Competitive Bioturbators in the Wadden Sea and Ariake Sound
161
could be subdivided into four zonal assemblages, parallel to the shoreline
(Tamaki and Kikuchi 1983; Fig. 7.6). They were identified according to their
most characteristic species, including N. harmandi and the epibenthic filterfeeding trochid gastropod, Umbonium (Suchium) moniliferum. On one representative transect, the Nihonotrypaea and Umbonium zones occupied the
upper one-third and lower half parts, respectively. Subsequent studies
revealed that biological interactions either associated with or incompatible
with N. harmandi and/or U. moniliferum were responsible for the determination of the distribution of several dominant species (e.g. Tamaki 1985a,b,
1987,1988,1994; Tamaki and Suzukawa 1991; Tamaki et ai. 1992).
From 1979, the distribution range of N. harmandi expanded seaward, and,
by summer 1983, the entire sand flat was densely populated by the ghost
shrimp (Tamaki 1994), which subsequently caused considerable bioturbation
effects on both the sediment and the benthic community. A population
explosion of both N. harmandi and N. japonica occurred also on several other
sand flats in the present estuarine system in these 20 years (indicated by
double circles in Fig. 7.5). From 1984, on the Tomioka Bay sand flat, although
no reduction in the ghost shrimp distribution range was observed, its population density varied.
Based on Tamaki and Ingole (1993) and Tamaki et ai. (1997; in preparation), the change in the population density of N. harmandi on the Tomioka
Bay sand flat over the course of 20 years (mostly in July or August each year)
is summarised in Fig. 7.7a. Three stations were located along the transect,
with each station number identical to the distance from the shoreline (Stns 60,
160, and 260 in Fig. 7.6). For the ghost shrimp collection, a corer of a 100-cm2
unit-area was used, with 16-20 (in most cases) sediment columns to the base
layer taken per station. Only adult members of the population are considered,
and juveniles recruited in the same year are excluded from the present
analysis. Accompanying the completion of the distribution expansion from
1980 to 1984, the mean density at Stn 60 increased by a factor of 3.6. After
1984, for a period of 10 years, the population density at each station was fairly
stable, with the mean numbers per 100 cm 2 of 10-14 at Stn 260,4-10 at Stn 160
(except in 1989), and 4-6 at Stn 60. Since 1995, the densities at Stns 260 and
160 have declined to 1.6-3.2/100 cm 2 in 1998, while that at Stn 60 has remained
more or less constant.
For the three phases of the N. harmandi population dynamics on the
Tomioka Bay sand flat (explosion, stability, and decline), several explanations
have been proposed concerning their driving forces (Tamaki et ai. 1992, 1997;
Tamaki et aI., in preparation; Tamaki and Ingole 1993). The population
explosion phase involved: (1) the colonisation of a new zone seaward of the
original habitat by emigrant adults prior to larval settlement period, and (2)
the highest settlement of post-larvae in that zone, followed by far better
survival of those newly recruited juveniles than those that settled in the zone
161
could be subdivided into four zonal assemblages, parallel to the shoreline
(Tamaki and Kikuchi 1983; Fig. 7.6). They were identified according to their
most characteristic species, including N. harmandi and the epibenthic filterfeeding trochid gastropod, Umbonium (Suchium) moniliferum. On one representative transect, the Nihonotrypaea and Umbonium zones occupied the
upper one-third and lower half parts, respectively. Subsequent studies
revealed that biological interactions either associated with or incompatible
with N. harmandi and/or U. moniliferum were responsible for the determination of the distribution of several dominant species (e.g. Tamaki 1985a,b,
1987,1988,1994; Tamaki and Suzukawa 1991; Tamaki et ai. 1992).
From 1979, the distribution range of N. harmandi expanded seaward, and,
by summer 1983, the entire sand flat was densely populated by the ghost
shrimp (Tamaki 1994), which subsequently caused considerable bioturbation
effects on both the sediment and the benthic community. A population
explosion of both N. harmandi and N. japonica occurred also on several other
sand flats in the present estuarine system in these 20 years (indicated by
double circles in Fig. 7.5). From 1984, on the Tomioka Bay sand flat, although
no reduction in the ghost shrimp distribution range was observed, its population density varied.
Based on Tamaki and Ingole (1993) and Tamaki et ai. (1997; in preparation), the change in the population density of N. harmandi on the Tomioka
Bay sand flat over the course of 20 years (mostly in July or August each year)
is summarised in Fig. 7.7a. Three stations were located along the transect,
with each station number identical to the distance from the shoreline (Stns 60,
160, and 260 in Fig. 7.6). For the ghost shrimp collection, a corer of a 100-cm2
unit-area was used, with 16-20 (in most cases) sediment columns to the base
layer taken per station. Only adult members of the population are considered,
and juveniles recruited in the same year are excluded from the present
analysis. Accompanying the completion of the distribution expansion from
1980 to 1984, the mean density at Stn 60 increased by a factor of 3.6. After
1984, for a period of 10 years, the population density at each station was fairly
stable, with the mean numbers per 100 cm 2 of 10-14 at Stn 260,4-10 at Stn 160
(except in 1989), and 4-6 at Stn 60. Since 1995, the densities at Stns 260 and
160 have declined to 1.6-3.2/100 cm 2 in 1998, while that at Stn 60 has remained
more or less constant.
For the three phases of the N. harmandi population dynamics on the
Tomioka Bay sand flat (explosion, stability, and decline), several explanations
have been proposed concerning their driving forces (Tamaki et ai. 1992, 1997;
Tamaki et aI., in preparation; Tamaki and Ingole 1993). The population
explosion phase involved: (1) the colonisation of a new zone seaward of the
original habitat by emigrant adults prior to larval settlement period, and (2)
the highest settlement of post-larvae in that zone, followed by far better
survival of those newly recruited juveniles than those that settled in the zone