Feeding & growth
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  Oysters filter-feed using their ctenidia to strain out phytoplankton cells. The process is similar to that used in other bivalves. If the process of ctenidial suspension-feeding in bivalves is unfamiliar, then a brief review can be found in CLAMS & RELATIVES: FOODS & FEEDING.  Shell growth is similar to that in other molluscs and is described below.
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Research study 1

schematic comparing inhalent, ctenidial ostial, and exhalent surface areas in an oysterschematic showing paths of water flow and particles in an oyster Crassostrea gigas

A study at the Pacific Biological Station, Nanaimo, British Columbia of particle sorting and retention in the Pacific oyster Crassostra gigas suggests that the first sorting occurs just after particles enter the mantle cavity.  The velocity of the feeding current markedly slows at this point, and heavier, inedible particles settle out by gravity (see diagram on Left). Note that the “total ostial aperture” in the schematic represents an estimate of total area of openings on all the ctenidial surfaces. This initial settling likely protects the delicate tissues of the ctenidia from impingement of heavy mineral particles. The fate of these heavy particles is to be gathered in mucous streams on the mantle surfaces and rejected as pseudofeces (#1 on the schematic above Right). Particles impinging on the ctenidia as the water flows through the ostia (#2) are either rejected (#4) or carried in mucous strings to the labial palps (#3).  Note that particles directed to the labial palps come from both upper and lower food grooves on the ctenidia. Mucus volume is reduced on the labial palps and the concentrated food-mucus mass is then passed to the mouth.  Oysters apparently cannot separate bacteria from unicellular algae, but rapid digestion results in a bacteria-free alimentary canal and feces.  The author concludes that ctenidial selectivity is limited to a simple acceptance or rejection of particles on a certain lamellar portion, and that the major function of the oral palps is to reduce mucous volume prior to ingestion.  Bernard 1974 Biol Bull 146: 1.

NOTE  common belief up to this time is that the labial palps are the primary area for sorting out unwanted particles prior to ingestion

Research study 2

photograph of oysters Crassostrea gigas on the shoreschematic of annual energy budget of an oyster reef Crassostrea gigas
The Pacific oyster Crassostrea gigas, introduced from Japan, does not normally reproduce in British Columbia except in locations with high summer water temperatures.  One large recruitment1 in 1958 resulted in the formation of extensive oyster reefs at the southern end of the Strait of Georgia.  Such reefs function as important consolidators of sediments and organic materials, and may host a rich diversity of infaunal organisms.  The oysters exhibit little somatic growth after 5yr of age, but their energy and nutrient contribution to the community may be large in the form of feces, pseudofeces, and reproductive products. 

A year-long study on energy partitioning in such a reef area shows that while the oysters harvest surprisingly little of the food available (11%), a relatively large proportion (7%) ends up as “deposits” (feces and pseudofeces2) and “gametes” (2%).  Note that the budget has no entry for somatic growth but, as noted above, this is considered to be “marginal” because the reef is comprised of mature individuals.  The estimate for shell growth is relatively small, as it is for most molluscs in such energy budgets, because it mainly represents the organic component of the shell matrix (calcium and carbonate ions are freely available in seawater).  Cost of assembling the shell components are represented in the “metabolism3” component of the budget.  Because there is no indication of how many individual oysters are involved, the expression of an energy budget for a 1-m2 area of the reef is meaningful only from a community-dynamics point of view.  Readers interested in single-oyster energetics can simply use the percentage values.  Bernard 1974 J Fish Res Bd Can 31: 185.

NOTE1  this recruitment was visible to shore-walkers for several decades afterwards, in part, because the eventual large size of the oysters made them less appetising for local consumption

NOTE2 although indicated here as being part of absorbed food energy, only the mucus part of pseudofeces is produced by the oysters; the remainder is inedible matter taken in as food, but shunted to the outside after rejection by the labial palps

NOTE3  the author remarks that certain methodological problems may have led to an order of magnitude underestimate in the magnitude of pumping costs.  This would affect the entry in energy budget for “metabolised”, but its effect on the other budget entries is not known

