THE CAUDAL APPENDAGE OF THE CLADOCERAN BYTHOTREPHES-CEDERSTROEMI AS DEFENSE AGAINST YOUNG FISH

Rainbow trout (Onchorhynchus mykiss), 40-80 mm in length, were fed the cladoceran Bythotrephes cederstroemi with and without its caudal appendage (spined or despined) to determine if the appendage makes ingestion of the zooplankter difficult for small fish. Handling time was measured as the period of opercular and buccal activity as fish ingested prey. In 1980 observations for 13 trout, fish spent 8 s longer or 800% more time handling spined Bythotrephes than despined. Fish ultimately rejected significantly more spined prey than despined prey. Average handling times of spined prey were negatively correlated with fork length and smaller fish rejected spined prey more often. All fish handled despined prey equally well. Eight of 13 fish changed their handling efficiency over time. However, only two fish decreased handling time on spined prey, while two fish increased handling time on spined prey. Four fish decreased their handling time on despined items. Bythotrephes' caudal appendage increases the likelihood that it will be rejected after capture by fish and decreases fish predation rate by increasing handling time. This is new evidence of a freshwater zooplankter having a structural defense effective against young fish.