STONER and ZIMMERMAN: FOOD PATHWAY ASSOCIATED WITH PENAEID SHRIMPS 



revealed when the full matrix of similarity indices, 

 including all species and size classes, was incor- 

 porated into a cluster analysis (Fig. 3). The greater 

 ontogenetic variation in diets of P. brasiliensis 

 resulted in the three size classes for the species fall- 

 ing into three distinct clusters, while all four size 

 classes of P. notialis were included under two 

 clusters. The largest P. subtilis were clustered with 

 the largest P. brasiliensis. Except for P. brasili- 

 ensis, all shrimps between 7 and 14 mm CL were 

 found in one trophic group and all between 15 and 

 22 mm CL were found in a second group. 



Species or generic level identification of prey 

 organisms indicated no species-specific differences 

 among the diets of the three shrimp species. For all 

 three species, all amphipods identifiable to species 

 were Grandidierella bonnieroides and all identifi- 

 able polychaetes were nereids (probably Nereis 

 occidentalis and Steninonereis martini). In P. sub- 

 tilis and P. brasiliensis, all harpacticoid copepods 

 were Euterpina spp. In P. notialis, 86% were Euter- 

 pina spp. and 14% were Microsetella sp. The cala- 

 noid copepods were a mixture ofAcartia tonsa and 

 Pseudodiaptomiis spp. 



Animals classified in groups 2 and 3 of the multi- 

 species cluster (Fig. 3) contained sufficient numbers 

 of individuals to make seasonal analyses of diets 

 (Figs. 4, 5). Although harpacticoid copepods were 

 taken in lower amounts by group 3 shrimps, by and 

 large both groups showed similar seasonal trends 

 in diet. Amphipods were taken in large numbers 

 from July to October 1985 and from March through 

 June 1986. Polychaetes and harpacticoid copepods 

 were consumed most abundantly in November 

 through March. Consumption of detritus was rela- 

 tively constant in both groups, with slightly higher 

 detrital intakes in group 2 individuals during 

 November and December 1984. 



SIMILARITY 



1 .0 0.8 0.6 0.4 0.2 

 I 1 1 1 1 1 1 1 1 — 



B-7 



Juv. 



N-7 



S-7 



B-11 



S-11 



N-11 



N-15 



S-15 



S-19 



N-19 



B-15 



S-23 





3 



Figure 3.— Cluster diagram for the diets of 

 shrimps incorporating all species and size classes. 

 The cluster strategy is the same as that in Figure 

 2. Food categories are identified in Table 2. 



100 



10 11 12 1 2 3 4 5 6 



MONTHS 



Figure 4.— Diets of shrimp trophic group 2 by sampling date. 



Carbon Isotopes 



The d^^C values for three different samples of 

 tissues from Penaeus spp. ranged from -15.0 to 

 -18.1"/oo (Table 3). These values were much higher 

 than the plankton fraction <35 /i (comprised primar- 

 ily of dinoflagellates; -26.8 to -27.2''/oo) or the 

 primary copepod species in the lagoon, Acartia 

 tonsa (-24.0 to -25.9°/oo). The d^^C values for the 

 shrimps are also much higher than the values for 

 either mangrove leaves (green or dead), or detritus 

 particles from the sediment, mostly of mangrove 

 origin. The only primary producers with d^^C 

 values within the range of Penaeus spp. were the 

 seagrass Thalassia testudinum (- 16.1°/oo) and the 



100 



7 8 9 10 11 12 1 2 3 4 5 6 

 MONTHS 



Figure 5.— Diets of shrimp trophic group 3 by sampling date. 



547 



