Background/Question/Methods - Cyanobacteria invasions are a threat to aquatic ecosystems that can impact species and community dynamics.
Aetokthonos hydrillicola (
Ah) is a novel cyanobacteria associated with submerged aquatic macrophytes (SAVs). The species forms dense colonies primarily on the invasive macrophyte
Hydrilla verticillata, and the direct or indirect ingestion of
Ah causes neurological impairment and death among birds, turtles, and fish. Toxicity to other aquatic fauna has not been established, and all evidence of the toxicity to wildlife come from studies of ingestion of
Ah-positive
Hydrilla during the fall. We do not know whether consumers exposed to
Ah during other seasons are at risk, and we do not know whether
Ah produces a similar toxicity when it occurs on other macrophytes. We used laboratory feeding studies to determine whether ingestion of
Ah positive
Hydrilla causes abnormal behavior and high mortality among tadpoles, whether
Ah toxicity to tadpoles varied seasonally, and whether
Ah toxicity might differ between host plants. Tadpoles are common, abundant grazers of epiphytic biofilms. We fed tadpoles of three ranid species
Ah positive or negative
Hydrilla or
Ah positive Southern Pond Weed (
Najas guadalupensis) collected in the summer or early fall and measured the frequency of abnormal behavior and mortality.
Results/Conclusions - Tadpoles in all trials showed high rates of abnormal behavior and rapid mortality when fed Ah positive Hydrilla harvested in the fall, but not when fed Ah negative Hydrilla harvested in the summer. Tadpoles also did not show abnormal behavior or mortality when fed Ah positive Naja collected in the fall from the same site where Ah positive Hydrilla caused high tadpole mortality. Our results demonstrate that ingestion of Ah positive Hydrilla causes similar neurological impairment and mortality as observed in other vertebrates, but this risk varies seasonally and between host plants. The mechanisms for seasonal and host plant specific toxicity are not known, but they have the potential to create differential vulnerability to Ah invasion among aquatic fauna and geographic variation in risk, which may have important consequences for community interactions in addition to creating conservation and management challenges.
