Physiology & physiological ecology
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Research study 1

Olympia oysters Ostrea lurida1  inhabit low intertidal to shallow subtidal regions along the west coast, and may be subject as adults to high temperatures during extreme low tides in summer.  Does exposure to high temperature induce a heat-shock response2 involving expression of protective “molecular-chaperone” proteins?  This aspect of O. lurida’s physiology is investigated by researchers at the Bodega Marine Laboratory, California along with the possibility that such heat-induced responses may also be present in early developmental3 stages.  Results show that expression of several Hsp proteins occurs in both adult and larval tissues (Hsp77, Hsp72, and Hsp69).  Expression in adults occurs at temperatures 33oC and above.  More importantly, expression of Hsp69 after a heat shock4 of 34oC leads to tolerance of otherwise lethal temperatures of 38-39oC.  As for the developmental stages, there is no heat-shock response in cleaving embryo or blastula stages, but Hsp69 is expressed in the veliger stage after exposure to a heat shock of 34oC.  Interestingly, the increased levels of Hsp69 in the larva persist for only a few hours, rather than for up to 3wk as in adults.  Brown et al. 2004 J Shellf Res 23 (1): 135.

NOTE1  the authors actually refer to this species as Ostreola conchaphila. However, as it was collected in Tomales Bay near San Francisco, it should be photograph of oysters Crassostrea gigas and Ostrea conchaphilareferred to as Ostrea lurida. The name Ostreola conchaphila is an older name for Ostrea conchaphila, the latter now thought to live only in Baja California, Mexico

NOTE2 expression of Hsp’s (heat-shock proteins) occurs in many intertidal invertebrates, including abalones, limpets, mussels, sea anemones, and others.  There are several types, categorized by their molecular masses

NOTE3 the authors note that Olympia oysters incubate their ealy life stages in the mantle cavity;  thus, these stages experience the same environmental conditions as do the brooding adults.  Later on, the veligers are released into the plankton 

NOTE4  this consists of 1h exposure to a given temperature, then a  return to ambient seawater

Japanese or Pacific oyster Crassostrea gigas
and Olympia oysters Ostrea lurida 0.6X

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Research study 2

photographs of veliger larvae and a 12d juvenile of the Olympia oyster Ostrea conchaphilagraphs showing effects of pH on growth of larval and juvenile Olympia oysters Ostrea conchaphilaOlympia oysters Ostrea lurida possess a free-swimming larval stage and therefore must contend with two different sets of environmental stresses, one during the pelagic phase; the other, during the benthic, post-metamorphic stage.  The question raised by a group of University of California scientists1 is the extent to which the effects of environmental stresses can “carry over” from one developmental stage to another.  The stress-factor considered by the researchers is increasing seawater acidity, of  current interest in view of increasing carbon-dioxide dissolution in the world’s oceans and resulting acidification as a component of climate change2.  The researchers rear larvae in seawater at 3 pH levels3, 8.0 (control, equivalent to oceanic seawater), 7.9, and 7.8, and compare growth rates before and after metamorphosis.  Results show that larval growth at 9d is significantly slower in the most acidic condition of 7.8, as compared with control seawater, and this effect carries over to about 40% reduction in growth of 7d juveniles regardless of the pH they are experiencing as juveniles (the researchers do reciprocal translocations from high to low, and low to high, pHs).  These adverse effects persist for 1.5mo in juveniles transferred to a common pH environment.  In showing these carry-over effects4 the researchers point to the need to  attend to all phases of an organism’s existence rather than focussing in such studies on what might be perceived as a critical “weak link”.   Hettinger et al. 2012 Ecology 93 (12): 2758.

