Differential grazing effects by isopods on Gracilaria gracilis and epiphytic Ceramium diaphanum in suspended raft culture

Large populations of epiphytic Ceramium diaphanum and the isopod Paridotea reticulata are found in association with commercially farmed Gracilaria gra cilis. When given a choice, P. reticulata consumed C. diaphanum rather than G. gracilis. Feeding rate experiments indicate that small isopods could co nsume 0.060 +/- 0.040 mg((dry)) C. diaphanum per gram((dry)) of isopod per day and that large isopods consumed 0.043 +/- 0.007 mg((dry)) C. diaphanum per gram((dry)) of isopod per day. In contrast, the amount of G. gracilis c onsumed was negligible. One year's population data indicated that P. reticu lata populations increased as C. diaphanum populations increased. However, P. reticulata's influence on C. diaphanum was only significant when C. diap hanum populations were low. Using the consumption rate of 0.060 +/- 0 040 g ((dry)) C. diaphanum consumed per gram((dry)) of isopod per day and biomass values obtained in the field, it was calculated that during some months P. reticulata is capable of consuming 7.8% of the total C. diaphanum populati on per day. (C) 1998 Elsevier Science B.V. All rights reserved.