Figure 2. — Paraffin section of Carcharhiniis milberti atrium; hematoxylin and eosin (100 x i; bright field illumination. Necrosis of 

 heart tissue demonstrated by basophilic degeneration of muscle fibers and loss of nuclei. Note Ciroluna borealis (delineated by darkly 

 stained exoskeleton) directly below necrotic tissue. 



carried near shore by an event (or repeated events) 

 of particularly strong upwelling, possibly the eddy 

 of April 1977. Gulf Stream intrusions are known to 

 exert major influences on populations of shelf 

 biota (Atkinson et al. 1978). The isopods were not 

 observed in the area by 1979, however, and may 

 not have been able to survive as a species in 

 coastal waters subject to extremes of temperature, 

 light, and salinity, parameters which remain 

 uniform in the deepwater environment. 



Factors indicate that the C borealis population 

 in nearshore waters was aberrant: the depths 

 were shallower than previously recorded for the 

 species in American waters; large-scale predation 

 on living animals was heretofore unknown. Addi- 

 tionally, the shark samples containing isopods 

 were collected during three seasons (winter, 

 spring, and summer) in 1978; and while the life 

 cycle of C. borealis is unknown, it was surprising 

 to not find a few juveniles or females with remnant 

 marsupia in any of the samples. 



Opportunistic feeding by C borealis occurred 

 when isopods and sharks were concurrently 



inhabiting nearshore waters. Both groups of ani- 

 mals may have been drawn to common areas as a 

 response to the olfactory stimuli of baited setlines. 

 Dead sharks on the lines were extensively scav- 

 enged, but living animals on the lines were 

 attacked as well. Predation by isopods on active 

 sharks on the lines would have been facilitated by 

 the restricted avoidance capabilities of the hooked 

 fishes. A deliberate preference by the isopods for 

 the heart of the shark is strongly indicated, both 

 in its predatory and scavenging activities. 



The pathologies observed in the shark hearts 

 suggest that C. borealis might have attacked some 

 free ranging sharks while in nearshore waters. 

 Shark setlines were retrieved by the fishery with- 

 in 10-12 h of set because prompt recovery of catch 

 was essential in order to maintain the quality of 

 shark meat marketed for human consumption 

 (Walker^). Yet, necrosis in heart sample 1 devel- 

 oped at least 18-24 h prior to the death of the 



•'Joe D. Walker, RO. Box 356. Cape Canaveral, FL 32920. 

 pers. commun. October 1978. 



381 



