FISHERY BULLETIN: VOL. 86. NO. 3 



algal mat comprised primarily of the filamentous 

 blue-green alga Spirulina sp. Because there are only 

 small patches of T. testudinum in Laguna Joyuda 

 and the blue-green algae dominated the lagoon floor, 

 the data suggest that the main source of carbon for 

 penaeid shrimps in Laguna Joyuda is benthic blue- 

 green algae. Blue-green algae also appear to be the 

 primary source of carbon for other decapod crusta- 

 ceans in the lagoon including Callinectes spp. and 

 hermit crabs (Paguridae) (Table 3). 



The carbon isotope data, suggesting algal sources 

 of carbon for Penaeus spp., are consistent with the 

 results of dietary analyses. The primary prey of 

 penaeids were the amphipod Grandidierella bon- 

 nieroides and capitellid polychaetes. The d^^C value 

 for G. bonnieroides was within the range for 

 Penaeus spp., -YI.TIoo. Similarly, when the bodies 

 of capitellids were analyzed after removal of the 

 guts, the d^^C value of - 18.8°/oo also agrees with 



shrimps after adjusting for trophic fractionation of 

 rVoo less (Fry et al. 1984). Other organisms with 

 d^^C values similar to that of blue-green algae were 

 the bubble snail Haminoea antillarum and the 

 bivalve Macoma brevifrons. The carbon isotope 

 ratios of these benthic feeders were more negative 

 than Spirulina (-1 A. V'/(m), implying modification of 

 d^^C values from other sources; however, the major 

 input appears to be from blue-green alga. 



By contrast, organisms associated with the man- 

 grove forest, Uca vocator (-23.0Voo) and Aratus 

 pisonii (-23.3*'/oo), closely resembled carbon 

 isotope ratios of detritus (-22.9 to -25.0°/oo). 

 Moreover, the <35 fi plankton fraction (-26.8 to 

 -27.0"/oo) and associated grazers, Acartia (-24.0 

 to -25.9%o), Balanus (-24.0''/oo), and Isognomon 

 (-24.4°/oo), were not separable from the mangrove 

 group. These data indicate that mangrove carbon 

 is likely being incorporated into plankton-based food 



Table 3. — Carbon isotope ratios in organisms associated with the penaeid shrimp food web in Laguna 



Joyuda, Puerto Rico. 



548 



