At the end of the Pointe des Minimes, in the southwestern end of La Rochelle, Charente-Maritime, France, a limestone layer advances at very little depth far from the shore. The tidal flow coming from the south encounters the stream produced by the river's channel off the harbour of La Rochelle. A very muddy water gently pushed from the north-east is thus brewed by the clearer waters from the south. The mixing area lodges a true intertidal filtering reef.

CARDIIDAE
HIATELLIDAE
MYTILIDAE
MYTILIDAE
MYTILIDAE
OSTREIDAE
PECTINIDAE
PETRICOLIDAE
PHOLADIDAE
VENERIDAE

Cerastoderma edule (Linnaeus, 1758)
Hiatella arctica (Linnaeus, 1767)
Mytilus edulis Linnaeus, 1758
Modiolus modiolus (Linnaeus, 1758)
Modiolus barbatus (Linnaeus, 1758)
Crassostrea gigas (Thunberg, 1793)
Mimachlamys varia (Linnaeus, 1758)
Petricola lithophaga (Retzius, 1786)
Pholas dactylus Linnaeus, 1758
Tapes philippinarum (Adams & Reeve, 1850)

common
rare
common
uncommon
uncommon
abundant
uncommon
uncommon
common
common

...Without forgetting the inevitable... Crepidula fornicata.

The big majority of these species lives attached ones on the others (Ostreas), clustered (Mytilidae), crimped in the mass of shells (Chlamys, Modiolus). The remainder lives under stones (Ostreidae, Cardiidae, Veneridae), or in stones and shells (Pholadidae, Petricolidae).

The shells are a little more rounded than most of those fished in Quiberon bay, with fewer but more pronounced ribs, and no or very few little spines which, when they exist, are concentrated near the margins. All the shells, without exception, have the same shape, colours and type of patterns:

A quite dark purplish brown, sometimes spotted of light caramelized brown on the ribs at the end of some of the scales, with a pattern of white blotches more present near the umbos than elsewhere.

Live collected in a mass of mussels and oysters, on top of a big rock, at extreme low tide. 52mm.
The shells which are sold on the fismarket of La Rochelle are similar to these samples, even if they are dredged offshore. I've never met any brown, all white or orange specimen. They range around this average form up to atra and down close to a less or more pronounced amelanism, but never pure white.