Muscle development in Antalis entalis (Mollusca, Scaphopoda) and its significance for scaphopod relationships

Abstract

We applied fluorescence staining of F-actin, confocal laser scanning microscopy, as well as bright-field light microscopy, SEM, and TEM to examine myogenesis in larval and early juvenile stages of the tusk-shell, Antalis entalis. Myogenesis follows a strict bilaterally symmetrical pattern without special larval muscle systems. The paired cephalic and foot retractors appear synchronously in the early trochophore-like larva. In late larvae, both retractors form additional fibers that project into the anterior region, thus enabling retraction of the larval prototroch. These fibers, together with the prototroch, disappear during metamorphosis. The anlagen of the putative foot musculature, mantle retractors, and buccal musculature are formed in late larval stages. The cephalic captacula and their musculature are of postmetamorphic origin. Development of the foot musculature is dramatically pronounced after metamorphosis and results in a dense muscular grid consisting of outer ring, intermediate diagonal, and inner longitudinal fibers. This is in accordance with the proposed function of the foot as a burrowing organ based on muscle-antagonistic activity. The existence of a distinct pair of cephalic retractors, which is also found in basal gastropods and cephalopods, as well as new data on scaphopod shell morphogenesis and recent cladistic analyses, indicate that the Scaphopoda may be more closely related to the Gastropoda and Cephalopoda than to the Bivalvia.

Mollusca

“…7.8E and 7.10 ). This muscle underlies the prototroch/velum and is present in all molluscan classes (including the aculiferans), except for the scapho-pods and cephalopods (monoplacophoran condition unknown), and thus most likely belong to the molluscan larval ground plan (Wanninger and Haszprunar 2002b ;Nielsen et al 2007 ;Wanninger et al 2008 ;Dyachuk and Odintsova 2009 ;Scherholz et al 2013 ). Far left column shows adult myoanatomy of polyplacophorans and the neomeniomorph Wirenia argentea in cross section ( top ) and dorsal view ( bottom ).…”

Evidence for a clade composed of molluscs with serially repeated structures: Monoplacophorans are related to chitons

“…Molecular attempts have not been conclusive, but they have aided to refute the ''Diasoma'' hypothesis (a clade uniting bivalves and scaphopods). Most recent molecular analyses suggest a relationship of scaphopods to cephalopods and gastropods (15)(16)(17), further corroborated through morphological and developmental studies (5,18). To date, the phylogenetic position of monoplacophorans remained untested using molecular data because of difficulties in collecting live samples of these enigmatic animals.…”

The scaphopod foot is ventral: more evidence from the anatomy ofRhabdus rectius(Carpenter, 1864) (Dentaliida: Rhabdidae)

“…The U-shaped gut has been compared to the arrangement in cephalopods and gastropods, both groups that have a turn in the digestive tract caused by ano-pedal flexion, such that the anus returns forward (Wanninger and Haszprunar 2002;Sumner-Rooney et al 2015). This is a trait that unifies scaphopods with cephalopods and gastropods (Ponder and Lindberg 1997;Lindgren et al 2004).…”

“…Bivalves are one putative sister group to scaphopods (Runnegar and Pojeta 1974); however most similarities seem to be convergent to an infaunal lifestyle (Waller 1998;Wanninger and Haszprunar 2002). Recent neuroanatomical comparisons and gene expression evidence have both indicated a closer relationship between scaphopods and cephalopods Wollesen et al 2015).…”