Condensing lenses and shell microstructure inCorculum(Mollusca: Bivalvia)

Abstract

The microstructure of the non-window portions of the shell of Corculum cardissa resembles other Fraginae, with predominantly fibrous prismatic outer, branching crossed lamellar middle, and complex crossed lamellar inner layers. Both the anterior and posterior windows in its shell reflect reduced pigmentation and incursion of the outer shell layer, but the posterior windows involve deeper incursion plus reduction of the outer and middle sublayers of the outer shell layer and microstructural modification of the middle shell layer to enhance light transmission. The planoconvex shape of the posterior windows has more likely evolved to direct and focus light toward the deeper, zooxanthellae-rich gills and anterior mantle, than to merely disperse light.

Characterizing Photosymbiosis Between Fraginae Bivalves and Symbiodinium Using Phylogenetics and Stable Isotopes

“…Some photosymbiotic fragines exhibit unique morphologies that are hypothesized to be adaptations to photosymbiosis, including large shell surface to volume ratio, highly flattened posterior shells, and transparent shell microstructures to increase light penetration (Watson and Signor, 1986;Ohno et al, 1995;Carter and Schneider, 1997;Schneider and Carter, 2001;Kirkendale, 2005;ter Poorten, 2009). Certain species also show behavioral strategies presumably to maximize sunlight exposure, such as posterior valve gaping (Persselin, 1998;Kirkendale, 2005).…”

The Cardiidae (Mollusca: Bivalvia) of tropical northern Australia: A synthesis of taxonomy, biodiversity and biogeography with the description of four new species

“…It is a member of the photosymbiotic fragines, a wholly photosymbiotic lineage of cardiids (Kirkendale and Paulay 2017). However, unlike many other closely related photosymbiotic species, C. cardissa is essentially a dorsoventrally flattened 'living' solar panel with transparent shell microstructural windows that facilitate light penetration through the shell (Carter and Schneider 1997). …”

The evolution of molluscan photosymbioses: a critical appraisal

“…Velates has been interpreted either as a suspension-feeder (Savazzi, 1992) or herbivore (Vermeij, 2011), although its large size, exposure of the mantle to light, and distribution in well-lit tropical carbonate-dominated environments are also consistent with photosymbiosis. Yonge, 1936;Rosewater, 1965;Hamner, 1978;Beckvar, 1981;Trench, Wethey & Porter, 1981;Lucas et al, 1991;Schneider, 1992Schneider, , 1998aGriffiths & Klumpp, 1996;Schneider & Ó Foighil, 1999;Keys & Healy, 2000;Harzhauser et al, 2008;Schwartzmann et al, 2011;deBoer et al, 2012 Fraginae Phylogenetic position: Bivalvia, Heterodonta, Cardiidae Placement of symbionts: Fragum and relatives with expanded inner fold of mantle in posterior part, often extending over exterior; Corculum: beneath windows on dorsal shell face Characteristics: Fragum partially enveloped posterior part of shell; all species with keel from umbo to posteroventral margin; Corculum dorsoventral flattened Ecology: tropical, infaunal in sand, Indo-West Pacific Maximum size: Fragum 65 mm; Corculum: 70 mm Age: Late Miocene to Recent References: Kawaguti, 1950Kawaguti, , 1983Watson & Signor, 1986;Ohno et al, 1995;Kobayashi, 1996;Carter & Schneider, 1997;Morton, 2000;Kirkendale, 2009;ter Poorten, 2007ter Poorten, , 2009 Clinocardium nuttalli Phylogenetic position: Bivalvia, Heterodonta, Cardiidae, Clinocaradiinae Placement of symbionts: in posterior mantle of adults Characteristics: no specializations Ecology: cool-temperate, infaunal in sand, northeastern Pacific Maximum size: 140 mm Age: Recent References: Jones & Jacobs, 1992; Coan et al, 2000 Controversy surrounds many of the groups listed in Table 1. Arguments for and against photosymbiosis in these groups are discussed below in the context of potential specializations to photosymbiosis in living species, with particular attention to the seemingly contradictory ecological distributions of many of the fossil candidates.…”