STONER and ZIMMERMAN: FOOD PATHWAY ASSOCIATED WITH PENAEID SHRIMPS 



chains in the lagoon. This can be supported using 

 Acartia as a link, since the copepod is known to feed 

 on detritus particles and because the growth of 

 Acartia is improved with detritus is included in the 

 diet (Roman 1984). Suspended detritus particles 

 from mangroves were a component of our <35 fi 

 plankton fraction, suggesting availability to plank- 

 tonic consumers. 



DISCUSSION 



Early studies of penaeid diets led to the general 

 conclusion that the shrimps were largely detritivor- 

 ous with incidental amounts of animal or plant 

 material in the guts (Williams 1955; Darnell 1958; 

 Odum and Heald 1972). Dall (1968) concluded that 

 several Australian penaeids were not predators, but 

 consumers of small-sized and disabled animals. Al- 

 though penaeid shrimps are known to sort sediments 

 for organic particles with their delicate pereiopods 

 (Dall 1968; Lindner and Cook 1970) it is now ap- 

 parent that penaeid shrimps are capable of taking 

 prey organisms such as large polychaetes, as well 

 as the more difficult prey such as gastropods and 

 bivalves, caridean shrimps, crabs, echinoderms, and 

 even fishes (Moriarty 1977; Marte 1980; Leber 

 1983). Lindner and Cook (1970) reported that 

 Penaeus setiferus is cannibalistic at times. The rela- 

 tively constant low amount of detrital particles in 

 the guts of shrimp from Laguna Joyuda suggests 

 that the less readily digestible detritus is taken in- 

 cidentally with animal prey or as a response to low 

 prey abundance. In addition, Gleason and Zimmer- 

 man (1984) showed that nematodes, oligochaetes, 

 polychaetes, and copepods were stripped from 

 detritus by P. aztecits during feeding. Reports of 

 shrimp filled with unrecognizable debris assumed to 

 be detritus may be a consequence of actual detritus 

 consumption or incomplete development of methods 

 sufficient to make animal remains identifiable. 



Despite considerable attention given to feeding in 

 the commercially significant penaeids, relatively few 

 investigators have examined ontogenetic variation 

 in diets. In this study, even within juvenile size 

 classes, there were clear patterns of dietary change 

 with shrimp size. All three of the subject species 

 abandoned smaller prey organisms such as foramini- 

 fera, nematodes, and harpacticoid copepods in favor 

 of amphipods, polychaetes, and shrimp as the pred- 

 ators became larger. Similar findings were reported 

 by Leber (1983) for P. duorarum. These ontogenetic 

 shifts in prey are undoubtedly related to increasing 

 size of the chelae and mouth parts and must be con- 

 sidered in any ecological interpretation of trophic 



position; Moriarty (1977) suggested that the great 

 opportunism of penaeid shrimps precluded their 

 being placed in one trophic group throughout their 

 life cycle. 



On the other hand, with the exception of acceler- 

 ated ontogenetic shifts in the diets of P. brasilien- 

 sis, interspecific variation in feeding in Laguna 

 Joyuda were relatively minor. Despite high abun- 

 dance and great temporal and spatial overlap in the 

 Penaeus spp. of the lagoon (Stoner 1988), coupled 

 with very low biomass values for macrofauna (<2 

 g dry wt/m^), there is no evidence for resource par- 

 titioning among the grooved shrimps. 



Seasonal diets in Penaeus spp. were coincident 

 with seasonal trends in the abundance of major prey 

 organisms. In the lagoon, the one important amphi- 

 pod species, Grandidierella bonnieroides, demon- 

 strated maxima between July and September and 

 again from May to June; this corresponded with 

 maximum amphipod consumption between July and 

 October and March through June in the same year 

 as benthic studies. Polychaete consumption was 

 highest between November and March (particular- 

 ly in the group 3 shrimps), corresponding with the 

 October to April peak in capitellid abundance in the 

 lagoon. Consequently, the oscillation in polychaete 

 and amphipod feeding appears to be related to the 

 availability of foods. 



The hypothesis that mangrove estuaries are fueled 

 primarily by carbon from mangrove detritus has 

 become established from the pioneering work on 

 mangrove-associated food webs conducted in the 

 North River estuary of south Florida (Odum and 

 Heald 1972, 1975). Mangrove litter inputs to Laguna 

 Joyuda are high (Levine 1981), the sediments are 

 rich in organic content, and detritus comprised a 

 portion of the gut contents of juvenile shrimp. It is 

 unlikely, however, that a large amount of carbon 

 derived from detritus or detritus-associated 

 microbes contributes in a large way to the tissues 

 of the shrimp in the lagoon. In fact, the only organ- 

 isms which had carbon isotope ratios similar to that 

 of detritus were those normally found in direct 

 association with the trees such as fiddler and man- 

 grove crabs. The d^^C values for shrimp in Laguna 

 Joyuda were, in fact, very similar to values for 

 penaeids from the open waters of the Gulf of Mex- 

 ico (Fry and Parker 1979). 



Benthic algal primary production in Laguna 

 Joyuda is probably highly significant because car- 

 bon isotope ratios in the algae and shrimp were 

 similar. Gleason (1986) also found that juvenile P. 

 aztecu^ penaeids in a Galveston Bay salt marsh 

 derived their carbon from blue-green algae, green 



549 



