Hello Conch-L'ers
 
Though my experience with Donax is very limited (the family is not
represented in our local fauna), I might offer a couple of general
observations, which might apply here.  In many cases, the location of
algal growth on a bivalve shell can shed some light on the natural
history of the animal.  My own observations lead me to believe that
algae generally do not tend to grow on shell surfaces that are
covered by sand or mud.  In living bivalves, this may be due in part
to the movement of the animal in the sediment, which causes a sort of
scouring action by the sand or mud; but probably the principle factor
is the exclusion of light below the surface of the sediment.  Algae
are photosynthetic organisms, and need sunlight to grow, just as
plants do.  In the first posting on this topic (sorry, I have deleted
it, and don't remember who posted it) the writer mentioned algae
attached to the posterior end of Donax shells.  This is typical of
many sand-dwelling or mud-dwelling species.  The siphons of almost
all bivalves are located at the posterior end; therefore they orient
themselves in the sediment with the anterior end down, and the
posterior end up.  In species which burrow deep into the sediment, so
that the shell is completely buried, algal growth I would guess is
unlikely (hey, Florida folks, do living angel wings ever have algae
on them??).  Many bivalves however, do not live completely buried,
but position themselves so that the upper (posterior) end of the
shell protrudes out of the sediment, in some species just barely, in
others 10% of the shell, and in others as much as a quarter of the
shell may be exposed.  This exposed portion of the shell may
accumulate algal growth (provided the water contains enough nutrients
to support such growth), while the buried portion tends not to do so.
You can often find dead shells with one green end, and a distinct line
between the green and non-green portions.  This is an indication of
how deeply the living animal inserted itself into the sediment.  The
fact that Donax (according to the posted report) do sometimes have
posterior algal growth suggests that they do have a habit of leaving
their little posteriors exposed, and I speculate that the reason they
seldom show such algal accumulations is a combination of their
vigorous activity, and their clean sand habitats, which typically
have reduced dissolved nutrients compared to other marine
environments.  Of course, species which live on the surface of the
sediment, like Pectens and Oysters, may have algae growing on them
anywhere, or everywhere.  But when a burrowing species has algae
anywhere other than the posterior end, I'd suspect that such growth
occurred after the death of the bivalve, when the shell was fully
exposed.
This tendency of algae to attach to exposed shell surfaces makes for
an interesting method of collecting, which some folks are no doubt
familiar with, but others may not be.  Here in New England (and
probably all the way down the coast, I'm not sure) we have a large
green alga (Codium sp.), commonly called "sponge seaweed".  It has
soft, thick green "branches", and may grow up to about 3 feet in
length.  It commonly attaches to the exposed parts of living
bivalves.  While the alga is small, there is no problem, but as it
grows bigger the constant back and forth action of the waves tugging
on the alga gradually works the bivalve loose from  the sediment (or
perhaps not so gradually if there is a storm).  The Codium, with
bivalve attached,  then floats in and gets deposited on the beach.
The bivalve, still alive, digs back down into the sand.  So, at low
tide, you can walk along the beach and look for stranded Codium which
appears to be growing out of the sand.  Since Codium doesn't grow in
sand, but only on hard surfaces, you know that some hard object is
buried there.  You grab the Codium and give a yank, and as often as
not you'll find a Mercenaria, Anadara, Tagelus, or some other bivalve
that wouldn't have been visible otherwise.  Just like picking carrots
in the garden.  Of course, sometimes the hard object will turn out to
be a rock.
 
Paul Monfils