The scaphopoda

Abstract

The Scaphopoda are marine infaunal carnivores that feed on foraminiferans and other microorganisms selected and manipulated by their unique feeding tentacles or captacula. Their tusk-like shell is open at both ends; the burrowing foot and captacula protrude anteriorly, while respiratory currents pass through the posterior opening. Although the scaphopods comprise one of the smallest molluskan classes in terms of species diversity, they have a worldwide distribution ranging from intertidal to depths in excess of 6000 m. Despite detailed monographic work from the late nineteenth and early twentieth centuries, however, the biology of scaphopods is today among the least understood of mollusks. To some extent, this is related to a significant departure from more typical molluscan organization inferred from studies of the larger classes. For example, the mantle cavity is elongate and restricted in space, and several associated organs are lost, including the osphradium and ctenidia; the latter is associated with the loss of auricles from the scaphopod heart. There is also little record of ontogeny in the adult shell, as the older portion is periodically decollated to maintain the posterior aperture for passage of inhalant and exhalant currents. This and other constraints on scaphopod diversification are reflected in the range limits of shell shape within the class. In contrast, the "Dentalium" egg has been a model system for experimental embryology, particularly in studies of cell lineage and early morphogenesis. Later larval development and most organogenesis is nonetheless poorly known, although recent studies have considerably enhanced our knowledge in this area. Global biogeographic patterns of scaphopod diversity have been studied only preliminarily. A general decrease in diversity with depth is broken by small diversity peaks at bathyal depths. There is also evidence for a marked latitudinal diversity gradient in the world's oceans, near equatorial in the Pacific but at approximately 20 degrees N in the Atlantic. Scaphopods have a wide diversity of ecto- and endo-symbiotic associations with other organisms, including commensal bacteria and ciliates, mutualistic anemones and corals, and parasitic algae and platyhelminths. Other documented associations include predation by naticid gastropods and ratfish, and the inhabitation of empty scaphopod shells by a variety of sipunculans and hermit crabs. Phylogeny within the class is still unresolved, although significant progress has been made recently in documenting morphological variation among families, genera, and species for application in a cladistic context. The evolutionary relationships of Scaphopoda within Mollusca are farther from resolution, with every conchiferan class a proposed sister group among modern analyses. Molecular avenues should provide significant progress in scaphopod phylogenetics, as indicated by recent comparative sequence analysis and gene expression patterns. Scaphopods comprise the most recent class of mollusks to appear in the fossil record,...

Retracted:Tusk shells in trouble? Physiology and behavior in response to changing temperature in a scaphopod (Mollusca: Scaphopoda: Dentaliida)

“…There is limited information on the anatomy, physiology, and natural history of scaphopods in general (Reynolds ; Reynolds & Steiner ), with confusion regarding even the fundamental axes (dorso‐ventral, anterio‐posterior) in the scaphopod body plan (Sumner‐Rooney et al. ).…”

“…This study documented for the first time the respiratory, and hence metabolic, rates of any scaphopods. The mechanisms of respiration for scaphopods may be more complex than previously understood; scaphopods have no gills and are presumed to respire through folds of the mantle (e.g., Reynolds ). However, like other invertebrates with a large surface area to volume ratio, they may acquire oxygen through absorption.…”

“…The normal position of scaphopods in the sediment is buried with the ventral (foot) side facing downwards and the narrower apical end of the shell slightly exposed at the sediment surface, although the animals can move deeper so that the whole shell is submerged (Gainey ). Scaphopods have no gills but absorb oxygen through respiratory folds in the mantle at the dorsal end of the shell (Reynolds ). Evidentially, a clear opening at the dorsal (apical) end of the shell is also required as animals in the lab whose apical ends become plugged with sediment become unhealthy and die (Shimek ; J.D.…”

Development and evolution of the metazoan heart

“…In cephalopods, the systemic heart and the two branchial hearts are similarly under neuroregulatory control, suggestive of common regulatory pathways. The activity of the heart in other mollusks strongly depends on the function of surrounding organs such as the gut and the excretory system, as well as on muscle activities related to burrowing movements, for example …”

A systematic review of animal predation creating pierced shells: implications for the archaeological record of the Old World

“…All shell beads belonging to this class have two anatomical holes, one in the anterior (allowing the burrowing foot and captacula to protrude) and a second one in the posterior part of the shell (responsible for respiration; Reynolds, 2002). As such, these shells can be threaded onto a cord without being pierced.…”