Environmental influences upon the expression of shell form in Rissoa parva (da Costa). [Gastropoda: Prosobranchia]

Abstract

The classification of the Rissoidae has been, and continues to be, based largely upon shell characters. In a group of small animals of variable form this has resulted in a state of considerable taxonomic confusion. Ponder (1965, 1967, 1968), in his works on the New Zealand Rissoidae, attempted to clarify, what he termed, the existing ‘pigeon-hole classification’ in the light of more detailed anatomical investigation. He observed (Ponder, 1967) that ‘many wrong assumptions can be drawn from the study of the shell alone’. In an earlier work Johansson (1939) had also tried to relate the taxonomy of the Rissoidae to anatomical features.

GROWTH AND SHELL VARIATION IN THE TROPICAL EASTERN PACIFIC INTERTIDAL GASTROPOD GENUSPURPURA:ECOLOGICAL AND EVOLUTIONARY IMPLICATIONS

“…muricids, cassids, cymatiids) (MacKenzie, 1961;Laxton, 1970;Spight, 1973;Spight et al, 1974). Rissoa parva exhibits a seasonal tendency to alternate between ribbed and smooth shell morphs (Wigham, 1975). In other snails growth ceases and a terminal morphology develops that typically includes increased laminar deposition forming a thickened apertural lip.…”

“…Variation in shell sculpturing and morphology has been postulated to be a response to one or more environmental conditions (Moore, 1934(Moore, , 1936Phillips el al, 1973;Spight, 1973;Kitching and Lockwood, 1974;Wigham, 1975;and others). Evidence has been found for some species to indicate direct genetic control of variation and that patterns of distribution reflect differential mortality or habitat selection (Struhsaker, 1968).…”

Ecology and systematics of the north European species of Rissoa and Pusillina (Prosobranchia: Rissoidae)

“…Borja (1986b) reported a life span of 3-4 months for specimens settled in March and 11 months for other specimens in north-west Spain. Fretter & Shale (1973) mentioned that settlement in inshore water had its peaks in March -May and Wigham (1975b) reported peaks of settlement in May, August, and November. This agrees well with personal observation of a slightly later reproduction (May -June) in Scandinavian waters, due to later onset of spring conditions.…”

“…This agrees well with personal observation of a slightly later reproduction (May -June) in Scandinavian waters, due to later onset of spring conditions. Wigham (1975b) considered in a slightly ambiguous statement that 'up to six generations may be produced during the course of one year', but this must mean settlements, which are the result of reproduction in other areas and transport by currents. As far as it is possible to determine, nobody has studied whether both overwintering and non-overwintering females die after reproduction, and the time interval from settlement to reproduction.…”

“…The soft parts of the two forms do not differ in pigmentation. Wigham (1975b) studied this in some detail and reported that the smooth 'interrupta form' was more common in winter and in exposed environments. He could see no reason for separating them.…”