164
E. Flach and A. Tamaki
7.6.2 Effects on Invertebrates
The expansion of N. harmandi over the Tomioka Bay sand flat also caused a
change in the distribution and abundance of most macrofauna. Regular
sampling of macrobenthos was carried out annually along the transect
(Fig. 7.6) during the end of July to mid-August. Every 20 m, a 25 x 25 cm
quadrat was sampled to a depth of 10 cm, and sieved through a 0.5-mm mesh
sieve. One of the most dramatic effects from 1979 onwards was the steady
decline of U. moniliferum from 2000 m- 2 to 0 in 1986 (Tamaki 1994; Fig. 7.7b).
It is likely that newly settled U. moniliferum larvae were buried and
smothered under the sediment deposited by ghost shrimps (Fig. 7.6). Interestingly, following the decline of the N. harmandi population from 1995, the
U. moniliferum population showed signs of recovery. Subsequent to the
extinction of U. moniliferum, nine of its associated species (those with check
marks in Fig. 7.6) also disappeared. In particular, those species using empty
shells suffered from the deep burial of shells by ghost shrimps.
The effects of the ghost shrimp bioturbation on the smaller-sized macrobenthos were both positive and negative. The cirolanid isopod, Eurydice
nipponica, which was confined to the Nihonotrypaea zone in 1979 (Fig. 7.6),
increased its distribution range and density over the entire sand flat in 1984
(Tamaki and Suzukawa 1991), but the population has declined since 1995 (A.
Tamaki, unpublished data). As the primary habitat of E. nipponica is exposed
sandy beaches facing the East China Sea, the isopod on the Tomioka Bay sand
flat appeared to have benefited from the destabilised substrate conditions
created by ghost shrimps. In contrast, sediment extrusion by ghost shrimps
caused active flight behaviour in the mobile, opheliid polychaete, Armandia
amakusaensis, by disturbing its surface deposit-feeding behaviour (Tamaki
1988), resulting in its much lower densities in the Nihonotrypaea zone in 1979
and 1980 (Tamaki 1985a,b). Since 1995, this polychaete has declined, probably partly due to stingray disturbance (A. Tamaki, unpublished data). The
abundance of the tube-building, spionid polychaete, Pseudopolydora paucibranchiata, which was initially the most dominant macrofaunal species,
occurring exclusively in the Solen to uppermost Umbonium zones (Tamaki
and Kikuchi 1983; Tamaki 1985a), has been quite low since 1984 (A. Tamaki,
unpublished data). Most probably, the destruction of the substrate surface
would have inhibited the establishment of the polychaete's tubes. The recent
slight recovery of the U. moniliferum population on the Tomioka Bay sand flat
(Fig. 7.7b) apparently owed its larval sources to the five extant local populations scattered along the east coast of Amakusa-Shimoshima Island (the sand
flats with 'u' marks in Fig. 7.5; Tamaki et aI., in preparation). The four local
populations of U. moniliferum in Ariake Sound became extinct (the sand flats
with 'X' marks in Fig. 7.5), and two of these cases were probably caused by the
population explosion of N. harmandi, which had colonised a considerable
E. Flach and A. Tamaki
7.6.2 Effects on Invertebrates
The expansion of N. harmandi over the Tomioka Bay sand flat also caused a
change in the distribution and abundance of most macrofauna. Regular
sampling of macrobenthos was carried out annually along the transect
(Fig. 7.6) during the end of July to mid-August. Every 20 m, a 25 x 25 cm
quadrat was sampled to a depth of 10 cm, and sieved through a 0.5-mm mesh
sieve. One of the most dramatic effects from 1979 onwards was the steady
decline of U. moniliferum from 2000 m- 2 to 0 in 1986 (Tamaki 1994; Fig. 7.7b).
It is likely that newly settled U. moniliferum larvae were buried and
smothered under the sediment deposited by ghost shrimps (Fig. 7.6). Interestingly, following the decline of the N. harmandi population from 1995, the
U. moniliferum population showed signs of recovery. Subsequent to the
extinction of U. moniliferum, nine of its associated species (those with check
marks in Fig. 7.6) also disappeared. In particular, those species using empty
shells suffered from the deep burial of shells by ghost shrimps.
The effects of the ghost shrimp bioturbation on the smaller-sized macrobenthos were both positive and negative. The cirolanid isopod, Eurydice
nipponica, which was confined to the Nihonotrypaea zone in 1979 (Fig. 7.6),
increased its distribution range and density over the entire sand flat in 1984
(Tamaki and Suzukawa 1991), but the population has declined since 1995 (A.
Tamaki, unpublished data). As the primary habitat of E. nipponica is exposed
sandy beaches facing the East China Sea, the isopod on the Tomioka Bay sand
flat appeared to have benefited from the destabilised substrate conditions
created by ghost shrimps. In contrast, sediment extrusion by ghost shrimps
caused active flight behaviour in the mobile, opheliid polychaete, Armandia
amakusaensis, by disturbing its surface deposit-feeding behaviour (Tamaki
1988), resulting in its much lower densities in the Nihonotrypaea zone in 1979
and 1980 (Tamaki 1985a,b). Since 1995, this polychaete has declined, probably partly due to stingray disturbance (A. Tamaki, unpublished data). The
abundance of the tube-building, spionid polychaete, Pseudopolydora paucibranchiata, which was initially the most dominant macrofaunal species,
occurring exclusively in the Solen to uppermost Umbonium zones (Tamaki
and Kikuchi 1983; Tamaki 1985a), has been quite low since 1984 (A. Tamaki,
unpublished data). Most probably, the destruction of the substrate surface
would have inhibited the establishment of the polychaete's tubes. The recent
slight recovery of the U. moniliferum population on the Tomioka Bay sand flat
(Fig. 7.7b) apparently owed its larval sources to the five extant local populations scattered along the east coast of Amakusa-Shimoshima Island (the sand
flats with 'u' marks in Fig. 7.5; Tamaki et aI., in preparation). The four local
populations of U. moniliferum in Ariake Sound became extinct (the sand flats
with 'X' marks in Fig. 7.5), and two of these cases were probably caused by the
population explosion of N. harmandi, which had colonised a considerable