Pictures and text by Erling Svensen
Life under the Eger Island brigde
At the Eger Island brigde the channels is 100 m wide. The normal tide
range is only 10-15 cm, but wind can pile up the sea along the coast.
The channel under the bridge is 7 m deep so the current can be
ectremely fierce. Nonetheless the seafloor has dense populations of
the horse mussel (Modiolus modiolus) and the edible mussel (Mytilus
edulis). These bivalves anchor to the substrate so they stay in one
place. They are filter feeders so, for them, living a current is an
advantage as more food is constantly available. There are outcrops of
rock that support groups of the plumose anomone (Metridium senile),
which feeds on small animals it traps with its tentacles. The largest
intertidal anemone - dahlia (Urticina felina) - lives here as well.
Although brightly coloured, with a pattern of red and grey marks on
the tentacles it does not have as many colour forms as the plumose
anemone. Being larger the dahlia anemone catches larger prey such as
crustaceans and fish.
At the edges of the channel, where the current is weaker, there are
larger populations of the whelk (Buccinum undatum) and hermit crabs.
These species thrive in each otheres company because when a whelk dies
a hermit crab will take over the empty shell. The hermit crab needs to
protect its soft abdomen and as it grows it needs to change house. So
when it finds a larger whelk house it moves in. This is not always
easy and hermit crabs that have outgrown their current home are
sometimes found.
Directly under the bridge where the current is most powerful colonies
of dead mans fingers (Alcyonium digitatum) have colonized the few
scattered rocks. This coelenterate can live in biologically polluted
waters such as those found in harbours and docks. These false (alcyonarian)
corals exist in white or yellow forms. It is worth noting that within
a colony all the poplyps are doing the same thing at the same time.
Either all are out filtering the water or all are retracted and
resting.
At the beginning of each year the cod come in under the bridge to
spawn. The coastal populatins prefer to spawn in brackish water and
for several weeks before the spawning you can see large numbers of cod
seeking refuge in the few stoneheaps under the bridge. Here they wait
until they are ready to spawn and one female may discharge up to 5
million eggs.
