Littorina neglecta Bean, a morphological form within the variable species Littorina saxatilis (Olivi)?

Abstract

The morphological variation of Littorina saxatilis and L. neglecta on a microscale (vertical transects down a shore) and on a geographical scale (Northeastern Atlantic) was examined to see if the sampled snails could be consistently separated into two groups on morphological criteria. Size, shell form and shell banding pattern, subopercular pattern and size at maturity were recorded for 21 samples from different shore levels (barnacle zone -usual habitat of L. neglecta, low littoral fringe and high littoral fringe -usual habitat of L. saxatilis) in Iceland, Norway, Sweden, Isle of Man and Wales. A multivariate approach (principal component analysis) was applied to analyse 14 quantitative shell characters. Between-shore variation in shell shape of samples from similar levels was generally larger than within-shore variation of samples from different levels, while size of snails was consistently smaller in barnacle zone compared to littoral fringe samples. The frequency of snails with qualitative characters, used in earlier studies to define L. neglecta, differed between samples from the littoral fringe and the barnacle zone at each site (except at the Swedish site), but the differences between geographical areas were generally larger. The results indicate that snails fitting earlier descriptions of L. neglecta are present in the barnacle zone in Iceland, Norway, Isle of Man and Wales, but that this form is not clearly distinguishable from L. saxatilis. Snails with intermediate shapes were common and, furthermore, the qualitative and the quantitative characters used to define L. neglecta were not closely coupled. No snails of L. neglecta form were found at the Swedish site, yet shape differences over the intertidal gradient were greatest at this site.The extensive morphological variation found in L. saxatilis, between shores and between microhabitats on the same shore, is likely to be due to low gene-flow within the species and a heterogeneous environment. The area over which mating can be random is probably small, thus a subpopulation will behave as a semi-isolate with more or less unique ecological and evolutionary factors influencing its detailed morphology.

Reconstructing past biological invasions: niche shifts in response to invasive predators and competitors

“…First, transplant experiments in New England have shown that, in the absence of competition from L. littorea, L. saxatilis grows 6 · faster in the low intertidal than in the high intertidal where it is most abundant (Behrens Yamada and Mansour 1987), suggesting that L. saxatilis could have occupied a more extensive tidal range in the absence of L. littorea. Second, in the northern part of its range where L. littorea and C. maenas do not occur, higher densities of L. saxatilis extend to the middle intertidal (Johannesson and Johannesson 1990;Reid 1996, p. 326) and subtidal (Reid 1996, p. 326;Gilkinson and Methven 1991). Third, Ganong (1886) reports that native littorines declined dramatically with the expansion of L. littorea.…”

Strong natural selection causes microscale allozyme variation in a marine snail.

“…It lacks a pelagic larva and young are produced as tiny benthic snails in the immediate vicinity of the female. This and an adult vagility in the range of a few meters per 3 months (25) suggest a generally low dispersal of members of this species, although occasionally long-distance migration takes place (26).…”

“…The morphological variation of this population has been documented and, as expected due to slow dispersal and a varying habitat, there is a slight gradual change in, for example, spire height, aperture size, and shell thickness from the surf to the splash zone (27). We analyzed five polymorphic allozyme loci-i.e., mannose-6-phosphate isomerase (Mpi; EC.…”

Morphological variation in Lacuna parva (Gastropoda: Littorinidae) from different European populations

“…6) and shell weight was used to quantify and illustrate the differences between the populations (Reyment et al 1984). The PCA analysis summarises the data by transforming them into an uncorrelated set of variables (principal components), arranged in decreasing order of explained variation (Johannesson and Johannesson 1990). A two-dimensional plot of the first two principal components, which accounts for most of the variation, offers a ready interpretation of differences between specimens (Sundberg 1988).…”

“…The measurements not indicated on the figure are abapertural edge (ae), width of penultimate whorl (sw 2 ), first whorl (apex) width and height (sw 1 and sh 1 ), umbilical "lip" thickness (ul) and weight (we). The measurements are partly modified from Johannesson and Johannesson (1990) characters, e.g. in having spire whorls with deep sutures, a relatively high second spire whorl, very thin shells (and small lip thickness), and a broad umbilical groove.…”