corresponding gill tissues gave significantly 

 lower heterotrophic counts. 



Dark blue crab gills harbored the same hetero- 

 trophic bacterial populations found in the sedi- 

 ments (Fig. 4). Similar results with total Vibrio 

 populations were obtained in these areas. As 

 shown in Figure 4, oysters and water contained 

 lower heterotrophic counts. 



Only 10 to 30% of the TCBS bacterial isolates 

 from individual samples of light and dark gills 

 could be identified to genus and species using 

 the API identification system. Aeromonas spp. 

 made up 28% of those TCBS isolates identifed. 



D SEDIMENT 



» Gill 



 OYSTER 



• WATER 



- 





5- 



1 1 1 1 1 — 



NOV 1979 FEB 1980 MAY 1980 AUG 1980 NOV 198 



Figure 4. — Average heterotrophic bacterial counts per gram 

 of sample collected quarterly from Foster Creek. 



Discussion 



Common external features which distinguish 

 blue crabs with brown to mahogany gills are 

 rust-spotted exoskeletons and, occasionally, at- 

 tached barnacles and algae. These conditions are 

 not considered abnormal for late intermolt 

 crabs. Johnson (1977) has also described a viral 

 disease in which blue crabs displayed similar 

 diagnostic signs: failure to molt, a brown-spotted 

 exoskeleton, and gills that were often red-brown. 



Sections made from dark gills collected during 



the survey showed that these gills were, to vary- 

 ing degrees, fouled by a layer of bacteria and 

 mucus (P. T. Johnson 3 ). Observations of similar- 

 ly fouled gills of rock crabs, using scanning elec- 

 tron microscopy, showed large numbers of bac- 

 teria residing on the gill tissue, similar to those 

 found on blue crab gills in this study using bac- 

 terial enumeration procedures (F. Thurberg 4 ). 



These enumeration data suggest that the gills 

 of blue crabs provide an ecological niche for the 

 growth and physiological activity of hetero- 

 trophs, Vibrio spp., and related organisms, 

 equivalent to that described for sediments and 

 zooplankton (Kaneko and Col well 1975 a, b, 1978). 

 The high fecal coliform population found in our 

 pristine area would indicate that gill surfaces 

 also provide a protective ecological niche much 

 like that reported for marine sediments, where 

 high fecal coliform populations can accumulate 

 and persist even in ecosystems where influx of 

 fecal coliforms is low (Rittenburg et al. 1958; 

 Van Donsel and Geldreich 1971; Babinchak et al. 

 1977). 



Urban and industrial pollution did not have an 

 effect on total Vibrio and aerobic, heterotrophic 

 bacterial counts on blue crab gills. Since these 

 two microbial populations are indigenous and 

 dependent on nutrient levels for growth, indus- 

 trial and domestic pollution would not necessar- 

 ily have shown an effect over the pristine areas 

 sampled. The vast marshlands which drain into 

 the St. Helena Sound area (Tiner 1977) would 

 introduce large natural levels of dissolved and 

 particulate organic material and other nutrients 

 which could support the high bacterial levels 

 observed. 



The low rates of identification by the API 20E 

 system can be attributed primarily to the high 

 percentage of clinical bacterial isolates which 

 form the API data base. Even some of our posi- 

 tive identifications are now in question, because 

 marine isolates identified as Aeromonas spp. 

 with the API system are known to be Group F 

 vibrios (Seidler et al. 1980). This group of Vibrio- 

 like organisms has been associated with diar- 

 rheal illness, although the epidemiology of the 

 disease has not been well-defined. 



3 P. T. Johnson, Northeast Fisheries Center, National Marine 

 Fisheries Service, NOAA, Oxford, MD 21654, pers. commun. 

 1981. 



4 F. Thurberg, Northeast Fisheries Center, National Marine 

 Fisheries Service, NOAA, Milford, CT 06460, pers. commun. 

 1981. 



889 



