The distribution of the invasive non-native gastropod Crepidula fornicata in the Milford Haven Waterway, its northernmost population along the west coast of Britain

Table 1 Habitat types classified from percentage cover values of the five main substrata types derived from 20 still image pictures taken during the subtidal surveys or five still images taken during the intertidal surveys

Habitat types	Description
Sediment	>80 % sediment
Sediment with gravel	>70 % sediment and 10–30 % gravel
Sediment with shell	>70 % sediment and 10–30 % shell
Boulder	>80 % boulder
Boulder with sediment and gravel	>10 % each
Mix of sediment and gravel	30–70 % sediment and 30–70 % gravel
Mix of sediment and shell	30–70 % sediment and 30–70 % shell
Mix of sediment, gravel, and shell	>10 % each
Gravel*	>80 % gravel
Gravel with boulder*	>60 % gravel and 10–30 % boulder
Mussel bed mixed with sediment, gravel, and shell*	>10 % each

Substrata types were: (1) Sediment (grain sizes up to ~16 mm, i.e. mud, sand, fine, and medium gravel on the Wentworth scale); (2) Gravel (grain sizes ~16–256 mm, i.e. coarse gravel and cobble); (3) Boulder (grain size >256 mm); (4) Shells (all empty shells); and (5) Live habitat-forming species (live Mytilus edulis, Ostrea edulis, and scallops). Asterisks indicate habitat types that were only present at intertidal sites and are therefore not displayed on the map (Fig. 3)

ISSN: 1438-3888