Of tests, trochs, shells, and spicules: Development of the basal mollusk Wirenia argentea (Solenogastres) and its bearing on the evolution of trochozoan larval key features

<p>Abstract</p> <p>Background</p> <p>The phylogenetic status of the aplacophoran mollusk taxon Solenogastres (Neomeniomorpha) is controversially discussed. Some authors propose the clade to represent the most basal branch within Mollusca, while others claim aplacophoran mollusks (Solenogastres and Caudofoveata) to be derived. Larval characters are central in these discussions, specifically the larval test (calymma, apical cap), the ontogeny of the epidermal scleritome, and the proposed absence of larval protonephridia. To date, developmental data are available for five solenogaster species, but most reports are incomplete and need confirmation.</p> <p>Results</p> <p><it>Wirenia argentea </it>deposit small batches of uncleaved embryos that are tightly enclosed by a smooth and transparent egg hull. Cleavage is spiral and unequal. The ciliated larvae hatch about 45 hours after deposition and swim actively in the water column. Within 48-60 hours after hatching they become mushroom-shaped with a pronounced apical cap partly enclosing a posterior trunk. The cells covering the apical cap are large and cleavage arrested. On the apical cap there is a prominent prototrochal band of compound cilia and an apical ciliary tuft and the trunk bears a terminal ciliary band (telotroch). Obscured by the apical cap, a ciliary band originates in the stomodaeal pore and surrounds the trunk. As development is proceeding, the trunk elongates and becomes covered by cuticle with the exception of a ventral ciliary band, the future foot. The larvae have a pair of protonephridia. At 5 days after hatching they begin to settle and within the following 7-9 days the apical cap is gradually reduced. Scattered epidermal sclerites form under the cuticle. <it>Wirenia argentea </it>lack iterated groups of sclerites at any developmental stage. At 40 days after hatching, the postlarvae have a fully developed foregut, but the midgut and hindgut are not yet interconnected.</p> <p>Conclusions</p> <p>Solenogastres develop via a trochophore-like lecitotrophic larva with a preoral apical cap that at least partly represents an enlarged prototrochal area. Homology of this larval type (pericalymma larva) to test cell larvae of other spiralian clades is doubtful. The ontogeny of <it>W. argentea </it>does not provide any evidence for a derived status of Solenogastres within Mollusca.</p

Diversity of Caudofoveata (Mollusca) Around Iceland and Description of Psilodens Balduri sp. n.

The first records from samples from the IceAGE cruise ME85/3 in 2011 include seven species of Caudofoveata with a distribution range in Icelandic waters. From this first cruise of the project, two new records for Iceland have been registered. Psilodens balduri sp. n. is new to science and Falcidens halanychi, with a known distribution in the American North-Atlantic, is new to Iceland. The current study thus increases the number of known caudofoveate species around Iceland to nine.publishedVersio

Fauna associated with non-native Sargassum muticum (Fucales, Phaeophyceae) vary with thallus morphology and site type (sounds and bays)

The invasive brown alga Sargassum muticum was first recorded in Norway in 1988 and is presently common in the sublittoral fringe on the sheltered coast in South Norway. Here, results from a study on mesofauna associated with S. muticum in an archipelago on the southwest coast of Norway are presented, with two stations placed in sounds exposed to tidal currents and two in sheltered bays. Sargassum muticum occurred with a patchy distribution and an estimated density of less than 7 individuals m−2 at three of the stations (two in sheltered bays and one in a sound) and 42 individuals m−2 at the fourth station (placed in a sound). A total of 102 distinct faunal taxa were identified, with an average of 31 taxa per sample. Crustacea and Gastropoda constituted around half of the identified taxa. Fauna richness showed no relation to site type but had a curvilinear correlation with habitat size (dry weight of thallus). Total and sessile fauna richness showed a significant negative correlation with branching density. Vagile faunal density showed a curvilinear correlation with weight of epiphytes and was significantly related to site type with higher densities in the bays. The community composition of vagile fauna reflected this and was significantly related to site type, but not to epiphyte weight or branching density. When including sessile taxa in a community analysis the stations became less separated, and the community was significantly influenced by branch density. Thus, both site characteristics and morphology of S. muticum influenced the associated fauna community.publishedVersio

Phylogenomics of Aplacophora (Mollusca, Aculifera) and a solenogaster without a foot

