Dear Art,
I'm sending this message to you since I deleted Peter Froehlich's message
about Donax variabilis by mistake after reading it.
 
I have also observed Donax variabilis with some type of growth on them.  It is
usually seen during the summer months and will involve large populations of
Donax found on local beaches here in Northeast Florida.  The Donax that I saw
with these growths were all alive.  I'll describe the growth attachments as
best I can.  They were allways located at the posterior end of the Donax.
They were  yellow-brown in color and seemed to be thin hair-like structures up
to 1 inch in length.  They were quite thick and spread out in the water when
the Donax tries to burrow into the sand.  As a matter of fact, the growths did
inhibit the Donax from burrowing effectively.  On many occasions the Donax
would be just under the sand with the growth protruding up into the water.  In
some Donax populations along our beach at least 50% of the animals would have
these seasonal growths.  I had always assumed that it was some sort of
attached colonial hydroid.  I guess I'll just have to collect some this summer
and get out the identification books to find out what type of growth it is.
Sincerely,
Charlotte Lloyd