https://creativecommons.org/publicdomain/zero/1.0/https://www.rawpixel.com/image/4024863
Calcareous red seaweed.
Corallina officinalis is a calcareous red seaweed which grows in the lower and mid-littoral zones on rocky shores.
It is primarily found growing around the rims of tide pools, but can be found in shallow crevices anywhere on the rocky shore that are regularly refreshed with sea water. It predominantly grows on the lower shore, especially where fucoid algae are absent, but is also found further up shore on exposed coasts. Original public domain image from Flickr
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