L. obtusata juveniles and L. fabalis adults.
Specimens from a sheltered shore with the most distinctly different forms of the two species, with adults of L. obtusata all far larger than adult L. fabalis.
But interspecific size difference can be removed or reversed when juvenile L. obtusata are compared with adult L. fabalis.
1 to 4:L. obtusata juveniles from distinct Ascophyllum zone.
1: thin juvenile aperture walls drawn into a spout; may also occur on the much smaller juveniles of L. fabalis.
2: citrina colour form, white interior usually forms with maturity as shell thickens.
3: olivacea colour form (dominant colour of L. obtusata on this shore) with purple/brown interior. The only colour that is possibly diagnostic of L. obtusata as absent from, or extremely rare on, L. fabalis in Britain, but beware of algal coating.
4: spire height is unreliable for differentiating the two species as these flat spires are more typical of L. fabalis on adults.
5 to 7: L. fabalis adults from distinct Fucus serratus zone.
5: adult aperture walls thickened and constricting aperture. Not drawn into a distinct spout.
6: citrina colour form with white interior on virtually all adults on this shore; yellow may show on interior before shell fully thickened. Some green algae on shell.
7: low spires of these are higher than flat spires on the juvenile L. obtusata; reversal of more frequent interspecific relationship; spire height is unreliable for differentiating the two species.
Menai Strait, North Wales. February 2011.
Full account at: 02 Differentiation L. obtusata/L. fabalis