Degradation of environmental DNA (eDNA) in aquatic habitats can affect the interpretation of eDNA data and the ability to detect aquatic organisms. The effect of temperature and trophic state on the decay of Common Carp (Cyprinus carpio) eDNA was evaluated using lake water microcosms and quantitative PCR for a Common Carp-specific genetic marker in two experiments. The first experiment tested the effect of temperature on Common Carp eDNA decay. Common Carp eDNA exhibited exponential decay that increased with temperature. The slowest decay rate was observed at 5 °C, with a T90 value (time to 90% reduction from initial concentration) of 6.6 days, as opposed to ∼1 day at higher temperatures. In a second experiment, decay was compared across waters from lakes of different trophic states. In this experiment, Common Carp eDNA exhibited biphasic exponential decay, characterized by rapid decay for 3-8 days followed by slow decay. Decay rate was slowest in dystrophic water and fastest in oligotrophic water, and decay rate was negatively correlated to dissolved organic carbon concentration. The overall rapid decay of eDNA and the effects of temperature and water quality should be considered in protocols for water sample storage and field sampling design.

Although environmental DNA (eDNA) has been used to infer the presence of rare aquatic species, many facets of this technique remain unresolved. In particular, the relationship between eDNA and fish distribution is not known. We examined the relationship between the distribution of fish and their eDNA (detection rate and concentration) in a lake. A quantitative PCR (qPCR) assay for a region within the cytochrome b gene of the common carp (Cyprinus carpio or ‘carp’), an ubiquitous invasive fish, was developed and used to measure eDNA in Lake Staring (MN, USA), in which both the density of carp and their distribution have been closely monitored for several years. Surface water, sub-surface water, and sediment were sampled from 22 locations in the lake, including areas frequently used by carp. In water, areas of high carp use had a higher rate of detection and concentration of eDNA, but there was no effect of fish use on sediment eDNA. The detection rate and concentration of eDNA in surface and sub-surface water were not significantly different (p≥0.5), indicating that eDNA did not accumulate in surface water. The detection rate followed the trend: high-use water > low-use water > sediment. The concentration of eDNA in sediment samples that were above the limit of detection were several orders of magnitude greater than water on a per mass basis, but a poor limit of detection led to low detection rates. The patchy distribution of eDNA in the water of our study lake suggests that the mechanisms that remove eDNA from the water column, such as decay and sedimentation, are rapid. Taken together, these results indicate that effective eDNA sampling methods should be informed by fish distribution, as eDNA concentration was shown to vary dramatically between samples taken less than 100 m apart.

The vertebrate olfactory system utilizes odorant receptors to receive and discriminate thousands of different chemical stimuli. An understanding of how these receptors encode information about an odorant's molecular structure requires a characterization of their ligand specificities. We employed an expression cloning strategy to identify a goldfish odorant receptor that is activated by amino acids-potent odorants for fish. Structure-activity analysis indicates that the receptor is preferentially tuned to recognize basic amino acids. The receptor is a member of a multigene family of G protein-coupled receptors, sharing sequence similarities with the calcium sensing, metabotropic glutamate, and V2R class of vomeronasal receptors. The ligand tuning properties of the goldfish amino acid odorant receptor provide information for unraveling the molecular mechanisms underlying olfactory coding.