Distributions & community interactions
  Not much is written about community interactions of the indigenous Pacific oyster Ostrea conchaphila, probably because so few of them remain in natural populations. In comparison, more is known about the introduced species Crassostrea gigas, but mostly from an aquacultural standpoint.
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Research study 1
 

graph showing number of meiofauna in mud, oyster, and eelgrass habitatsTo what extent does the introduction of oysters Crassostrea gigas to a community affect number and extent of habitat refugia for motile animals such as fishes and decapods? This is assessed for different habitats in Willapa Bay, Washington by researchers from the University of Washington.  Results show, in fact, that species composition of fishes and decapods is more strongly related to location within the estuary than to specific habitat.  Thus, while both eelgrass and oysters significantly increase habitat availability over bare mudflat, there is no significant difference between them (see graph).  Hosack et al. 2006 Estuaries & Coasts 29: 1150;  for a recent review of impacts of oysters Crassostrea gigas on west-coast intertidal communities see Padilla 2010 Integr Comp Biol
50: 213.

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Research study 2
 

photograph of wave-exposed island site in Barkley Sound, B.C.The Pacific oyster Crassostrea gigas has enjoyed limited natural-colonisation success after its repeated introduction for aquaculture purposes from the early 1900s.  Does this owe to abiotic factors such as temperature and salinity, or to biotic factors such as predation and competition?  With respect to biotic factors, to what extent does a community “resist” invasion by a non-indigenous species like C. gigas?  Is resistance greater in species-rich community assemblages, where many ecological roles are already filled, than in species-poor ones?  This interesting subject is investigated by a researcher at the Bamfield Marine Sciences Centre, British Columbia by transplanting newly settled juvenile oysters (<1cm) into 2 different west-coast habitat-types with differing native guilds (for example, more or less predators and competitors) and monitoring their growth and survival1.  One habitat2 type is mid-intertidal level on a shore with low wave exposure, an environment where the species is often common; the other is low-intertidal level on a shore with high wave exposure,  an environment where oysters are generally rare.  The “transplants” consist of several juveniles attached to a large oyster shell or cultch, the latter of which is screwed to the rock substratum.  Four separate transplants are done during spring/summer over a 2-yr period, each monitored for roughly 2mo.  Predator intensity is controlled in different factorial-treatment combinations through use of stainless-steel “exclusion” cages.  Results are predictably variable with so many interacting factors, but the author provides a bit of a summary. After 2mo, growth is faster at lower tidal elevations owing presumably to increased feeding time, but survival is less owing to greater exposure to predators.  Growth is almost 60% more at wave-protected sites than at wave-exposed sites (11.4mm vs. 7.2mm, respectively), a result possibly of reduced feeding in high rates of water flow.  Some reduction of growth, according to the author, owes to physical interference from neighbouring organisms3.  This takes the form, in the case of mussels, of them “leaning” in to cover the oyster transplants.  Thus, both guilds of native species appear to exert biotic resistance to the oyster presence, predation in the one, and competition for space in the other.  Interestingly, the presence of close neighbouring native species in some wave-exposed sites actually improves oyster survival, possibly owing to protection from water and/or debris impact.  Predation is about the same at both wave-exposed and wave-protected sites, but with different predatory species involved.  The author summarises the results of this interesting paper by saying that abiotic and biotic factors jointly affect the success of the oyster “invasion”, sometimes additively (growth) and sometimes not (survival).  Ruesink 2007 Mar Ecol Progr Ser 331: 1.

NOTE1   growth and survival are analysed as a function of 2 abiotic variables (wave exposure and tidal elevation) and 2 biotic variables (predation intensity and competition by neighbours)

NOTE2   rocky-shore habitats are selected on each of 3 islands in Barkley Sound, British Columbia, with the wave-exposed one in each pair fronting a 6km fetch, and the wave-protected one a 0.3km fetch.  In each set of habitats, water temperatures are consistently >15oC during summer, meeting the minimum required for survival and growth of larval oystersphotograph of wave-protected island site in Barkley Sound, B.C.

