Competitive Bioturbators in the Wadden Sea and Ariake Sound
165
part of each smaller sand flat (Tamaki et aI., in preparation). Densities of 160
or more ghost shrimps per m 2 seemed to be crucial to inhibit the recruitment
success of U. moniliferum. This density might well explain the recent recovery
of the gastropod population on the Tomioka Bay sand flat, especially at Stn
160 (Figs. 7.6, 7.7). On four of the five relatively large sand flats where the U.
moniliferum populations remained on the east coast of Amakusa-Shimoshima Island, the expansion of N. harmandi was not complete and a fairly
large part of each sand flat was left as a refuge for the U. moniliferum recruitment (Tamaki et aI., in preparation). Umbonium moniliferum larvae have a
short planktonic period (within 2 days? cf. Berry 1986), so that larvae released
from the east coast of the island could swiftly come to the Tomioka Bay sand
flat by riding on a strong ebb tide current which can attain speeds as high as
3.6 m/s at spring tides (Fig. 7.5). Given the larger population size of U. moniliferum on the Tomioka Bay sand flat recorded in 1979, the above source-sink
relationship would be reciprocal. This relationship is in stark contrast to the
situation for N. harmandi. Tamaki and Miyabe (2000) have demonstrated
that, in Tachibana Bay, larvae were retained only in the southern part of the
bay, being flushed out of the northern part of the bay westward to the East
China Sea (Fig. 7.5). Because of the longer planktonic period for N. harmandi
(3-4 weeks), those originating from the local populations other than the
Tomioka Bay sand flat and its neighbours would eventually be entrained by
the lower-salinity water mass in the northern Tachibana Bay and hence would
not reach the Tomioka Bay sand flat. Thus the N. harmandi population on the
Tomioka Bay sand flat would be a source rather than a sink in the metapopulation of the Ariake Sound estuarine system.
7.7 Comparisons Between Biogeographic Regions
We have shown here that the bioturbative activities of two species, even
belonging to two different major taxa (Polychaeta and Crustacea), can cause
similar effects on benthic communities composed of different species on
different sides of the world. Both the lugworm and the ghost shrimp provide
good examples of phenomena denoted by Posey's mobility-mode hypothesis.
Both species are relatively large and have a strong impact on sediment
structure by their bioturbating activities producing a similar mountain-like
landscape on the sediment surface, which in turn has a strong influence on the
densities of other species. These affected species are often smaller and/or live
in more or less permanent tubes and the effect of the large biotubators are in
this case always negative and thus good examples of amensalism. For the
lugworm, only negative influences were found in the present study. However,
Reise (1985) also found positive effects of lugworms on meiofauna species.
165
part of each smaller sand flat (Tamaki et aI., in preparation). Densities of 160
or more ghost shrimps per m 2 seemed to be crucial to inhibit the recruitment
success of U. moniliferum. This density might well explain the recent recovery
of the gastropod population on the Tomioka Bay sand flat, especially at Stn
160 (Figs. 7.6, 7.7). On four of the five relatively large sand flats where the U.
moniliferum populations remained on the east coast of Amakusa-Shimoshima Island, the expansion of N. harmandi was not complete and a fairly
large part of each sand flat was left as a refuge for the U. moniliferum recruitment (Tamaki et aI., in preparation). Umbonium moniliferum larvae have a
short planktonic period (within 2 days? cf. Berry 1986), so that larvae released
from the east coast of the island could swiftly come to the Tomioka Bay sand
flat by riding on a strong ebb tide current which can attain speeds as high as
3.6 m/s at spring tides (Fig. 7.5). Given the larger population size of U. moniliferum on the Tomioka Bay sand flat recorded in 1979, the above source-sink
relationship would be reciprocal. This relationship is in stark contrast to the
situation for N. harmandi. Tamaki and Miyabe (2000) have demonstrated
that, in Tachibana Bay, larvae were retained only in the southern part of the
bay, being flushed out of the northern part of the bay westward to the East
China Sea (Fig. 7.5). Because of the longer planktonic period for N. harmandi
(3-4 weeks), those originating from the local populations other than the
Tomioka Bay sand flat and its neighbours would eventually be entrained by
the lower-salinity water mass in the northern Tachibana Bay and hence would
not reach the Tomioka Bay sand flat. Thus the N. harmandi population on the
Tomioka Bay sand flat would be a source rather than a sink in the metapopulation of the Ariake Sound estuarine system.
7.7 Comparisons Between Biogeographic Regions
We have shown here that the bioturbative activities of two species, even
belonging to two different major taxa (Polychaeta and Crustacea), can cause
similar effects on benthic communities composed of different species on
different sides of the world. Both the lugworm and the ghost shrimp provide
good examples of phenomena denoted by Posey's mobility-mode hypothesis.
Both species are relatively large and have a strong impact on sediment
structure by their bioturbating activities producing a similar mountain-like
landscape on the sediment surface, which in turn has a strong influence on the
densities of other species. These affected species are often smaller and/or live
in more or less permanent tubes and the effect of the large biotubators are in
this case always negative and thus good examples of amensalism. For the
lugworm, only negative influences were found in the present study. However,
Reise (1985) also found positive effects of lugworms on meiofauna species.