166
E. Flach and A. Tamaki
Lackschewitz and Reise (1998) report a positive relationship between lugworms and the amphipod Urothoe poseidonis, and Beukema et al. (1999)
between lugworms and the polychaete Harmothoe sarsi. On the intertidal flats
of Tomioka Bay a positive relationship was found between the ghost shrimp
and the isopod Eurydice, expansion of the ghost shrimp being followed by
expansion of the isopod to the seaward zone. Thus, on both sides of the world,
examples of both amensalism and of commensalism were found. In both
cases the large bioturbators themselves did not seem to be influenced by the
interactions. The reduction in numbers of the ghost shrimp in the years after
the increase of the stingray population in Tomioka Bay appears to be an
example of predator-prey interaction that is of importance in structuring the
benthic community.
Predation seems to be important in the Wadden Sea as well, but as an
additional factor reducing further densities of species that are already
negatively influenced by the bioturbating activities of the lugworms. This is
an example of interaction modification (Wootton 1993), where the relationship between a predator and its prey is changed by the effect of a third species
(the lugworm in our case) that alters the behaviour of the prey (flight reaction
in Corophium) therefore increasing its predation risk. In shallow water bays in
the Gullmarsfjord on the Swedish west coast, this combined interaction was
shown to strongly influence the distribution pattern of Corophium (Flach and
De Bruin 1994). In the Finnsbovik a similar zonation pattern was observed to
that in the Wadden Sea, Corophium dominating the most shallow zone and
Arenicola strongly dominating the intermediate zone (Flach and De Bruin
1993). Removal of the lugworms and excluding predators by placing cages at
two different levels in the lugworm zone resulted in roughly similar high
Corophium densities as those found in the shallowest zone. The exclusion of
predators and lugworms had a much stronger effect in the deeper part, where
predation pressure was higher, than in the shallower part, thus demonstrating
the strong impact of both types of interactions working together (Flach and
De Bruin 1994).
The strong impact that expansion of the large bioturbator has on the
benthic community, as was observed for the ghost shrimp in Tomioka Bay,
was also found for expansion of the lugworm in the Finnsbovik in Sweden. In
1992 the lugworm was restricted in high densities to the intermediate part of
a transect from the shore to the low water line. During summer 1992, high
numbers of juvenile lugworms settled both in the shallow and the deep part,
where adult numbers were low (Flach and Beukema 1994). The juvenile lugworms stayed in the shallow zone and survived the winter resulting in
shoreward expansion of the lugworm zone (Fig. 8 in Flach and Beukema
1994), which had strong effects on the species that were previously living in
this shallow zone. The numbers of Corophium were strongly reduced (Flach
and De Bruin 1994) as were the numbers of the tube-building polychaete,
E. Flach and A. Tamaki
Lackschewitz and Reise (1998) report a positive relationship between lugworms and the amphipod Urothoe poseidonis, and Beukema et al. (1999)
between lugworms and the polychaete Harmothoe sarsi. On the intertidal flats
of Tomioka Bay a positive relationship was found between the ghost shrimp
and the isopod Eurydice, expansion of the ghost shrimp being followed by
expansion of the isopod to the seaward zone. Thus, on both sides of the world,
examples of both amensalism and of commensalism were found. In both
cases the large bioturbators themselves did not seem to be influenced by the
interactions. The reduction in numbers of the ghost shrimp in the years after
the increase of the stingray population in Tomioka Bay appears to be an
example of predator-prey interaction that is of importance in structuring the
benthic community.
Predation seems to be important in the Wadden Sea as well, but as an
additional factor reducing further densities of species that are already
negatively influenced by the bioturbating activities of the lugworms. This is
an example of interaction modification (Wootton 1993), where the relationship between a predator and its prey is changed by the effect of a third species
(the lugworm in our case) that alters the behaviour of the prey (flight reaction
in Corophium) therefore increasing its predation risk. In shallow water bays in
the Gullmarsfjord on the Swedish west coast, this combined interaction was
shown to strongly influence the distribution pattern of Corophium (Flach and
De Bruin 1994). In the Finnsbovik a similar zonation pattern was observed to
that in the Wadden Sea, Corophium dominating the most shallow zone and
Arenicola strongly dominating the intermediate zone (Flach and De Bruin
1993). Removal of the lugworms and excluding predators by placing cages at
two different levels in the lugworm zone resulted in roughly similar high
Corophium densities as those found in the shallowest zone. The exclusion of
predators and lugworms had a much stronger effect in the deeper part, where
predation pressure was higher, than in the shallower part, thus demonstrating
the strong impact of both types of interactions working together (Flach and
De Bruin 1994).
The strong impact that expansion of the large bioturbator has on the
benthic community, as was observed for the ghost shrimp in Tomioka Bay,
was also found for expansion of the lugworm in the Finnsbovik in Sweden. In
1992 the lugworm was restricted in high densities to the intermediate part of
a transect from the shore to the low water line. During summer 1992, high
numbers of juvenile lugworms settled both in the shallow and the deep part,
where adult numbers were low (Flach and Beukema 1994). The juvenile lugworms stayed in the shallow zone and survived the winter resulting in
shoreward expansion of the lugworm zone (Fig. 8 in Flach and Beukema
1994), which had strong effects on the species that were previously living in
this shallow zone. The numbers of Corophium were strongly reduced (Flach
and De Bruin 1994) as were the numbers of the tube-building polychaete,