Hyalogyrinidae, Waren & Bouchet, 1993

Hess, Gerhard Haszprunar Erika Speimann Andreas Hawe Martin, 2011, Interactive 3 D anatomy and affinities of the Hyalogyrinidae, basal Heterobranchia (Gastropoda) with a rhipidoglossate radula, Organisms Diversity & Evolution (New York, N. Y.) 11 (3), pp. 201-236 : 233

Affinities of Hyalogyrinidae

Generic division of Hyalogyrinidae

Marshall (1988) noted the close affinity of Hyalogyrina with Hyalogyra and distinguished the two genera mainly on radula characters: Hyalogyra expansa (formula n 6 1 6 n) has considerably more lateral and marginal teeth than Hyalogyrina (formula n 1 1 1 n). Xenoskenea pellucida has a third kind of radula (n-3-1-3-n according to Warén et al. 1993) and is also clearly different from both other genera in showing eyes, distal appendages of the snout, the single tentacle at the posterior end of the foot, the large mantle pad, and a small but true copulatory organ. Accordingly, Hyalogyra and Hyaloyrina appear more closely related to each other than to Xenoskenea .

Position of Hyalogyrinidae among the Ectobranchia and Heterobranchia

We regard Linnean ranks solely as expressions of a relative hierarchy, thus we do not argue pro or contra superfamilial ( Valvatoidea ) versus higher rank ( Ectobranchia ). Ectobranchia is preferred, because (1) Valvatoidea (often cited as Valvatida or Valvatacea) is also a major group of sea-stars, and (2) Ectobranchia is independent of ranking and refers to a clear synapomorphy of the group, the ectobranch gill condition (the inclusion of the gill-less Orbitestellidae cannot be substantiated by any morphological character and is contradicted by the molecular analyses; Dinapoli and Klussmann-Kolb 2010). Among the Heterobranchia (see below), Ectobranchia ( Valvatoidea ) are clearly characterized by plesiomorphic and apomorphic characters:

Plesiomorphic traits mainly concern the alimentary tract, where paired jaws composed of teeth and a dorsal food channel in the buccal roof and the anterior oesophagus are still present and the stomach is still equipped with a cuticularized gastric shield and tooth as well as with ciliary sorting areas. Concerning the radula there is a clear trend to reduce the number of teeth per row: Hyalogyrinidae still show the rhipidoglossate type; Cornirostridae and Valvatidae have a taenioglossate radula with 7 or 9 teeth per row (see Warén et al. 1993 for discussion); Xylodisculidae (formula 2 1 0 1 2) lack a rhachidian tooth; and Orbitestellidae have only three teeth per row (1-1- 1). This transformation series of the radula type character places Hyalogyrinidae at the basis of Ectobranchia by outgroup comparison, since a re-establishment of the rhipdioglossate type looks very improbable.

Van den Biggelaar and Haszprunar (1996) showed that Valvata piscinalis differentiates its mesentoblast 4d cell at the 40-cell stage, whereas opisthobranch and pulmonate taxa do that at the 24-cell stage already. This acceleration has occurred in parallel in the evolution to architaenioglossate (44- to 48-cell stage) and sorbeoconch (40- to 24-cell stage) Caenogastropoda. Together with the other plesiomorphies of the gut (see above) this suggests Ectobranchia as the most basal extant offshoot among the Heterobranchia.

Ectobranch synapomorphies are particularly the diagnostic gill (for Orbistestellidae see above) and the pallial tentacles used for exhalant water current. In addition, a couple of sperm characters (see Healy 1993 for details) separate Ectobranchia from Architectonicoidea and all other heterobranch groups. Also the recent study of Dinapoli and Klussmann-Kolb (2010) placed Cornirostra and Valvata (but not Orbitestella ) in a single clade close to the origin of Heterobrachia. All current data on the Hyalogyrinidae support the concept of monophyly of the Ectobranchia .

The taxon sampling among the ectobranch taxa is still quite poor, and it is likely that several more taxa currently seen as skeneids, vitrinellids or cyclostrematids are in fact ectobranchs. In addition, the Xylodisculidae and several other basal heterobranch taxa (e.g. Tjaernoiidae , Murchisonellidae , Aclididae , Cimidae ) remain unstudied. Therefore, an unambiguous phylogenetic analysis of the other families among the Ectobranchia cannot be provided. Nevertheless, Hyalogyrinidae can be reasonably placed at the very basis of Ectobranchia and Heterobrachia, suggesting an origin of Heterobranchia from the rhipidoglossate level of gastropod evolution. However, this does not necessarily mean that the hypothesis of monophyletic Apogastropoda (Caenogastropoda and Heterobranchia) is obsolete: although a synapomorphic taenioglossate condition of the radula is no longer supported, in particular sperm characters ( Healy 1993; Ponder and Lindberg 1997) still suggest a common origin of Caenogastropoda and Heterobranchia; the rhipidoglossate sister group for both remains unresolved at the present stage of knowledge. Thus, the story will continue: anatomical, spermatological and molecular studies on more basal heterobranch taxa are needed to provide a robust phylogenetic framework of the origin and early evolution of the Heterobranchia.