Comparative studies on the histology and ultrastructure of the siphons of two species of Tellinidae (Mollusca: Bivalvia) from Brazil

Abstract

Despite the great importance of the siphons for infaunal bivalves, only a few studies have examined their tissues using histology techniques or scanning electron microscopy. In the present study, the siphons of Tellina lineata Turton, 1819 and Macoma biota Arruda & Domaneschi, 2005 were investigated. The siphon walls are composed by a series of muscle sheets of longitudinal ("L"), circular ("C") and radial ("R") fibers, with a clear pattern common to both species: there is a main median longitudinal layer (Lm), and two peripheral circular layers, one inner (Ci) and one outer (Co), near the epithelia. A median circular layer (Cm) separates an internal (Lmi) from an outer (Lmo) median longitudinal layer. Further, the Co is split by a thin outer longitudinal layer (Lo), forming Coi and Coo layers, the former being obliquely oriented. Thin radial fibers (R) delimit clear packages of Lmi and Lmo fibers. In each siphon, there are six longitudinal nerve cords, running within the Lmi layer, adjacent to the Cm. The inhalant and exhalant siphons of M. biota are very similar in structure, but the Lmo of the exhalant siphon is almost twice as thick as its Lmi, while in the inhalant siphon these layers have similar thicknesses; the Coi is very thick, especially in the exhalant siphon. The inhalant siphon of T. lineata is very similar to that of M. biota, differing only with respect to the thickness of the Coi, which in the former species is not as well developed as in the latter. The Lmo of the exhalant siphon of T. lineata is by far the most developed layer, with the Lmi represented only by uniseriate small cells; in the vicinities of the nerve cords, the Cm is split in two layers. The epithelia of both siphons of M. biota and T. lineata bear ciliated receptors, which were difficult to observe as they were frequently covered with mucus. It was possible to observe that cilia are present in both species, differing in length and in the number per receptor between the inhalant and exhalant siphons, and between the species. Detailed comparisons among the siphons of M. biota and T. lineata and other bivalve species are very difficult, because of at least two reasons. First, each investigator has used different methods to prepare and observe the siphons through histological sections; and second, different nomenclatural schemes are used to describe the musculature of the siphons, causing confusion when the same layers are compared among different species. In order to unify the nomenclature of tissue layers of the bivalve siphons, we now propose a scheme to name these layers based on topological homology

Form and function of tentacles in pteriomorphian bivalves

“…Sensory ciliated receptors are also common at the tip of siphons and on siphonal tentacles, as observed in Donacidae and Tellinidae (Hodgson & Fielden, ; Vitonis et al, ), and on papillae along the middle fold, as in Mysella charcoti (Lasaeidae; Passos, Domaneschi, & Sartori, ). In the present study, a dense ciliary cluster associated with abundant innervation was consistently observed at the tip of MFT in I. bicolor , O. equestris , P. imbricata , and P. colymbus .…”

“…In infaunal and semiinfaunal bivalves, IFT are common on the siphons as sensory and protective structures distributed close to their apertures (Yonge, ). A great diversity in shape and number of siphonal tentacles was described for infaunal bivalves from the families Veneridae, Donacidae, and Tellinidae, which have multiple branched tentacles on the incurrent siphon (Fishelson, ; Narchi, ; Piffer, de Arruda, & Passos, ; Sartori & Domaneschi, ; Sartori, Printrakoon, Mikkelsen, & Bieler, ; Vitonis, Zaniratto, Machado, & Passos, ). Despite the absence of a siphon, a similar condition is present in P. colymbus and P. imbricata , both species have well‐developed, branched tentacles restricted to the ventral, incurrent region.…”

“…In contrast, profound anatomical differences, such as tentacle innervation, cilia organization, and size distribution along dorsal to ventral regions (Table ), indicate IFT and MFT are not homologous across Pteriomorphia. In addition, given that tentacle diversification may be associated with similar selective pressures, as suggested by the recurrent evolution of protective IFT in numerous bivalve lineages (e.g., Sartori et al, ; Vitonis et al, ), evolutionary convergence should be expected when exploring tentacle evolution in Pteriomorphia.…”

Mantle margin morphogenesis in Nodipecten nodosus (Mollusca: Bivalvia): new insights into the development and the roles of bivalve pallial folds

“…The mantle of adult bivalves has been extensively studied, mainly regarding aspects of shell secretion and siphon anatomy (e.g. [ 7 , 12 , 32 , 69 , 70 ]). The bivalve mantle epithelium generally comprises cuboidal to columnar cells, with numerous secretory cells spread across both surfaces.…”

Comparative and functional anatomy of the mantle margin in ark clams and their relatives (Bivalvia: Arcoidea) supports association between morphology and life habits

“…Mantle ciliated cells may also perform sensory functions, although ciliated chemo-and mechanoreceptors can hardly be distinguished by morphological criteria alone (Owen & McCrae, 1979). The cilia organized in tufts on the pallial tentacles of scallops (Pectinidae) and file clams (Limidae) are regarded as sensory receptors (Moir, 1977;Owen & McCrae, 1979;Audino et al, 2015a), as well as those at the tip of the siphons of several bivalves (e.g., Vitonis et al, 2012). In Arcopsis adamsi and Barbatia candida, we found long ciliumbearing cells sparsely distributed on the inner mantle fold at the region of the inhalant aperture ( Fig.…”