while working in Belize and looked at the effects of various fertilization regimes, water quality and cyanobacteria. I have also worked with molasses and looked at the correlation's between pond water parameters, bacterial counts, cyanobacteria and occurrences of vibrosis.
Helen Dixon
References:
Lightner, D.V. , 1988. Hemocytic enteritis (HE) Disease of Penaeid Shrimp. In. S.K. Johnson (ed), Disease Diagnosis and Control in North American Marine Aquaculture. Elsevier, Amsterdam, Netherlands, pp.89-92.
Herman, R.L., and F.P. Myer, 1992. U.s. Fish and Wildlife Service.
Resource Publication 177. Chapter 5, p. 42.