[Oyster Anatomy Laboratory]

External Anatomy: Observation and Investigation


This activity uses:
  • A set of valves from an oyster in which the animal has been removed
  • Stereomicroscope (optional)
  • Large magnifying glass (optional)


An oyster is composed of shells called valves. When working with the oyster, be careful — some edges of the valves can be very sharp (Figure 1).

1.   How many valves are there?
1     2     3     4     5     Clear your answer

2.   What general name is given to a mollusk with this many valves?
Univalve     Bivalve     Cephalopod     Clear your answer

The oyster is more pointed at one end. This is the anterior end — called the umbo (Figure 2). The umbo is the oldest part of the oyster.

The posterior end is the larger, curved end — called the bill (Figure 3).

The height of the oyster is the distance from the umbo to the bill (Figure 4).

3.   Determine the height of your oyster (1 in = 2.54 cm).

Observe the size of the two valves and how they fit together (Figure 5).

4.   Are the valves different in size?
Yes     No     Clear your answer

The shorter of the two valves is the right valve. Notice how the umbo end of the longer valve, the left valve, extends beyond the umbo end of the right valve (Figure 6).

Note — Typically, the right valve of an oyster will be shorter and less concave than the left valve. However, since the size and shape of an oyster can be influenced by its environment, observing the umbo end of the oyster is the most reliable way of differentiating between the right and left valves.

5.   Locate the right and left valves.

Separate the two valves to expose the internal surfaces (Figure 7).

On the inside surface of the valves you will observe a dark area. This is the scar from where the oyster's adductor muscle was attached to the valves (Figure 8).

6.   What is the function of this muscle?
Moving from place to place     Opening and closing     Pump for filtering food     Clear your answer

Take A Closer Look

On the inside of surface of the right valve you may see a small indentation no bigger than the head of pin. (Figure 9).

This indentation is the scar from the Quenstedt muscle.

Flip the two valves over to expose the external surfaces of the valves (Figure 10).

The oyster reef is home to a number of different organisms. How those organisms utilize the oyster reef can have a lasting impression on an oyster.

7.   Examine the external and internals surfaces of the valves and identify any artifacts that may remains for some of the commensal and predatory organisms that inhabit oyster reefs.

Common examples of artifacts left behind:

  • Barnacle (Balanus spp.) — commensal
    Empty barnacle shell on an oyster.

  • Barnacle (scar) (Balanus spp.) — commensal
    Footprint scar of a barnacle on an oyster shell. These schares are typically round and have striations radiating from the center. A "footprint" scar is evidence of previous barnacle attachment to the oyster shell.

  • Boring Sponge (Cliona spp.) — predatory
    Holes created by the boring sponge on an oyster.

  • Lacy Crust Bryozoan (Conopeum tenuissimum) — commensal
    Encrusting structure created by the colonial lacy crust bryozoan on an oyster. These colonies have a hony-comb appearance.

  • Hooked Mussel (Ischadium recurvum) — commensal
    Byssal threads remaining after a hooked mussel has been detached from an oyster.

  • Oyster Mud Worm (Polydora websteri) — commensal
    Burrows created by the oyster mud worm can be seen on the inside of an oyster valve. The dark spot that is formed is called a "blister."

  • Limy Tube Worm (Hydroides dianthus) — commensal
    Calcareous tube remains of a limey tube worm on an oyster valve.

  • Oyster Spat Scar (Crassostrea virginica) — commensal
    Scar of the left valve of an oyster spat on an oyster. These scars are smooth and irregular in shape.