Two images of same individual male Littorina fabalis with penis expanded differently. Two further images of same specimen in image 5aDof 05a Differentiation L. obtusata/L. fabalis . The motile penis of L. fabalis varies in length on any individual unsedated specimen, and the appearance on photographs is affected by positioning/angle of view, so it is advisable to take several observations or photographs. The four views of this specimen show the filament (glandless tip) variously as 38%, 43%, 30% and 50% of total length of penis; all within the accepted limits for L. fabalis, 30% to over 60% . Because of variation on any individual, precise measurements are difficult, but mean figures for dead material graphed by Reimchen (1981) showed that small specimens, like this one, usually had a filament length 30% to 40% of total penis length, while larger specimens with shell height about 13mm, had on average proportionally longer filaments over 60% of penis length.
Full account at: 02 Differentiation L. obtusata/L. fabalis