5.3 Settlement and Dispersal of Macrofauna
143
observed, with no or only a short slack water around high tide. In these periods the
abundances of several macrofauna species (Crangon crangon, L. conchilega,
U. poseidonis, C. capitata) showed a short-term reduction. This reaction was observed with a delay of about 1-2 days and was possibly due to deposit-feeders
responding to a short-term shortage of food related to the changed hydrodynamic
conditions. Specific hydrodynamic parameters such as current velocities as well as
tidal levels at slack tide and inundation periods could not be related to the occurrence of driftfauna in the water column (Steuwer, Kaiser unpublished data). But a
link between the number of abundant species and the residual current was assumed
to exist. In one case in May 1994 the vector of the residual current at the Groninger
Plate (GP3) pointed westwards, affecting the net transport direction of the animals
in the water column.
For the import experiments carried out in 1995, azoic sediments were exposed
either for one day or one week. Comparable to the results from driftfauna studies
these experiments indicate that a part of the variability in macrofauna abundance is
caused by mobility. Unfortunately, this part could not be quantified due to the
generally low abundance of macrofauna in 1995. Plots exposed for one week were
not colonized in higher abundances as plots exposed for one day. As the in situ
staining experiments this implies a high turnover rate of benthic macrofauna at the
study site.
5.3.6
Lanice conchilega
L. conchilega is a polychaete species with separate sexes. In its life history this
species passes through two larval phases, of which the last one, the so-called auloph ora larva, lives about 4-6 weeks in the water column (Kessler 1963). The hypothesis that the distribution of larvae at high tide above the Groninger Plate determines the distribution of juveniles in the sediment was falsified by the field
data. The larvae showed a random distribution pattern, whereas the distribution of
juveniles was determined by the distribution of tubes of adult L. conchilega.
Experimental results as well as data from field studies revealed that in the Wadden Sea the existence of hard substrate, preferentially tubes of adults is a requirement for initial settlement of L. conchilega larvae (Heuers 1998). Thus,
L. conchilega is one of the few dominant benthic organisms in the Wadden Sea
settling on hard substrate. The preferred settlement sites are adult tubes, tube-like
structures as they were used in the experiments and, to a small extent, shells (Heuers et a!. 1998). As after the ice winter 1995/96 initial settlement took place in the
absence of adults in the intertidal or even shallow subtidal, intraspecific settlement
cues are not in operation in this species in contrast to the tube-building polychaete
Phragmatopoma californica. Adults of this species release chemical cues which
are of importance in initiating settlement behaviour and metamorphosis of the
larvae (Pawlik 1986).
The distribution of juvenile L. conchilega was determined by a combination of
settling behaviour of the larvae and the small and large scale distribution of adults.
Modifications were due to varying abundances of adults and variations in local
hydrodynamics (Heuers, 1998). Harvey & Bourget (1995) observed that in a dense
tube lawn the current velocity has to reach a critical value upon which turbulence
143
observed, with no or only a short slack water around high tide. In these periods the
abundances of several macrofauna species (Crangon crangon, L. conchilega,
U. poseidonis, C. capitata) showed a short-term reduction. This reaction was observed with a delay of about 1-2 days and was possibly due to deposit-feeders
responding to a short-term shortage of food related to the changed hydrodynamic
conditions. Specific hydrodynamic parameters such as current velocities as well as
tidal levels at slack tide and inundation periods could not be related to the occurrence of driftfauna in the water column (Steuwer, Kaiser unpublished data). But a
link between the number of abundant species and the residual current was assumed
to exist. In one case in May 1994 the vector of the residual current at the Groninger
Plate (GP3) pointed westwards, affecting the net transport direction of the animals
in the water column.
For the import experiments carried out in 1995, azoic sediments were exposed
either for one day or one week. Comparable to the results from driftfauna studies
these experiments indicate that a part of the variability in macrofauna abundance is
caused by mobility. Unfortunately, this part could not be quantified due to the
generally low abundance of macrofauna in 1995. Plots exposed for one week were
not colonized in higher abundances as plots exposed for one day. As the in situ
staining experiments this implies a high turnover rate of benthic macrofauna at the
study site.
5.3.6
Lanice conchilega
L. conchilega is a polychaete species with separate sexes. In its life history this
species passes through two larval phases, of which the last one, the so-called auloph ora larva, lives about 4-6 weeks in the water column (Kessler 1963). The hypothesis that the distribution of larvae at high tide above the Groninger Plate determines the distribution of juveniles in the sediment was falsified by the field
data. The larvae showed a random distribution pattern, whereas the distribution of
juveniles was determined by the distribution of tubes of adult L. conchilega.
Experimental results as well as data from field studies revealed that in the Wadden Sea the existence of hard substrate, preferentially tubes of adults is a requirement for initial settlement of L. conchilega larvae (Heuers 1998). Thus,
L. conchilega is one of the few dominant benthic organisms in the Wadden Sea
settling on hard substrate. The preferred settlement sites are adult tubes, tube-like
structures as they were used in the experiments and, to a small extent, shells (Heuers et a!. 1998). As after the ice winter 1995/96 initial settlement took place in the
absence of adults in the intertidal or even shallow subtidal, intraspecific settlement
cues are not in operation in this species in contrast to the tube-building polychaete
Phragmatopoma californica. Adults of this species release chemical cues which
are of importance in initiating settlement behaviour and metamorphosis of the
larvae (Pawlik 1986).
The distribution of juvenile L. conchilega was determined by a combination of
settling behaviour of the larvae and the small and large scale distribution of adults.
Modifications were due to varying abundances of adults and variations in local
hydrodynamics (Heuers, 1998). Harvey & Bourget (1995) observed that in a dense
tube lawn the current velocity has to reach a critical value upon which turbulence