Strong winds and high tides battering the shorelines of the North-West Highlands this October have washed up a host of small pebbles to ponder. Encrusted with the hard chalky forms of the shells of filter-feeding marine organisms the surfaces of these pebbles appear richly decorated.

Anchored shells, protective living spaces for small organisms with static lifestyles, are located predominantly in the turbulent littoral zones and are built by the larval stages of organisms to anchor themselves to rocks, concrete seawalls, boats, marine litter, and even seaweeds. 

Secreting calcium rich substances they model tiny, calcified homes, in conical, tube and spiral forms. Some, including barnacles, have ‘trap doors’ that close at low tide for protection against predation and water loss.  

Acorn barnacle colonies can cover and change the appearance and surface texture of hundreds and even thousands of square metres of the most exposed shoreline rocks.

For several days I have observed three annelid worms (RHS image) extending their fan-shaped arrangement of tentacles out of their tubes to filter the water to feed.

There are many variations of marine ‘calcified houses’. One of the annelid worms makes longitudinal structures with a top ridge; the shell is up to 20mm in length, roughly triangular in cross section and the segmented worm lives within the inner circular tube.

Magnifying collected seaweed samples on my observation table revealed the tiny calcified spiral shells of the filter feeder Spirorbis spirorbis. Each sinistrally coiled tube is only 2mm in diameter, appearing as tiny white specks on the fronds of wracks, which I have often missed. I now see them as perfect examples of calcified shelters, bespoke marine homes demanding closer inspection and study. Sculpturally inspiring.

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