A reconsideration of epipodial and cephalic appendages in basal gastropods: homologies, modules and evolutionary scenarios

Abstract

Basal gastropods have various types of body appendages. Besides pallial or cephalic tentacles, subocular tentacles, neck lobes and (occasionally) copulatory structures, there are epipodial tentacles and epipodial sense organs (ESOs), which have often been confused in the past. We provide clear definitions of these two different epipodial structures, describe various examples and reconsider literature data on their occurrence throughout basal gastropod clades, i.e. Patellogastropoda, Cocculiniformia, Neritimorpha, Neomphalina and (in particular) Vetigastropoda. So-called 'epipodial tentacles' of Patellogastropoda, Cocculiniformia, Neomphalina and of several vetigastropod subgroups are considered to represent a distinct and apomorphic gastropod organ, the ESO. In contrast, true epipodial tentacles are probably serial or iterative homologues of cephalic tentacles and are restricted to Vetigastropoda and certain caenogastropod taxa. In the light of these new data and interpretations, an evolutionary scenario is presented for epipodial structures in basal gastropods: a single pair of ESOs in a posterior position is considered as an apomorphic character of the gastropod stem lineage, which is retained in early juvenile Patellogastropoda, many Cocculiniformia and early juvenile Vetigastropoda. The various conditions in Neomphalina and Vetigastropoda can be explained by considering modularity of ESOs, cephalic/epipodial tentacles and sensory papillae. Each of these modules has become serially repeated in evolution and occurs in various combinations among basal gastropod clades.