Exercise 8
Crassostrea virginica: Recruitment in the Lafayette River
This exercise focuses on conservation of an ecologically important oyster species whose populations have
become decimated due to overharvesting, disease and degradation of habitat.
SUGGESTED ELEMENTS FOR AN INTRODUCTORY LECTURE
Crassostrea virginica is an important species ecologically and has been harvested since early
settlers arrived to the Chesapeake Bay
In the past century, the population of Crassostrea virginica has fallen dramatically due to
overharvesting, disease and poor water quality.
Numerous programs are underway to restore oysters to the Chesapeake Bay and its tributaries;
these include constructing reefs made of fossilized oyster shells and creating reef balls to provide
substrata for oyster larvae to settle.
ACTIVITIES
1. Set up shell strings for collection of oyster spat
2. Deposit and monitor shell strings at 2 sites in the Lafayette River
3. Analyze shell strings for spat settlement
VOCABULARY
Reef ball
Larvae
Recruitment
Spat
Settlement
Shell string
Oyster reef
MATERIALS FOR ALL PROCEDURES
Equipment
Compound Microscope
Supplies
Lab notebook
Ruler
Counter
Drill press (for drilling holes in valves)
Four wire lines (shell strings)
Pliers
Rope to attach shell strings to docks
YSI meter
Refractometer
Note: The end of the wire can be bent at one end to prevent valves from sliding off line. The end
of the shell string to be attached to the rope (and dock) can be curved to serve as a handle.
Organisms
Crassostrea virginica shells (washed)
SUPPLEMENTAL MATERIALS
http://web.vims.edu/mollusc/monrestoration/monoyster.htm
VENDORS FOR MATERIALS
Shells can be collected locally from restaurants. Additional information on obtaining used oyster shells
can be found at http://www.oysterrecovery.org/Content/ContentDisplay.aspx?ContentID=88
Drill press can be purchased at www.sears.com or www.amazon.com
Crassostrea virginica: Recruitment in the Lafayette River
The native oyster species, Crassostrea virginica, is commonly found in the Chesapeake Bay and
its adjoining rivers. This species has been significant since the settlement of the Chesapeake Bay in the
1600’s when early settlers were dependent on it as a staple food item. During recent times, however, the
oyster has undergone a serious decline. With the advancement of fishing techniques, equipment and
technology the decimation of populations has occurred that were once very abundant. In efforts to
replenish oyster populations many organizations such as the Virginia Institute of Marine Science (VIMS)
and Chesapeake Bay Foundation have planted oyster reef balls (Fig 3) in tributaries of the Chesapeake
Bay. The oyster reef balls are cement structures that provide substrata on which oyster spat (Fig. 4) can
settle and form oyster reefs (Fig. 3). It is therefore important to understand where the largest amount of
oyster recruitment is in the Chesapeake Bay and its adjoining rivers (Chester River, Choptank River,
Nanticoke River, Pocomoke River, Patapsco River, Patuxent River, Potomac River, Rappahannock River,
York River, James River, and Lafayette River). By understanding where and when oyster recruitment
occurs in these rivers, researchers will be able to successfully plant reef balls where oysters can thrive and
accrete.
Figure 1. Satellite view of the Chesapeake Bay.
Figure 2. Adult Crassostrea virginica shells.
Figure 3. Two oyster reef balls(l); oyster reef (r).
Lab Objectives:
1. To set up shell strings for collection of oyster spat
2. To deposit and monitor shell strings at 2 sites in the Lafayette River
3. To analyze shell strings for spat settlement
In today’s lab you will examine recruitment and settlement of Crassostrea virginica in the Lafayette
River. To do this, you will need to create shell strings on which larvae can settle, or attach. To create the
shell string, thread ten predrilled, dried oyster valves on a wire line (See Fig. 5). The shell strings will be
placed at two sites (attached to docks) in the Lafayette River for 5 days. At each site also record water
temperature and dissolved oxygen using the YSI meter and salinity using the refractometer.
After 5 days the shell strings will be retrieved, returned to the laboratory, and hung in open air to dry
for 24 hours. (The wet shells can create a sheen appearance that makes it difficult to spot the spat).
Remove each valve from the string, place it under the compound microscope and count the number of
spat on both sides of valve (Fig. 4).
Figure 4. Crassostrea virginica spat.
Figure 5. Shell string for collection of oyster spat.
Questions
1.
2.
3.
4.
5.
How many oyster spat did you count on each valve?
Were there differences in larval recruitment between site 1 and 2?
What other organisms were observed on the valves?
How would you describe the density of spat on the valve shells?
Would you expect to see differences in the density of larvae settling during different seasons?
Explain your answer.