The Daily Bucket is a regular feature of the Backyard Science group. It is a place to note any observations you have made of the world around you. Rain, sun, wind...insects, birds, flowers...meteorites, rocks...seasonal changes...all are worthy additions to the bucket. Please let us know what is going on around you in a comment. Include, as close as is comfortable for you, where you are located. Each note is a record that we can refer to in the future as we try to understand the patterns that are quietly unwinding around us.(All photos by me. In Lightbox...click to enlarge)
Salish Sea, Pacific Northwest
The Purple Varnish Clam (Nuttalia obscurata) is a new arrival to the waters of the Salish Sea, first noticed in 1991 in the Vancouver area, likely having been carried accidentally in ballast water of a ship from Asia, its native origin. In the years I've been walking the beaches of the Salish Sea, it's showed up here and has become abundant in some places. The shells are a distinctive purple (or lavender when faded), covered by a thin shiny brown varnish-like coating. These are small clams, rarely growing larger than 2.5" (70 mm).
An introduced species. And here to stay. Will this new species create problems for the existing community? Most of the time new species do, like outcompeting or preying on natives, exploiting nutrients, damaging the structure of a habitat. If it does, it's considered an "invasive" species. If a new species doesn't significantly alter the established ecosystem, it can be considered an introduced non-invasive, in general parlance referred to as an "introduced" species (even though all non-natives are introduced).
Like most clams, the PVC is a broadcast spawner, spewing eggs and sperms into the water where fertilization occurs, forming planktonic larvae. Even by clam standards, the PVC is a prodigious spawner, and its larvae develop somewhat longer than many (6-8 weeks), giving them a good chance to drift far and colonize new areas. In 15 years, this clam was throughout the Northwest, as seen in the range map for 2005 (arrows represent ocean currents). At 25 years, now probably further.
However, while the PVC is abundant on some beaches, it is absent on others. Aleck Bay beach has none.
Neighboring Barlow Bay is littered with empty purple PVC shells. What gives?
The difference is due to the lower salinity at Barlow Bay, where two culverts pour fresh water from a wetland out into the bay. PVCs do well in less salty water, and even if their larvae enter another bay - even with the same kind of gravelly-sandy-muddy substrate, and wave action - for some reason they are not successful there.
If we look at the common shells on these two typical beaches, we can see something else about what species live here. At Aleck bay, we have (clockwise from top) a Butter clam (native), Bentnose clam (native), Pacific oyster (non-native), Softshell clam (non-native), Cockle (native), and various fragments:
On Barlow Bay, we have a Softshell clam, a Cockle, several Manila clams (non-native), a PVC (non-native), a Blue Mussel (native), a Butter clam, and a Pacific oyster in the middle. Plus a couple of limpets:
The Pacific oyster was brought to America for a commercial shellfish industry. The others came over accidentally, but are eaten recreationally and commercially (Manila clams are Steamers). A high proportion of the bivalves in Washington are already non-native. This is the established ecosystem the PVC has entered.
What role does the Purple Varnish clam have in the beaches where it's found? It burrows fairly deep, the literature says, although I see it within the top few inches here. It filters water for plankton and sweeps the sediment surface for detritus particles, unlike most clams which are more specialized.
The PVCs concentrate in the upper third of the intertidal, which makes them more accessible to terrestrial predators than clams further down the beach. These clams are one of the most common foods I see foraging birds collect.
Gulls reach down though shallow water into soft sand. We can see the "varnish" covering on the live clam in this Glaucous-Winged gull's beak. The shells are fragile enough so the gull can often crush it open directly, as we see in the second photo.
If it doesn't break easily, the gull flies up a short ways and drops it, usually onto the road next to the beach. The crows do that too.
Purple Varnish clams are edible and tasty to people too, but like many clams, they can be toxic, filtering and concentrating dinoflagellates that produce neurotoxins. The Red Tide hotline prohibits collecting shellfish on our beaches more often than not. County beaches are closed right now. I'd be leery of eating PVCs in any case because they are especially likely to filter freshwater runoff which can contain herbicides, pesticides, petroleum products, etc.
So is it invasive? PhD dissertation conclusions of Sarah Dudas, based on experimental and field research:
Currently, the varnish clam does not appear to be causing measurable declines in co-occurring bivalve species in coastal BC (Chapter 3). Varnish clams bury deeper than co-occurring species, decreasing competition for space, and use different feeding modes (suspension, deposit and pedal feeding), likely reducing direct competition for resources (Gillespie et al., 1999). Therefore, the varnish clam is unlikely to exert intense direct competitive pressure on local species (i.e. as compared to an invader species that occupies the same depth and utilizes identical food resources). (p. 122)
However, if interspecific competition is currently low (e.g. perhaps due to the relatively recent nature of the invasion), then competitive effects on native species may simply not yet be detectable.So far, they have not displaced existing species, finding some marginal niche in intertidal range, depth and water salinity. They have become a popular food source for birds, and, by report, some land mammals (eg. raccoons). Perhaps they are not an invasive species. However these are early days. The jury is still out on their long term effect.
In their native range, varnish clams have been shown to decrease the organic content of the substratum by deposit feeding (Tsuchiya and Kurihara, 1980). Thus, if varnish clams can utilize food resources more effectively than native species in coastal BC, they may yet have a significant impact. Given their very high densities, varnish clam bioturbation and biodeposition activities may be substantial, and have the potential to change nutrient fluxes, chemistry, oxygen content and stability of beach sediments (Ahn et al., 1993; Lelieveld et al., 2004; Lenihan and Micheli, 2001; Vaughn and Hakenkamp, 2001). Through these activities, the varnish clam may alter the composition of the benthic communities they have invaded in BC (Vaughn and Hakenkamp, 2001). (p.122)
Invasion Dynamics of a Non-indigenous bivalve, Nuttallia obscurata in the Northeast Pacific by Sarah Elizabeth Dudas, 2005 University of Victoria
Observations of nature in your backyard? What's happening where you are today?
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