EFFECTS OF PRODUCTS RELEASED BY APHANIZOMENON FLOS-AQUAE AND PURIFIED SAXITOXIN ON THE MOVEMENTS OF DAPHNIA-CARINATA FEEDING APPENDAGES

Experiments were run to determine the effect of filtrate from incubated Aphanizomenon flos-aquae on the thoracic appendage beat rate and frequency of postabdominal rejections by Daphnia carinata. Comparisons were made between the response to Aphanizomenon filtrate and to purified saxitoxin (STX). The short-term response pattern to the filtrate included an immediate 30-50% depression of thoracic appendage beat rate and elevation of postabdominal rejection rate followed by slowly decreasing thoracic appendage beat rate and increasing postabdominal rejection rate during the IO-min exposure. Animals recovered to pretreatment activity rates in <5 min. A similar response was observed in STX, except that Daphnia began to recover immediately from the thoracic appendage beat rate inhibition. Animals recovered to similar to 60% of pretreatment activity after 1-2 h in Aphanizomenon filtrate, whereas in STX they returned to pretreatment activity levels after similar to 1 h of exposure to the neurotoxin. Water from a lake with a winter bloom of A. flos-aquae produced the same pattern of thoracic appendage beat rate and postabdominal rejection rate response as seen with the cultured Aphanizomenon. A comparison of responses to STX and the Aphanizomenon filtrate indicates the effect on D. carinata is through chemosensory stimulation rather than neuromotor inhibition.