FISHERY BULLETIN: VOL. 70, NO. 4 



Ingested material enters the proventriculus 

 where it is ground between the teeth of the gas- 

 tric mill and forced through the filter press into 

 the digestive gland (Dall, 1967) . Passage of the 

 ingested material through the proventriculus is 

 governed in part by the time required for this 

 trituration. As the digestive gland fills, inges- 

 tion slows. Thus the ingestion rate is governed 

 by the filling of the digestive gland which is in 

 turn regulated by the time required to render 

 the food small enough to enter this organ. In 

 this respect the shrimp is viewed as a conser- 

 vative grazer, ingesting no more than it can ef- 

 fectively assimilate at one time (see Corner and 

 Cowey, 1968, on the question of superfluous 

 feeding.) 



The assimilation of an organic moiety is a 

 function of (1) the activity and concentration of 

 the digestive enzymes of the shrimp and (2) 

 the form of the moiety in a specific diet. Dif- 

 ferences in rates of assimilation of different or- 

 ganic moieties indicate that lipases and protein- 

 ases are more active in the shrimp's digestive 

 processes than are carbohydrases. The form of 

 the moiety determines its susceptibility to cat- 

 alytic attack. For instance, the diatom lipids 

 were assimilated almost entirely (999f ); where- 

 as assimilation eflRciency of the detritus lipids, 

 which occurred at about the same concentration 

 in the diet, was 86 '^r . We suggest that lipids 

 in the diatom occur primarily as oil droplets in 

 the cell and are easily leached and attacked by 

 lipolytic enzymes. Incorporation of the lipids 

 into structural materials such as cell membranes 

 in bacteria could make them less accessible by 

 enzymes and reduce assimilation efficiency. For 

 structural components of a diet, then, assimila- 

 tion efficiency would be related to the degree of 

 maceration or trituration of the food. 



Material not entering the digestive gland pas- 

 ses from the proventriculus to the intestine to be 

 voided at the anus. The physical requirement in 

 the shrimp for material to occupy the space in 

 the proventriculus above the filter press probably 

 places a mechanical limit upon its assimilation 

 efficiency. 



The results suggest three generalizations con- 

 cerning the type of diet shrimp may be expected 

 to feed on in the natural environment. (1) Ju- 



venile shrimp exhibit a high and comparable 

 assimilation efficiency on a variety of plant and 

 animal materials. (2) Proteins and lipids are, 

 in general, assimilated more efficiently than car- 

 bohydrates. (3) Rates of assimilation on dif- 

 ferent material vary, and this variability is con- 

 sidered to be related to how rapidly the diet can 

 be ground and filtered for assimilation. These 

 generalizations show the shrimp to be a true 

 omnivore in the sense that it is able to assimilate 

 a wide variety of foods. The most digestible 

 food is high in protein and lipids, low in carbo- 

 hydrates. 



Since the animal occurs seasonally in high 

 concentrations in the marsh, its natural diet 

 must be present in large volumes and must be 

 rapidly replenished. Data from our laboratory^ 

 indicate that the benthic meiofaunal biomass in 

 the Louisiana saline bays is insufficient to com- 

 prise a major volume of the shrimp's diet. On 

 the other hand, the rates of production of detri- 

 tus from the marsh grass Spartina, and of the 

 periphyton and benthic communities, have been 

 found to be high (Kirby, 1971; Stowe et al, 

 1971; Pomeroy, 1959). Thus, of the naturally 

 occurring foods, the benthic algal communities 

 and the detrital microbial communities on dead 

 Spartina, because of their rapid turnover rates, 

 appear to be the most likely sources. 



ADDENDUM 



An excellent article by Forster and Gabbott 

 (1971) was published during final revision of 

 this publication. They investigated the nitrogen 

 and carbohydrate assimilation of certain com- 

 mercial foodstuffs by Palaemon sermtus and 

 Pandahis platyceros. Nitrogen assimilation of 

 the diflferent diets varied from 80 to 95 "^r where- 

 as the assimilation of simple carbohydrates var- 

 ied from 66 to 102 9^ . Thus the nitrogen assim- 

 ilation efficiencies reported in our paper agree 

 with their findings while our carbohydrate as- 

 similation efficiencies are lower. 



' H. J. Bennett, Louisiana State University, Baton 

 Rouge, La. (Unpublished.) 



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