Welsh, B.L. 1975. The role of grass shrimp, Palaemonetes pugio , in a 

 tidal marsh ecosystem. Ecology 56:513-530. 



The grass shrimp, Palaemonetes pugio , is a dominant species uniquely 

 adapted to a highly stressed tidal marsh embayment. Monthly sampling of 

 length and dry weight revealed that its life cycle was a single year, 

 with spawning in May, June, and July and most rapid growth in late 

 summer and fall. Mark and recapture estimates conducted quarterly 

 and quadrat net estimates calculated monthly indicated that shrimp 

 were present throughout the year and that densities peaked in the fall 

 (over 1.2 million in 0.01 km^ in October). Production of biomass 

 (growth) equaled loss to predation (including decomposition) over the 

 annual cycle, averaging 0.2 kcal/m^/day. Respiration averaged 1.1 

 kcal/m^/day. 



Average daily production per square meter of "total consumables" 

 (fecal pellets = 0.8 kcal , dissolved organic matter [DOM] = 0.7 kcal , 

 biomass = 0.2 kcal) was 60 percent of total ingestion (2.9 kcal); 

 production of feces and DOM thus outweighed biomass production 15:2. 



Microcosm studies and observations by scanning electron microscope 

 revealed that shrimp macerated detritus into a heterogeneous assortment 

 of uneaten particles by plucking away the cellular matrix from surfaces 

 of large detrital fragments. This action provided cavities that became 

 heavily invaded by pennate diatoms, and particles that became suspended 

 in the water column and populated by bacteria. 



Nutrient analyses indicated the shrimp excreted large quantities of 

 ammonia and phosphate which together with DOM release were presumably 

 responsible for heavy growth of microflora and increased protein fraction 

 in both feces and large and small uneaten detrital fragments. 



Palaemonetes pugio . while supporting its own trophic requirements, 

 accelerated breakdown of detritus, preventing blockages of accumulations 

 that might have occurred from pulses of emergent grass and macroalgal 

 detritus in the embayment. This repackaging into feces, heterogeneous 

 fragments, DOM, and shrimp biomass made detrital energy available at 

 a variety of trophic levels, smoothing out organic pulses over time 

 and space, and raising the efficiency of transfer to the food web. 

 The special adaptation of P. pugio to the low-oxygen environment of 

 the decomposer system appeared to limit predation and competition, 

 allowing the shrimp to develop large populations necessary to carry 

 out its role effectively. (A, A.) 



Keywords: grass shrimp, tidal marsh, ecosystem, biomass, detritus, 

 U.S. general 



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