Switching Between Deposit and Suspension Feeding in Coastal Zoobenthos
87
that the net is composed of an irregular mesh made up of long, relatively thick
filaments (up to 300 nm) inter-connected with a variety of shorter and
thinner filaments with diameters ranging from 5 to about 25 nm. The average
mesh size, measured directly on the EM pictures, was found to be between 0.5
and 1.0 flm, but due to shrinkage during preparation for EM these values
represent only approximately 75 % of the actual dimension of the intact net.
Subsequent measurements performed by Riisgard et al. (1992) showed that
particles as small as 2 to 3 flm can be cleared from the water with near 100 %
efficiency. This efficiency is comparable to that found in other suspensionfeeding invertebrates (M0hlenberg and Riisgard 1978; Riisgard 1988; J0rgensen et al. 1984).
4.2.3 Filtration Rates
The filtration rate of Ne re is diversicolor individuals was measured by Riisgard
(1991) using three different methods. Results were given as pumping rate,
measurement of clearance of suspended algal cells, and by means of the socalled 'suction' method. Further, the filtration rate of N. diversicolor was
measured as a function of dry weight. Using this relationship and knowing
the density and size distribution at the collecting site the population pumping
rate was estimated to be about 10 m 3 m- 2 d- I , i.e. a volume corresponding to 10
times the above water column daily. Vedel et al. (1994), Riisgard et al. (1996b),
and Vedel (1998) found similar population filtration capacities. In the shallow
Odense Fjord, Denmark, the population filtration capacity was found to
increase with rising water temperature from nearly 2 m 3 m- 2 d- I in late March
to 7 m 3 m- 2 d- I in early August, or a water volume equivalent to 3 to 14 times
the water column per day (Vedel1998).
4.2.4 Energy Cost of Pumping
In Nereis diversicolor the pumping action is a result of the undulating movements of the body in the confining tube. Observations indicate N. diversicolor
has three different phased posteriorly directed propagating waves, one of
which creates an effective stroke responsible for the pumping action. During
the effective wave, the worm's body makes firm contact with the tube wall on
the dorsal and ventral sides, forming good seals. On the lateral sides, the
parapodia make contact with the wall of the tube, to form less perfect seals
that may leak with increasing backward pressure, resembling a positive
displacement pump. Back-pressure characteristics of the Nereis pump were
measured, and hence the pump was modelled as a leaking positive displacement unit by Riisgard et al. (1992) and Riisgard and Larsen (1995). The
87
that the net is composed of an irregular mesh made up of long, relatively thick
filaments (up to 300 nm) inter-connected with a variety of shorter and
thinner filaments with diameters ranging from 5 to about 25 nm. The average
mesh size, measured directly on the EM pictures, was found to be between 0.5
and 1.0 flm, but due to shrinkage during preparation for EM these values
represent only approximately 75 % of the actual dimension of the intact net.
Subsequent measurements performed by Riisgard et al. (1992) showed that
particles as small as 2 to 3 flm can be cleared from the water with near 100 %
efficiency. This efficiency is comparable to that found in other suspensionfeeding invertebrates (M0hlenberg and Riisgard 1978; Riisgard 1988; J0rgensen et al. 1984).
4.2.3 Filtration Rates
The filtration rate of Ne re is diversicolor individuals was measured by Riisgard
(1991) using three different methods. Results were given as pumping rate,
measurement of clearance of suspended algal cells, and by means of the socalled 'suction' method. Further, the filtration rate of N. diversicolor was
measured as a function of dry weight. Using this relationship and knowing
the density and size distribution at the collecting site the population pumping
rate was estimated to be about 10 m 3 m- 2 d- I , i.e. a volume corresponding to 10
times the above water column daily. Vedel et al. (1994), Riisgard et al. (1996b),
and Vedel (1998) found similar population filtration capacities. In the shallow
Odense Fjord, Denmark, the population filtration capacity was found to
increase with rising water temperature from nearly 2 m 3 m- 2 d- I in late March
to 7 m 3 m- 2 d- I in early August, or a water volume equivalent to 3 to 14 times
the water column per day (Vedel1998).
4.2.4 Energy Cost of Pumping
In Nereis diversicolor the pumping action is a result of the undulating movements of the body in the confining tube. Observations indicate N. diversicolor
has three different phased posteriorly directed propagating waves, one of
which creates an effective stroke responsible for the pumping action. During
the effective wave, the worm's body makes firm contact with the tube wall on
the dorsal and ventral sides, forming good seals. On the lateral sides, the
parapodia make contact with the wall of the tube, to form less perfect seals
that may leak with increasing backward pressure, resembling a positive
displacement pump. Back-pressure characteristics of the Nereis pump were
measured, and hence the pump was modelled as a leaking positive displacement unit by Riisgard et al. (1992) and Riisgard and Larsen (1995). The