Korrie, you're right about them being similar to burrowing estuarine prawns (mudprawns and sandprawns). These guys, as Chris said, 'pump' water through their burrows. The large tusks are used for burrowing. The Ephemeriidae also burrow and have tusks, but the points face the opposite way. Also only in more tropical rivers.
If you think that your find is a first for the Vaal, you should by all means collect some in alcohol and get in touch with the entomologists at the Albany Museum's Department of Freshwater Invertebrates or any other relevant parties (i'm sure listed in you book).
I wonder how they stand up to a carp's 'grunter-like' diggings?
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