Research study 3
  photograph of diatoms beinG moved along the food groove in the ctenidia of an oyster Crassostrea gigas, courtesy Cognie et al 2003 Mar Ecol Prog Ser 250: 145A study on particle selectivity in oysters Crassostrea gigas concludes that there is little or no selection at the level of the ctenidial filaments.  As long as a particle is small enough to fit into the ciliated grooves between the ctenidial filaments, it will be transported.  Larger particles, even inedible ones, can be transported along the edges of the filaments, and edges of the food grooves at the dorsal and ventral edges of the gills.  Both large and small particles are sorted by the labial palps. Those that are palatable are directed along ciliated grooves into the mouth, and those that are unpalatable are shunted away and rejected as pseudofeces.  When fed on diatoms Coscinodiscus perforatus almost 100% of the pseudofeces in Crassostrea is made up of empty diatom husks. The authors use an endoscopic video system to observe directly the mechanism of processing and transport.  Cognie et al. 2003 Mar Ecol Progr Ser 250: 145.
Research study 4

photograph of sea louse developmental stages Lepeophtheirus salmonisAlthough phytoplankton is the mainstay dietary component of oysters Crassostrea gigas, any particle of appropriate size and palatability may be taken in and eaten. Surprisingly, this includes even such seemingly oversized particles as larvae1 of invertebrates, such as molluscs, echinoderms and, in the vicinity of salmon farms, copepod “sea lice” Lepeophtheirus salmonis, the adults of which infest the penned salmon stock. Scientists from several Canadian governmental, university, and commercial research facilities collaborate on a study to show that C. gigas2 of juvenile to adult size will readily consume sea-lice developmental stages (copepodid as well as naupliar stages3). The parasites are filtered from the water column in the normal way just as for phytoplankton, and then ingested and digested (see photograph on Left). Infestations of sea lice in salmon farms globally cause hundreds of million dollars in damage to stock and in “de-lousing” treatments. The authors discuss the significance of their findings in the context of combining bivalve culture in the proximity of salmon farms as a natural prophylactic. Webb et al. 2013 Aquaculture 406-407: 9. Photograph of oyster gut courtesy the authors; photograph of sea-lice larva and copepodid courtesy Eichner et al. 2015 Exp Parasit 151-152: 39 SLRC Sea Lice Research Center, U Bergen, Norway.photograph of the stomach and associated parts of an oyster Crassostrea gigas showing the presence of many ingested sea-lice developomental stages

NOTE1 the authors refer to the developmental stages eaten by the oysters generally as “larvae”, both nauplii and copepodids, but this of course is not correct. Only the nauplii are larvae, while the copepodids are juvenile developmental stages. There are 2 stages of each, and it is after the second copepodid stage that the parasite ceases swimming and seeks out a salmon host to complete its life cycle

NOTE2 also included in the study are cockles Clinocardium nuttallii, mussels Mytilus spp., and scallops Patinopecten caurinus (actually a hybrid form), all species also ingesting sea-lice larvae and copepodid stages. The study also involves effects of body size and temperature on ingestion rates, results of which are of interest to salmon aquaculturists, but not considered further here

NOTE3 even a modest-sized oyster can consume a suprisingly large number of developmental stages (>300.h-1), with >300 being present in the gut at any given time (stomach, crystalline style, intestine)


Oyster Crassostrea gigas gut showing semi-digested larvae
and copepodids (circled) of sea lice Lepeophtheirus salmonis

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Research study 1

drawing of shell valves of a pediveliger larva of an oyster CrassostreaTwo types of calcium carbonate are employed in the construction of molluscan shells, including those of oysters Crassostrea gigas.  The first is calcite, with the formula CaCO3.  The second is aragonite, which has the same chemical formula, but a different crystalline structure.  Aragonite is slightly denser and harder than calcite, and has greater strength as a structural material.  It is interesting, then, that the shell of larval oysters Crassostrea is composed of aragonite, while the adult shell is composed of calcite, with only minor amounts of aragonite. Aragonite is heavier than calcite, so the advantage to the free-swimming veliger larva may be in its extra strength.  However, most or all other bivalves have aragonitic shells in both larval and adult stages, so what advantage accrues to adult oysters in having a calcitic shell?  The author suggests that calcitic shells may be advantageous to an non-motile bivalve such as an photograph of new growth on the shell of an oyster Crassostreaoyster because calcite is more stable in seawater (less prone to leaching) and because it can be secreted more economically than aragonite.  Thus, while a stronger, sleeker shell is beneficial to free-swimming larvae and to other free-living adult bivalves, a thicker more economical shell is more beneficial for protection of exposed, non-motile oysters.  Stenzel 1964 Science 145: 155. 

