3D-anatomy and systematics of cocculinid-like limpets (Gastropoda: Cocculiniformia): more data, some corrections, but still an enigma

New material and new methodologies substantially widen the anatomical knowledge on cocculinid limpets. We first provide 3D-anatomies of Fedikovella caymanensis and Teuthirostria cancellata based on serial sections. Both species differ in several major points (mainly the gill-type and several features of the alimentary tract) from typical cocculinids, accordingly they are classified in a new clade, Teuthirostriidae fam. nov. Specimens studied by McLean and Harasewych (LA County Mus Contrib Sci 453:1–33, 1995) under “Fedikovella beanii” probably represent another species new to science. Additional investigations of original (type) section series of Cocculina laevis Thiele, 1904 (type species of Paracocculina Haszprunar, 1987) and of Cocculina radiata Thiele, 1904 (type species of Coccocrater Haszprunar, 1987) imply some nomenclatorial revisions: Cocculina cervae Fleming, 1948 is designated as type species of Pedococculina gen. nov. Anatomical characters confirm the subsequent placement of Cocculina viminensis Rocchini, 1990 into Coccopigya Marshall, 1986, whereas the original generic status of the whale-fall inhabitant Cocculina craigsmithi McLean, 1992 is confirmed despite the unusual habitat. The latter species probably has symbiotic bacteria in the midgut gland; if so this might be due to the environmental and feeding conditions at whale cadavers or hydrothermal vents. Contrary to Lepetelloidea, the Cocculiniformia cannot be included in Vetigastropoda. Recent molecular data support a sistergroup relationship of Cocculiniformia with Neomphalida, and we add the phenotypic perspective on this so-called “Neomphaliones”-hypothesis. In particular, more phylogenomic data are needed to specify the position of Cocculinida among the rhipidoglossate Gastropoda.

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.

Expression of six3 and otx in Solenogastres (Mollusca) supports an ancestral role in bilaterian anterior-posterior axis patterning

The homeodomain transcription factors six3 and otx are involved in patterning the anterior body and parts of the central nervous system (CNS) in bilaterians. Their similar expression patterns have been used as an argument for homology of heads, brains, segmentation, and ciliated larvae. We investigated the developmental expression of six3 and otx in the aplacophoran mollusk Wirenia argentea. Six3 is expressed in subepithelial cells delimiting the apical organ of the solenogaster pericalymma larva. Otx is expressed in cells of the prototroch and adjacent regions as well as in posterior extensions of the prototrochal expression domain. Advanced larvae also show pretrochal otx expression in the developing CNS. Comparative analysis of six3 and otx expression in bilaterians argues for an ancestral function in anterior-posterior body axis patterning but, due to its presence in animals lacking a head and/or a brain, not necessarily for the presence of these morphological structures in the last common ancestor (LCA) of bilaterians. Likewise, the hypothesis that the posterior border of otx expression corresponds to the border between the unsegmented head and the segmented trunk of the LCA of protostomes is not supported, since otx is extensively expressed in the trunk in W. argentea and numerous other protostomes.