Morphological, ecological and molecular characterization of the enigmatic planispiral snail genus Adeuomphalus (Vetigastropoda: Seguenzioidea)

Abstract

Adeuomphalus Seguenza, 1876 is a little known genus among the skeneimorph vetigastropods, with very few specimens previously reported alive from the deep sea. We examined newly collected and museum-stored specimens from upper to lower bathyal depths in the Atlantic, Mediterranean, Pacific and Indian Oceans and recognize seven recent species in the genus: A. ammoniformis Seguenza, 1876, A. densicostatus (Jeffreys, 1884), A. trochanter Waren & Bouchet, 2001, A. sinuosus (Sykes, 1925) n. comb., A. guillei n. sp., A. elegans n. sp. and A. collinsi n. sp., along with a fossil species, A. bandeli (Schroder, 1995) from the Lower Cretaceous, Poland. These species are characterized by a minute and colourless shell with almost perfectly planispiral whorls, an orthocline aperture, distinct radial ribs and a deeply concave apex and base. At least three species are confirmed to be radula-less, while A. guillei n. sp. has a simplified (3 -2-1-2-3) rhipidoglossate radula. Anatomical investigations of A. collinsi n. sp. and A. trochanter revealed the following traits: a monopectinate ctenidium, blunt and tapering cephalic tentacles with sensory papillae, a cylindrical snout, a simple right neck lobe, a large foot with the anterior corners drawn out into finger-like projections, a smooth ESO-tentacle and a single, micropapillate epipodial tentacle on each side of the foot; absence of pigmented eyes, eye lobes, cephalic lappets and subocular peduncles. Three species collected by submersibles in the vicinity of hydrothermal vents co-occurred with carnivorous sponges of the family Cladorhizidae; a parasitic mode of life is suggested based on the lack of the radula and the peculiar, tube-like shape of the snout. Separate and combined phylogenetic analyses of mitochondrial (COI and 16S rRNA) and nuclear (histone H3 and 18S rRNA) gene sequences revealed six monophyletic groups in Seguenzioidea: Seguenziidae, Chilodontidae, Calliotropidae, Cataegidae, Spinicalliotropis and skenei- morph seguenzioids. Three included skeneimorphs (A. elegans n. sp., Xyloskenea sp. and Ventsia tricari- nata) were ambiguously grouped together with long branches and low statistical supports, possibly suggesting a vast, undiscovered phylogenetic diversity of the group. Taxonomic composition, mor- phological characteristics and evolutionary history are discussed for the skeneimorphs and five other groups in the superfamily.