NOTE  calcite is the commonest mineral form on the earth’s surface, being found as limestone, marble, stalactites, and in skeletons of echinoderms and other marine organisms. Aragonite appears to be the more common crystal in mollusc shells but, in species such as oysters, both calcite and aragonite are present, and proportions may vary under different circumstances of physiology and locality

NOTE  this Research Study deals with the eastern oyster Crassostrea virginica, but it is assumed that west-coast species are constructed similarly

New growth on the shell of a juvenile oyster Crassostrea gigas 2X

Research study 2

The shell of oysters Crassostrea gigas begins to grow just after metamorphosis.  Different regions of the mantle secrete the shell.  Consider first the upper, freely moving, right shell valve. The outer fold of the mantle secretes the chalky part of the shell, while the general mantle tissue secretes the nacreous layer.  A protective protein-composite sheet known as the periostracum is secreted from a groove drawings showing the orientation of mantle folds to shell of an oyster Crassostrea gigaslying between the outer and middle folds of the mantle.  The periostracum is a part of most mollusc shells but is often worn off, as it is in oyster. 

The lower shell valve is formed in the same way, and crystal seeds just at the growing edge can be seen through the periostracum, which is transparent at that stage (see drawings lower Left and Right). Cementation occurs by the sheet of periostracum being pressed into the substratum so tightly that it adheres to the minute hollows and ridges, and forms a strong attachment layer.  The attachment is usually so tight that the cemented valve can rarely be detached without breaking the shell.  Yamaguchi 1994 Mar Biol 118: 89.

NOTE  lit. “around shell” G.

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Research study 3
  Oysters Crassostrea gigas and barnacles Balanus glandula inhabit the same rocky shores at similar intertidal levels, and interaction between them is extensive. Here are some examples:
photograph of oyster Crassostrea gigas overgrowing barnacles Balanus glandula
Oyster Crassostrea gigas over-grows barnacles Balanus glandula. The shingle-like shell growth of the oyster likely represents seasonal growth 3X
photograph close view of oyster Crassostrea gigas overgrowing barnacles Balanus glandula
Close view of B. glandula being over-grown. Note the transparency of the new shell growth of the oyster C. gigas 2X
photograph of barnacle Balanus sp. settled on new growth of oyster shell Crassostrea gigas
Unidentified barnacle on new growth of oyster. The out-come may be that the barnacle is undercut and ends up growing on the oyster shell 5X
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Research study 4

photos of pigmented shells of oysters Crassostrea gigasDuring growth the shell and mantle edge of oysters become pigmented.  Investigation of the underlying causes for this in Pacific oysters Crassostrea gigas by researchers at the Hatfield Marine Science Center, Newport shows a strong genetic component, and little or no correlation with phenotypic characters such as body mass or survival in culture (see photographs).  The findings are important from an oyster-breeding standpoint where certain types or degrees of pigmentation may be desirable traits for marketability.  The authors note that theirs is the first report on the genetics of pigmentation in C. gigas.  Brake et al. 2004 Aquaculture 229: 89.

NOTE  inherited colour traits in other aquaculture species include flesh and skin colour in salmon, colour patterns of koi carp, and colour of pearl oysters

Research study 5

map of study site at Tomales BayInterestingly, in certain shallow estuarine bays in California, oysters Ostrea conchaphila close to the mouth of the bay and further into the bay may grow more slowly than ones in the middle, a result of food limitation through features of water circulation. Thus, although water temperatures are generally higher in the inner bay and therefore potentially better for growth, phytoplankton concentrations may be low.  By outplanting oysters at 10 locations in Tomales Bay (see map), and measuring tidal movements, and seasonal nutrient and chlorophyll concentrations at intervals over 1yr, researchers from Bodega Bay Laboratories determine that growth is, in fact, greater in the middle part of the bay as a result of intermediate levels of mixing that allow phytoplankton to accumulate.  In the inner bay, tidal replenishment is low and nutrient levels are low, while in the outer bay, despite higher nutrient levels for phytoplankton growth, strong tidal movements preclude phytoplankton from accumulating.  Kimbro et al. 2009 Limnol Oceanogr 54: 1425.

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