InEx a direct in situ method to measure filtration rates, nutrition, and metabolism of active suspension feeders

Abstract:
Sponges, bivalves, and tunicates play an important role in the trophic dynamics of many benthic communities. However, direct in situ measurements of their diet composition, filtration, and excretion rates are lacking for most species, and knowledge of these rates is based mostly on indirect, in vitro measurements.

This paper presents and evaluates an in situ, nonintrusive technique of direct measurement of the rate and efficiency by which an active suspension feeder removes (or discharges) substances from (to) the water it filters. The technique, termed “InEx,” is based on the simultaneous, pair-wise collection of the water inhaled and exhaled by the animal. It was specifically adapted to allow reliable sampling of common, small suspension feeders with excurrent aperture as small as 2 mm. The difference in concentrations of a certain substance between a pair of samples provides a measure of the reten- tion (or production) of the substance by the animal. Calculations of feeding (or production) rates are obtained recording the movement of a dye front in a transparent tube positioned within the excurrent jet. An important qual- ity of the InEx technique is that it does not manipulate the studied organisms and thus allows realistic estimates of the organism’s performance under natural conditions. Preliminary results showing the diet composition, feeding rates, and removal efficiencies of some coral reef sponges, bivalves, and tunicates are presented and discussed.

See the 13 page PDF: gitai_yahel_0046

Gitai Yahel1*, Dominique Marie2, and Amatzia Genin1
1The Interuniversity Institute for Marine Sciences of Eilat and Department of Evolution, Systematics & Ecology, The Hebrew University of Jerusalem, Israel 2Station Biologique, CNRS, INSU et Université Pierre et Marie Curie BP 74, Roscoff Cx 29682, France

Three locations were used to evaluate the method. Our primary location was the coral reef in front of the Steinitz Marine Laboratory of Eilat, Gulf of Aqaba, Red Sea (29°30′N, 34°56′E). Further evaluation was carried out at the reefs of Alphonse Island, Seychelles (06°59′S, 52°43′E), and at Race Rocks (123°32′W 48°18′N), British Columbia, Canada. The latter site is characterized by high currents (>2 m s–1) and cold water (6°C to 10°C). All field samples were collected by two divers using standard SCUBA techniques. A flow cytometer (FACSort, Bec- ton Dickinson) was used to estimate the concentrations of 4 groups of ultra-plankton: nonphotosynthetic bacteria (Bact, median size ~0.4 μm) and the three major autotrophic groups, Synechococcus (Synechococcus,~1 μm), Prochlorococcus (Pro, ~0.6 μm), and pico-eukaryotes (Eukaryotic algae, ~2 to 3 μm), as described in Yahel et al. (1998).

See also the video referred to on the Race Rocks website: http://www.racerocks.ca/in-situ-measurement-of-benthic-community-trophic-dynamics-at-race-rocks/