Competitive Bioturbators in the Wadden Sea and Ariake Sound
151
reworking corresponding to a sediment layer of -6.2 cm (Cadee 1976).At high
lugworm densities, however, this annual sediment reworking can amount to a
layer up to 33 cm. Lugworms also cause sediment instability at the surface,
change the particle composition and stratification, irrigate the anoxic
subsurface sediment with overlying water, release their excretions into the
sediment, and produce large amounts of mucus (Reise 1985).
On the other side of the world, on the tidal flats of Japan, another large
deposit-feeding species is found. The ghost shrimp, Nihonotrypaea harmandi
(Bouvier) (Crustacea: Decapoda: Callianassidae), formerly misidentified as
Callianassa japonica Ortmann, is an important member of the benthic community on shallow estuarine sand flats (Figs. 7.5, 7.6). It lives in a Y-shaped
burrow occupying the entire sediment column (30-60 cm deep), under which
lies an accumulation of large shell remains. Each adult burrow is inhabited by
a single shrimp only. The ghost shrimp extrudes sediment through one
burrow opening, which is deposited as a mound on the sand flat surface; on
average, about 5-10 cm 3 of sediment (3-5-cm diameter mound) per shrimp is
discarded per day.
7.3 Lugworms in the Wadden Sea
The lugworm Arenicola marina is found almost everywhere on the tidal flats
in the Dutch Wadden Sea (Dankers and Beukema 1983) and accounts for
about 20 % of the benthic biomass (Beukema 1976). Densities of adult lugworms are usually of few tens per m 2 ; although locally they may reach densities up to about 100 per m 2 , they generally do not surpass 50 per m 2
(Dankers and Beukema 1983). Adult lugworms show their highest densities at
1-4 km from the shore and are significantly less abundant both at shorter and
longer distances from the coasts. These high densities fall within a zone from
about MTL to about 6 dm below MTL and a silt content of the sediment between -2 to 10 % (Beukema and De VIas 1979). The distribution of lugworms
over the tidal flats is remarkably even and shows very little year-to-year
variation (Beukema et al. 1983). Also, in the German part of the Wadden Sea,
Reise (1985) reports a stable adult population at the tidal flats in Konigshafen.
This stable population size raises the question: how is this maintained? Reise
(1985) reports two mechanisms; firstly a density-dependent mobility of the
adults and, secondly, a migratory pattern of the juveniles in four steps
avoiding physical stress, predation and competition with adults to guarantee
a high survival rate. During the third phase, juvenile lugworms occupy, in very
high densities, the upper tidal zone, the so-called nursery bed or Brutwatten
(Reise 1985). Beukema and De VIas (1979) observed high densities of young
lugworms only on relatively high tidal flats where adults occurred only in low
151
reworking corresponding to a sediment layer of -6.2 cm (Cadee 1976).At high
lugworm densities, however, this annual sediment reworking can amount to a
layer up to 33 cm. Lugworms also cause sediment instability at the surface,
change the particle composition and stratification, irrigate the anoxic
subsurface sediment with overlying water, release their excretions into the
sediment, and produce large amounts of mucus (Reise 1985).
On the other side of the world, on the tidal flats of Japan, another large
deposit-feeding species is found. The ghost shrimp, Nihonotrypaea harmandi
(Bouvier) (Crustacea: Decapoda: Callianassidae), formerly misidentified as
Callianassa japonica Ortmann, is an important member of the benthic community on shallow estuarine sand flats (Figs. 7.5, 7.6). It lives in a Y-shaped
burrow occupying the entire sediment column (30-60 cm deep), under which
lies an accumulation of large shell remains. Each adult burrow is inhabited by
a single shrimp only. The ghost shrimp extrudes sediment through one
burrow opening, which is deposited as a mound on the sand flat surface; on
average, about 5-10 cm 3 of sediment (3-5-cm diameter mound) per shrimp is
discarded per day.
7.3 Lugworms in the Wadden Sea
The lugworm Arenicola marina is found almost everywhere on the tidal flats
in the Dutch Wadden Sea (Dankers and Beukema 1983) and accounts for
about 20 % of the benthic biomass (Beukema 1976). Densities of adult lugworms are usually of few tens per m 2 ; although locally they may reach densities up to about 100 per m 2 , they generally do not surpass 50 per m 2
(Dankers and Beukema 1983). Adult lugworms show their highest densities at
1-4 km from the shore and are significantly less abundant both at shorter and
longer distances from the coasts. These high densities fall within a zone from
about MTL to about 6 dm below MTL and a silt content of the sediment between -2 to 10 % (Beukema and De VIas 1979). The distribution of lugworms
over the tidal flats is remarkably even and shows very little year-to-year
variation (Beukema et al. 1983). Also, in the German part of the Wadden Sea,
Reise (1985) reports a stable adult population at the tidal flats in Konigshafen.
This stable population size raises the question: how is this maintained? Reise
(1985) reports two mechanisms; firstly a density-dependent mobility of the
adults and, secondly, a migratory pattern of the juveniles in four steps
avoiding physical stress, predation and competition with adults to guarantee
a high survival rate. During the third phase, juvenile lugworms occupy, in very
high densities, the upper tidal zone, the so-called nursery bed or Brutwatten
(Reise 1985). Beukema and De VIas (1979) observed high densities of young
lugworms only on relatively high tidal flats where adults occurred only in low