Switching Between Deposit and Suspension Feeding in Coastal Zoobenthos
81
water it was observed that many A. filiformis extend their arms into the water,
but at moderate flow velocities the proportion of individuals with at least one
extended arm increases. From video recordings in flume experiments, A. filiformis was observed to capture suspended food items. Particles encountered
and retained on the tube feet are transported between adjacent tube feet in
the proximal direction along the arm. Small papillate protrusions on the tube
feet act to improve retention efficiency by increasing adhesion to encountered
particles (Fig. 4.1e). Reorientation of active arms at increasing flow velocities
may be an attempt to adjust the height of arm extension to match an optimal
flow velocity in the boundary layer with respect to encounter rate and
retention efficiency. During transport, captured particles become entangled
in mucus and are rolled into a bolus by the tube feet. Eventually, the bolus is
transported by tube feet below the sediment surface to the mouth. Occasionally, individuals may also pick deposited particles from the sediment surface, and this behaviour is most common in still water (Loo et al. 1996).
Buchanan (1964) also observed in directional bottom-current flows that A.
filiformis holds the arms up into the current flow with a rheotactic response
to current direction and feeds by trapping suspended particles. A. chiajei does
not show this response and feeds exclusively on deposited matter on the
sediment surface. Miller et al. (1992) found that the brittle-star Amphipholis
squamata in still water holds the arms on the sediment surface, but in a
moderate oscillatory current the arms are held in flow, waving to and fro. In
high flows, however, suspension feeding ceases.
4.1.3 Bivalves
Only a small group of bivalves is dealt with in this section, viz. the tellinids.
Yonge (1949) discerned a continuum in the Tellinaceae from real deposit
feeders (e.g. Macoma) to real suspension feeders (e.g. Donax). Although
primarily a deposit feeder (Fig. 4.1d), Scrobicularia plana may obtain some of
its food by filtering suspended matter from the ambient water (Hughes 1969).
Bradfield and Newell (1961) proposed that Macoma balthica is a deposit
feeder at low water and a suspension feeder when covered by the tide. Thus,
M. balthica and S. plana may not rely on deposit feeding only, because they are
able to filter food from near-bottom resuspended material while the inhalant
siphon is just at the sediment surface (Brafield and Newell 1961; Hughes 1969;
de Wilde 1975; Earll 1975; Hummel 1985a).
Reid and Reid (1969) examined eight species of Macoma which were
classified into three feeding categories: two deposit feeders, five suspension
feeders, and one feeding on the surface films of bacteria on sand grains.
Kamermans (1994a) found that high concentrations of suspended algal cells,
between 180 to 1300 jlg chI a 1-1 (chla: chlorophyll a), are required to sustain
81
water it was observed that many A. filiformis extend their arms into the water,
but at moderate flow velocities the proportion of individuals with at least one
extended arm increases. From video recordings in flume experiments, A. filiformis was observed to capture suspended food items. Particles encountered
and retained on the tube feet are transported between adjacent tube feet in
the proximal direction along the arm. Small papillate protrusions on the tube
feet act to improve retention efficiency by increasing adhesion to encountered
particles (Fig. 4.1e). Reorientation of active arms at increasing flow velocities
may be an attempt to adjust the height of arm extension to match an optimal
flow velocity in the boundary layer with respect to encounter rate and
retention efficiency. During transport, captured particles become entangled
in mucus and are rolled into a bolus by the tube feet. Eventually, the bolus is
transported by tube feet below the sediment surface to the mouth. Occasionally, individuals may also pick deposited particles from the sediment surface, and this behaviour is most common in still water (Loo et al. 1996).
Buchanan (1964) also observed in directional bottom-current flows that A.
filiformis holds the arms up into the current flow with a rheotactic response
to current direction and feeds by trapping suspended particles. A. chiajei does
not show this response and feeds exclusively on deposited matter on the
sediment surface. Miller et al. (1992) found that the brittle-star Amphipholis
squamata in still water holds the arms on the sediment surface, but in a
moderate oscillatory current the arms are held in flow, waving to and fro. In
high flows, however, suspension feeding ceases.
4.1.3 Bivalves
Only a small group of bivalves is dealt with in this section, viz. the tellinids.
Yonge (1949) discerned a continuum in the Tellinaceae from real deposit
feeders (e.g. Macoma) to real suspension feeders (e.g. Donax). Although
primarily a deposit feeder (Fig. 4.1d), Scrobicularia plana may obtain some of
its food by filtering suspended matter from the ambient water (Hughes 1969).
Bradfield and Newell (1961) proposed that Macoma balthica is a deposit
feeder at low water and a suspension feeder when covered by the tide. Thus,
M. balthica and S. plana may not rely on deposit feeding only, because they are
able to filter food from near-bottom resuspended material while the inhalant
siphon is just at the sediment surface (Brafield and Newell 1961; Hughes 1969;
de Wilde 1975; Earll 1975; Hummel 1985a).
Reid and Reid (1969) examined eight species of Macoma which were
classified into three feeding categories: two deposit feeders, five suspension
feeders, and one feeding on the surface films of bacteria on sand grains.
Kamermans (1994a) found that high concentrations of suspended algal cells,
between 180 to 1300 jlg chI a 1-1 (chla: chlorophyll a), are required to sustain