Copepod feeding quantified by planar laser imaging of gut fluorescence

Abstract

We present a new method for quantifying the feeding of individual copepods, using a planar sheet of laser light to stimulate the fluorescence of phytoplankton ingested by the copepod. The fluorescence is imaged with a sensitive CCD camera, giving two-dimensional images of the copepod's gut with 20 × 20 μm spatial resolution. Using tethered copepods, we have obtained > 3 h long time series of copepod gut fluorescence with images every 15-20 s. The same individual copepod can be used for multiple experiments, obviating the problems of individual variability as a source of error. Initial data reveal two distinct patterns of variability as material moves through two functionally different gut compartments. These patterns reflect processes occurring in each compartment. The upper (anterior) mid-gut shows higher variability and less repeatability than the posterior midgut where undigested material is aggregated into a fecal pellet and evacuated at regular intervals. Variability in the upper mid-gut is likely due to factors such as intermittence of feeding and relatively complex mixing dynamics. In the posterior mid-gut, mixing dynamics are much simpler, and the variability of the upper compartment is integrated over the time scale of pellet formation.

Biased measurements by stationary turbidity‐fluorescence instruments due to phototactic zooplankton behavior

“…Due to the different orientation of the sensors with respect to the interrogation volume, peaks of fluorescence and turbidity did not always match. In addition, fluorescence is influenced by the recent feeding history of individuals traversing the light beam, as fluorescence of phytoplankton ingested by zooplankton is detected from the outside the animal's body (Karaköylü et al ).…”

The effect of phytoplankton properties on the ingestion of marine snow by Calanus pacificus

“…The smaller sample sizes for stable isotope analysis resulted in larger standard error for these data ( Figs 2B and 3B ), which was likely the reason that the same statistical patterns were not observed. High levels of individual variability in consumption rates, while not the focus of our study, have been reported from in-depth studies of C. pacificus in the field and lab ( Karakoylu et al, 2009 ; Karakoylu and Franks, 2012 ). Understanding the general species patterns that hold despite the large variability related to individual feeding thus is challenging, and statistically significant results (when present) indicate important patterns that are robust given that our sample size was low.…”

Fluorometer optical path interference via zooplankton phototaxis: Implications for high‐frequency data collection

“…While phototaxis may be a primary behavior driving FIZ bias, this response should not be taken out of the broader context of DVM and its relation to zooplankton grazing. Tanaka et al (2019) linked sensor bias with the fluorescence of ingested phytoplankton, and given previous work using fluorometers to detect fluorescence through the carapace of zooplankton (Karaköylü et al 2009), it can be assumed that the quality, quantity, and residency times of florescent compounds within the digestive tract of a zooplankton will influence the magnitude of FIZ bias. Gut residency times of the freshwater cladoceran Daphina pulicaria were found to range from 4 to 106 min (Murtaugh 1985).…”

“…Typical residency times of phytoplankton in their digestive tracts are 0.5–10 h (Murtaugh 1984; Chipps 1998; Karaköylü and Franks 2012) and gut extractions have found viable photosynthetic pigments (Nemoto 1968; Conover et al 1986; Pandolfini et al 2000) that can be measured using standard lab fluorometric techniques (Mackas and Bohrer 1976). Karaköylü et al (2009) measured F chl through the carapace of the live copepod Calanus pacificus , demonstrating the potential for a detectable fluorescence response by viable photosynthetic pigments within the digestive tract of zooplankton when exposed to excitation wavelengths. While the optical characteristics of a carapace likely vary by species, many species of zooplankton have a transparent carapace, allowing for the transmission of light at excitation and emission wavelengths used for detection of Chl a (Kerfoot and Lynch 1987).…”