NOTE1  joined by colleagues from Kalamazoo College, Michigan and University of North Carolina

NOTE2  as with many similar studies on effects of seawater acidification, the treatment protocol used here involves acute, not chronic change in acidity.  Change in ocean acidity related to climate change will occur slowly, over many generations, and one might expect to see at least some degree of physiological adaptation to the progressively changing conditions

NOTE3  created by increasing seawater carbon-dioxide levels from 700ppm (control = pH 8.0) to 800ppm and 1100ppm by bubbling CO2 gas in 20liter carboys

NOTE4  another comparatively well-studied environmental parameter with carry-over effects is nutrition

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Research study 3

Another interesting study with implications for climate change is presented by a consortium of aquacultural, fisheries, and ocean and atmospheric scientists from several North American institutions.  The study, involving oysters Crassostrea gigas cultured at Whiskey Creek Hatchery, Oregon, attempts to correlate larval production and growth with aragonite concentrations that vary naturally with coastal upwelling graph showing effect of aragonite concentration in initial seawater used during early development of oyster larvae Crassostrea gigas on later larval growthcycles.  The protocol involves spawning and growing larvae for 48h in a range of aragonite-saturation waters, then measuring growth from D-hinge stage to a size of 120µm  and then from 120-150µm (4-8d in culture). Results show that larval production as well as growth from 120-150µm are significantly negatively correlated with aragonite saturation levels of seawater in which eggs and larvae are initially reared (see graph).  No effect, however, is seen on growth from the D-hinge stage to 120µm in size, suggesting a delayed effect of initial water chemistry on later larval life.  Barton et al 2012 Limnol Oceanogr 57 (3): 698.

NOTE  a type of CaCO3  found in young stages of oysters.  Aragonite is about twice as soluble as calcite, which is the material used for shell calcification after settlement.  Aragonite saturation in seawater is dependent upon bicarbonate concentration [CO3=].  Depending upon season and extent of upwelling, pHs resulting from varying [CO3=] may range from 7.6-8.2 in this area of the Oregon coast.  Aragonite saturation at a pH of 8.2 is about 5-fold greater than at pH 7.6

NOTE  these results parallel those shown for Ostrea lurida in Research Study 2 above.  Based partly on this assumed knowledge, one wonders why the researchers do not follow growth for longer than just a few days to determine if this effect extends into later larval life

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Research study 4

graphs showing effects of carbon dioxide concentrations on growth of oyster larvae Crassostrea gigas to 3d of ageStudies on the effects of increasing ocean acidification associated with climate change on growth and survival marine invertebrates, most notably their developmental stages, are becoming increasingly popular (see Research Study 2 above).  Investigations at Friday Harbor Laboratories, Washington on effects of pCO2 on calcification and growth of oyster larvae Crassostrea gigas add to this fund of knowledge.  The researchers culture eggs from field-collected adults to the 3d larval stage under 3 pCO2 conditions: AMBIENT (400µatm), MID (700µatm) and HIGH (1000µatm).  Initial 24h results show a pulse of greater calcification in larvae in the HIGHC02 treatment than in AMBIENT, but this is credited by the researchers to a higher metabolic rate coupled with ample energy reserves.  By 3d, however, although larvae in all treatments have reached the D-hinge stage in development, those in the HIGH treatment are significantly smaller (shell height, see histogram) that those in the AMBIENT and MID treatments, and calcification is less.  Survival to 3d is 93% in AMBIENT, 87% in MID, and 86% in HIGH.  The study is useful but leaves the reader wanting more, and it is puzzling that the researchers chose not to extend their data collection into later larval life. Timmins-Schiffman et al. 2013 Mar Biol 160: 1973.

NOTE  these pCO2 treatments yield pHs of 8.0 for AMBIENT, 7.8 for MID, and 7.7 for HIGH

