Cyndi,

Re:  why little D. lumholtzi has such long spines?

There is a paper in Evolution and Ecology of Zooplankton Communities by
Henri Jean Dumont on biogeography of African zooplankton, I think it gives a
lot of range maps for different taxa, maybe it will be a starting place for
you.
         I am surprised that the spines are not a deterent to predation by
small fish, did this include larval fish?  Maybe lumholtzi big relative to
others where it is native?

Good luck

Chris Mayer
Cornell Biological Field Station