Bennelongia cf. nimala, Deckker & Martens & Act, 2013

Deckker, Patrick De & Martens, Koen, 2013, Extraordinary morphological changes in valve morphology during the ontogeny of several species of the Australian ostracod genus Bennelongia (Crustacea, Ostracoda), European Journal of Taxonomy 36, pp. 1-37 : 6-7

Bennelongia cf. nimala nov. sp.

Figs 2-10

Remarks

We are describing the last 4 instar stages (A-4 to A-1) of this species first, because, among the taxa we examined, Bennelongia cf. nimala nov. sp., is the species that displays the largest array of morphological features. The subsequent species will be described using the terminology developed for the present species.

Description

Three major types of ‘ornamentation’ are clearly visible, going from large to small:

1. The ‘ dome ’ is the largest and most prominent feature and lies more or less in the upper central region of the juvenile ostracod valve. It is clearly visible when viewed in a dorsal position ( Figs 2 , 3 ). The French word ‘ coupole ’ seems to be more suitable to describe this feature, which is faintly reticulated and consists of pseudo-pores (viz. that they do not penetrate through the entire valve) and also bears a normal pore with a seta in its middle ( Figs 4 , 5 ). The dome is not met by a depression inside the ostracod valve and is therefore considered to be an (external) ornamental feature. It is also clear that the type of ornamentation recognised on the dome is not unique to that area, as it appears also on most of the valve ( Fig. 5 ). In some instances, there appears to be a second feature in the shape of a dome, but which is additionally set with 2 to 4 large cones of the types discussed below. This is visible in Fig. 4 , Fig. 5 , upper left panel, and Figs 7-10 . The second domal feature is located posteroventrally to the first dome.

2. the cones with an outer lip and a seta at their apex. The seta is substantially larger than the one located on the dome. These features are clearly visible in Figures 5 and 6 . Such type of conical feature has already been recognised in other ostracods, such as the ‘pustules’ illustrated by De Deckker (1981b) and Smith & Martens (1996) near the anterior margin of Eucypris virens , which belongs in Eucypridinae , a family most likely phylogenetically quite close to the Bennelongiinae . It appears that some of the setae penetrate through the entire shell, and could therefore have a different function, as holes are visible inside the carapace of some specimens.

3. the rest of most valves can either be smooth (as seen in the anterior region of the valves) or consist of pseudo-pores (pitted). Nevertheless, normal pores are also present (see Fig. 5 , left panel) and are different from the radial pore canals (see van Morkhoven 1962). The juvenile valve surface is pitted, while the adult valves are set with small pustules.

Finally, a clear depression is seen on the outside of the valves posterior to the largest dome (arrowed in Fig. 7 ). We have been unable to determine whether it is linked with a muscle attachment inside the valve. This feature was not seen in juvenile of other Bennelongia species.

Although the youngest instars (A-4) of B. cf. nimala nov. sp. characterisitically display proportionately larger and more obvious ornamentation ( Figs 3-4 ) than the older specimens (A-3 to A-1 instars), the same pattern of nodes, elevated pores and the domes (see description below) are repeated in all juvenile stages. This is best seen in dorsal views of the carapaces ( Fig. 3 ). Note that none of these features are visible in adult specimens as shown in Fig. 2 (top).

Examination of Figs 7 to 10 enabled us to recognise that the domes and conical pores are located mostly at the same sites on both valves in the same instar stages. Many of the cones with pores are arranged either in rows or in clumps. There are some very slight differences between opposite valves, but this could be an artefact of carapace orientation under the SEM when the rounded specimens are stuck on the double-sided sticky tape. Finally, the positioning of the domes and also cones with pores remain similar from one juvenile stage to the other, thus indicating that the placement of these ornaments may be genetically controlled.

In the case of B. cf. nimala nov. sp., well over 30 cones with the outer lips and setae have been counted in A- 1 specimens and slightly lower numbers were seen in A- 4 specimens, but this is not surprising, knowing that the external surface area of the valves of older instar ostracods is much more extensive than for the juvenile ones. Although there were less cones in A-4 than in A-1, they were more pronounced.

We did not examine inner views of juveniles in this species.

Figures

Fig. 2. External lateral views of Bennelongia cf. nimala nov. sp. from Muggon Lake. Top. adult carapace in right lateral view. Note the pronounced asymmetry of the valves, especially in the anterior region. Middle. LV of A-1 and A-2 specimens. Bottom. LV of A-3 specimen and RV of A-4 specimen. Domes are arrowed.

Fig. 3. External dorsal views of juvenile carapaces (A-1 to A-4) of Bennelongia cf. nimala nov. sp. from Muggon Lake. Domes are arrowed.

Fig. 4. Lateral external views of carapaces of A-1 juveniles of Bennelongia cf. nimala nov. sp. from Muggon Lake. Top in left lateral view and bottom in right lateral view.

Fig. 5. Details of external lateral views of carapaces of Bennelongia cf. nimala nov. sp. juveniles from Muggon Lake. Top left. A-1 showing the dome and many of the cones with outer lips. Note the large pseudo-pores that are elongated over what appears to be a second protuberance. Top right. A-2 showing the cones with the outer lips at their apex. Many of the pores yield a seta. Bottom. RV of A-2 in dorsal view, showing the assemblage of cones with outer lips and the dome, on top of which a seta occurs. Note the hinge at the bottom.

Fig. 6. Details of external views of carapaces of Bennelongia cf. nimala nov. sp. juveniles from Muggon Lake. Top left. anterior margin of A-2 carapace in left lateral view, showing the arrangement of the radial pores and their setae. Top right. enlargement of the cone of A-3 valve, showing the fine seta near its top. Bottom left. ventral view of the two valves, showing the radial pores and setae as well as the very fine and narrow external flange in both valves, but more pronounced in the RV seen on the left here. Note also that claws of some limbs are protruding from the closed valves. Bottom right. enlargement of cone and associated lip and protruding seta of an A-3 specimen. Note the presence of fine particulate material stuck to the valve.

Fig. 7. External views of carapaces of Bennelongia cf. nimala nov. sp. juveniles from Muggon Lake, showing left valves in order to compare the ornamentation between an A-1 specimen (top) and A-2 specimen (bottom). The arrows indicate a clear depression postero-dorsally to each dome, that are linked by a white line.

Fig. 8. External views of carapaces of Bennelongia cf. nimala nov. sp. juveniles from Muggon Lake, showing left valves in order to compare the ornamentation between an A-2 specimen (top) and A-3 specimen (bottom). The circles, ellipses and polygons are used to define the ornamentations that are common to both specimens. The arrows indicate a clear depression, postero-dorsally to each dome, that are linked by a white line.

Fig. 9. External views of carapaces of Bennelongia cf. nimala nov. sp. juveniles from Muggon Lake. The top image shows an A-3 left valve, whereas the bottom image shows a RV of an A-4 specimen that has been ‘flipped over’ digitally, so as to permit comparison of the type of external ornamentation and respective location. The white lines (straight as well as circles and ellipses) permit direct comparison between features. Note the presence of particulate matter as well as diatoms, stuck to the anterior portion of the (flipped over) A-4 RV.

Fig. 10. External views of A-1 carapaces of Bennelongia cf. nimala nov. sp. juveniles from Muggon Lake. The top image shows an A-1 LV, whereas the bottom image shows a RV of an A-1 specimen that has been ‘flipped over’ digitally so as to permit comparison of the type of external ornamentation and respective location. Same comments as for Figure 9 concerning the white lines.