Functional morphology with particular reference to hinge and ligament inSpondylusandPlicatulaand a discussion on relations within the superfamily Pertinacea (Mollusca: Bivalvia)

Abstract

Excluding the isomyarian family Dimyidae, the Pectinacea comprise the families Propeamussidae, Pectinidae, Spondylidae and Plicatulidae. Present investigations are primarily concerned with species of the last two, both of which are cemented by the right valve, with secondary teeth and sockets which form ball and socket joints between the valves. Neither has previously been examined in life or the hinge and ligament critically studied. Comparing throughout with conditions in the Pectinidae, the ctenidia inPlicatulaare simpler (like those ofPropeamussium) but both here and inSpondylusthe ciliary pattern (type B (1a)) is more primitive.Spondylusresembles the Pectinidae in that it has elaborate arborescent lips and pallial eyes;Plicatula(andPropeamussium) has neither, and the inner mantle folds (velum) are reduced (though enlarged inPropeamussium). The foot is lost inPlicatulaand inSpondylushas solely to do with cleansing; the Pectinidae display a range of pedal form and function - from locomotion to byssal attachment and to cleansing. The ratio of ‘quick’ to ‘catch’ muscle in the adductor is associated with habit, being greatest where need for rapid adduction is greatest, primarily in connexion with cleansing, a matter of particular urgency in horizontally disposed bivalves. Pallial eyes - as well developed in permanently attached as in swimming species - are most probably concerned with immediate response to predatory attack on pallial tissues widely exposed when the valves gape. The ligament in bothSpondylusandPlicatulais surprisingly different from that in the Pectinidae (and Propeamussidae). The long anterior and posterior outer ligament layers found in the two last which unite the valves at either end of the condensed rounded inner ligament layer are replaced inSpondylusby fused periostracum . The outer ligament layers have migrated inwards and, after dividing on either side of the unchanged inner ligament layer, unite (topographically) above and below it, forming morphologically left and right areas composed equally of anterior and posterior outer ligament layers. The inward extensions of the fused periostracal grooves which form the secondary extensions to the primary ligament may well be associated with the change in nature of the hinge plate (and thus of teeth and sockets) to crossed-lamellar aragonite instead of the foliated calcite present in the Pectinidae. The combined inner and outer ligament layers produce the more powerful ligament demanded by the more massive valves; the secondary periostracal extensions serve only to unite the valves which are maintained in alinement by way of the secondary teeth and sockets. The conspicuous bilateral asymmetry in the hinge and ligament is a result of cementation; similar conditions exist in the cemented pectinid,Hinnites. InPlicatuladifferences are much greater. Inward growth of the mantle margins results in union above the now submarginal ligament. This is extremely compressed in the transverse plane becoming hoop-like with the right limb the longer. Basally it fractures, although the two halves remain in contact and function is unaffected. As in Spondylus, the halves of the anterior and posterior ligament layers unite on the two sides of the inner ligament layer. Owing to dorsal overgrowth by the hinge plate, the epithelia secreting the outer ligament layers form the two sides and roof of a chamber the base of which is the mantle isthmus (forming the inner ligament layer). Contact with the valves is exclusively by way of the outer ligament layers. The periostracum fuses in the mid-line dorsally and does not contribute to the ligament from which it is separated. Owing to the division of the inner ligament layer into right and left halves, union of the valves is effectively by way of the secondary teeth, here more dorsally extended than inSpondylusbut, as there, composed of crossed-lamellar aragonite. Evolution of these four families starts in Palaeozoic stocks with modifications of organs in the mantle cavity - ctenidia, lips, pallial eyes, etc. - proceeding along lines distinct from those involving modifications in the ligament. The former particularly concern the primitive Propeamussidae, largely confined to deep water, and the universally distributed Pectinidae, the latter the Spondylidae and the Plicatulidae. Modifications of the foot have to do with final habit which is invariable freedom in the Propeamussidae, byssal attachment, freedom or cementation in the Pectinidae, and invariable cementation in the Spondylidae and Plicatulidae, the process occurring earlier in the latter and involving loss of the foot. Separation of the Spondylidae from the Pectinidae is more fully established with the present demonstration of the totally different ligamental structure; the difference is so profound in the Plicatulidae as to raise the question of elevating this to superfamily status.