Comparison of dissected penes.
1 & 2: penis of L. obtusata.
3 & 4: penis of L. fabalis.
4: mesial face, often held against body when alive, of L. fabalis penis with long vermiform filament (5) measuring 36% of penis length (species range 30% to 60%), and single row of 10 large mamilliform glands (6) along 64% of penis length (species range 3 to 17 glands along 40% to 70% of penis) .
2: mesial face, often held against body when alive, of L. obtusata penis, with short (19% of penis length, species range 10% to 25%), stout, triangular filament (7) and irregular, peripheral row of 12 small mamilliform glands (8), and a secondary row of 8 indistinct, whitish, small mamilliform glands (9); total 20 glands along 81% of penis length (species range 10 to 54 glands along 75% to 90% of penis length).
1 & 3 distal faces of penes, visible when penis is held against body in normal live position; can be difficult to see mamilliform glands and limit of filament on this face, especially on L. obtusata.
Size of penis depends on size of individual male and is not diagnostic of species. On the sheltered Menai Strait most adult male L. fabalis are larger, and so have larger penes, than most male L. obtusata. On more exposed shores in North Wales the sizes of the two species can be almost equal.
Menai Strait, North Wales. February 2011.
Full account at: 02 Differentiation L. obtusata/L. fabalis