NOTE3   these include potential space-competitors such as mussels, barnacles, and sea anemones.  The author as well considers these species and some filter-feeding crabs as potential food competitors, but realistically only mussels of the species listed would befiltering out particles similar to those used by oysters.  Also, food-competition among filter-feeders, whether inter- or intra-specific, has not often been convincinglydemonstrated.  In the present study,only indirect evidence of food competition is provided, and other explanations may exist

 

 

Typical wave-exposed (upper Left) and wave-protected
(lower Right) island sites in Barkley Sound, B.C.
of the type chosen for study of oyster "invasions" 

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Research study 3
 

photograph showing oyster-bed inhibition on growth of eelgrassInterestingly, the presence of Pacific oysters Crassostrea gigas appears to be deleterious to survival and growth of eelgrass Zostera marina.  Note in the photograph that eelgrass grows only where oysters are absent higher up the shore (see "no oysters" in the photograph). Where oysters are present, eelgrass is absent (see "below-oyster zone" in the photograph). The effects may be both physical, in altering patterns of water flow, and chemical, in increasing sulphide content of the sediments.  However, although sulphide is known to be toxic to eelgrass, the connection between oysters and increased sulphide content of the sediments is still speculative.  Researchers from the University of Alberta, Edmonton perform experiments at Cortez Island, British Columbia where eelgrass plugs are transplanted at varying distances below oyster beds to determine the distance and magnitude of the inhibitory effects.  Results indicate significant inhibition of eelgrass growth in the “below-oyster” zone after an 85-d exposure period.  In view of their results, the authors express concern that predicted warming trends associated with climate change may exacerbate eelgrass loss through increased natural production of oysters on British-Columbian shores.  Eelgrass beds form important nursery areas for crabs, shrimps, clams, and fishes such as salmon and herring, and their loss from human-induced disturbances is already great without adding to it by further expansion of oyster-culture beds.  Kelly & Volpe 2007 Botanica Marina 50: 143.

NOTE  oysters add considerable amounts of organic matter to the sediments in the form of feces and pseudofeces.  This promotes growth of anaerobic bacteria that release sulphides, in amounts up to 10 orders of magnitude greater than at oyster-free reference sites.  Such sulphides are detectable by yellowish-black colours and a rotton-egg smell

NOTE  these include dredging, waste spills, sediment re-suspension, eutrophication from agricultural runoff, and other activities

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Research study 4
 

map showing historical distribution of Olympia oysters along the west coast of N.A.The historic range of the Olympia oyster Ostrea (lurida) conchaphila was from Alaska to Baja California (see map), but over the last 2 centuries the species has suffered massive declines in numbers.  Possible causes include overharvesting (see graph below), pollution, habitat loss, and others, but no one factor from a long list seems to have been paramount.  Over the past decade, U.S. federal financing of restoration programmes has renewed maricultural and general interest in the species.  McGraw 2009 J Shellf Res 28: 5. Map courtesy Marti McGuire NOAA Restoration Center.

NOTE this Research Study is actually an introductory paper to an entire issue (No. 1) of Volume 28 (2009) of the Journal of Shellfish Research dedicated to Olympia oystersgraph showing landings of Olympia oysters Ostrea conchaphila in the U.S. from 1888-2006

photograph of Olympia oysters courtesy Royal Provincial Museaum, Victoria, British ColumbiaNOTE  in Canada in 2000, O. lurida was listed as a species of special concern by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC).  In 2009, the Department of Fisheries and monitoring of abundance levels at designated “index sites” over a defined period.  Recently, the Canadian Science Advisory Secretariat has recommended 13 such index sites for monitoring, most on Vancouver Island, British Columbia (see map).  DFO 2010 DFO Can Sci Advis Sec Sci Advis Rep 2010/05; for information on the status of Olympia oysters in British Columbia see Gillespie 2009 J Shellf Res 28: 59 and Meyer et al. 2010 J Shellf Res 29: 181; for a consideration of factors preventing natural recovery of O. lurida on the west coast see Trimble et al. 2009 J Shellf Res 28: 97. Photograph courtesy Royal British Columbia Museum, Victoria.


