The Rensch\'s rule of sizes in littorinids: an ecologist\'s point of view

• Main Authors: Sacchi, Cesare F., Sconfietti, Renato
• Other Authors: Università del Salento - Coordinamento SIBA
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Coordinamento SIBA - Università del Salento 1970
• Subjects: size variations; ecological interpretation; littorinids; Pacific; North Atlantic
• Online Access: http://siba-ese.unile.it/index.php/thalassiasal/article/view/i15910725v23p5; https://doi.org/10.1285/i15910725v23p5

EnAn ecological rule - proposed by Rensch for terrestrial Gastropods - states that intraspecific variations in shell size are related to environmental quality. Maximum sizes are attained at environmental optima. Due to their semi-continental way of life as adults, some examples may be looked for in littorinids.In Littorina obtusata (L.) two main size gradients are apparent, one macroclimatic and the other local: from the Channel southward to the Iberian shores, and from sheltered to exposed sites. Shells from Galicia are smaller than from Brittany and Devon. This is probably related to an aerial climate becoming Mediterranean-like southway, while waters conserve North-Atlantic traits. Local gradients are likely to be the result of biomass reduction and morphological modifications of the sheltering and nourishing patches of the brown seaweeds inhabited by snails. Ecomorphes reflecting kinetic energy variations shall also be considered, as well as a genetic selection expressed by the predominance of morph oliuacea in calm waters versus reticulata in sites fully exposed to waves. L. mariae Sacchi et Rast. follows parallel biogeographic variations.As for local gradients, however, it behaves in a less schematic way. Samples of Melaraphe neritoides (L.) collected in Naples from breakwaters of a different lithological nature show differences in adult sizes and population densities related more to mechanical structure (substratum unevenness) than to limestone content. Hydrodynamic and trophic gradients may account for size in confined environments. In the Venice lagoon-the specific topotypic waters-adult specimens of L. saxatilis (Olivi) on sheltered, very eutrophicated shores, are twice as large in both sexes than in sites with more stirring waters or lower degrees of trophism.In insular Indo-pacific Littoraria size gradients from sheltered to exposed shores were also observed.ItUna regola ecologica, proposta dal Rensch per i Gasteropodi terrestri, afferma che le variazioni intraspecifiche nella mole del guscio sono ...