Shell tubules in Condylocardiinae (Bivalvia: Carditoidea)

Shell tubules are microscopic canals in the shells of various molluscs. In polyplacophorans and fissurellid gastropods, the canals form by shell growth around multicellular extensions of the mantle epithelium (caeca). Bivalves have unicellular caeca which etch tubules into prefabricated shell (Reindl & Haszprunar, 1996). These occur in numerous independent bivalve lineages, although more frequently in Pteriomorphia and Carditoida (Taylor, Kennedy & Hall, 1969, 1973). Tubules are a taxonomically constant feature of all postlarval Arcoida and most likely of all Carditidae, Pisidiidae, Sphaeriidae and Corbiculidae (Araujo, Ramos & Bedoya, 1994; Araujo & Korniushin, 1998; Malchus, 2006; Taylor et al., 1973; Waller, 1980; this study). Unionidae (Palaeoheterodonta) seem to be the only taxon with shell tubules in the larval, glochidial shell (Lima et al., 2006; Roe, Simons & Hartfield, 1997; Malchus personal observation). The feature has never been mentioned for the microbivalve family Condylocardiidae (e.g. Middelfart 2002a, b and references therein; Coan, 2003), although this group was probably derived from tubulate Carditidae (Taylor et al., 2007). The present study examines the shells of three Condylocardia species (cf. Coan, 2003) from the National History Museum in Los Angeles (LACM) and compares them to Cardita calyculata (Linnaeus, 1758) (Carditidae), Adacnarca limopsoides (Thiele, 1912) (Philobryidae) (Swedish Museum of Natural History, SMNH) and Striarca lactea (Linnaeus, 1758) (Noetiidae) (own collection, NMA). Additional photographic support material is available from Morphobank (http://morphobank.org/permalink/?P343; image reference numbers begin with letter M).