Echonitor Iredale, 1937

Hyman, Isabel T. & Ponder, Winston F., 2010, A morphological phylogenetic analysis and generic revision of Australian Helicarionidae (Gastropoda: Pulmonata: Stylommatophora), and an assessment of the relationships of the family 2462, Zootaxa 2462 (1), pp. 1-148 : 72-73

Echonitor Iredale, 1937

Echonitor Iredale, 1937a: 27 . Type species Thalassia cyrtochila Gude, 1905 (OD).

Description. External morphology: Shell of 4.3–4.6 whorls, spire and apex slightly raised. Protoconch sculptured with incised spiral grooves; teleoconch with microscopic spiral grooves (only examined in E. cyrtochila ). Whorl profile rounded above and below ( E. cytrochila ) or flattened above ( E. albumenoidea ) an evenly convex periphery. Internal walls of early whorls complete; umbilicus very narrowly open. Mantle laps narrow, finger-shaped, not fused, uniform in colour. Mantle lobes very small (only examined in E. cyrtochila ). Caudal apparatus as for family; caudal foss diamond-shaped ( E. cyrtochila ) or vertical ( E. albumenoidea ) slit in tail.

Mantle cavity (only examined in E. cyrtochila ): As for family. Mantle with no visible minor blood vessels; pigmentation of white spots.

Digestive system: Oesophageal crop absent.

Genital system: As for family. Carrefour and talon not examined. Free oviduct long; capsular gland present; internal longitudinal pilasters present. Bursa copulatrix inserted on vagina, moderately long, more than half spermoviduct length ( E. cyrtochila ), or moderately short, about half spermoviduct length ( E. albumenoidea ); duct of bursa copulatrix wide, distinguishable from bursa copulatrix, internally smooth. Epiphallus enters penis through simple pore; penis internally covered in flat, ridged, diamond-shaped pustules ( E. cyrtochila ) or flat ( E. albumenoidea ), pilasters absent ( E. cyrtochila ) or present ( E. albumenoidea ), diverticulum on penis absent. Penial sheath present, enclosing only penis; penis retractor muscle attached to epiphallus (attached to penis in one specimen of E. cyrtochila ). Epiphallus internally with longitudinal pilasters. Epiphallic retractor caecum absent. Flagellum and lime-sac absent. Spermatophore not present in dissected material.

Radula (only examined in E. albumenoidea ): Relatively long and narrow. Central tooth with small ectocones; mesocone short and rounded, much shorter than tooth base. Lateral and marginal tooth fields distinguishable. Lateral teeth with endocones much smaller than central tooth ectocones; ectocone equal in size to those on central tooth; mesocone slightly longer than tooth base. Marginal teeth with endocones absent; ectocones shorter and narrower than the mesocone, not subdivided into extra teeth.

Remarks. Echonitor was introduced by Iredale (1937a) for Thalassia cyrtochila Gude, 1905 , Helix albumenoidea Cox, 1868 , Helix waterhousei Cox, 1868 and Echonitor euroxesta Iredale, 1937 , all from South Australia. Iredale noted that the type locality given for Echonitor waterhousei (given only as South Australia) might have been incorrect, as no other shell matching the figure is known from South Australia. Echonitor waterhousei was introduced as a replacement name for Helix (Thalassia) subangulata Angas, 1863 . The holotype was never illustrated and is now missing and this taxon was appropriately listed under Incertae Sedis by Smith (1992).

Two species previously placed in Echonitor were examined: E. cyrtochila and E. albumenoidea . Echonitor cyrtochila was found to be very similar anatomically to Periclocystis ardeni , sharing the distinctive internal structure of the penis and the extremely thick penial sheath. However, there are considerable differences between the shells of the two species, so the two genera have not been synonymised. Echonitor albumenoidea shares several characters of the shell with E. cyrtochila but differs anatomically. However, only one specimen was available for dissection, so E. albumenoidea is tentatively retained in Echonitor . A third species, E. euroxesta , was not examined in the current study but probably belongs in Echonitor on the basis of shell characters.

This genus shares with Periclocystis , Tarocystis , Sheaia and Levidens the lack of a flagellum and lack of an epiphallic caecum. The absence of these features could suggest that this genus belongs to Euconulinae (Euconulidae) rather than Helicarionidae . However, the shell shape and sculpture indicate a closer affinity to Helicarionidae . The flagellum and epiphallic caecum are both characteristic features of Helicarionoidea but appear to have been secondarily lost in a number of different taxa [e.g., the flagellum in some Urocyclidae ( Van Mol 1973) and the caecum in the southeast Australian taxa Helicarion , Mysticarion , Parmavitrina , Peloparion and Brevisentis ]. There is high probability that the absence of the flagellum and caecum in Echonitor , Periclocystis , Tarocystis , Sheaia and Levidens is due to secondary loss, although this is not supported by the phylogenetic analyses, which consistently show these genera to be the most basal members of Helicarionoidea ( Figures 1–4 ). One of these species, Echonitor cyrtochila , was included in a phylogenetic study based on mitochondrial DNA ( Hyman et al. 2007). In all their analyses this species was included in Helicarionidae , indicating that it must have lost the flagellum and caecum secondarily ( Figure 44A ; see Discussion).

Figures

FIGURE 1. Majority-rule consensus tree (1,743 trees, length 117 steps) from the morphological analysis containing representatives of all groups. Families and subfamilies are shaded and are labelled in bold if the group is not monophyletic.

FIGURE 2. Majority-rule consensus tree (646 trees, length 106 steps) from the morphological analysis with Dyakiidae and Urocyclidae excluded. Families and subfamilies are shaded and are labelled in bold if the group is not monophyletic.

FIGURE 3. Majority-rule consensus tree (770 trees, length 117 steps) resulting from the morphological analysis with two additional mantle lobe characters. Families and subfamilies are shaded and are labelled in bold if the group is not monophyletic.

FIGURE 4. Australian Helicarionidae. Majority-rule consensus tree of 13,839 trees, length 99. Where two or more species from a genus are included, the genera are shaded. Groups A and B are referred to in text.

FIGURE 44. Trees resulting from the phylogenetic analysis of two molecular datasets. Families and subfamilies are shaded. A. Mitochondrial gene cytochrome c oxidase subunit I [after Hyman et al. (2007)]. B. Nuclear gene 28S rRNA [after Wade et al. (2006)].