Graham & Fretter (1947) had a ‘suspicion’ that the leaflets on frond-dwellers are smaller than those on holdfast-dwellers, and that this can be correlated with the greater exposure to water movement of frond dwellers. This seems to be supported by these images that show large leaflets projecting from the shell of a holdfast dweller (1), but none projecting from the shell of a frond dweller (2). But this is insufficient evidence as measurement of live, expanded, gill area is difficult, and individuals can extend or retract the mantle and gills at will.
Concise
Part 1, SHELL FORMS: 02 Patella pellucida. Growth Stage 2 (GS2). July 2012. Littoral. North Yorkshire, England. © P. Lightfoot.
Part 2, BODY & ANATOMY: 03 Patella pellucida. Growth Stage 3 (GS3). 2009. Sublittoral, Farne Islands, Northumberland, England. © P.Lightfoot.
Part, 3 HABITS & ECOLOGY: 04 Patella pellucida. Length 4.6mm. Growth Stage 3 (GS3). September 2016. Yorkshire, England. © P.Lightfoot.