STONER and ZIMMERMAN: FOOD PATHWAY ASSOCIATED WITH PENAEID SHRIMPS 



grams were then constructed by complete linkage 

 classification. 



Plant and animal materials were collected from 

 the lagoon for stable carbon isotope analyses on 

 several occasions between 1981 and 1984. These 

 were taken to give an indication of the range of 

 d^^C values between organisms associated with 

 mangroves, plankton, and benthos and to test the 

 efficiency of using the technique to identify food 

 pathways. Materials were collected with trawl, 

 sieve, plankton net, and by hand. All materials were 

 fresh and not exposed to chemical preservatives. In 

 the field, samples were placed in plastic bags on ice 

 as temporary storage. Within 24 hours, these sam- 

 ples were flushed free of salt using deionized water, 

 dissected to acquire tissue uncontaminated by gut 

 contents and outside shells, treated for 5 to 10 

 minutes with 5% phosphoric acid to remove carbon- 

 ates, double rinsed in deionized water, and oven- 

 dried at 70°C until brittle. Dried samples were 

 ground to a fine powder with a mortar and pestle, 

 packaged in plastic bags, labeled, and stored for 

 later mass spectrometer analyses. The homogenized 

 samples were combusted at 550 °C for 24 hours in 

 the presence of CuO in evacuated sealed pyrex tubes 

 using techniques modified from Stofer (1980). CO2 

 gas was analyzed from the combusted samples on 

 a Finnigan - MAT 25 P isotope ratio mass spectrom- 

 eter. d^^C was calculated according to Craig (1957) 

 and methods and definitions generally followed 

 those reviewed by Fry and Sherr (1984). For small 

 animals and all plants, the entire organism (with 

 guts removed where possible) was used in analyses. 

 For large animals, muscle tissue was removed from 

 the body and used separately for analyses. In most 

 cases, organisms were pooled to acquire mean d^^C 

 values for n individuals. Unlike more complex 

 marine systems (Fry et al. 1982), our d^^C were 

 well separated between groups of species and dis- 

 tinctions were relatively clear-cut. Since diversity 

 was also relatively low in the lagoon, d^^C signa- 

 tures in food pathways based on the dominant 

 primary producers were easily detected. Some 

 organisms with known restricted diets, that were 

 closely associated with particular plants in the 

 system, were selected as controls to follow trophic 

 fractionation of d^^C. These included Uca vocator 

 from the intertidal forest floor (a mangrove detritus 

 feeder), Haminoea antillarum from the middle of 

 the lagoon (an algae grazer), and Balanns ebumetcs 

 and Isognomon alatus (filter feeders). Based on 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



results of Fry et al. (1984), we assumed that ^^C 

 enriched the isotope ratio by about l^/oo from one 

 trophic level to the next. 



RESULTS 



The Shrimp Community 



The shrimp community composition during the 

 study period was similar at the three sampling sites, 

 except that Penaeus schmitti were not collected at 

 station 3 (Table 1). Penaeus notialis dominated the 

 shrimp assemblage in the lagoon (50.4%), followed 

 by P. subtilis (41.1%), and P. brasiliensis (8.1%). 

 Only five individuals of P. schmitti (0.3%) were 

 collected in 12 months of sampling. Catch per unit 

 effort over time was highly variable for each of the 

 three primary species, with no distinct seasonality 

 (Fig. 1). Penaeus subtilis, however, showed a 

 general increase in abundance over the sampling 

 period. Penaeus brasiliensis populations remained 

 relatively low and stable throughout the year ex- 

 amined. Population and community structure of the 

 penaeids in Laguna Joyuda has been examined in 

 detail elsewhere (Stoner 1988) and the important 

 point to be made here is that the three most abun- 

 dant Penaeus spp. in Laguna Joyuda are sympatric 

 in both time and space. 



Q 

 UJ 



I- 

 O 



o 

 o 

 w 



D. 

 1 

 DC 

 I 

 CO 

 _l 



< 



H 



O 



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

 MONTHS 



Figure 1.— Abundance of the three dominant Penaeus species in 

 Laguna Joyuda, Puerto Rico, during the study period. The unit 

 of effort for each sampling date is 18 trawl samples, pooled over 

 all three stations. P.n. = Penaeus notialis; P.s. = P. subtilis; P.b. 

 = P. brasiliensis. 



Shrimp Diets 



Over 1,300 Penaeus guts were examined for the 

 present study. The proventriculi of most individuals 

 contained freshly consumed food items; 89% of all 



545 



