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Life on the Upper Shore
Living at the Limits
The highest section of the seashore is not guaranteed
a daily visit by the sea and some parts may go many days before the next
brief return of the tide. Despite this, the upper shore is populated mostly
by marine plants and animals that are able to cope with prolonged periods
of drying, rather than by land species able to survive the occasional
salty dowsing.
Why bother trying to survive in such an extreme
environment, why should a seaweed, dependent on its connection with the
sea, choose to live almost beyond its reach?
The answer is simple - all species living on the
shore are struggling against each other for a share in a limited supply
of resources. One of the most significant of these resources is space
amongst all other shore inhabitants. To avoid this struggle, one tactic
is to be tough enough to move up the shore where no other species can
survive, thereby gaining as much space as you need.
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Channel wrack is a good example of this. It can survive
longer out of water that any other seaweed, often looking so black
and shrivelled it could be given up for dead, yet quickly reviving
on the next tide. There are limits to its tolerance - it will not
grow above a certain level on the shore, but at that level it has
the shore itself. |
A little lower down the shore, another seaweed spiral wrack can also survive and is able to outcompete the channel wrack, restricting
it to a narrow band at the top of the shore.Spiral wrack is itself
then replaced lower down by a further brown seaweed and the sequence
continues down the shore, giving rise to the classic zonation pattern.
Each species is living at or near its limits, gaining the slight
advantage necessary to dominate its own particular zone. |
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The upper shore is not well represented at Kimmeridge,
often disappearing under sand or merging into the cliffs, neither of which
are solid enough to support much life. The Flats, beneath the oil
well is a good place to study this zone, and the big advantage
is that you don't need to wait for a good tide.
Much of the rock surface here looks barren at first,
but if you investigate closely, there is life amongst the many cracks
in the rock surface. Small rough winkles and the occasional hardy barnacles gain enough shelter here to survive.
Higher on the shore that the rough winkle,
the aptly named small winkle hides out in crevices on vertical
surfaces. These tiny winkles are marine snails, yet spend most of their
time out of the water. Their gill cavity has developed into an efficient
lung for breathing air.
Rock pools this high on the shore tend to be brackish,
diluted by rain water in between being replenished by the tide, that is
if they are big enough not to dry up altogether in dry weather.
Green seaweeds tend to dominate these pools, but
if you look into some of them very closely you should see tiny specs
darting about, in the water - these are copepods - tiny shrimp-like
animals, relatives of the planktonic copepods that make up an essential
part of the food chain in the open sea.
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If the weather is damp, or if you visit this part
of the shore at night, you could come across surprisingly large
crustaceans, similar to woodlice, scuttling over the rocks. These
are sea slaters, and to find them on dry day you need to
peer into likely nooks and crannies.
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Another crustacean active at night is the sand
hopper, which can be found all along the top of the beach. During
the day they shelter under stones and rotting seaweed, and will spring
in all directions if you disturb them, not stopping until they are out
of the light. |
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