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Dive Dry with Dr. Bill

#128: Winged Pearl Oyster... Invasive, or Not?

Diving a few weekends ago posed some interesting issues. No, the sediment from the rains had settled and the plant plankton bloom was over so visibility was fairly good. Sure it was windy and there was a good current kicking up as well as swell. These come with "the territory." I can't complain about a "hostile" work environment based on that. For the first time in a number of years the problem had to do with an "alien intruder" coming across the channel floating on the ocean surface. The record rains had washed lots of giant reed (mistakenly called bamboo by many who saw it) from riparian areas on the mainland like the Ventura River.

Giant reed (Arundo donax) is believed to be a native of Asian coastal regions, but has been cultivated in southern Europe, north Africa and the Middle East throughout history. It has also been planted in Australia. It was not known naturally from North America, but became a serious problem in California about 25 years ago. This aggressive invasive weed has reached our shores several times in the past few decades after similar heavy rains. Because it can easily root and take hold here, potentially out-competing our native willows and other plants, I e-mailed Ann Muscat at the Conservancy. She replied that they were making plans to deal with this threat so I was relieved. The last time it happened, when I was still V.P. at the Conservancy, we had many Conservancy staff and "weed warrior" volunteers out on the island's beaches to prevent this plant from establishing here.

Fortunately the giant reed (and a few large pieces of driftwood) stayed at the surface in the park. However when I submerged, I encountered an alien species of a different sort. Several years ago I started seeing a mollusc growing on our local soft corals or gorgonians. It was a bivalve, a two-shelled species like the clams, mussels and scallops. It bore a resemblance to some I'd seen in the waters off Baja California. After some investigation, I learned it was probably the winged or rainbow lipped pearl oyster (Pteria sterna). This is one of the pearl-producing oysters that native peoples like the Seri Indians harvested for ornamental use well before European explorers entered the Gulf of California (Sea of Cortez). Following contact, these oysters were cultivated in the La Paz area by the Spanish for the multi-colored pearls they produced. This industry collapsed in 1940 as documented by John Steinbeck and Ed "Doc" Ricketts. There is still one commercial facility near Guaymas that uses this species to produce cultivated pearls. These oysters are also found along the Pacific coast of Baja.

The winged pearl oyster differs from the oysters normally found in our area. As the name suggests, it has two unequal extensions (or wings) at either end of its shell. One wing is long and narrow and the other is short and stout (like my teapot). The shell itself is somewhat thin and has lines incised on its surface. The length of the shells is said to be 3 1/2" but I've seen a few right in the Dive Park that were longer than that. These oysters prefer subtropical and warm temperate waters. They are usually found in relatively shallow areas (down to about 15 ft). In its native habitat around La Paz, it spawns during the winter when water temperatures are closer to those experienced here. This suggests it may survive and reproduce in our waters, especially if ocean temperatures increase slightly.

The individuals of this species that I find in the Dive Park are all attached to soft corals (gorgonians). In their native habitats they are known to attach to either living organisms or inanimate objects including the rocks. I have only seen one here on anything but a soft coral. Perhaps on the rocky reefs they are out competed by our native invertebrate species, and the soft corals were the easiest way to gain a foothold.

This "alien invader" (as it is called by many marine biologists) poses an interesting philosophical question. The oyster has "invaded" our Channel Islands from its native habitats in the Gulf of California and Pacific coast of Baja. It is possible that the species drifted north as planktonic larvae with the counter-current which flows along the coast in our region. Its appearance shortly after an El Nino condition suggests this is a good possibility. If true, then it would be hard to consider this an "alien invasion" in the biological sense. It dispersed naturally from its native habitat.

There is also a second possibility... that this species hitchhiked to our island. Certainly there are private vessels that travel between La Paz and Avalon. Since vessels from our area might be more likely to be in La Paz or other warm spots during the winter, and this species reproduces then, larvae may have become attached to the vessel or be taken into the bilge and later discharged here. If a few oysters attached to the hull of a boat, and it traveled to Catalina as our waters warmed, then this would become an alien invasive. Although it traveled from its natural place of origin, it did so through human intervention (albeit unintentional).

Perhaps if I finally meet the "mermaid of my dreams," I will collect these "alien intruders" and give her any exotic pearls I find inside! We have all heard that pearls form inside such bivalves when an irritant like a grain of sand gets inside the shell. Apparently this is not the case and the pearls actually form when organic material like uneaten food gets trapped inside the shell. Although diamonds may have been Marilyn Monroe's best friend, I think the love of my life will prefer something that originates from the sea. Besides, it's a lot less costly to harvest a few pearl oysters while underwater (all in the name of science and ecological purity) than to visit the jewelry shop, and I am a starving marine biologist!

Winged oyster on soft coral; one "wing" of the oyster; edge on-views of the shell partially and fully open.