The shell of this common mussel is encrusted with the calcareous tubes of keelworms, which have a prominent ridge, so that they’re triangular in cross section.
The carapace of this shore crab is encrusted with barnacles, these are the barnacle Semibalanus balanoides, which have a diamond-shaped aperture. Between the barnacles at the front of the crab’s shell there’s a flat, pockmarked whitish crust, which looks like sea mat, a marine bryozoan, a colonial animal, filter-feeding from tiny individual cells, like coral.
Like this small frond of bladderwrack seaweed, I picked these up on the beach near the harbour at Bridlington when we spent the day there last month.