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First published online March 2, 2007
Journal of Experimental Biology 210, 1006-1014 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.000570

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Oxygen dynamics around buried lesser sandeels Ammodytes tobianus (Linnaeus 1785): mode of ventilation and oxygen requirements

Jane W. Behrens, Henrik J. Stahl, John F. Steffensen and Ronnie N. Glud^*

Marine Biological Laboratory, University of Copenhagen, Strandpromenaden 5, DK-3000 Helsingør, Denmark

View larger version (18K): [in this window] [in a new window]   Fig. 1. The experimental chamber (height 21 cm, width 14 cm, depth 3 cm) filled with sediment (height 7–12 cm) and overlying water (height 9–14 cm). The yellow square indicates the position of the transparent planar optode sensor.

View larger version (16K): [in this window] [in a new window]   Fig. 2. A schematic illustration of the experimental set-up. GA, glass aquarium; EC, experimental chamber; P1, flow pump; P2, central mixing pump; P3, recirculation pump; DeO2, deoxygenation tower; LSW, laboratory seawater supply/outlet; O2M, oxygen meter; O2R, oxygen regulating meter; N2, nitrogen gas supply; O2E, oxygen electrode; CB, thermostated cooling bath; CC, cooling coils; CCD, digital camera; LED, excitation light source; ExF, excitation filter; EmF, emission filter; TD, trigger device.

View larger version (73K): [in this window] [in a new window]   Fig. 3. Digital images showing the sandeel-mediated movement of Rhodamine-coloured water deposited at the sediment surface. During ventilation, the fish dragged the coloured water through the interstice and into its mouth. The exhalent water formed a lightly coloured plume around the gills that gradually became diluted.

View larger version (36K): [in this window] [in a new window]   Fig. 4. (A) A black and white image showing the position of the sandeel (arrow) in the sediment. Broken line indicates the sediment water interface. (B) The corresponding oxygen image, showing the O2 distribution in the sediment around the buried sandeel. The fish sustained its oxygen requirements by advection of normoxic water from above the surface and into the mouth by gill ventilation, thus creating a `funnel' of oxygenated sediment. During this mode of ventilation a mean of 86.2±4.8% (N=7) of the oxygen in the inspired water was extracted, and a strongly O2-depleted (average of 11.4±6.0%, N=7) plume surrounded the gill area. 1 and 2 refer to profiles in C. (C) Extracted oxygen profiles from the oxygen image showing the vertical O2 distribution and penetration depths at two different positions indicated by the vertical lines in B. The oxygen penetration in profile 1 is clearly affected by the actively ventilating fish whereas profile 2 is only shaped by the diffusive mediated microbial O2 consumption of the sediment.

View larger version (51K): [in this window] [in a new window]   Fig. 5. On rare occasions a phenomenon termed `plume ventilation' was seen, which caused significant temporal and horizontal variations in the sediment oxygen distributions. Here the fish (arrow) made a wriggling body movement, which channelled oxygenated water down along the body creating a `pocket' of oxygenated sediment around the fish. The plume, which typically lasted 20–30 min (the time before anoxic conditions were re-established around the fish), penetrated into the interstice as the oxygenated water was replenished by oxygen-depleted water leaving the gills. The oxygen was gradually consumed by microbes and chemical oxidation processes during sediment percolation.

View larger version (25K): [in this window] [in a new window]   Fig. 6. Planar optode images illustrating the behavioural response of a buried sandeel (arrow) to a progressive decrease of the oxygen saturation in the overlying water. The upper row of greyscale photos shows sandeel position and the lower row the corresponding oxygen images. In the illustrated case, the fish stayed completely submerged until oxygen saturation in the above water had declined to 40%, whereafter its head emerged from the sediment. Despite increasingly lower levels of oxygen in the above water, the fish stayed in this position until it finally fully emerged after approximately 140 min at 5–8% air saturation. The time (min) is time elapsed from the onset of the experiment.

View larger version (74K): [in this window] [in a new window]   Fig. 7. An example of the sterile, aerated sediment used for determinations of the oxygen uptake rates of buried fish by method 1. (A) Before burial of the fish, (B) alignment of the fish (arrow) in the sediment, (C,D) progressive deoxygenation of the sediment due to oxygen-depleted water leaving the gills, percolating into the adjacent interstice and replenishing the oxic porewater. C is approximately 1 h after the fish buried, while (D) is taken about 4 h later. To minimize the likelihood of cutaneous oxygen uptake from the sediment, only images obtained after the fish had been buried for 4–5 h were used for calculations of oxygen uptake rates, i.e. when local anoxia had evolved around the fish, as illustrated by D.