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Life on the Mid Shore
Limpets and Ledges
Life in the middle of the shore is
considerably easier that it is for those living higher up, though it is
still a harsh environment for marine creatures, with regular and prolonged
periods of exposure to air.
Animals have developed many ways of coping with
the extreme conditions encountered during the low water period - from
the risk of drying out and overheating during the summer to the possibility
of freezing during a hard winter. The methods of overcoming these problems
vary, but generally fall into one of two categories:
Shut up tight and sit it out
Take refuge somewhere cool and
damp
The best examples of the more stoic approach
are limpets and barnacles - two very different animals
that look superficially alike because of their similar approach
to solving the problems of prolonged exposure to air.
The limpets attachment seems permanent but when
covered by the sea, or perhaps on a cool, damp night, the grip is
relaxed and the limpet crawls over the rock surface, grazing as
it goes on an almost microscopic layer of algae covering the rocks.
The short compressed video below shows the night-time feeding activity of limpets at Kimmeridge Bay at low tide over a 6 hour period. A fascinating look into movements on the rocky shore!
Scratch marks left by the limpet's file-tongue can be clearly seen
on the rock surface, betraying its extensive feeding range.
Before the retreating tide leaves it stranded
the limpet is able to find its way "home" - the one spot
where its shell perfectly fits the underlying rock, thereby reducing
the risk of it drying out or being dislodged. Frequently scars can
be seen on the rocks marking the position of old limpet "homes".
The barnacle, by contrast, is permanently
glued to one spot. Inside the shell is a shrimp-like animal lying
on its back. The top of the shell is fitted with tightly closing
doors which can be opened for feeding when the barnacle is submerged.
Free living barnacle larvae are chemically attracted
to barnacle shells and tend to settle amongst established adults.
This can lead to a very dense covering.
The barnacle relies on the sea to bring it food,
straining out microscopic particles and plankton with its hairy
legs. This is a common way of making a living in the sea, letting
the rich, food-bearing currents come to you rather that hunting
about for something to eat. It explains why there are so many attached
plant-like animals in the sea.
Strangely enough, another animal which closes
up tight and stays put, doesn't even have a protective shell. The beadlet anemone is frequently found alongside limpets on
open patches of rock, yet appears very vulnerable to drying out.
It is, in fact, very hardy. By completely withdrawing its tentacles
until it looks like a blob of coloured jelly, it is able to reduce
the rate of water loss. Secretion of a mucus coating is a further
safeguard and its elastic body allows it to swallow a reservoir
of seawater before the tide retreats.
Looking at these shrivelled, lifeless blobs
on the shore it is difficult to imagine that this is an active and
indeed quite vicious, creature. Not only will it capture live prey
with its stinging tentacles, it will engage in slow motion, but
deadly serious, territorial battles with any other anemones that
wander too close.
- two very different animals
that look superficially alike because of their similar approach
to solving the problems of prolonged exposure to air.
ght and stays put, doesn't even have a protective shell. The beadlet anemone is frequently found alongside limpets on
open patches of rock, yet appears very vulnerable to drying out.
It is, in fact, very hardy. By completely withdrawing its tentacles
until it looks like a blob of coloured jelly, it is able to reduce
the rate of water loss. Secretion of a mucus coating is a further
safeguard and its elastic body allows it to swallow a reservoir
of seawater before the tide retreats.
