Multiple approaches to the taxonomical status of Cardium edule and C. glaucum

Nossier, Musa Attia (1980) Multiple approaches to the taxonomical status of Cardium edule and C. glaucum. University of Southampton, Doctoral Thesis.

Abstract

Ecological, morphological, physiological, immunological and electrophoretical studies have been carried out in this investigation on C.edule and C.glaucum in order to clarify their taxonomical status. The ecological survey at sites in southern England has revealed that C.edule colonise estuarine habitats whereas C.glaucum were found only in lagoons. The disappearance of C.glaucum from Southampton Water and the Crouch Estuary, where they were previously reported to coexist with edule, has been discussed. The morphology, sculpture, crystallography, microstructure and organic and inorganic compositions of the shells of C.edula and C.glaucum were found to be similar. The gross anatomy, with the exception of variation in pigmentation of the siphons, mantle edge and digestive gland, are the same. The condition indices, bivariate and multivariate growth analyses showed variations within and between the populations of edula and C.glaucum. These variations are due to environmental factors with the type of substrata having the major effect. Multiple discrimination analysis utilising twelve phenetic characteristics clustered the populations of C.edule and C.glaucum into three phenotypes, C.edule, with thick shells, from exposed sandy habitats, occupied one side of the phenetic spectrum and C.glaucum, with thin shells, from soft muddy substrata, occupied the other side, The sublittoral populations of C.edula occupied the transition of this spectrum. Although they are both osmoconformers, the estuarine C.edula and the lagoonal C.glaucum respond differently to changes in salinity or osmolality of the ambient medium. C.glaucum utilised the shell gaping process more effectively to postpone the effects of the environmental salinity and osmotic stresses than did C.edule. During the acclimation period in a tidal regime simulating that of Southampton Water, C.edule and C.glaucum demonstrated different responses of shell gaping and adduction. Both C.edule and S.glaucum gape during exposure to air at low tide, suggesting they both breathe in air, contrary to previous reports. This is the first time the Continuous Automatic Salinity Cycling Apparatus and the tidal system have been used for comparative studies on C.edule and C.glaucum. The soluble muscle proteins of C.edule and C.glaucum from the different populations studied are immunologically identical. The similarity of the electrophoretic patterns of the shell and muscle proteins and the haemolymph and extrapallial fluid indicate the similarity of their molecular protein weights. In view of the results of the present studies, the reviewed literature and numerical and biological species concepts, it would be premature, at this stage in their evolution, to accept the estuarine C.edule and the lagoonal C.glaucum as two distinct species.

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