I believe these are Shanny (Lipophrys pholis), also called Blenny. As the tide pulled out it left these crystal clear pools of water and in them they teemed with these young fish which moved nimbly through the shallow water. They are so well adapted to their environment you would hardly notice them until they moved.
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Shanklin Beach, Isle of White, England, August 2018 © Pete Hillman.
Also called the ‘Common Blenny’, its colouration is variable from dark brown to greenish depending on its surrounding environment. It has no scales but is covered in a protective slime which stops it from drying out during periods when it is out of the water. It has an elongated body with a long single dorsal fin. It has sharp teeth and can bite humans if handled incorrectly. Length up to 13cm.
It feeds on small invertebrates, including the Acorn Barnacle (Semibalanus balanoides). The males guard the eggs until they hatch. It can live up to 15 years.
Found in pools, amongst seaweed and under rocks on the lower shore. Widespread and locally common except in the south-east.
Photographs of juvenile taken August 2015, Meadfoot Beach, Torquay, Devon.