We are moving through phylum Mollusca, and with the gastropods covered, we turn to the bivalves.
Class Bivalvia includes the mussels, clams, scallops, oysters, and shipworms. Most of these
organisms are sedentary filter feeders as adults. Unlike the gastropods, they have no radula,
and very little cephalization, or formation of a head. Most bivalves are marine, but there are
many species that live in brackish water and in freshwater streams, ponds, and lakes. All
bivalves have a compressed body enclosed within a hinged shell. The two sides of these shells are
known as valves, hence the name “bivalves.” In order to discuss these organisms, we will look
at their economic and ecological impact, form and function, feeding, locomotion, eyesight,
and finally reproduction and development. Bivalves are incredibly economically important
and are often lauded as one of the most sustainable seafoods on the planet. This
is because the environmental footprint of bivalve aquaculture is lower than many terrestrial crops,
due in part to the fact that they can be grown rapidly on submerged substrate such as ropes,
wires, and other structures that increase habitats for other creatures. In addition,
since bivalves are filter-feeders, they reduce algal blooms and eutrophication relatively
quickly. A single mussel can filter about 1.75 liters of seawater per hour, and can be raised
humanely at a density of 500 mussels per square meter. In addition, many bivalves
have higher protein content than many meats and plant crops, as well as high levels of essential
omega-3 fatty acids and micronutrients, like iron, zinc, and magnesium. However, the carrying
capacity for bivalve aquaculture is still being researched, and such practices are likely not
feasible in coastal waters the world over. However, since bivalves are so prolific,
there are a great deal of invasive species, often carried unknowingly in the ballast of ships,
that have colonized ecosystems all over the world. In the United States, for example, the Asian clam,
zebra mussel, golden mussel, and the Chinese pond mussel have outcompeted native species,
and in some cases, literally encrusted the corpses of their native competitors, many of which are now
listed as threatened and endangered species. However, in addition to being fast growers,
bivalves are also extremely adept at detecting pollution. They’re so good at it,
that at least one water pump in Poland known as Gruba Kaska, which means Fat Kathy, and a second
water distribution service in Minneapolis, use bivalves hooked up to computers to monitor the
city’s drinking water. When the mollusks encounter heavy metals, pesticides, or other pollutants,
they close their shells, which triggers an alarm. Both locations claim that this biomonitoring
method is one of the most effective technologies for water quality testing. In addition,
the mollusks are unharmed by the process and are released after they serve their time.
In terms of form and function, adult bivalves range in size from the microscopic “seed shells”
that max out at about 1 to 2 millimeters, to the Giant South Pacific Clams that can
reach lengths of over 1 meter and weigh as much as 225 kilograms, or 500 pounds.
Bivalves are laterally compressed, meaning they’re flattened from side to side. This means that the
valves are the sides of the animal, not the top and bottom as they are often incorrectly
illustrated. These two valves are held together dorsally by a hinge ligament and drawn together
using powerful abductor muscles. The umbo is the oldest part of the shell, and the bivalve’s growth
can be viewed as concentric rings around it. All species of bivalves are capable of producing
pearls when a foreign object, such as a grain of sand or parasite, becomes stuck
between the shell and mantle. The animal will secrete layers of nacre, or mother-of-pearl,
to isolate the irritation. However, only two species, freshwater pearl mussels,
and marine pearl oysters, secrete a nacre coating that has the attractive luster of a gem pearl.
The bivalve visceral mass is suspended from the dorsal midline, and the foot attaches to the
visceral mass. The gills, or ctenidia, hang down on both sides. An important anatomical
point to remember is that the ligament and hinge are located on the dorsal or “back” end of the
animal, while the valve opening and feeding structures are located on the ventral end.
Gas exchange in bivalves occurs across both the mantle and gills. The gills of most bivalves
are highly adapted for filter feeding and are comprised of highly folded, lengthened filaments.
Filaments lying beside each other form platelike lamellae through numerous interfilamentous
junctions that increase surface area. In order to feed, most bivalves create respiratory
currents that bring both dissolved oxygen and organic materials to the gills. Here, the gland
cells and labial palps secrete a great deal of mucus in order to entangle particles suspended in
the water column. Most bivalves have an incurrent and excurrent siphon and are sometimes the only
part of the animal that is visible above the substrate. Water is drawn in through the incurrent
siphon and expelled through the excurrent siphon. Some bivalves, like the shipworms, which are
actually a group of saltwater clams with long, soft, naked bodies, feed by burrowing into wood.
Symbiotic bacteria live within their bodies and produce cellulase, which allows them to break down
the cellulose of wood into digestible material. They bore into the wood using a pair of valves
covered in microscopic teeth. If you’ve ever seen a piece of drift wood, or a damaged pier or boat
that looks like it has been drilled full of holes, you’re likely seeing the impact of shipworms,
which are not actually worms at all. Other species of clams, like the piddock or angelwing,
can even bore into stone. They do so using strong valves covered in spines that they
use to slowly cut away at the rock while remaining anchored with their muscular foot.
Adult bivalves that do not anchor themselves permanently to the substrate initiate movement
by extending a slender muscular foot between their valves, pumping blood into their foot, causing it
to swell and then shorten, which pulls the animal forward or down into the substrate. In terms of
locomotion, some bivalves, like some scallops and file shells, can actually swim, albeit in a
jerky motion, by clapping their valves together in order to create a type of jet propulsion. Though
it might look clumsy, the mantle edges can direct the stream of expelled water so the animals can
swim in almost any direction, and since they have eyes, they can actually see where they’re going.
Bivalve eyes vary greatly by taxa. Some have photoreceptive cells in the mantle,
others have pit eyes, mirror eyes, and even compound eyes. However, the only bivalve eyes
that are known to form images are the mirror eyes of scallops, and swimming scallops possess better
vision than sessile scallops. Bivalves also differ in the number of eyes. Some species have eyes that
may number in the tens, while the giant clams have eyes that number in the thousands. This
is interesting because bivalves have a greatly decentralized nervous system, and hardly anything
beyond a series of several ganglia that can be called a brain. How exactly they process
visual information is still being researched. In terms of reproduction, most marine bivalves
discharge millions of eggs in a single season and fertilization occurs externally. The resulting
trochophores and subsequent juvenile stages swim weakly or are carried by the current
into a new resting place where they settle onto the substrate and begin their lives as
sessile individuals. Unsurprisingly, a great deal of the planktonic juveniles are eaten
or carried out to sea, never to become adults. Many freshwater bivalves, especially freshwater
clams and mussels, brood fertilized eggs internally. During the breeding season,
females filter sperm from the water and use it to fertilize their eggs. The eggs develop into
glochidium larva which need to attach to specific fish hosts and live parasitically on their kills
for several weeks to complete their development. Many species simply discharge their larvae into
the water column, but in some species, a unique mantle flap develops in brooding
females to closely resemble a lure that the female wiggles to attract predators. Within that flap,
the female holds her young in a gelatinous pouch known as a conglutinate. So the unsuspecting fish,
thinking it’s going to get a meal, actually gets a mouth full of parasitic larvae that promptly
attach to its gills. The parasitic juveniles then encyst and feed until they complete their
development and drop to the substratum to begin their lives as sessile adults.
And that wraps up our introduction to class Bivalvia. Let’s move forward
and go over some of the most intelligent invertebrates on our planet, the cephalopods.