Rogneda westbladi, Brunet, 1969

Artois, Tom J., 2008, Revision of Rogneda Uljanin, 1870 (Rhabditophora, Eukalyptorhynchia, Polycystididae) with the description of seven new species, Zoological Journal of the Linnean Society 153 (1), pp. 1-28 : 7-8

THE WESTBLADI -GROUP

The westbladi -group consists of three species: two unpigmented ones ( R. colpaerti and R. westbladi ), and one with two dorsal pigment stripes ( R. anglica ). Stylet A of these species is again very simple, consisting of two simple plates. In R. anglica ( Figs 11C , 12C ) and R. colpaerti ( Figs 11B , 12D, E ) it is almost identical, with an S-shaped plate A1, distally ending in a serrated rim and with a rather broad hook at about its midpoint. Plate A2 ends in a point, just before the distal end of plate A1. In R. westbladi ( Fig. 11A ), plate A1 is not S-shaped and does not have a hook, and plate A2 is relatively much shorter. Stylet B of the three species is very complex and apparently both constituting plates have grown together at some places, which is most apparent in R. anglica . The construction of this stylet is, however, comparable in the three species. It consists of a curved plate B1, which distally is rather narrow and rectangular, ending in a serrated rim. In R. colpaerti and R. westbladi this plate has two extensions (x and y in Figs 11A, B , 12D ). Extension y is apparently lacking in R. anglica , but extension x could be represented in this species by the broad plate that seems to be connected to plate B2 (x? in Figs 11C , 12C ). In all three species, plate B2 projects extensively at one side of plate B1 (z in Figs 11 , 12C, D ) and in R. colpaerti and R. anglica also a smaller projection at the other side is observed (arrow in Figs 11B, C , 12C, D ). Plate B 2 in R colpaerti also has a sharp-ending projection near the proximal rim of plate B2 (indicated by a dashed arrow in Figs 11B , 12D ). In all three species, B2 is relatively broad with a broad, serrated distal rim. Thus, also in this group there is conflicting evidence for establishing sister group relationships. The detailed construction of stylet B, especially the presence of the projections at either side of the distal part of plate B1, and the lack of pigment suggest a close relationship between the two Mediterranean species: R. colpaerti and R. westbladi . The almost identical construction of stylet A, however, suggests a relationship between R. anglica (from the Atlantic) and R colpaerti .

As already mentioned in the Introduction and as is obvious from the discussion above, it is extremely difficult to make correct assessments of homology between the different (parts of the) stylets within the taxon Rogneda . In some particular cases, however, it seems fairly straightforward to make homology assessments. For instance, stylet B of the species within the westbladi -group is so typical that it hardly can be imagined not to be homologous among the three species of this group (of course the logical deduction is that stylets A of these three species are also homologous with each other). The same applies for stylet A in the capulata -group, stylets A and B in the hibernica -group and probably stylet B in the falcata -group. Whether stylets A (and B) of the two species of the polyrhabdota -group are homologous with each other is doubtful, a problem discussed above. Within the steueri -group, homologies are also very unclear, and there is little evidence with which to assess homologies. Stylets A of R. cincta , R. palula and R. reticulata are possibly homologous; they all have both plates A1 and A2 with a serrated distal rim. Also plate A2 of R. steueri has the distal rim serrated, but (apparently) plate A1 does not. None of the plates of stylet B has a serrated rim in these species, although Karling (1953) draws a plate B2 with a serrated rim in R. steueri . I, however, cannot confirm this observation, not even after studying the same material he studied. The other two species of this group, R. martensi and R. gallica , have a stylet A with plates that do not have a serrated distal rim. Moreover, stylet A of R. gallica is clearly almost identical to stylet B of R. steueri , which suggests a homology of these two structures.

It becomes even more difficult to search for homologies between the stylets of species of different groups. Karling (1953) considered stylet A of R. hibernica homologous with stylet A of R. steueri , because of the ‘whip-like’ appearance of what I now call plate A2. However, stylet B of the species of the hibernica - group has plates B1 and B2 with a serrated distal rim, thus completely comparable with stylet A of, for example, R. cincta or R. reticulata .

It is almost impossible to assess homologies between groups with one or two aberrant stylets ( falcata -group, tripalmata -group) and the other groups. As already mentioned, there is some resemblance between stylet A of R. falcata and stylet B of R. reticulata . If one imagines plate B1 of R. reticulata without its distal part (i.e. the part distally from the junction with B2), it would be almost identical to stylet A of R. falcata . Whether this reflects a common origin of the two structures remains, however, an open question.

Figures

Figure 11. Stylets of the representatives of the westbladi-group. A, R. westbladi (from the lectotype). B, R. colpaerti sp. nov. (from the holotype). C, R. anglica (A, B stylets A and B, respectively, from the same specimen from Kristineberg, B′ stylet B from another specimen from Kristineberg). Stylet A is indicated by an A, stylet B by a B. For each stylet plate 1 is indicated by a 1, plate 2 by a 2. Scale bar = 25 Mm. Abbreviations: x–z, see text. Arrows: see text.

Figure 12. Habitus and stylets of R. valckei sp. nov. and stylets of representatives of the westbladi-group. A, B, R. valckei sp. nov. (A, habitus from a paratype, B stylets from a paratype). C, stylets of R. anglica (from a specimen from Kristineberg). D, E, R. colpaerti sp. nov. (D, stylet B. E, stylet A). Abbreviations: x–z, see text. Arrows: see text.