What the Bay HINGES on!
Background Information: Shellfish Basics
Shellfish
What are “shellfish” anyway? Commonly, the term shellfish refers to an aquatic
invertebrate animal with a shell; especially: an edible mollusk or crustacean. So,
shellfish are classified as animals, which have no backbone (invertebrate), and live in
water. Usually, we refer most often to commercially important species that fit the above
description as “shellfish”, although there are many more species that are inedible or not
commonly harvested. There are many species of shellfish distributed all over the world,
but we will focus in this guide on animals that can be found along the Atlantic Coastline,
and specifically within the Barnegat Bay area. Furthermore, we will concentrate on
molluscan shellfish in this guide. The term mollusk represents a group of animals with
no backbone and soft, unsegmented bodies. Because the Barnegat Bay Shellfish
Restoration Program focuses on Hard Clams and Oysters, the crustacean shellfish,
such as shrimp and crabs, will be minimally discussed in this guide.
Biological Classification
Alpha taxonomy is a term used to describe the naming and organization of living things
into hierarchical groups of similarities. Organisms are grouped into the following
categories: Kingdom, Phylum, Class, Order, Family, Genus, Species. Each of these
categories are based on certain characteristics that all members share, such as the
Phylum Mollusca, which have soft, unsegmented bodies, and no backbone. All mollusks
also possess a mantle, a thin layer that surrounds their internal organs, and often
produces a hard, calcareous outer covering, called a shell, or valve.
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What the Bay HINGES on!
All the shellfish discussed in this guide belong the Phylum Mollusca. Nearly all of these
organisms belong to the Class Bivalvia, which means they are surrounded by two
shells. The Whelk, our New Jersey State Shell, belongs in the Class Gastropoda, and
is more similar to a snail or univalve.
Commercially Important Species
The molluscan shellfish that are commercially important species in New Jersey are the
hard clam, soft shell clam, blue mussel, eastern oyster, surf clam, and ocean quahog.
These species are regulated by the Department of Environmental Protection, Division of
Fish and Wildlife, Bureau of Shellfisheries. Whelk are also edible, but are not
commercially harvested from this geographic region. More detail on each of these
species follows:
Hard Clam or Quahog (Mercenaria mercenaria) – The Hard Clam is the most abundant
edible shellfish in Barnegat Bay, and one of the most valuable species commercially.
Hard clams have thick shells, with purple marks inside, and are slightly curved from the
hinge to the front of the clam. They are found in soft mud/sand substrates. Commercial
categories are named by size – top neck, little neck, cherry stone, chowder, and
prepared in a variety of ways. Because of its prevalence in Barnegat Bay, this is the
main species discussed in this guide.1
Oyster (Crassostrea) – These shellfish have white, black, or gray misshapen shells.
Oysters attach to hard substrates, such as other shells. They are a valued food species,
often served raw. Oysters were nearly completely eliminated from Barnegat Bay by
over harvesting and two diseases: MSX and Dermo; Rutgers University Haskin
Shellfish Laboratory on Delaware Bay has developed a disease resistant oyster. One
two-inch oyster can filter up to 40 gallons of water per day.
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What the Bay HINGES on!
Soft-Shelled Clam (Mya) - These are small clams with elongated thin, white shells. They
are also called steamers, or long necks, because their siphons protruding way out of the
shell. They are found several inches deep in sandbars, and are often harvested
recreationally.
Blue Mussels (Mytilus Edulis); Mussels have elongated black shells with a silvery-blue
inside lining. They are often found in the ocean, in colonies, and may grow up to three
inches long. Use byssal threads to attach to rocks, piers, ropes, or other shellfish. Can
filter 10-15 gallons of water per day.
Bay Scallops (Aequipectin Irradians) - As the name implies, these shellfish have
scalloped or ribbed shells, of varying color. Scallops swim freely using jet propulsion.
They prefer to live in eel grass beds with sandy bottoms. Humans eat the adductor
muscle from this species, sometimes referred to as the “eye”.
Surf Clam - This clam is strongly triangular in shape, and may be 6 to 8 inches across.
