CONDREY, GOSSELINK, and BENNETT: SHRIMP DIETS 



Penaeids were able to remove sections of the 

 algal mat from the surface of S. alterniflora with 

 either the mandibles, or the mandibles and the 

 first and second walking legs. These sections 

 were then rotated in the mandibular area by the 

 maxillipeds. Rejection of uningested particles 

 occurred in the mandibular region. At no time 

 was regurgitation of particles entering the pro- 

 ventriculus observed. 



Despite rejection of portions of the mat, the 

 concentration of oxidizable carbon in the algal 

 mat presented to each shrimp was employed as 

 an approximation of oxidizable carbon ingested. 

 Thus, in this test we were unable to account for 

 selection. Table 8 shows the concentration of 

 oxidizable carbon occurring in the mat and feces. 



Table 8. — Net assimilation efficiencies of individual 

 Penaens setiferus fed on algal mat. 



' A ratio of organic weight to oxidizable carbon of 2 was chosen to 

 convert oxidizable carbon to total organic weight. 



- Individual values recorded have a coefficient of variation of 10%. 



Assimilation efficiency ranged from 52 to 81 Cf , 

 with a mean of 68%. The feeding efficiency 

 (Figure 2) was 33% . Thus, digestibility of the 

 algal mat appears to be lower than that of two 

 of its constituents, i.e., C. fusifoiynis and detri- 



Figure 2. — Organic budget of shrimp fed on algal mat. 



tus. The lower (68''; ) IV for the algal mat com- 

 pared to the defined diets may be explained by 

 the following considerations. First, the green 

 and red algae present in the mat have cellulose 

 cell walls which are probably not digestible, or 

 may be more resilient to trituration, by the 

 shrimp. Second, the ash content of the mat was 

 about 50%, and Conover (1966b) has reported 

 an inverse relationship between ash content and 

 digestibility of various algal diets ingested by 

 Calanus hyperboreus. A third possibility con- 

 cerns the rejection of portions of the mat as a 

 means of selection. The occurrence in the re- 

 jected material and in the feces of the same algal 

 species indicated that rejection was not a method 

 of selection for specific organisms. However, 

 the rotation of the detached mat in the mandib- 

 ular area close to the excurrent respiratory 

 stream appeared to result in removal of adherent 

 silt. If this observation is valid, the organic 

 fraction in the algal mat as it occurs on Spartina 

 would be lower than that of the ingested mate- 

 rial, and efficiencies of net assimilation and feed- 

 ing would have been underestimated in this 

 study. 



CONCLUSIONS 



The results of this study and others begin to 

 form a picture of the feeding process in shrimp, 

 which is still far from complete, but does ex- 

 plain some of the observed phenomena. We de- 

 scribe this process for a shrimp feeding on an 

 algal-microbial community such as that described 

 above, not as a definitive statement but as a hy- 

 pothesis for further testing. 



A portion of the mat is torn oflf by the first 

 and second walking legs and passed to the maxil- 

 lipeds. Rotation of the mat in the proximity of 

 the excurrent respiratory stream results in 

 washing most of the silt away from the periph- 

 ery of the mat. Once cleaned, the peripheral 

 area is torn oflF by the mandibles and ingested. 

 Thus selection for organic material occurs. Con- 

 tinued rotation of the mat entangles the inner 

 algal filaments and silt into a ball. This knotting 

 of the filaments impedes further silt removal. 

 At this point, the uningested portion is rejected. 



1289 



