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Rocky Shore Trail
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Plankton - an introduction

Plankton refers to organisms that live in open water and have little or no control over where they go. Fish are not planktonic as they can move and determine which way to go. Animals of open water that have the ability to swim purposefully and powerfully are called nekton . Most plankton is microscopic and requires very fine mesh nets to collect. Marine plankton have, exclusively, whole groups of organisms that are very visible to the naked eye, e.g. Ctenophores (sea gooseberries or comb jellies).

All plankton is divided into the plant-type plankton, the phytoplankton , and the animal plankton, zooplankton . The latter can be further sub-divided into the temporary and permanent zooplankton. The temporary tends to be those which are larval forms of adults living on the seashore, e.g. barnacle larvae called nauplii. Permanent plankton have all stages of the life cycle living in the open water, e.g. copepods.

As planktonic creatures are so small they will get everywhere and will become trapped in rock pools and crevices and phytoplankton will stick to stones and become caught in mud deposits. Look carefully at low tide in small, clear pools and you may see some tiny black dots darting about. These are calanoid copepods that are trapped.

Plankton is a vital part of seashore ecology even though much of it appears to be off-shore. Many of the larval stages live in the plankton but more importantly it is a food resource for creatures living on the shore.


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