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Size distribution of the surfing gastropod Olivella semistriata along a cross-shore gradient in an Ecuadorian sandy beach
Vanagt, T.; Vincx, M.; Degraer, S. (2007). Size distribution of the surfing gastropod Olivella semistriata along a cross-shore gradient in an Ecuadorian sandy beach, in: Vanagt, T. De rol van swash in de ecologie van macrofauna op Ecuadoriaanse zandstranden, met speciale aandacht voor de surfende gastropode Olivella semistriata = The role of swash in the ecology of Ecuadorian sandy beach macrofauna, with special reference to the surfing gastropod Olivella semistriata. pp. 95-112
In: Vanagt, T. (2007). De rol van swash in de ecologie van macrofauna op Ecuadoriaanse zandstranden, met speciale aandacht voor de surfende gastropode Olivella semistriata = The role of swash in the ecology of Ecuadorian sandy beach macrofauna, with special reference to the surfing gastropod Olivella semistriata. PhD Thesis. Universiteit Gent. Faculteit Wetenschappen. Vakgroep Biologie: Gent. XVII, 289 pp., more

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    VLIZ: Open Repository 125756 [ OMA ]

Keywords
    Beaches; Ecological zonation; Surfing; Wave runup; Olivella semistriata (Gray, 1839) [WoRMS]; ISE, Ecuador [Marine Regions]; Marine

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Abstract
    The gastropod Olivella semistriata is by far the most dominant taxon on mesotidal, intermediate sandy beaches in Ecuador. Its surfing behaviour, in which the snails follow the swash with the tide, makes it very difficult to determine the distribution pattern of this species. In this paper, transect sampling of the swash at different stages in the tidal cycles and at both upcoming and falling tide was used to obtain swash and crossshore zonation patterns of O. semistriata. The swash zonation showed a uniform unimodal curve, independent of the tidal state and of ebb sampling or flood sampling. Maximum densities were typically found in the upper half of the swash zone. Although the swash zonation pattern did not change towards the low intertidal, densities dropped dramatically. A large part of the population, mainly small individuals, remained stranded on the dry intertidal. Hence, there was a clear increase in shell length towards the subtidal, with only animals larger than 4 mm on the low beach. The same trend of increasing length was also found within the swash: the closer to the surf, the larger the snails. Along with density data, biomass distribution was calculated. This allowed for population size estimation in terms of both abundance and biomass. Therefore, two measures were compared: IST (individuals per strip transect) and IPE (integration population size estimation). Although no significant differences were found, IPE is proposed as the more precise approach. With abundance values of up to 96257 ind./m beach length, Olivella semistriata seems to be an extremely abundant species for sandy beaches.

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