Little-known and phylogenetically obscure South African estuarine microgastropods (Mollusca: Truncatelloidea) as living animals

ABSTRACT

Areas of the Knysna estuarine bay in the Western Cape are dominated by three endemic South African truncatelloid microgastropods, temporarily known as ‘Hydrobia’ knysnaensis (Krauss), ‘Assiminea’ capensis (Sowerby) and ‘Assiminea’ globulus Connolly. Although first described 80–170 years ago and present in abundance (up to 100,000 m⁻²), they remain surrounded by confusion and still await taxonomic assignment, largely because they appear most atypical members of their groups by virtue of anatomy and/or biogeography and/or habitat. This study contributes in-life perspectives to morphological and phylogenetic analyses known to be on-going. At Knysna, they are syntopic: at least two occurring in >85% and all three in >40% of individual 0.0026 m² samples from their region of dominance. Nevertheless, they tend to greater abundance in divergent microhabitats; ‘A.’ globulus dominating higher tidal levels, and ‘A.’ capensis and ‘Hydrobia’ lower ones; the former especially unvegetated sediment, the latter, if anything, seagrass. Interspecific feeding interactions appear unlikely to be responsible for these patterns, other evidence suggesting that all are maintained below carrying capacity. Field biology of ‘H.’ knysnaensis generally appears equivalent to that of northern-hemisphere intertidal hydrobiids and that of ‘A.’ globulus is typically assimineid, albeit at atypically low shore height. Unlike assimineids, however, ‘A.’ capensis is truly aquatic. The success of these truncatelloids in unusual circumstances may be consequent on the absence from South Africa of other microgastropod groups that fill their niches elsewhere in the southern hemisphere.

KEYWORDS:

• Assimineidae
• benthos
• feeding
• Gastropoda
• Hydrobiidae
• Knysna
• saltmarsh
• seagrass

Acknowledgements

I am most grateful to Rhodes University Research Committee for financial support; to SANParks Scientific Services at Rondevlei and the Knysna Area Manager, Johan de Klerk, for permission to undertake fieldwork in the Knysna Section of the Garden Route National Park under permit BARN-RSK/2009-002a; to Bronwyn Tweedie, Rhodes University Graphic Services Unit, for producing Figure 1; to Ruth Farre, South African Navy Hydrographic Office, for advance information on the Knysna Tide Tables; to Nelson Miranda for valuable liaison; and to Brian Allanson for his support.

Disclosure statement

No potential conflict of interest was reported by the author.

Notes

1. The molecular analyses of Wilke et al. (2013) suggest that the endemic African genus Tomichia and the related Australian salt-flat Coxiella may belong to a Gondwanan subfamily or family separate from the remaining (Holarctic and Oriental) Pomatiopsidae.

Additional information

Funding

This work was supported by the Rhodes University Research Committee.