Importance of Trophic Mediation Through Predation 



Predation on animals can also be important for brown shrimp. Like many Penaeus species, Penaeus a^ecus can 

 feed on different prey taxa including oligochaetes, polychaetes, crustaceans, mysids, moUusks and meiofauna 

 (Hunter & Feller 1987, Dall et al. 1990). In fact, the utilization of detritus derived from terrestrial and marsh 

 vascular plants could occur indirecdy through infaunal prey or through bacterial mediation (Gleason & 

 Zimmerman 1984). For example, juvenile Penaeus merguiensis tany use mangrove leaf detritus as a food source 

 indirecdy through predation on small detritivorous invertebrates (Newell et al. 1995). Detritus from Spartina 

 altemiflora salt marshes may be predonunant in the diet of meiofauna when an accumulation of detrital Spartina 

 is associated with the development of an important microbial biomass (Couch 1989). Then, meiofauna may also 

 have a role in the trophic mediation between plant detritus and brown shrimp in these habitats. Likewise, 

 benthic diatoms can be an indirect food source for juvenile shrimp through direct grazing or through 

 intermediate infaunal prey (Stoner & Zimmerman 1988). In the present study, Corixidae {Corix sp) and mysids 

 {Leptomis sp and Mysidopsis almyra), which were collected frequendy at the Nueces River and the Rincon Bayou 

 marsh, could be part of the diet oi Penaeus a^ecus. 6"C values (mean: -17.9%o) and 6'*N (-0.2%o) of Conxsp 

 may explain partly the enrichment in "C and the depletion in '^N observed for individuals of Peneaus a^ecus in 

 Rincon Bayou marsh (Table 2). In contrast, isotopic values measured for both for zooplankton near Rincon 

 Bayou mouth and Mysidopsis almyra in Rincon Bayou marsh were too "C-depIeted and '*N -depleted to 

 contribute significandy to the feeding of brown shrimp in these two habitats. 



Variation in Trophic Level of Brown Shrimp 



Brown shrimp may vary their food sources and feed at different trophic levels during their migratory life cycle. 

 Variations in brown shrimp feeding can be direcdy related with habitats. However, the habitat variations may be 

 associated with a variation in the trophic level of shrimp. Variations in the diet of Penaeus sp with size and age 

 were partly attributed to the variation from a herbivorous to a carnivorous feeding mode as shnmp grow 

 (Chong & Sasekumar 1981). Therefore, shrimp 6'^N should become more positive as size increases. This effect 

 should be more obvious in animal tissue with 6"N than 6''C values, due to a higher 6'^N fractionation during 

 assimilation. The relationship between 6'''N and shrimp length varies in each feeding habitat occupied by 

 migratory brown shrimp, because nitrogen isotopic values at the base of the food chain can vary among the 

 different habitats (Fig.4). At Aransas Pass, this effect should have been especially clear, because brown shrimp 

 size range was largest there. However, there was no significant Pearson correlation between 6'*N and shrimp 

 size at Aransas Pass (r=-0.22, p>0.5), at Rincon Bayou mouth (r=0.12, p>0.5), at Rincon Bayou marsh 

 (r=0.18, p>0.1) and at Nueces River (r=0.35, p>0.1). These results mdicate that differences in food sources of 

 Penaeus a^ecus thoughout its growth and migration are not associated with an increase in trophic level with size 

 but mosdy related to feeding habitats, as indicated by 6"C. 



In addition, large ranges of o'^N observed for shrimps within the different habitats (Fig.4) suggest that shrimps 

 may use the different sources both direcdy and indirecdy through predation. This hypothesis could explain a 

 higher variability for shrimp 6'^N values compared to corresponding 6"C (Table 2) due to the higher trophic 

 enrichment during nitrogen assimilation. Therefore, the results of the present study are consistent uith an 

 omnivore feeding mode for migratory juvenile brown shnmp. This is concordant with gut content analyses, 

 which indicate that animal prey is part of the food ingested by Penaeus a^ecus, but that there is no variation in 

 dietary breadth and prey preference as shrimp grow (Hunter & Feller 1987). 



In conclusion, 0"C values suggest that the main food sources for juvenile brown shrimp {Penaeus a^ecus) in the 

 Rincon Bayou marsh are Spartina detritus, benthic diatoms and blue green algae. Tissue turnover rates in these 

 marsh habitats are apparendy high, because shrimp isotopic signatures change rapidly. Moreover, O'^C and 

 6'^N results show that terrestrial inputs carried by the freshwater inflow can contribute significandy to the diet 

 of juvenile brown shrimp in the Nueces River. Finally, these results show that the restoration of coastal 

 marshes through the introduction of freshwater inflow can provide nursery areas favorable for feeding and 

 growth of juvenile brown shrimp. 



\Jtilit(ation ofEstuarine Organic Matter During Growth 

 and Migration hy Juvenile Brown Shrimp 



