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32 American Seashells
tide. The Coquina Clam, Donax, is faced with much the same problem of
making the most of high tides but, in contrast to the sedentary hfe of its
oyster cousin, it leads a very active existence on the sandy beaches along the
open ocean. It is an attractive sight when a scouring wave suddenly studs
the white beach with dozens of brightly hued clams. The tumbling motion
and sudden exposure to light act as a stimulus to the clam which instantly
thrusts out its small muscular foot and rapidly pulls itself down into the
sand again. During the three or four hours in which the waves are sweeping
the middle and upper sections of the beach, the tiny clams may be uncovered
and obliged to burrow down again several hundred times.
While many clams prefer clean sand as a habitat, others are habitual
mud-dwellers. The handsome Angel Wing, Barnea costata, is usually found
in mud so soft and deep that Florida collectors find it extremely difficult to
reach them. The Angel Wing is usually located one or two feet below the
mud surface and maintains its connection with the bay's waters with its long
siphon. Because of its popularity as a souvenir and collector's item, methods
have been devised to collect them at high tide from a boat or barge. Power-
ful jets of water are forced through hoses, the mud is swept away from the
clams, and then hand-nets are employed to gather them. In more shallow
regions where a mixture of sand in the bay bottom permits walking, the
exposed Angel Wings are gathered by hand at the next low tide.
The majority of marine clams live in a substrate of sandy mud, but a
few have become specialized to the extent of making burrows in exceedingly
compact clay, as in the case of the Arctic Saxicave, Hiatella arctica, and
the False Angel Wing, Petricola. A few groups such as the Date Mussels,
Lithophaga, and the Piddocks, Pholas, burrow into corals, other shells or
soft rocks such as sandstone and limestone. The shipworms, Teredo and
Bankia, are expert at drilling out their long, tube-like homes in wooden
planks of ships, wharf pilings, and manila hemp. So too is the Wood Piddock
or Martesia.
A large proportion of bivalves are found in shallow water, but many
others are typically deep-water dwellers. The bathymetric range for some
species may be narrowly defined in the case of certain scallops. Dipper
Clams {Cuspidaria) and astartes. On the other hand, some species found in
a few feet of water may also occur in depths of over two miles. One species
of Abra Clam, Abra projundorum E. A. Smith, has been dredged in the
mid-North Pacific at a depth of 2,900 fathoms—over three miles!
HOW THEY GROW
The shelly valves of clams are the product of the fleshy mantle. This
thin, leaf-like organ covers the animal as the flyleaves cover the body of a