Cementation in Cleidothaerus albidus (Lamarck, 1819) (Bivalvia: Anomalodesmata: Pandoroidea)

Abstract

The pandorid anomalodesmatan Cleidothaerus albidus cements to hard substrata by the anterior half of the right valve. Cementation begins after settlement of a lecithotrophic larva during the dissoconch stage (at a mean height of 1.2 mm), presumably after a brief period of byssal attachment. The aragonitic shell comprises a thick inner nacreous layer, with ’chalky deposits’, and a thin outer prismatic layer. The persistent periostracurn is relatively thick. It is bi-layered with the outer layer being thinner (1–2 μm) on the right cemented valve than the ’upper’ left valve ( 10 μm). The inner layer, secreted by the outer component of the periostracal groove, is probably more mucoid. There are no obvious differences in the character of the right valve periostracum between areas of cementation and non-cementation. An extra-pallial cement was discovered under some parts of the attached valve where it had accumulated in ‘valleys’ in the substratum, providing the necessary smoothing to allow adhesion. The cement was mainly unmineralised but in some patches showed spherulitic growth of (?) aragonite.

Cleidothaerus albidus shows Type B pallial fusion, i.e. fusion of the inner and inner surfaces of the middle mantle folds. Significantly, no arenophilic glands were observed, however, we did identify a large gland within the mantle cavity of the right lobe that was restricted to the anterior portion, i.e. the part of the mantle directly involved with cementation. This structure comprises two types of secretory cells. Since the gland was restricted to only those parts of the mantle that were involved in cementation, we suggest that it may be responsible for the production of the cement that drains, under gravity; to exit the mantle cavity at the pedal gape, coating the attaching periostracum.