Turning over stones in rockpools at low tide is a popular pursuit of holidaymakers, usually hoping to find a crab feverishly scuttling for cover under a mass of seaweed, or a small fish darting in the blink of an eye into some nook or cranny. If only they looked more closely at the rock itself, because some truly strange creatures can be found clinging to its surface.
Bryozoans are tiny colonial animals that live surrounded by a cuticle of resilient material, rather like the rooms in a block of flats. Usually less than a millimetre in length, each individual is known as a zooid. Sometimes the colonies form encrustations on rock surfaces ( hence they are sometimes known as sea mats ), but some species form branching structures often mistaken for seaweed.
Hornwrack: Flustra foliacea
A close-up of a frond of Hornwrack ...
The photographs above show Hornwrack. This is often cast up on the beach in large quantities after a rough sea. It looks for all the world like a piece of dried up seaweed, but it feels quite different. It has a rough, sandpaper-like surface. On closer examination the seperate compartments each bryozoan inhabits can clearly be seen.
A colony of Electra pilosa on a stone
Electra pilosa showing the oval zooecia
This encrusting bryozoan has more oval shaped compartments, or zooecia as they are correctly called, than hornwrack's rectangular ones. All the members of a colony are the progeny of a single individual known as the ancestrula. In its free swimming larval stage the ancestrula chooses a suitable surface on which to settlefor the remainder of its adult life. The growth of the colony occurs by budding.
It goes without saying that every stone that's turned over in any pool should be put back as near as possible into the same position as it was found. Each one is a tiny ecosystem. Rocky shores such as ours on the North Yorkshire coast support a multitude of fascinating and beautiful plants and animals, and they deserve our respect. They've been around a lot longer than we have. Fossil bryozoans are known from rocks 470 million years old. Humans emerged about 650,000 years ago.