Shape variation in the Australian freshwater mussel Alathyria jacksoni Iredale (Bivalvia, Hyriidae)

Abstract

Different forms of the hydriid mussel Alathyriajacksoni occur in riverine habitats of the Murray-Darling river system, south-eastern Australia. Electrophoretic data for three enzymes (glucose phosphate isomerase, malate dehydrogenase and malic enzyme) support the assumption that these are morphological variants of one species. Shells from the Murray in South Australia are elongate-oval, with the dorsal margin extended posteriorly as a blade or 'wing', whereas shells from Victoria and New South Wales have a dorsal arch and a postero-ventral inflection. The variation appears to be associated with water velocity. The arched shells have comparatively large, strong adductor muscles; the posterior adductors are elongate and both anterior and posterior adductors are displaced ventrally. These internal modifications are reflected in changing shell shape as size increases. The changes apparently enhance the mussel's ability to maintain an anchorage in a strong current, but lessen the effectiveness of its valve seal. An incidental observation is that the related species A. condola Iredale, known from the Murrumbidgee and Lachlan rivers, may also occur in the middle reaches of the Murray.

Identification of ecophenotypic trends within three European freshwater mussel species (Bivalvia: Unionoida) using traditional and modern morphometric techniques

“…Adductors are known to play an important role in bivalve burrowing and anchorage (Trueman, 1966a, b, 1968), and larger and therefore stronger adductors would be expected to be more valuable in stronger water movement. Our observation of proportionately larger muscles in slower flowing habitats is clearly in conflict with this theory and other observations (Sell, 1907–1908; Balla & Walker, 1991). It is quite possible that internal anatomy is not independent of shell morphology, and so adductor size might also be affected by the shape of the shell.…”

“…It is quite possible that internal anatomy is not independent of shell morphology, and so adductor size might also be affected by the shape of the shell. However, in contrast to our observations, in the Australian hydriid unionoid Alathyria jacksoni Iredale, 1934, larger, more powerful adductors were found to be associated with dorsal arching (Balla & Walker, 1991). Without experimental data, it is not possible to identify whether adductor size is adaptive in its own right, or whether it reflects physical constraints by shell morphology.…”

“…lotic versus lentic) or mean/maximum current velocities might be determining the shape of the dorso‐posterior margin of unionoids in the River Thames and its marinas. Similar observations have been made in other unionoid families, including Hyriidae (Balla & Walker, 1991), Margaritiferidae, and Carboniferous Anthracosiidae (Eagar, 1948, 1978). These shape differences might have adaptive significance.…”

Disparity pattern of unionoid bivalves from Lake Malawi (East-Africa): a case study for adaptive strategies to heterogeneous environment

“…A lot of work has been done in the past to elaborate on the reasons for this variability. It is consensus that the environment has a major influence on shell shape (Jordan, 1881;Ortman 1920;Haas 1922;Altnaeder 1926;Baker 1928;Agrell 1949;Green 1972;Eagar 1978;Balla & Walker 1991;Savazzi & Peyi 1992;Watters 1994;Walker et al 2001;Scholz 2003). Several key observations have been made describing the shape-environment relation.…”

Comparing size of morphospace occupation among extant and cretaceous fossil freshwater mussels using Elliptical Fourier Analysis

“…Confounding the use of any quantitative shape measure for these purposes is the noted morphological plasticity of unionoids with regard to habitat, leading to repeated convergence in shape of either closely or distantly related unionoid phylogenies (Balla and Walker, 1991;Eager, 1948Eager, , 1974Eager, , 1977Hinch and Bailey, 1988;Watters, 1993;Burton-Kelly, 2013).…”