FISHERY BULLETIN: VOL. 70, NO. 4 



Table 6. — Minimum fecal release rates' of Penaeus 

 setiferus fed on Cylindrotheca fusiformis and Chow. 



Diet 



Fecal release 

 rata 



Dry wt 

 scrimp 



Percent 



organic wt 



shrimp 



Fecal release 

 rate 



Diatom 

 Chow 



mg/hr 



5.79 

 9.06 



^ Calculations give minimum fecal release rates because some small 

 loss of feces may have occurrecJ. 



on C. fusiformis proceeded at a minimal rate of 

 1.3 mg dry weight per gram organic weight of 

 P. setiferus per hour. With the Chow, the re- 

 lease rate was more rapid, 3.4 mg hr~^ 



The minimal fecal release rates were employed 

 to determine the rates at which total organic as- 

 similation and assimilation of organic moieties 

 occurred. These values (Table 7) reflect all 

 three assimilation parameters, that is, digest- 

 ibility, concentration in the food, and ingestion 

 rate. The similarity in digestibility and total 

 organic and protein composition of the two diets 

 (see Figure 1) was obscured by the higher in- 

 gestion rate on the Chow. When viewed in time, 

 net assimilation of organic material proceeded 

 more than twice as fast on the Chow as on the 

 diatom. 



The protein assimilation rate followed a pat- 

 tern similar to the rate of assimilation of total 

 organic material ; however, because of the high 

 Chow carbohydrate analysis, sugars were assim- 

 ilated more than 13 times as fast from the Chow 

 than from the diatom. Conversely, even though 

 ingestion of the diatom diet was relatively slow, 

 its high concentration of easily digested lipids 

 resulted in a faster assimilation of lipids than 

 from the Chow. 



We have reported assimilation rates in terms 

 of the organic content of the feeding penaeids 

 so that these rates are indicative of rates of re- 

 plenishment. Johannes and Satomi (1967) re- 

 ported the rate of net assimilation of Nitzschia 

 closterium by Palaemonetes pugio, the grass 

 shrimp, as 1.3 mg organic carbon per gram dry 

 weight P. pugio per hour. This is somewhat 

 lower than the rate we measured for Penaeus 

 setiferus on C. fusiformis (2.1 mg oxidizable 

 carbon per gram dry weight per hour) . Jo- 

 hannes and Satomi employed quantitative re- 



Table 7. — Minimum rates of ingestion, net assimilation, 

 and egestion of total organic weight, protein, lipid, and 

 carbohydrate per gram organic weight Penaeus setiferus. 



Diet 



Organic moiety 



Ingested 

 I 



Assimilated 

 (net) 



A 



Egested 

 N 



Diatom 



Chow 



covery of food and feces rather than the ratio 

 method. 



The rate of organic assimilation is not nec- 

 essarily proportional to the rate of incorpo- 

 ration into body tissue. However, for an om- 

 nivore feeding on a richly diverse flora and 

 fauna, mutual compensation should prevent the 

 nutritional imbalance of any specific food from 

 exerting a profound eflfect (Provasoli, Shiraishi, 

 and Lance, 1959) . The data make clear the dis- 

 tinction between digestibility (V) and assimi- 

 lation rate. The latter is a function of both U' 

 and ingestion rate. A rapid ingestion rate can 

 compensate for low digestibility. For example, 

 Meyers" has preliminary growth experiments 

 which suggest that the growth rate on AF-1 is 

 high although U' on this diet is low (Table 4). 



ALGAL MAT 



In order to estimate the ability of the shrimp 

 to graze a naturally occurring food material, 

 white shrimp were allowed to feed individually 

 on the algal mat coating Spartina alterniflora 

 culms. This mat consisted mostly of diatoms 

 and filamentous green algae growing on and 

 among the red algae Polysiphonia and Bostrich- 

 ia. Such communities are found on the bottom- 

 most foot of the streamside Spartina in many 

 shallow brackish water bays along the south- 

 eastern Louisiana coast. 



" Personal communication, Dr. Samuel P. Meyers, 

 Louisiana State University, Baton Rouge, La. 



1288 



