Effects of products released by Aphanizomenon Jlos-aquae and purified saxitoxin on the movements of Daphnia carinata feeding appendages

Experiments were run to determine the effect of filtrate from incubated Aphanizomenonflos-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 1 O-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 -60% of pretreatment activity after l-2 h in Aphanizomenon filtrate, whereas in STX they returned to pretreatment activity levels after 1 h of exposure to the neurotoxin. Water from a lake with a winter bloom of A. Jlos-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. Filaments or colonies of cyanobacteria can reduce the feeding rates of cladoceran zooplankton by mechanically disrupting the movement of filtering appendages (Burns 1968; Gliwicz and Siedlar 1980). The cells of smaller cyanobacteria may also contain endotoxins that reduce feeding and result in elevated mortality of Daphnia species (Lampert 198 1; Fulton and Paerl 1987). Substances can also be released by cyanobacteria into lake water that cause inhibition of feeding rates of zooplankton (Ostrofsky et al. 1983; Burns et al. 1989). Forsyth et al. (1992) also determined that dissolved feeding inhibitors were produced by Anabaena in laboratory incubations. Little is known, however, about the chemical nature of the feeding inhibitors produced by cyanobacteria. There is also question as to whether the cyanobacterial toxins are the substances that cause feeding inhibition in zooplankton (e.g. Nizan et al. 1986; Jungmann et al. 199 1). Toxicity of Aphanizomenon to zooplankton has been reported (Gentile and Maloney 1969), but may be less common than for other cyanobacteria (Lampert 1987). The chemical composition of aphantoxins produced by Acknowledgments Special thanks go to Maura Callahan and Kenneth Kinder whose extreme patience and acute eyes facilitated the, collection of these data. We also acknowledge two anonymous reviewers whose suggestions improved the text and added valuable ideas. This research was supported by the U.S. Geological Survey under USGS Award 14-08-00 1 -G2 10 1. Support for this study was also provided by Hatch Project 205 and University of New Hampshire Agricultural Experiment Station. We also acknowledge the use of the plankton research facilities at the Anadromous Fish and Aquatic Invertebrate Research Laboratory at the University of New Hampshire. Contribution 1906 of the New Hampshire Agricultural Experiment Station. strains of Aphanizomenon flos-aquae isolated from New Hampshire lakes have been identified as neosaxitoxin (neoSTX) and saxitoxin (STX) neurotoxins that are also produced by marine dinoflagellates (Ikawa et al. 1989). In this study we examine the effects of substances released by A. Jlos-aquae on the frequency of thoracic appendage beats and postabdominal rejection movements of Daphnia carinata. Feeding responses are compared to the patterns produced by purified STX to determine whether inhibition could be accounted for by this toxin.