Identifying natural groups within the caenogastropod family Vermetidae has proven challenging. The sessile lifestyle of vermetids, with associated xenomorphically distorted, overgrown and corroded shells, has resulted in a long and confused taxonomic history based primarily on adult shell characters. In this study, we use morphological, anatomical and molecular data to clarify systematics and phylogenetic relationships within the genus Dendropoma s.l. We assess generic names previously used in the Dendropoma group for availability and recognize Veristoa Iredale, 1937, as a junior synonym of Dendropoma Mörch, 1861. We describe 21 species, eight of which are new, place them into four robustly supported genera (Dendropoma s.s.; Novastoa Finlay, 1926; Ceraesignum n. gen.; Cupolaconcha n. gen.), and outline the current state of knowledge of the distribution of these taxa. The genus Dendropoma s.s. is well supported in our phylogenetic analysis and is additionally supported by features of the operculum, reproductive traits and a novel mitochondrial gene order. Among the four genera, members of Dendropoma s.s. alone maintain unstalked egg capsules in the mantle cavity rather than attaching them to the shell via a slit in the female mantle. The opercula of examined species within the genus Novastoa are characterized by a well-developed mammilla on the internal surface and upright tightly packed spiral lamina on the external surface. In Ceraesignum n. gen., the operculum lacks a mammilla and displays a fingerprint-like texture on its inner surface. The genera Ceraesignum n. gen. and Novastoa form a well-supported monophyletic group with the genus Dendropoma s.s., although sister group relationships among these genera are not resolved. The fourth genus, Cupolaconcha n. gen. is more closely related to the vermetid genera Eualetes Keen, 1971a, Thylaeodus Mörch, 1860, and Petaloconchus Lea, 1843, demonstrating that Dendropoma s.l. is not a monophyletic group. The calcified operculum of Cupolaconcha n. gen. is unique in the Vermetidae and examined species in this genus are also characterized by a translocation of the valine mitochondrial tRNA. Further study will encompass the full range of morphological diversity in the Vermetidae to clarify the major lineages within this remarkable family of snails.

Freshwater mussels (Mollusca: Bivalvia: Unionoida) are a species-rich group of parasitic bivalves comprising approximately 843 nominal species in six families, including 300 species of Unionidae and five of Margaritiferidae in North America. Unionid shells have been studied extensively for the purposes of taxonomy, but less information exists about the cellular anatomy of their “soft tissues” (mantle cavity tissues and visceral tissues). No systematic histological atlas of any unionid has been published in the peer-reviewed literature, and this lack of information hinders basic and applied research topics involving freshwater mussels. Herein, we describe the tissue and cell anatomy of a representative species from each of three lineages (tribes) of Unionidae sensu Graf & Cummings (2006) ranging in North America: Villosa nebulosa (Ambleminae: Lampsilini), Fusconaia cerina (Ambleminae: Pleurobemini) and Strophitus connasaugaensis (Unioninae: Anodontini). Based on necropsy observations and light microscopy of serial histological sections, for each species we describe and compare mantle cavity tissues (i.e., tissue enclosed by mantle: mantle, adductor muscle, pedal protractor, pedal retractor, gill, foot, labial palp) and visceral organs (i.e., internal organs: esophagus, digestive diverticulum, stomach, crystalline style sac, intestine, heart, nephridium, cerebral ganglia, pedal ganglia, visceral ganglia, ovaries and testes). We also present a synoptical review of pertinent literature on histological anatomy of unionids. The present study (i) represents the first histological atlas for freshwater mussels, (ii) comprises a baseline for monitoring mussel health in aquatic ecosystems, and (iii) could assist future workers studying freshwater mussel physiology, life history, toxicology, pathology, systematics and ecology.