Abstract
BackgroundDespite extensive knowledge on bivalve anatomy and development, the formation and differentiation of the mantle margin and its associated organs remain largely unclear. Bivalves from the family Pectinidae (scallops) are particularly promising to cast some light on these issues, because they exhibit a complex mantle margin and their developmental stages are easily obtained from scallop farms. We investigated the mantle margin of the scallop Nodipecten nodosus (L. 1758) during larval and postmetamorphic development.MethodsA thorough analysis of the mantle margin development in Nodipecten nodosus, from veliger larvae to mature adults, was conducted by means of integrative microscopy techniques, i.e., light, electron, and confocal microscopy.ResultsInitially unfolded, the pallial margin is divided into distal and proximal regions by the periostracum-forming zone. The emergence of the pallial musculature and its neural innervation are crucial steps during bivalve larval development. By the late pediveliger stage, the margin becomes folded, resulting in a bilobed condition (i.e., outer and inner folds), a periostracal groove, and the development of different types of cilia. After metamorphosis, a second outgrowth process is responsible for emergence of the middle mantle fold from the outer surface of the inner fold. Once the three-folded condition is established, the general adult features are rapidly formed.ConclusionsOur data show that the middle mantle fold forms from the outer surface of the inner fold after metamorphosis and that the initial unfolded mantle margin may represent a common condition among bivalves. The first outgrowth process, which gives rise to the outer and inner folds, and the emergence of the pallial musculature and innervation occur during larval stages, highlighting the importance of the larval period for mantle margin morphogenesis in Bivalvia.
Highlights
Despite extensive knowledge on bivalve anatomy and development, the formation and differentiation of the mantle margin and its associated organs remain largely unclear
Exceptions to this three-folded pattern do exist, e.g., four pallial folds have been described for the Veneridae [10,11,12], duplication in the middle fold may occur in Donacidae [13], and two pallial folds may be present in some Arcidae, with the outer one being duplicated [14, 15]
The pallial margin of live larvae cannot be properly visualized by means of conventional light microscopy because of its transparency and location underneath the shell edge, being partly obliterated by the large, antero-ventrally located ciliated velum (Fig. 1a, b)
Summary
Despite extensive knowledge on bivalve anatomy and development, the formation and differentiation of the mantle margin and its associated organs remain largely unclear. The mantle margin is the free edge of this organ and bears tissue extensions named mantle (or pallial) folds, which can bear muscular, sensory or secretory structures [1]. The periostracum is formed in a deep groove between the outer and middle folds, while the shell layers are secreted by the outer mantle epithelium [8, 9]. Exceptions to this three-folded pattern do exist, e.g., four pallial folds have been described for the Veneridae [10,11,12], duplication in the middle fold may occur in Donacidae [13], and two pallial folds may be present in some Arcidae, with the outer one being duplicated [14, 15]
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