the hypothesis that a high diversity of food sources is used by brown shrimp throughout the migration. 

 However, during each sampling date from January 25 to July 29 (Table 2, Fig.3), there was little variation in 

 shrimp 6'^C values (sd < 2 %o), indicating that the shrimps had a similar diet at specific sites and times (de Niro 

 & Epstein 1978). Wide ranges of 6"C for invertebrates have been previously observed along salinity gradients 

 (Incze et al. 1982, Hughes & Sherr 1983). The oyster Crassostna gigas exhibited significandy different 6"C 

 variations between sites along an estuarine gradient, reflecting the preferential utilization of different food 

 sources, namely, terrestrial detritus, benthic diatoms and marine phytoplankton (Riera & Richard 1996). The 

 present study suggests that individual 6''C variation of a migrating species (Penaeus a^ecus) along an estuarine 

 gradient can be as large as inter-individual o"C variation of a sedentary species {Crassostna gigas). 



Contribution of salt marsh sources to brown shrimp diet 



Salt marsh habitats appear to be important food sources for young brown shrimp. When entering Corpus 

 Christi Bay through Aransas Pass, brown shrimp larvae had typically O C values (-21.7 to -20.7 %o) 

 characteristic of animals feeding primarly on an oceanic planktonic food soiurce (Fry & Parker 1979, Incze et al. 

 1982). However, as they entered the mouth of Rincon Bayou through the Spartina altemiflora and Salicomia sp 

 marsh, brown shrimp became more "C -enriched (-19.4 to -16.8 %o) indicating a significant contribution of a 

 "C-enriched source to shrimp diet. At Rincon mouth, suspended POM, SaUcomia sp and zooplankton were too 

 "C-depleted (-27.3 to -24.2 %o) to explain the enrichment in "C of migratory brown shrimp (Table 1, Fig.3). 

 Moreover, assuming phytoplankton is the main food source for zooplankton, and that the trophic O C 

 enrichment is about 1 %o per trophic level (de Niro & Epstein 1978), phytoplankton O C should be about -27 

 %o, which is too negative to be a major contribution to brown shrimp diet. The "C-enrichment of brown 

 shrimp at the mouth of Rincon Bayou can be explained by a significant contribution of carbon derived from 

 Spartina altemiflora detritus (-16 to -14.5 %o). Consistent with these results, plant detritus derived &om Zostera sp, 

 Vhragmites sp or Spartina sp were found in gut contents of post-larval penaeids indicating that marsh detritus may 

 be a food source for these shrimps when they occupy that habitat (see Dall et al., 1990 for a review). Likewise, 

 mangrove detritus has been shown to contribute to the diet of juvenile Penaeus merguiensis inhabiting tidal creeks 

 in Peninsular Malaysia (Newell et al. 1995). In contrast, from feeding experiments Gleason & Wellington (1988) 

 reported that Spartina altemiflora detritus and its epiphytes contributed only a small part of Penaeus a^ecus 

 assimilated carbon. Finally, Dall et al (1990) concluded that plant detritus itself is not a major food source for 

 prawns. 



The results of the present study indicated that detritus derived from Spartina altemiflora can be an important 

 carbon source to juvenile brown shrimp. However, Penaeus a^ecus may assimilate carbon that is ultimately 

 derived from Spartina via several routes other than direct feeding on plants detritus. It is possible that Penaeus 

 a^^ecus may obtain part of its carbon derived from Spartina altemiflora detritus through microbial mediation. For 

 example, in '''C labelling experiments, the grass shrimp Palaemonetes puffo could assimilate carbon from detrital 

 Spartina altemiflora with 38,4 % efficiency via bacterial mediation between non-living organic detritus and shrimp 

 (Crosby 1985). In fact, bacteria associated with debris of refractory plant material can facilitate the carbon 

 transfer from plant sources to bivalves (Langdon & Newell 1990, Crosby et al. 1990). 



As brown shrimp occupied the down and up marsh, they remained "C-enached (-19 to -17 %o), indicating a 

 persistence in their utilisation of a relatively heavy ''C source (Fig.3). Although there is no Spartina altemiflora 

 within the Rincon Bayou marsh, these 6"C values may be explained by utilization of benthic diatoms, blue 

 green algae and/or detritus derived from Spartina spartinae as carbon source (Fig.3). However, the respective 

 contributions of these '^C-enriched sources to brown shrimp feeding cannot be established from 6'^C values 

 alone. It is known that benthic microalgae from mudflats represent one of the dominant food source for 

 juvenile penaeids within tidal creeks in Peninsular Malaysia (Newell et al. 1995). Also, a positive growth rate of 

 postlarval brown shrimp can be supported over 16 days by feeding only on the planktonic diatom, Skeletonema 

 costatum (Gleason & Zimmerman 1984). Although living microalgae can be more readily used than detritus of 

 vascular plants, as shown for marine bivalves (Bayne et al 1 987, Crosby et al 1 989), a significant contribution of 

 detritus derived from Spartina spartinae to shrimp diet may also accoimt for the observed carbon isotope values. 

 These results are in accordance with recent isotopic data of Deegan and Garritt (1997) showing a preferential 

 utilisation of local sources organic matter in coastal marsh areas by invertebrates. 



Appendix E ♦ E-5 



