Copepod feeding and digestion rates using prey DNA and qPCR

Copepod feeding and digestion rates were measured using quantitative polymerase chain reaction (qPCR) to amplify the prey Thalassiosira weissflogii and Heterocapsa triquetra from the guts of Acartia tonsa. Using species-specific primers, prey 18S rDNA could be detected routinely and quantified in the guts and fecal pellets of A. tonsa.

Recovery of gut contents DNA using two fixation methods was compared. Prey 18S copy numbers were >10-fold higher in copepods fixed in 95% ethanol (3260 ± 822 copies copepod–1) compared with anesthetized and frozen copepods (210 ± 19 copies copepod–1). Experiments using 95% ethanol fixation showed rapid prey DNA digestion rates during the initial 2 min after ingestion (0.7 min–1) after which they slowed ∼10-fold.

Chlorophyll pigment disappearance rates were slower (∼0.015 min–1). Rates of gut filling measured by DNA and gut pigments differed, reaching 95% of the asymptote, Imax, in 3 and 58 min, respectively, likely reflecting differences in rates at which biomarkers were digested. Gut fullness measured by DNA increased with prey concentration, reaching Imax at 9760 copies copepod–1 and a critical concentration (Icrit) at 1530 cells mL–1.

These results demonstrate that qPCR analysis of prey DNA in copepod guts can be used to provide a quantitative index of feeding rates.