Mussel Case Study

CaseStudyATaleofTwoStressors—OceanAcidificationinaWarmingWorld

Background:Biologistswhostudyoceanacidification(OA)inthelaboratoryoftenfocusontheeffectsofpHchangeinisolationfromotherenvironmentalvariables.Thiscanbeausefulapproach,becauseitallowsresearcherstodefinitivelylinkcause(lowerpH)andeffect(forexample,smallerurchinlarvae).Thisisnotthewaymarineorganismsexperienceacidificationintheirnaturalenvironment,however.Itismuchmoretypicalforanorganismtoexperiencemultiplestressorssimultaneously.ThisCaseStudydescribeshowtheCarringtonlabattheUniversityofWashington(http://depts.washington.edu/ecarring)istryingtounderstandthecombinedeffectsofacidificationandincreasedseawatertemperatureonmussels,atypeofbivalve.Italsoconsidersthefindingsofanothergroupofresearchers(http://pfisterlab.uchicago.edu/andhttp://woottonlab.uchicago.edu/)whoarestudyinghowoceanchangeisaffectingtheintertidalcommunityonremoteTatooshIsland,offthenorthwestcornerofWashingtonState.LessonPlan(1-2days):

Reviewthefollowingresourcesabouthowmusselsareabletosurviveinthestressfulintertidalenvironment,andhowoceanacidificationmaybemakingitharderforthemtomaintaintheirgrip(literally):

• BBCvideo(4.5-minutes)aboutthecoastalintertidalzone:http://www.bbc.co.uk/nature/habitats/Intertidal_zone#p00l2t0n

• UWDailyarticle:http://www.dailyuw.com/science/article_cbf0d545-3e7e-55ce-a58c-58d5142ec2a4.html

• UWpressrelease:www.eurekalert.org/pub_releases/2013-02/uow-mcb020813.php

• CarringtonlabwebpageonOAandMusselByssalAttachment:http://depts.washington.edu/nucella/mussel-byssal-threads/

• CarringtonlabgraduatestudentLauraNewcomb’spresentationonthecombinedeffectofacidificationandtemperatureonmusselbyssalthreadstrength(15minutevideo:https://youtu.be/vkHyrL4_nU8).Notetoinstructors:youmaywanttoreviewtheconnectionbetweendissolvedCO2(pCO2)andpH:inseawater,amoleculeofCO2combineswithamoleculeofH2Otoformcarbonicacid(H2CO3),whichlowersthepH.

Usethefollowingquestionsandexercisesasabasisforin-classdiscussionorgroupwork(possibleanswersinbold):

1. Howwouldyoudescribemussels’habitat?Intertidal,rocky,strongwavesandcurrents,extremelychangeableduetotidalcycle…Whatadaptation(s)allowmusselstowithstandthemechanicalstress(i.e.waveaction)typicalofthatenvironment?Gluethemselvestorocks,hardshells,…Howdomusselsattachthemselvestoasurface?Themussel’sfootsecretesbyssalthreads—strong,stretchyfiberswithwhichitattachesitselftothesubstrate.

2. Whatothertypesofstressdointertidalorganismsexperience?Predation,competitionforspace,competitionforfood,watertemperaturechangesrelatedtoseasonandtides,unabletofeedorbreathewhenexposedatlowtide,desiccation,freezing,logdamage,parasites…Whatnewstressesareoccurringasaresultofanthropogenic(human-generated)CO2?Increasedseawatertemperaturesandoceanacidification.WhatistheconnectionbetweenCO2,seawatertemperatureandpH?ExcessatmosphericCO2trapsheat,warmingtheoceansurface;excessdissolvedCO2(pCO2)lowersseawaterpH.

3. Youalreadyknowthatmarinecalcifierslikeoysters,urchins(andmussels!)havetoworkhardertoobtainthecarbonatetheyneedfortheircalciumcarbonateshellsandskeletonsfromacidifiedseawater.TheCarringtonlabhasfoundevidencethatmusselscouldalsobevulnerabletoanothertypeofdirectimpactfromoceanacidification.Describe.Inlaboratoryexperiments,highpCO2/lowpHconditionsweakenbyssalthreads.Specifically,theweakeningseemstooccurintheplaqueregionwherethethreadadherestorockysurfaces.Whatothersenvironmentalstressaffectstheabilityofmusselstomaintaintheirholdonasubstrate?Byssalthreadsformedattemperaturesabove65F(18C)aresignificantlyweaker.

4. Usingthebyssalthreaddiagrambelow,indicatehowbothelevatedtemperatureandhigherpCO2/lowpHconditionsaffectedbyssalthreadstrengthinLauraNewcomb’slaboratoryexperiments.

5. UsethediagrambelowtosummarizehowtemperatureandpCO2

independentlyaffectbyssalthreadstrength.Usetherockclimberanalogytoexplainhowthesetwostressorsaffecttheoverallstrengthofabyssalthread.

