Table Of ContentReference: Btol Bull 181: 289-297. (October. 1991)
Acid-Base and Ionic Regulation, During and Following
Emersion, in the Freshwater Bivalve, Anodonta grandis
simpsoniana (Bivalvia: Unionidae)
ROGER A. BYRNE* AND BRIAN R. McMAHON
Department ofBiologicalSciences, The University ofCalgary, 2500 UniversityDr. NW.,
Calgary, Alberta, Canada T2N 1N4
Abstract. Specimensoftheboreal clam.Anodontagrandis behavioral responses have been documented (see reviews
simpsoniana were emersed at 10C for 6 days and then by McMahon, 1988; Shick etai, 1988). Inthefreshwater
reimmersed for 24 h. The clams lostwaterat a rate of 1.6% environment, however, changes in water level and emer-
total water per day. After 144 h ofemersion, water weight sion events are unpredictable; their timing and duration
haddeclinedbyalmost 15%,whileextracellularfluid(ECF) isdependent on such factorsasrainfall, temperature, and
osmolality had increased 30% to 52 mOsm kg"1. Control physical changes in the watershed. The responses to
levels were reattained after 6 h reimmersion. ECF P , de- emersion ofthe freshwater bivalves that inhabit the shal-
clined rapidly in the first 24 h ofemersion, but remained lower regions ofsuch systems are less well known. As an
near 20 Torr for the full 6-day exposure. After an initial adaptation tothisstress, somespeciesoffreshwaterbivalve
rapid fall, pH declined at a slower rate, reaching 7.494 can withstand emersion for up to a year(Hiscock, 1953).
0.037 (mean SEM) at 144 h. Pco, was elevated from An emersed clam is faced with opposing needs: water
0.6 0.6 to 12.4 1.1 Torr after 96 h, but no further conservation, accomplished by valve closure, and respi-
increase was noted. ECF [Ca] increased threefold to 13.1 ration, requiringvalveopening. Inaddition,theproblems
0.8 mmol T1, while [HCO,app] rose from 5.4 0.3 to a ofacid-base balance, ion regulation, and excretion areall
maximum of 12.9 0.8 mmol 1 ' after 144 h. ECF [Na] exacerbated by aerial exposure. Intertidal bivalves need
and [Cl] were not affected by emersion. On reimmersion, only withstand approximately 12 h emersion before the
recoverywasrapid,withpH, P ,and Pco,returningtocon- next tidal inundation. Even so, many intertidal bivalves
trolwithin 2 h,while[Ca]and [HCO,app] remainedelevated haveevolved compensations, chiefly respiratory, that en-
until 24 h after reimmersion. A 1:1 stoichiometry between ablethem toconserveenergyduringemersion. Examples
[Ca] and [HCO,app] existed throughout the emersion and include valve gaping and aerial gas exchange (Boyden,
reimmersion periods. In the absence ofprotein buffers, the 1972; Widdows et a/., 1979) and the use ofshell CaCO,
fall in ECFpH wasarrested by the mobilization ofcalcium for maintaining acid-base balance (Crenshaw and Neff,
carbonate, presumably from the shell. By 96 h emersion 1969; Crenshaw, 1972; Akberali et a/., 1977; Booth and
PCO: and P , had stabilized, suggesting that diffusion gra- Mangum, 1978; Jokumsen and Fyhn, 1982; Booth eta/.,
dients sufficient to allow limited gas exchange had been es- 1984).
tablished. Theeffectsofemersion on freshwaterclamsare known
primarily from studies ofthe responses ofthe corbiculid
Introduction
bivalve, Corbicu/a fluminea (Miiller) (McMahon, 1979,
Marine intertidal bivalves experience cyclical episodes 1983; McMahon and Williams, 1984; Byrne et al.. 1988,
ofemersion and reimmersion. and theirphysiological and 1989, 1990, 1991a,b). Cfluminea, however, isarelatively
Received 2 February 1991;accepted 20June 1991. recent invaderoffreshwater (Keen and Casey, 1969) and
* Currentaddress: DepartmentofBiology.SUNYCollegeatFredonia, so displays many responses thought to be intermediate
Fredonia, NY 14063. between those ofestuarine clams and those ofmore an-
289
290 R. A. BYRNE AND B. R. McMAHON
cient freshwater species (McMahon, 1979; Byrne et ai, loweredat 10-min intervalssothatthesubsequentsamples
1988). Dietz(1974)described aerial O: consumption and could be taken at the same nominal exposure time. After
responses to dehydration in the unionid, Ligumia sub- an emersion period, at constant temperature, of 144 h,
rostrata, and Heming et al. (1988) examined acid-base the clams were reimmersed by gently filling the basins
changes in extrapallial (mantle cavity) fluid ofMargari- with temperature-equilibrated APW. The reimmersion
tifera margaritifera during emersion. These studies indi- period lasted 24 h. No mortality resulted during either
cate that freshwaterbivalves possessadaptationstoemer- the emersion or reimmersion periods.
