Apertural Lamellae, Epiphragms, and Aestivation of Planorbid Mollusks

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Abstract
Summary The occurrence of six apertural lamellae in Australorbis glabratus and three species of Tropicorbis was studied. In laboratory cultures both lamellate and non-lamellate offspring without intermediate forms, were derived in a single clutch from either a lamellate or non-lamellate parent. In Tropicorbis albicans, Tropicorbis obstructus, and Tropicorbis janeirensis lamellae were formed only once, and associated with the onset of egg laying. In A. glabratus lamellae were associated with a characteristic growth stage long before sexual maturity at an average size of 3.5 mm diameter. By rearing lamellate A. glabratus, it was demonstrated that in some cases the lamellae remained intact while in others they were resorbed leaving only a parietal papilla. Lamellate snails showed a tendency to climb out of water and suspend feeding and growth. Laboratory tests gave no evidence that lamellae aided the snails to resist desiccation. Apertural lamellae were commonly associated with a prominent shell growth ring. Apertural ribs were found to be similarly associated with growth rings in all planorbids studied. It was concluded that apertural lamellae or ribs are supporting structures for the vulnerable aperture during aestivation, or other phenomena involving interruption of shell growth, such as internal changes as sexual maturity. Aestivating planorbid snails produced epiphragms. In A. glabratus, T. albicans, T. obstructus, T. riisei, and Helisoma transparent, water soluble, mucous, “froth” epiphragms were formed; in Drepanotrema simmonsi and D. cimex transparent partitions; and in D. hoffmani Baker opaque, paper-like partitions. Lamellate A. glabratus were not uncommon, involving 25 (50%) of 50 collection localities and 116 (18%) of 640 snails examined. Most of the A. glabratus found aestivating in nature were small (average diameter 3.5 mm), lamellate, and with froth epiphragms. Such snails survived up to 4 months out of water. The simultaneous tendency of certain A. glabratus to form lamellae, climb out of water, produce epiphragms, and aestivate may be of concern in the effective use of molluscicides.
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