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Quahog Development
and Growth
The quahog, Mercenaria
mercenaria, develops from eggs and spends the beginning of its life
as plankton,
floating near the surface of the water.
It goes through a series of development stages that ends with the form
with which we are familiar in the hard-shell clam. The following information
and images are taken from The Northern Quahog: The Biology of Mercenaria
mercenaria by Michael Rice. Check out a dissection
of a quahog.
Eggs
and fertilization
 |
The eggs of quahogs are 70 to 73 microns (1 micron = 1/25,400 inch) in diameter and are surrounded by a gelatinous membrane that is 50 microns thick. Both eggs and sperm (a) of adults are expelled in the water current of the excurrent siphon. Fertilization proceeds externally in the water column. |
Embryonic stages
The embryonic stages
include the fertilized
egg (b), early
cell divisions (c,d,e), and the morula
(f). These stages end with the blastula, a hollow ball of cells.
It takes from 18 to 48 hours to complete these stages, depending on the
temperature.
Larval stages
The first larval stage
is the trochophore
(g,h). After these stages, the larva is called a veliger, because it has
a velum (literally, a "curtain"), a large sail-shaped organ
that extends from the small larval shell. The velum is ciliated, meaning
that it has hairlike structures on it. The velum does several things.
It captures small particles of food. It also makes a current that passes
water over the clam's gills so it can breathe. And it even helps the clam
move a bit in the water column.
Larval stages include the straight-hinge
veliger (i), the umbonate
veliger (j), and the pediveliger
(k). The straight-hinge veliger (also known as the "D-hinge"
veliger) is given its name from the shape of the larval shell. The larval
shell of the of the umbonate veliger has the characteristic triangular
shape of quahogs.
Most of the larval stages tend to swim toward light (or opposite the force
of gravity), so they tend to be concentrated in the surface water. During
this time, they are dispersed by wind, waves, and currents.
The pediveliger (ped = foot) stage is the final stage prior to settlement
and eventual metamorphosis to juveniles. Pediveligers have a foot that
extends from the shell. Eventually, the pediveligers begin to settle on
the bottom of the bay.
Depending on water temperature, the time from trochophore to settlement
may last from eight days to two weeks.
See below for:
a. Unfertilized egg and sperm
b. Fertilized egg and polar body formation
c. First cell division
d. Four-cell embryo
e. Eight-cell embryo showing spiral cleavage
f. Morula
g. Early trochophore larva (post-gastrulation)
h. Fully developed trochophore larva
i. D-hinge veliger larva
j. Unbonate veliger larva
k. Pediveliger larva
l. Developed post-set juvenile
Growth
Once the quahog settles
to the bottom of the bay, it uses its foot to dig into the mud. Its secretes
a calcium carbonate shell that grows as the clam's internal organs grow.
Shell deposition, or growth, depends on temperature. It occurs mostly
between the temperatures of 10° C (50° F) and 25° C (77°
F), with growth ceasing below 9° C (48° F) and above 31° C
(88° F). In most areas within the geographic range of the quahog,
the winter water temperature drops below 9° C, so growth ceases and
there is an annual "winter break." This can be seen as a translucent
layer in the shell. So if you slice a cross-section of the shell, you
can find out how old the clam was. Quahogs may live to be more than 50
years old.
