[Lord_Bob]

I don't think clam hoarding can actually get out of hand unless it is very late game, or the power starts as a nation that gets water gems from it's national site, but here are a few ideas:

Mental Drain!
Each clam "creates" it's pearl from the mental and magical energy of it's holder. While this has little effect on the battlefield, the daily drain slows research considerably.
-1 or -2 rp per turn
(thus clams DO COST RESEARCH to have on those research mages)

This change can be added by simply adding a negative Sage bonus to the clams item description.

The Care And Feeding Of Clams:
Clams cost gold upkeep

Hungry, Hungry Clams!
Each clam "consumes" one water gem and produces 1-2 astral gems per turn
clam cost adjusted appropriately

Power Drain:
Clams have a negative effect on holders... lower MR for example. Making them more vunerable to spells and assasins.

Bloodstones:
Horror marking at a real level of power
or
The research penalty.