This is the largest clam on the Atlantic seaboard, and is found in the surf (where waves
crash) area of the ocean as the name implies. The meat of this clam is used in
processed seafood products such as chowders and clam strips.
Ocean Quahog – This slow growing clam is often 3 to 6 inches across and may live to
be 100 years old. This clam has a stronger flavor than most, and is often minced and
added to other foods.
Whelk – There are two kinds of whelk in this area, channeled and knob bed. The Knob
bed Whelk is the state shell of New Jersey. Whelk is a predator of clams, and is eaten
by humans as scungilli.
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What the Bay HINGES on!
Hard Clam Biology
The hard clam will be the model organism for this guide, because of its abundance in,
and importance to the Barnegat Bay. The scientific name of the Hard Clam, or Northern
Quahog (pronounced Co-Hog) is Mercenaria mercenaria (Linnaeus 1758). This animal
is a mollusk within the class Bivalvia, which means that its shell has two valves. The
hard clam can be found naturally from Canada to Florida along the Atlantic east coast,
and is an important commercial species.
The full classification of the hard clam
follows:
Kingdom: Anamalia
Phylum: Mollusca
Class: Bivalvia
Subclass: Lamellibranchiata
Order: Heterodonta
Family: Veneruidae
Genus: Mercenaria
Species: Mercenaria
A simplistic model of the hard clam life cycle is illustrated below. The eggs (A) of M.
mercenaria are buoyant and float with the currents. Fertilization is external in clams, and
each female can produce millions of eggs during a spawn. This process is controlled by
water temperature, has been observed between 24-26°C (75-79 °F) in the Barnegat
Bay area. As larvae develop in the planktonic stage (B), many are consumed by
predators. They continue to develop into swimming trochophore larvae (C) and feed on
phytoplankton drifting within the water. The trochophore will develop into a veliger (D)
that will develop a foot and begin to crawl on the bottom sediment. In the veliger-state
the clam will swim or crawl searching for suitable bottom habitat. Hard clams that
survive to settlement-stage (E) will locate on the bottom, attaching themselves to sand
grains with byssal threads. At this time they develop siphons and the shell develops
ridges.
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What the Bay HINGES on!
Simplified clam life cycle. Adapted from Rice 1992.
They will eventually burrow into the sediment and remain there, living for over 50 years
if conditions are favorable. Every year, the clam adds a layer to its shell in order to
contain its growth. By observing the pattern of ridges on the shell, clams can then be
aged, just like trees. Predation, sediment suitability, water quality and contaminant
levels influence survivorship of clams. Poor water quality conditions may retard growth
and impact clam survivorship and can increase the chances of predation on the early
life stages of hard clams.
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What the Bay HINGES on!
Example of a shell cross-section, illustrating growth lines. Adapted from Rice 1992.
Feeding
The hard clam gathers food by taking in water from its surroundings via an incurrent
siphon (Figure 2). Water and particulate matter, sand, silt and planktonic food all pass
over the gills of the clam. The mucous coating of the gills traps the suspended
materials in the water. Cilia on the outer wall of the gills are used to pass trapped
particles to labial palps where food and other particles are separated. The food is
passed onto the mouth and to the stomach. There is a digestive organ called the
Crystalline style, which contains digestive enzymes and help grind food, such as silicon
diatom shells. The unused particulate matter along with some missed food items are
recycled to the mantle and carried toward the siphon. When enough waste is collected
the clam contracts the adductor muscle, forcibly ejecting water that contains waste from
the incurrent siphon (Kellogg 1903). These wastes are deposited as pseudofeces
outside the shell.
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What the Bay HINGES on!
Model of the interior structures of the hard clam. reprinted by Rutgers Cooperative Extension from the
Atlantic States Marine Fisheries Commission
Oyster Biology
The eastern oyster is similar to the hard clam in many respects. The primary
differences are in the shape and texture of its shell. The fertilization, growth, and
development are similar as well. Oysters prefer to set on hard surfaces, such as other
shell, while clams like to dig themselves into soft sandy bottoms. Oysters are no longer
a primary commercial harvest in the Barnegat Bay area, although Monmouth and
Ocean counties were historically the sites of large, rich oyster beds.
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