6. LauraconcludedthatpCO2andtemperaturedonotact‘synergistically’to

weakenbyssalthreads(inotherwords,thecombinedeffectofthesetwostressorsisnotgreaterthantheeffectofeitheronebyitself).However,thesetwoclimatechange-relatedstressorsmaystillcombinetoreducetheoverallabilityofmusselstocolonizeasurface,whetherthey’regrowingonarockyshoreoraculturerope.Whatisonepossiblemechanism,andhowwouldyoutestyouridea?IfthetimingofhighpCO2episodesandwarmertemperaturesdonotcoincide,thenthesetwostressorscouldcombinetolengthentheperiodoftimewhenmusselbyssalthreadsareweaker,leadingtoalongerwindowwheredetachmentisaproblem.

7. HowdidLauraapplywhatshelearnedinherlabexperimentstoherfieldworkatPennCovemusselfarm?AfterconcludingthattemperaturewasamoreimportantdeterminantofbyssalthreadstrengththanpH,shetestedthebyssalthreadstrengthofmusselsgrowingatdifferentdepthsalongthecultureropes,toseeiftenacitycorrelatedwithseasonaltemperaturepatterns.

8. Ifyouwereamusselgrower,wouldyouchangeanythingaboutyourculturepracticesbasedonLaura’sfindings?

• Optional(advanced):Asagroup,readanddiscusstheCarringtonlabpublicationMusselByssusAttachmentWeakenedbyOceanAcidification;NatureClimateChange;March2013;doi:10.1038/nclimate1846.Directlink:http://rdcu.be/F9G0(subscriptionrequired);alsoavailablefromtheCarringtonlabwebsite:https://depts.washington.edu/nucella/mussel-byssal-threads/.Notetoinstructors:Readingandinterpretingascientificarticlewillbechallengingformosthighschoolstudents,butit’spossibletogainsomethingfromtheexperiencewithoutgettingboggeddownbytechnicallanguageorcomplexfigures.Thereareseveralwaystoapproachthismanuscript:

• Usethepaperasanopportunitytoseehowscientistssharetheirfindingswiththeirpeers;

• Explorethestructureofascientificpaper(framingtheresearchquestionintheintroduction;presentingtheresults;sharingthemethods;backingupstatementswithreferences);

• Simplytotrytosummarizetheauthors’hypothesis,majorresultsandconclusions;

• Carefullyanalyzeafigureanddiscusswhetherthedatasupporttheauthors’conclusions;

• FocusontheMethodssection.Howweretheexperimentscarriedout?Discusswhytheauthorsmighthavechosentoconducttheexperimentsastheydid[Note:reference#26(MoeserandCarrington,2006;Seasonalvariationinmusselbyssalthreadmechanics;JournalofExperimentalBiologyvo.209)providesamorethoroughexplanationofthemethodsusedinthisstudy]

ReviewthefollowingresourcesaboutscientistsTimWooton’sandCathyPfister’soceanacidificationfieldworkonTatooshIsland,offthecoastofWashington:

• NewYorkTimesarticle(Oct.2012):http://www.nytimes.com/2012/10/07/us/scientists-in-washington-state-adopt-tiny-island-as-climate-change-bellwether.html

• OregonPublicBroadcasting9-minutevideo(Feb2010):http://www.opb.org/television/programs/ofg/segment/ocean-acidification/

Usethefollowingquestionsandexercisesasabasisforin-classdiscussionorgroupwork(possibleanswersinbold):

• ScientistshavebeenmonitoringtheintertidalhabitatonTatooshIslandforover40years.Recently,they’veobservedsomestartlingtrends.Whatarethey?Declinesinseabirdandcalcifier(barnacles,musselsandcorallinealgae)populations;mussels—thinnershells&weakerattachmenttosubstrate;

• Whatdothescientistsbelieveisbehindthesechanges?Human’suseoffossilfuelsisincreasingthelevelofCO2inseawater.Inseawater,CO2combineswithwatermoleculesformcarbonicacid,whichismakingseawatermoreacidic(i.e.lowingthepH).DecliningpHisdirectlyimpactingsomespecies,andindirectlyaffectingothers(throughfoodwebinteractionsandchangesinhabitat).

• Whatisthe“KeystoneSpeciesHypothesis”,andhowdoesitapplytowhatscientistsareseeingonTatooshIsland?ThekeystonespeciesconcepthasbeenamainstayoftheecologicalandconservationbiologyliteraturesinceitsintroductionbyUniversityofWashingtonzoologyprofessorRobertT.Painein1969.Akeystonespeciesisonewhoseimpactsonitscommunityorecosystemarelargeandgreaterthanwouldbeexpectedfromitsrelativeabundanceortotalbiomass.Keystonespeciesareusuallynoticedwhentheyareremovedortheydisappearfromanecosystem,resultingindramaticchangestotherestofthecommunity.Thephenomenonhasbeenobservedinawiderangeofecosystemsandforawiderangeoforganisms.OnTatoosh,musselsandbarnaclesarekeystonespecies;theydominatetheintertidalzone,andinfluencemanyotherTatooshspeciesthroughfoodwebinteractions,andasarchitectsoftheintertidalhabitat.