Olympia oysters Ostrea conchaphila 0.5X

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Research study 5
 

histogram showing chemical-signature data for shells of oysters Ostrea conchaphilaA chemical-fingerprint method1 for determining sources and interconnectivity2 of populations is tested for Olympia oysters Ostrea (lurida) conchaphila by a research group in California based out of California State University, Fullerton. The procedure in theory can be used with any animal that lays down a calcified structure such as an otolith (fishes), statolith (cephalopod), or shell (oysters).  In each case, the structure carries a chemical signature3 of its site of origin based on the elemental composition of seawater at that formative site, that may differ from the seawater encountered at a later stage in life.  In the case of oysters, the larval shell formed at the adult site during brooding remains embedded in the new juvenile shell formed at the site where metamorphosis occurs.  All that is required is to extract a portion of the larval shell and compare its chemical signature with that of the new juvenile shell.  With thorough sampling of larvae or adults from the “home” population and even more extensive sampling of juveniles from “downstream” populations, the method can be used to track larval movements.  Results show, indeed, that there are significant ontogenetic shifts in the shell chemistry of O. lurida from larva to settler with respect to Mg and Sr, but not for Mn, Ba, Pb, or Ce (see sample data for Mg and Sr in histogram).  Interestingly, the experiments show that the ratio of Cu to Ca in the shell does not change with increase in seawater Cu, indicating that this element would not be a useful “chemical-tag” discriminator for O. lurida.  The authors conclude that larval O. lurida shells have potential as natural indicators of natal origin, providing data that may be useful in future restoration programmes.  Zacherl et al. 2009 J Shellf Res 28: 23.

NOTE1  the method in various forms has been used previously for tracking fishes and invertebrate larvae, such as clams

NOTE2  the extent to which a certain population relies on neighbouring populations for sustained recruitment

NOTE3  the “signature” tested here is based on the ratios of magnesium, strontium, barium, lead, manganese, cesium, and copper to calcium.  A laser is passed over the area of interest on the shell, and the analysis of ablated material is done  automatically in a mass  spectrometer (the method is known as laser-ablation inductively coupled plasma-mass spectrometry: LA-ICPMS)

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Research study 6
 

photograph of newly settled Olympia oyster Ostrea conchaphiladiagram showing larval transfer of oyster larvaeThe chemical-signature1 method described in Research Study 3 above is used by a researcher at the Scripps Institution of Oceanography, La Jolla to assess interconnectivity of 3 populations of Olympia oysters Ostrea conchaphila in southern California.  The researcher first collects shelled larvae from brooding adults at sites in 3 estuarine regions (San Diego Bay, North County, and Mission Bay) distributed over 75km of southern Californian coastline.  Settlement plates at various sites in each region collect recruits whose embedded larval shells are then analysed and identified as to origin.  Results2 for early summer recruitment at the 3 regions are shown in the schematic on the Right.  Note that most of the San Diego Bay recruits are self-recruits (48%), compared with 40% for North County and only 4% for Mission Bay.  The last is really a “sink” region for the other two, supplying only about 30% of the other regions, a finding that, once the technique is refined, should provide useful information for conservation and management of Olympia oysters.  Carson 2010 Limnol Oceanogr 55: 134.

NOTE1  the “signature” in this case is based on the ratios of copper, barium, lead, and uranium to calcium (other elements are measured but not used).

NOTE2  results are only for identifiable recruits, and these represent only a small portion of all recruits at a given site.  Also, water chemistries vary over time from site to site, and this introduces other variability.  Overall, recruit signatures could be identified as to origin with about 60% accuracy

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