Relation of Shell Form to Life Habits of the Bivalvia (Mollusca)

Abstract

Study of 95 Western Atlantic bivalve mollusk species representing 29 families has demonstrated that morphologic features of the bivalve shell other than hinge type and microstructure primarily reflect life habits and habitat preferences. Many morphologic features represent potentially powerful tools for paleoecologic and evolutionary interpretation.Byssally attached groups living with the sagittal plane vertical are characterized by elongation and flattening of the ventral margin. Non-burrowing, byssally attached arcids and carditids, for example, are more elongate than closely related burrowing species. Epifaunal mytilids can be distinguished from related infaunal and semi-infaunal species by cross-sectional shape, degree of anterior reduction, and configuration of the byssal musculature and sinus. Tridacnids show special adaptations for exposure of algae-infested siphonal tissue to sunlight.Byssally attached groups living with the sagittal plane nonvertical are characteristically inequivalve, with the lower valve flattened for maximum substratum contact. Pectinids and pteriids utilize a special auricle-sinus configuration for firm byssal anchorage. The posterior "wing" of many pteriids apparently shelters the exhalent current to increase its discharge distance.Epifaunal groups attached by cementation are commonly thickshelled. The spines of certain cemented groups apparently serve a defensive function.Many reclining taxa employ a thick shell and a flattened upper valve for stability. The lower valve is commonly convex, if partly buried in life position, and nearly flat, if lying at the substratum surface.Most swimming species have thin shells. Pectinids with a freeliving adult stage differ from those with a byssally attached adult stage in their high degree of auricle symmetry and large umbonal 1 Evolutionary divergence, convergence, and homeomorphy support functional interpretations and provide strong arguments for the adaptive nature of shell form in the Bivalvia. Divergent representatives of the Mactracea, for example, show remarkable convergence in form and habit with species belonging to other superfamilies. 7 8 SHELL FOKM AND LIFE HABITS OF THE BIVALVIA locomotion or attachment is very useful for dividing bivalves into meaningful ecologic groups. Seven major categories are recognized here:Byssally attached. Employing a byssal apparatus for fixation to the substratum, and normally lacking the capacity for pedal burrowing. Cemented. Attached by secreted shell material to a hard substratum.Reclining. Occupying a position on or partially buried in a soft substratum and lacking the capacity for attachment.Swimming. Moving freely through the water by self-propulsion. Burrowing. Penetrating a soft substratum by means of pedal locomotion and maintaining a life position of at least partial burial.

Notes on growth and behaviour of the American razor clamEnsis directus in the Wadden Sea and the predation on it by birds

“…In North America, E. directus lives in the low intertidal and subtidal zone in sand and sandy mud in bays and estuaries, where it is most abundant in areas swept by moderately strong currents such as tidal channel margins and tidal flats exposed to wave action (Drew, 1907;Bousfield, 1960;Gibbons, 1964;Stanley, 1970}. Thus, in the Wadden Sea, /7.…”

Storm-influenced deposition and cyclicity in a shallow-marine mudstone succession - example from the Middle Jurassic ore-bearing clays of the Polish Jura (southern Poland)

“…As their shells are not as so ci ated with re sid ual ac cu mu la tions of sand, silt and shell de bris, it seems that the sed i ment, in which they were orig i nally bur ied, had to be re moved by bot tom flows, not sim ply re set tled from the sus pen sion. Con sid er ing that mod ern bi valves may bur row as deeply as 30 cm be low the sea-floor (Stan ley, 1970;Kondo, 1987), the depth of this ero sion might be sig nif i cant. The ex humed shal low-bur row ing bi valves may re cord less in tense ero sion than that in ferred from the deep-bur row ing ones.…”

The benthic macrofauna from the Lower Maastrichtian chalk of Kronsmoor (northern Germany, Saturn quarry): taxonomic outline and palaeoecologic implications

“…The guild assignments proposed herein (Text- fig. 10) are based on available autecological literature and databases (e.g., Paleobiology database: fossilworks.org), on the interpretation of functional morphologies, and/or on comparison with closely related extant relatives (Ager 1965;Carter 1968Carter , 1972Ernst 1970;Rudwick 1970;Stanley 1970;Surlyk 1972;Thayer 1975;La Barbera 1981;Abdel-Gawad 1986;Aberhan 1994;Fürsich and Hurst 1994;Machalski and Robaszewska 2003;Wilmsen et al 2007;Lauridsen and Surlyk 2008;Lauridsen et al 2009;Hansen and Surlyk 2014).…”

“…Pervasive bioturbation, the lack of black shales and laminations indicate normal oxygenation of the water column and the upper part of the sediment layer (e.g., Oschmann 1991). All identified bivalves lived epifaunally although many Mesozoic bivalves had an infaunal lifestyle (Stanley 1970;Aberhan 1994). In the absence of any indications for oxygen depletion, a possible explanation could be the secondary, diagenetic removal of their aragonite shells (Voigt 1996;Wright et al 2003).…”