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

Sexual dimorphism in the parasitic snail Nanobalcis worsfoldi: a histological and morphometric approach with insights for the family Eulimidae

Sexual dimorphism has long been stressed as a remarkable characteristic of Eulimidae snails. This idea is so common for this family that, even without information on the soft parts, some species were already identified as male or female, as observed in Nanobalcis worsfoldi Warén, 1990. Here, we investigated whether larger and smaller individuals of N. worsfoldi are female and male, respectively, and what are the most useful characteristics to differentiate the sexes. We analyzed shell morphology and morphometry, along with coloration of the soft parts of large and small individuals, and crosschecked our findings through histological analysis. Our results confirmed that (i) larger and smaller individuals are females and males, respectively; (ii) shell morphology is not useful, while morphometry is important to address sexual dimorphism; (iii) since shell length and width showed the most remarkable differences, they are considered the best parameters to predict the sex. Furthermore, conchological characteristics associated with histological analysis suggest that this species could be the first non-sessile eulimid known as a sequential hermaphrodite (protandric) with environmental sex determination, but this remains to be confirmed. Considering that most studies on eulimids are based on shells, the morphometric parameters used here may support future ecological/biological analyses in the family, such as parasitological prevalence, or parasitic burden related to sex.

New data from Monoplacophora and a carefully-curated dataset resolve molluscan relationships

Relationships among the major lineages of Mollusca have long been debated. Morphological studies have considered the rarely collected Monoplacophora (Tryblidia) to have several plesiomorphic molluscan traits. The phylogenetic position of this group is contentious as morphologists have generally placed this clade as the sister taxon of the rest of Conchifera whereas earlier molecular studies supported a clade of Monoplacophora + Polyplacophora (Serialia) and phylogenomic studies have generally recovered a clade of Monoplacophora + Cephalopoda. Phylogenomic studies have also strongly supported a clade including Gastropoda, Bivalvia, and Scaphopoda, but relationships among these taxa have been inconsistent. In order to resolve conchiferan relationships and improve understanding of early molluscan evolution, we carefully curated a high-quality data matrix and conducted phylogenomic analyses with broad taxon sampling including newly sequenced genomic data from the monoplacophoran Laevipilina antarctica. Whereas a partitioned maximum likelihood (ML) analysis using site-homogeneous models recovered Monoplacophora sister to Cephalopoda with moderate support, both ML and Bayesian inference (BI) analyses using mixture models recovered Monoplacophora sister to all other conchiferans with strong support. A supertree approach also recovered Monoplacophora as the sister taxon of a clade composed of the rest of Conchifera. Gastropoda was recovered as the sister taxon of Scaphopoda in most analyses, which was strongly supported when mixture models were used. A molecular clock based on our BI topology dates diversification of Mollusca to ~546 MYA (+/- 6 MYA) and Conchifera to ~540 MYA (+/- 9 MYA), generally consistent with previous work employing nuclear housekeeping genes. These results provide important resolution of conchiferan mollusc phylogeny and offer new insights into ancestral character states of major mollusc clades.