Switching Between Deposit and Suspension Feeding in Coastal Zoobenthos
85
the metabolic efficiency of gills/whole organism is about 20 0/0, and, using this
figure as a measure of energy, it is obvious that the energetic costs of suspension feeding by means of enlarged specialised gills is considerable. Clearly, the
enlarged gill structures are expensive to maintain irrespective of whether the
mussel is pumping water or not, and this means that energy for functions
other than pump work can only be justified when the part of the organism
responsible for the pumping action is dimensioned for continuous feeding.
From these arguments it can be realised why deposit-feeding bivalves have
small gills and a low specific 'filtration' rate and no ability to switch to suspension feeding. Further, it may be realised that the adaptation of N. diversicolor to an occasional suspension feeder (see Example I, Sect. 4.2) is exceptionally cheap in terms of no conspicuous anatomic alterations and accompanying metabolic investments.
4.2 Example I: Switching to Suspension Feeding
in Nereis diversicolor
H. U. RnSGARD
The polychaete Nereis diversicolor is a common benthic species of shallow
areas in north-western Europe, where it often penetrates far into estuaries
(Theede et al. 1973; Wolff 1973; Chambers and Milne 1975). The ragworm is
almost entirely restricted to the littoral zone where it lives in aU-shaped
burrow in the sediment. N. diversicolor has been described as a carnivore, a
scavenger, a suspension feeder and a surface-deposit feeder, feeding partly by
ingesting detritus and microphytobenthos around the openings of the burrow
(Wells and Dales 1951; Goerke 1966, 1971; Evans 1971; Smith 1977; Olafsson
and Persson 1986; R6nn et al. 1988).
The occurrence of a suspension-feeding mechanism in Nereis diversicolor
was first described by Harley (1950), and later confirmed by Goerke (1966),
but quantitative measurements have only recently been made. N. diversicolor
may meet its metabolic requirements on a pure diet of suspended phytoplankton at naturally low (i. e. usually
worm may contribute to the control of phytoplankton production in many
shallow brackish water areas - a role previously overlooked and thus undervalued (Riisgard 1991; Riisgard et al. 1992, 1996b; Vedel et al. 1994; Nielsen et
al. 1995; Vedel1998). The aim of the present account is to give an overview of
the current knowledge concerning suspension feeding in N. diversicolor and
its ecological implications.
85
the metabolic efficiency of gills/whole organism is about 20 0/0, and, using this
figure as a measure of energy, it is obvious that the energetic costs of suspension feeding by means of enlarged specialised gills is considerable. Clearly, the
enlarged gill structures are expensive to maintain irrespective of whether the
mussel is pumping water or not, and this means that energy for functions
other than pump work can only be justified when the part of the organism
responsible for the pumping action is dimensioned for continuous feeding.
From these arguments it can be realised why deposit-feeding bivalves have
small gills and a low specific 'filtration' rate and no ability to switch to suspension feeding. Further, it may be realised that the adaptation of N. diversicolor to an occasional suspension feeder (see Example I, Sect. 4.2) is exceptionally cheap in terms of no conspicuous anatomic alterations and accompanying metabolic investments.
4.2 Example I: Switching to Suspension Feeding
in Nereis diversicolor
H. U. RnSGARD
The polychaete Nereis diversicolor is a common benthic species of shallow
areas in north-western Europe, where it often penetrates far into estuaries
(Theede et al. 1973; Wolff 1973; Chambers and Milne 1975). The ragworm is
almost entirely restricted to the littoral zone where it lives in aU-shaped
burrow in the sediment. N. diversicolor has been described as a carnivore, a
scavenger, a suspension feeder and a surface-deposit feeder, feeding partly by
ingesting detritus and microphytobenthos around the openings of the burrow
(Wells and Dales 1951; Goerke 1966, 1971; Evans 1971; Smith 1977; Olafsson
and Persson 1986; R6nn et al. 1988).
The occurrence of a suspension-feeding mechanism in Nereis diversicolor
was first described by Harley (1950), and later confirmed by Goerke (1966),
but quantitative measurements have only recently been made. N. diversicolor
may meet its metabolic requirements on a pure diet of suspended phytoplankton at naturally low (i. e. usually
worm may contribute to the control of phytoplankton production in many
shallow brackish water areas - a role previously overlooked and thus undervalued (Riisgard 1991; Riisgard et al. 1992, 1996b; Vedel et al. 1994; Nielsen et
al. 1995; Vedel1998). The aim of the present account is to give an overview of
the current knowledge concerning suspension feeding in N. diversicolor and
its ecological implications.