94
H.U. Riisgard and P. Kamermans
4.3.4 Feeding on Siphon Tips
Feeding on siphon tips by carnivorous bottom feeders such as fish, shrimps
and crabs is another factor suggested to influence the feeding behaviour of
Macoma balthica (Levinton 1971; de VIas 1985). Deposit feeding requires
exposure of the inhalant siphon above the sediment in order to be able to
graze on the surrounding sediment. For suspension feeding the siphon is
directed towards the water column. It is not known at which height the opening of the inhalant siphon should be above the sediment for the most favorable suspension feeding. Keeping the siphon level with the sediment surface
to allow contact with the overlying water column could be sufficient. A switch
to a suspension-feeding mode involving only a short siphon exposed above
the sediment has therefore been interpreted as a defence against feeding on
siphon tips (Levinton 1971; de VIas 1985). Kamermans and Huitema (1994)
carried out a series of experiments which included both removing part of the
inhalant siphon of M. balthica and exposing the bivalves to siphon-nipping
shrimps. They demonstrated a reduction in the sediment surface area that
could be grazed by M. balthica following siphon nipping, and a decrease in
siphon activity in M. balthica under nipping conditions. These results demonstrate a decrease in deposit feeding when exposed to siphon nipping and suggest that it is favourable to switch to suspension feeding under such conditions. A switch from deposit feeding under conditions without nippers to
suspension feeding when subjected to nipping was, however, not observed
(Kamermans and Huitema 1994). Exclosure and fish-inclosure experiments
also showed that siphon nipping can induce a reduction in deposit-feeding
activity (Peterson and Skilleter 1994; Skilleter and Peterson 1994). The
authors used the amount of sediment in the mantle cavity as a measure of
deposit-feeding activity, but did not demonstrate a switch to suspension
feeding.
4.3.5 Protection Against Lethal Predation
Another advantage of the fact that the siphon does not need to be stretched
over the sediment when suspension feeding is the possibility for the bivalve to
increase its burying depth. In this way the clam can increase its protection
against lethal predation by birds (Zwarts and Wan ink 1989) and crabs
(Blundon and Kennedy 1982). To date, studies that determine how the risk of
lethal predation influences the feeding behaviour of Macoma balthica are
absent in the literature.
H.U. Riisgard and P. Kamermans
4.3.4 Feeding on Siphon Tips
Feeding on siphon tips by carnivorous bottom feeders such as fish, shrimps
and crabs is another factor suggested to influence the feeding behaviour of
Macoma balthica (Levinton 1971; de VIas 1985). Deposit feeding requires
exposure of the inhalant siphon above the sediment in order to be able to
graze on the surrounding sediment. For suspension feeding the siphon is
directed towards the water column. It is not known at which height the opening of the inhalant siphon should be above the sediment for the most favorable suspension feeding. Keeping the siphon level with the sediment surface
to allow contact with the overlying water column could be sufficient. A switch
to a suspension-feeding mode involving only a short siphon exposed above
the sediment has therefore been interpreted as a defence against feeding on
siphon tips (Levinton 1971; de VIas 1985). Kamermans and Huitema (1994)
carried out a series of experiments which included both removing part of the
inhalant siphon of M. balthica and exposing the bivalves to siphon-nipping
shrimps. They demonstrated a reduction in the sediment surface area that
could be grazed by M. balthica following siphon nipping, and a decrease in
siphon activity in M. balthica under nipping conditions. These results demonstrate a decrease in deposit feeding when exposed to siphon nipping and suggest that it is favourable to switch to suspension feeding under such conditions. A switch from deposit feeding under conditions without nippers to
suspension feeding when subjected to nipping was, however, not observed
(Kamermans and Huitema 1994). Exclosure and fish-inclosure experiments
also showed that siphon nipping can induce a reduction in deposit-feeding
activity (Peterson and Skilleter 1994; Skilleter and Peterson 1994). The
authors used the amount of sediment in the mantle cavity as a measure of
deposit-feeding activity, but did not demonstrate a switch to suspension
feeding.
4.3.5 Protection Against Lethal Predation
Another advantage of the fact that the siphon does not need to be stretched
over the sediment when suspension feeding is the possibility for the bivalve to
increase its burying depth. In this way the clam can increase its protection
against lethal predation by birds (Zwarts and Wan ink 1989) and crabs
(Blundon and Kennedy 1982). To date, studies that determine how the risk of
lethal predation influences the feeding behaviour of Macoma balthica are
absent in the literature.