Competitive Bioturbators in the Wadden Sea and Ariake Sound
167
Pygospio elegans (E. Flach, unpublished data), and both species were in 1993
restricted to a very small zone close to the shore. It can thus be expected that
an expansion of the lugworm in the Wadden Sea could have a similarly
dramatic effect as that observed for the ghost shrimp in Ariake Sound. The
American lugworm species Abarenicola pacifica also has a negative influence
on Pygospio (Wilson 1981) and Woodin (1985) reports an active avoidance of
the spionid Pseudopolydora of sites inhabited by Abarenicola. Interspecific
interactions can thus be regarded as strongly influencing the benthic community on tidal flats.
Most of the reports on the effect of the bioturbation by callianassid
shrimps on the abundance pattern of other macrobenthos have pointed out
asymmetrical effects, not to mention shrimps harbouring obligate commensals within or close vicinity to their burrows (e.g. Ricketts et al.1985), for
example: (1) amensalism - against filter-feeding bivalves (Peterson 1977;
Murphy 1985), corals (Aller and Dodge 1974), seagrass (Suchanek 1983),
cultured penaeid shrimps (Nates and Felder 1998), and smaller macrobenthos
with limited mobility such as tanaids and spionid polychaetes (Dayton and
Oliver 1980; Posey 1986a); and (2) commensalism - for highly mobile species
such as ostracods (Dayton and Oliver 1980; Riddle 1988), amphipods (Posey
1986a; Riddle 1988; Wynberg and Branch 1994), bivalves (Aller and Dodge
1974; Tudhope and Scoffin 1984), and polychaetes (Riddle 1988; Wynberg and
Branch 1994). The effects on meifauna also seem to be dual (Branch and
Pringle 1987; Dobbs and Guckert 1988; Wynberg and Branch 1994; Dittmann
1996). Although the abundance patterns of these affected species were quite
distinct, some of the relationships with callianassid shrimps seem to be facultative. As regards the biological agents that limit the distribution of callianassid shrimps, only a few papers have been published (by predation, Posey
1986b; through inhibition of burrowing by seagrass, Brenchley 1982; Harrison
1987).
Intraspecific competition in key-species, such as the large bioturbating
lugworm and ghost shrimp, can be important as well. For the lugworm both
adult-adult, juvenile-juvenile and adult-juvenile interactions have been observed (Flach and Beukema 1994). In the case of the adult-adult and juvenilejuvenile interactions there seems to be competition for food (thus real competition, - - interaction Arthur and Mitchell 1989), keeping the adult population at the carrying capacity of a particular site and resulting in low mean
individual weights for juveniles when they occur in high densities (Flach and
Beukema 1994). Adult-juvenile interactions can be regarded as a form of
amensalism; the adults do not seem to be influenced by the juveniles, but the
juveniles are strongly negatively influenced by the adults probably again due
to disturbance by the bioturbating activity of the adults. These intraspecific
interactions seem to be responsible for the stable lugworm population in the
Wadden Sea within the boundaries set by the physical environment (Beukema
167
Pygospio elegans (E. Flach, unpublished data), and both species were in 1993
restricted to a very small zone close to the shore. It can thus be expected that
an expansion of the lugworm in the Wadden Sea could have a similarly
dramatic effect as that observed for the ghost shrimp in Ariake Sound. The
American lugworm species Abarenicola pacifica also has a negative influence
on Pygospio (Wilson 1981) and Woodin (1985) reports an active avoidance of
the spionid Pseudopolydora of sites inhabited by Abarenicola. Interspecific
interactions can thus be regarded as strongly influencing the benthic community on tidal flats.
Most of the reports on the effect of the bioturbation by callianassid
shrimps on the abundance pattern of other macrobenthos have pointed out
asymmetrical effects, not to mention shrimps harbouring obligate commensals within or close vicinity to their burrows (e.g. Ricketts et al.1985), for
example: (1) amensalism - against filter-feeding bivalves (Peterson 1977;
Murphy 1985), corals (Aller and Dodge 1974), seagrass (Suchanek 1983),
cultured penaeid shrimps (Nates and Felder 1998), and smaller macrobenthos
with limited mobility such as tanaids and spionid polychaetes (Dayton and
Oliver 1980; Posey 1986a); and (2) commensalism - for highly mobile species
such as ostracods (Dayton and Oliver 1980; Riddle 1988), amphipods (Posey
1986a; Riddle 1988; Wynberg and Branch 1994), bivalves (Aller and Dodge
1974; Tudhope and Scoffin 1984), and polychaetes (Riddle 1988; Wynberg and
Branch 1994). The effects on meifauna also seem to be dual (Branch and
Pringle 1987; Dobbs and Guckert 1988; Wynberg and Branch 1994; Dittmann
1996). Although the abundance patterns of these affected species were quite
distinct, some of the relationships with callianassid shrimps seem to be facultative. As regards the biological agents that limit the distribution of callianassid shrimps, only a few papers have been published (by predation, Posey
1986b; through inhibition of burrowing by seagrass, Brenchley 1982; Harrison
1987).
Intraspecific competition in key-species, such as the large bioturbating
lugworm and ghost shrimp, can be important as well. For the lugworm both
adult-adult, juvenile-juvenile and adult-juvenile interactions have been observed (Flach and Beukema 1994). In the case of the adult-adult and juvenilejuvenile interactions there seems to be competition for food (thus real competition, - - interaction Arthur and Mitchell 1989), keeping the adult population at the carrying capacity of a particular site and resulting in low mean
individual weights for juveniles when they occur in high densities (Flach and
Beukema 1994). Adult-juvenile interactions can be regarded as a form of
amensalism; the adults do not seem to be influenced by the juveniles, but the
juveniles are strongly negatively influenced by the adults probably again due
to disturbance by the bioturbating activity of the adults. These intraspecific
interactions seem to be responsible for the stable lugworm population in the
Wadden Sea within the boundaries set by the physical environment (Beukema