NOTE  larvae of C. gigas spend 2-3wk in the plankton before settling

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Research study 5

histograms showing growth rates of oysters Ostrea conchaphila of 127d of age after having been treated as larvae to different levels of CO2Few studies on acidification effects on oysters extend beyond the first few days of larval development.  This is remedied by researchers at Bodega Marine Laboratory, California who not only monitor postCO2-exposed larvae of Olympia oysters Ostrea lurida through metamorphosis, but also the juveniles through 127d in Tomales Bay, California.  Larvae are treated for 11d at 1000 or 400µatm CO2 to yield seawater pHs of 7.8 and 8.1 (control), respectively.  At this time the larvae are moved into normal seawater in special jars with PVC plastic bottoms, and allowed to settle and metamorphose.  At day 14 when 95% of the larvae have settled, the plates with attached juveniles are set out at 2 intertidal levels, mid and low, at 2 separate sites and monitored at intervals over 127d for carry-over effects on growth and survival.  Results show that larvae subjected to control pCO2 conditions are 80% more successful in metamorphosing and are about 10% larger than ones cultured in elevated pCO2 conditions.  Similarly, juveniles exposed to elevated pCO2 as larvae grow significantly less than controls suggesting a larval carry-over effect.  Interestingly, this carry-over effect is not exacerbated by harsher conditions at mid-tide levels, at least not initially.  However, by day 27 post-settlement all juveniles are dead in the mid-tide zone. At low-tide levels initial poorer growth of high pCO2 individuals is not compensated over time, at least not by the end of the 127d observation period (see histograms for the 2 sites).  Thus, even relatively brief exposure to lower pH seawater during development may have significant long-term effects on growth.  Hettinger et al. 2013 Global Change Biol 19: 3317.

NOTE  this mid-tide level is chosen to add elements of emersion and temperature stress (outplanting takes place in autumn), factors that may act additively or synergistically to the effect of lower pH.  Data show that individuals at the mid-tide level are air-exposed for 12% more time than ones in the low-tide zone

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Research study 6

photograph of eastern oyster drill Urosalpinx cinerea with egg caseshistograms comparing Ocean acidification associated with climate change has potential risk for any marine invertebrate with a calcareous skeleton. Because it is potentially the most susceptible life stage, many studies have dealt with larvae (see Research Study 5 above), but much has also been written on adult stages of west-coast species. One recent investigation by scientists at Bodega Marine Laboratory, California is novel in determining whether thinner-shelled Olympia oysters Ostrea lurida, known to be produced in other oyster species when larvae are cultured under elevated pCO2 conditions (see Research Study 4 above), suffer enhanced risk of predation by eastern oyster drills Urosalpinx cinerea, an introduced oyster predator from the east coast that co-occurs with oysters in California estuaries. The snails hatch in late summer/autumn just as oyster larvae are settling out, and these recruits represent a good food source for the hatchling snails. Experimental hatchlings are obtained from field-deposited egg cases and maintained for 2wk in the laboratory under the same CO2 levels as the oysters, with food (small barnacles) being provided for the first week only. Results show that after 1wk exposure to elevated pCO2 (pH 7.8) shell growth of Ostrea is significantly less than in controls (pH 8.1) by about 40%, but shells on a 1:1 comparison are not thinner. The smaller oysters are eaten by Urosalpinx hatchlings (2mm length) at rates about 20% higher than larger control oysters (see histograms), possibly because they are quicker to drill into and absolute yield of nutrients less. In choice tests smaller oysters are preferentially selected as prey by the drills. The latter seem not to be affected by elevated pH in culture, at least not in feeding behaviour (compare Left vs. Right bars in graph). The authors remark that their results support a general concern that the physiologically greater resiliance of non-native species over native ones may allow the former to “weather” better the effects of impending climate change. Sanford et al. 2013 Proc Roy Soc B 281: 2681. Photograph of oyster drill Urosalpinx cinerea with less than healthy-looking egg cases courtesy Courtenay Janiak, Connecticut.

NOTE oysters are reared from larvae to early juveniles under pCO2 conditions of 500µatm (control, similar to current atmospheric conditions) and 1000µatm (predicted for year 2100 at current rates of atmospheric increase). Microalgal food Isochrysis galbana is provided both to oyster larvae and resulting juveniles

NOTE in the Discussion section the authors themselves “fall prey”, in this case, to over-interpretation of results. Thus, their summary comment “that ocean acidification led to a striking 20-48% increase in predation by invasive snails on native oysters” greatly exaggerates results from what is really just a short-term acute test of effects of seawater pH on growth and behaviour of the test species in the laboratory which lacks, as in all similar protocols, the potential for acclimation to changing pH through generations of development of the oysters and snails from now to the end of the century. However, apart from some kind of decades-long series of rearing experiments that no one would undertake willingly, there is not much that can be done about this - just a suggestion that the rhetoric in such studies be tailored to fit reality

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