Native epifaunal communities on invasive Crassostrea gigas (Mollusca, Bivalvia) in Argyle Lagoon

The Pacific Oyster, Crassostrea gigas (Fig. 1), was introduced from Japan to western Washington in the 1920s to replace the dwindling populations of the much smaller, slow-growing native oyster, Ostrea lurida. C. gigas is more robust and economically viable and outcompetes what little population of O. lurida remains in western Washington (White 2009).

big w-scale

Figure 1: Crassostrea gigas individual collected from Argyle Lagoon, San Juan Island, Wa. This non-native species can grow to 45 cm in length, whereas the native Ostrea lurida only grows to approximately 9 cm.

Crassostrea gigas is abundant in the northwest portion of Argyle Lagoon Biological Preserve on San Juan Island, Washington. The oysters are cemented to the small cobbles that make up the bed of the lagoon and are therefore effectively “free living.” The exterior of C. gigas shells are highly fluted with large, irregular furrows and the inner surface is smooth. The unique texture of the valves provides a heterogeneous microhabitat suitable for many of the native organisms that populate the lagoon. During low tide on July 10th, 2013, I collected from Argyle Lagoon ten C. gigas individuals (7 alive, 3 dead) ranging in size from 8.5 to 21 cm in length. I brought them back to the lab where I used Kozloff’s Keys to the Marine Invertebrates and Lamb and Hanby’s Marine Life of the Pacific Northwest to identify all macrofauna living on and within the non-native oyster shells. Diverse communities of organisms representing five metazoan phyla were found on the surface of the ten C. gigas individuals (Table 1).
   

Table

Table 1: Species found on C. gigas shells in Aryle Lagoon.
*Non-native species
+ Only egg masses found (no adults)

Limpets (Lottia spp.) were present on every oyster examined and occurred in densities of up to 26 Lottia per C. gigas individual. Mossy chitons (Moplia spp.), acorn barnacles (Balanus glandula), and burrowing spionid worms were also found on all ten oysters collected from the lagoon (Fig. 2).

anemone

Figure 2: Members of three different phyla (Mollusca, Annelida, and Cnidaria) occupying the same C. gigas shell.

The high abundance of grazers Mopalia and Lottia on the surface of C. gigas is likely due to the presence of algae such as Enteromorpha, Ulva, chain diatoms, and microalgae on the same shells (Fig. 3). Sessile organisms Balanus glandula, Serpula columbiana, and Mytilus trossulus use the C. gigas shells as a hard substrate for settlement while gastropods Haminoea and Nucella use them as sites for laying egg masses. The empty shells of dead Pacific Oysters provide a potentially safe hiding space for the Pagarus spp. and Hemigrapsus oregonensis found within and even more surface area for grazers (Fig. 4).

algae

Figure 3: Surface of C. gigas valve covered with a layer of algae.

dead

Figure 4: Empty C. gigas shell. Small hermit crabs, shore crabs, tanaid shrimp and amphipods were found between the two valves.

Although C. gigas provides substrate for native fauna, contributes to local plankton populations, and is preyed upon by native gastropods and echinoderms, it has been a problematic species for Puget Sound ecosystems because it competitively excludes the native Olympia Oyster from much of its historical range. When C. gigas was introduced to Western Washington for aquaculture, many other non-native Asian species such as Sargassum, Schizoporella unicronis and Ocinebrinus inornata were introduced with it and are now prevalent and in some cases problematic in our marine habitats (White 2009; Lamb and Hanby 2005; Harbo 2011)(Fig. 5).

limpet and bryozoan (1)

Figure 5: C. gigas with encrusting Schizoporella unicornis. S. unicornis was introduced to Western Washington with Pacific Oyster seed (Harbo 2011; Lamb and Hanby 2005).

Sources:
Harbo, R.M. 2011.Whelks to Whales: Coastal Marine Life of the Pacific Northwest. Harbour Publishing Co., Madeira Park, BC, Canada.

Kozloff, E.N. 1974. Keys to the marine invertebrates of Puget Sound, the San Juan Archipelago, and adjacent regions. University of Washington Press, Seattle, Washington.

Lamb, A. & B.P. Hanby. 2005. Marine Life of the Pacific Northwest. Harbour Publishing, Madeira Park, BC, Canada.

White, J., J.L. Ruesink & A.C. Trimble. 2009. The Nearly Forgotten Oyster: Ostrea lurida Carpenter 1864 (Olympia Oyster) History and Management in Wasington State. Journal of Shellfish Research 28(1): 43-49.

Bailey Craig- University of Washington (B.S. 06/2011)

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