Observations on the morphology and behaviour of Chilina fluctuosa Gray (Chilinidae), with a discussion on the early evolution of pulmonate gastropods

Abstract

An account is provided of (i) the general morphology, (ii) the vascular, nervous and feeding systems, and (iii) aspects of the behaviour of a representative species of the pulmonate genus Chilina . Examination of this genus has aided in understanding the pattern of early evolution of freshwater pulmonates from their marine prosobranch ancestors and of their divergence from opisthobranchs. The gross morphology of Chilina is adapted to ploughing through the surface layers of soft substrata. Although C. fluctuosa was found on rocks, it was nevertheless capable of burrowing into sand, using a stereotyped digging cycle. Streamlining to facilitate burrowing was achieved by nuchal shortening and by a ‘detorsion’ of the mantle complex as in opisthobranchs. The nuchal shortening incurred loss of a major vascular pathway which, in monotocardian prosobranchs, drains the head-foot. ‘Detorsion’, however, provided a substitute which incorporates the anterior roof of the mantle cavity where, in Chilina , the precursor of the rich pulmonary plexus characteristic of later pulmonates may be discerned. The suprapallalial vascular system is described in detail. In contrast, it is apparent that increased reliance on burrowing in opisthobranchs brought about loss of the anterior region of the mantle cavity, and thus any potential for development of a plexus in that region. Enclosure of the mantle cavity, which clearly pre-adapted the mantle complex to function also as a ‘lung’, was necessary to reduce clogging of the mantle cavity by particulate material. Only water is held in the cavity of Chilina flutuosa , whose habitat is rivers, but both air and water may be held in the mantle complex of other species found in Chile. The nervous system is extremely similar to that of the primitive opisthrobranch Acteon , but the posterior section of the visceral loop is shorter owing to greater nuchal reduction. The loop is uncrossed, though the supraoesophageal ganglion lies only a little right of the suboesophageal ganglion. The musculature of the buccal mass is extremely similar to that of Lymnaea and Planorbarius , and is designed for moving and tensing a broad radular ribbon. It is argued that the buccal morphology characteristic of these snails evolved to manipulate quantities of particulate material during the early infaunal phase, and only later was used to great effect in both microphagous and macrophagous feeding. Collectively, the observations made on C. flluctuosa (and also on estuarine species found in Chile) support the contention that the Basommatophora invaded freshwater habitats via estuarine niches. Continuity during this progression was apparently provided by soft substrata, and probably by diatoms as a food source. Previous arguments purporting a terrestrial or semi-terrestrial origin for the limnic basommatophorans are refuted.