Recent molecular phylogenetic investigations strongly supported the placement of the shell-less, worm-shaped aplacophoran molluscs (Solenogastres and Caudofoveata) and chitons (Polyplacophora) in a clade called Aculifera, which is the sister taxon of all other molluscs. Thus, understanding the evolutionary history of aculiferan molluscs is important for understanding early molluscan evolution. In particular, fundamental questions about evolutionary relationships within Aplacophora have long been unanswered. Here, we supplemented the paucity of available data with transcriptomes from 25 aculiferans and conducted phylogenomic analyses on datasets with up to 525 genes and 75 914 amino acid positions. Our results indicate that aplacophoran taxonomy requires revision as several traditionally recognized groups are non-monophyletic. Most notably, Cavibelonia, the solenogaster taxon defined by hollow sclerites, is polyphyletic, suggesting parallel evolution of hollow sclerites in multiple lineages. Moreover, we describe Apodomenia enigmatica sp. nov., a bizarre new species that appears to be a morphological intermediate between Solenogastres and Caudofoveata. This animal is not a missing link, however; molecular and morphological studies show that it is a derived solenogaster that lacks a foot, mantle cavity and radula. Taken together, these results shed light on the evolutionary history of Aplacophora and reveal a surprising degree of morphological plasticity within the group.publishedVersio

Abyssal fauna of the UK-1 polymetallic nodule exploration area, Clarion-Clipperton Zone, central Pacific Ocean: Mollusca

The file attached is the Published/publisher’s pdf version of the article. This is an OpenAccess article.Copyright Helena Wiklund et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Development of the nervous system in Solenogastres (Mollusca) reveals putative ancestral spiralian features

Background: The Solenogastres (or Neomeniomorpha) are a taxon of aplacophoran molluscs with contentious phylogenetic placement. Since available developmental data on non-conchiferan (that is, aculiferan) molluscs mainly stem from polyplacophorans, data on aplacophorans are needed to clarify evolutionary questions concerning the morphological features of the last common ancestor (LCA) of the Aculifera and the entire Mollusca. We therefore investigated the development of the nervous system in two solenogasters, Wirenia argentea and Gymnomenia pellucida, using immunocytochemistry and electron microscopy. Results: Nervous system formation starts simultaneously from the apical and abapical pole of the larva with the development of a few cells of the apical organ and a posterior neurogenic domain. A pair of neurite bundles grows out from both the neuropil of the apical organ and the posterior neurogenic domain. After their fusion in the region of the prototroch, which is innervated by an underlying serotonin-like immunoreactive (−LIR) plexus, the larva exhibits two longitudinal neurite bundles - the future lateral nerve cords. The apical organ in its fully developed state exhibits approximately 8 to 10 flask-shaped cells but no peripheral cells. The entire ventral nervous system, which includes a pair of longitudinal neurite bundles (the future ventral nerve cords) and a serotonin-LIR ventromedian nerve plexus, appears simultaneously and is established after the lateral nervous system. During metamorphosis the apical organ and the prototrochal nerve plexus are lost. Conclusions: The development of the nervous system in early solenogaster larvae shows striking similarities to other spiralians, especially polychaetes, in exhibiting an apical organ with flask-shaped cells, a single pair of longitudinal neurite bundles, a serotonin-LIR innervation of the prototroch, and formation of these structures from an anterior and a posterior neurogenic domain. This provides evidence for an ancestral spiralian pattern of early nervous system development and a LCA of the Spiralia with a single pair of nerve cords. In later nervous system development, however, the annelids deviate from all other spiralians including solenogasters in forming a posterior growth zone, which initiates teloblastic growth. Since this mode of organogenesis is confined to annelids, we conclude that the LCA of both molluscs and spiralians was unsegmented

New records for the solenogaster Proneomenia sluiteri (Mollusca) from Icelandic waters and description of Proneomenia custodiens sp. n

During August-September 2011, scientists aboard the R/V Meteor sampled marine animals around Iceland for the IceAGE project (Icelandic marine Animals: Genetics and Ecology). The last sample was taken at a site known as "The Rose Garden" off northeastern Iceland and yielded a large number of two species of Proneomenia (Mollusca, Aplacophora, Solenogastres, Cavibelonia, Proneomeniidae). We examined isolated sclerites, radulae, and histological section series for both species. The first, Proneomenia sluiteri Hubrecht, 1880, was originally described from the Barents Sea. This is the first record of this species in Icelandic waters. However, examination of aplacophoran lots collected during the earlier BIOICE campaign revealed additional Icelandic localities from which this species was collected previously. The second represents a new species of Proneomenia, which, unlike other known representatives of the genus, broods juveniles in the mantle cavity. We provide a formal description, proposing the name Proneomenia custodiens sp. n. Interestingly, the sclerites of brooded juveniles are scales like those found in the putatively plesiomorphic order Pholidoskepia rather than hollow needles like those of the adults of this species. Cytochrome c oxidase subunit I (COI) DNA barcode sequences are provided for both species of Proneomenia