sion that mayaccount forsome oftheobserved tolerances At intervals of0, 1, 7, 25, 48, 96, and 144 h emersion,
toaerial exposure. But the respiratory, acid-base, and ion and at 2, 6, and 24 h reimmersion, five clams (one per
regulatory consequences of emersion and subsequent basin) were removed and an extracellular fluid (ECF)
reimmersion have not been comprehensively examined sampletakenanaerobicallybypericardiacpunctureusing
in a unionid clam. cooledglass syringes(afterthe method ofFyhn andCost-
Here we report an examination ofsuch events during low, 1975). ECF P ,, PCO, and pH were measured im-
an extended bout of aerial exposure and for 24 h after mediately on 500 n\ samples using a Radiometer BMS 3
reimmersion in the unionid bivalve Anodonta grandis Mk 2 Blood Microsystem maintained at experimental
PHM
simpsoniana. Although this species inhabits the shallow, temperature and connected to a Radiometer 73
littoral region of lakes, and members ofthe population pH/Blood gas monitor. P , and PCo,were determined by
under study could become aerially exposed with minor means ofa Radiometer P (E5047-0) and Pro (E5037-
O: ,
changes in lake level, such occurrences would be rare. 0) electrode respectively; pH was measured using a Ra-
Therefore, any compensations to aerial exposure and diometerglasscapillary pH electrode (G299A). Total CO;.
subsequent reimmersion that maybenotedinthisspecies (CCOJ was also measured immediately on 45 n\ samples
maybe interpreted asan inherent ability ofunionid clams by meansofaCorning965 CO: Analyzercalibrated with
to withstand periods ofemersion and not any particular standard NaHCO, solution. The remaining ECF (500-
response ofan often exposed species. 800 ^1) was stored at 4C for subsequent ion analyses.
Immediately following sampling, the soft tissue of the
Materials and Methods clams was removed from the shell and dried to constant
weight (>96 h) at 80C.
Animals
ECF [Na] and [K] were measured usinga Perkin-Elmer
Anodonta grandis simpsoniana is a northern nearctic model 5000 atomic absorption spectrophotometer on
species inhabitingthe littoral region ofboreal lakes. Spec- samples diluted with 1% CsCl, and ECF [Ca] was deter-
m
imens were collected at depths of between 1-7 by mined on samples diluted with 1% LaClj. ECF [Cl] was
SCUBA divers from Narrow Lake in north-central Al- measured by means of a Radiometer CMT10 chloride
berta. Clams were returned to the laboratory within 6 h
titrator.
and maintained, unfed, at 10C in aerated aquaria con- Inanotherexperiment, 60specimenswereindividually
tainingartificial pondwater(APW; composition in mmol numbered, blotted of excess water, and weighed to the
1': 0.5 NaCl, 0.4 CaCl:, 0.2 NaHCO,. 0.05 KC1; Dietz nearest 0.1 mg. Animals were placed in containers filled
and Branton, 1975) for at least 2 weeks prior to experi- with APW at 10C for24 h, emersed for 144 h, and reim-
mentation. mersed for 24 h, as described previously. At the same
time intervals as in the previous experiment, a sample of
Emersion and reimmersion five animals was removed, blotted ofexcess water, and
Seventy animals were numbered, and groups of 14 in- reweighedtodeterminewaterlossorgain. An ECFsample
dividualswereplacedon plasticmeshplatformssuspended was drawn from each animal and its osmolality deter-
3cm abovethebottom in each of5 covered plasticbasins mined on 50 n\ aliquots usinga freezingpoint depression
(30 X 25 X 18 cm). Theclamsweresubmerged toadepth osmometer(Precision Systems^Osmette). Thesofttissue
of2 cm above the animal with aerated APW at 10C. was excised, dried (80C, >96 h), and weighed; the shell
A preliminary experiment indicated that a 144 h (6 was blotted dry and weighed. Total body water [total
-
days)periodofemersion resulted in asignificant response weight at the beginningofthe experiment (shell weight
without mortality. Thus, after a 24 h acclimation period + dry tissue weight)] was calculated. All weight changes
in the containers, the clams were emersed by emptying were assumed tobedueto waterlossorgain. The change
the container to a level of2 cm from the bottom thereby in body water content was calculated from the weight
retaining a water reservoir to maintain the humidity in change at the experimental time and was expressed as a
thebasin nearsaturation. Thewaterin thefivebasinswas percentage ofthe initial body water content.
EMERSION AND RE1MMERSION IN ANODONTA 291
In vitrodeterminations ofCO2 combiningcurves were after 1 hand2 hincubation. Theclamswerethen returned
performed on ECFdrawn from the pericardia! cavities of to an aquarium containing aerated pondwater. At 6, 12,
sixanimals. Thesamplefrom each animal wascentrifuged 24, 36, and 48 h reimmersion, thesameclamswere again
at 6000 X g, and the supernatants from all samples were placed in 50 ml APW and samples ofbathing medium
then pooled and maintained on ice. Aliquots (100 ^1) of taken initially and 1 h later. Handling and other distur-
pooled ECF were placed in equilibration tubes and to- bances were minimized throughout. Undiluted samples
nometered at 10C in a Radiometer BMS2 Mk 2 Blood wereassayed forNaand Kbyemission spectroscopy with
Micro System with CO2/O2/N2 mixtures of0.05%, 0.1%, a Perkin-Elmer model 5000 atomic absorption spectro-
0.4%, 1.0%, 2.0% and 3.0% CO2 in 50% O2 and balance photometer. Calcium concentration was determined by
N2 supplied by Wosthoff precision gas mixing pumps. atomic absorption spectroscopy on samples diluted with
After equilibration, ECF pH and Cco, were determined LaClj, while Cl content was measured by coulometric
as described previously. titration on a Radiometer CMT10 adjusted to assay low
concentrations(0.05 mmol-l'). Thesofttissuesofeach
Calculations clam were excised and dried to constant weight at 80C
ThepKapp(apparent pK oftheCO2/bicarbonate buffer w(9a6shc).alTchuleanteetdifornofmlutxh(eJncoth)adnugreinignetahcehisoanmipclcionngtpeenrtioodf
system) ofclam ECFwas calculated from the PCo equil- the bathing medium over time, and normalized to the
ibration tension (Torr) and the corresponding measure- dryweight oftissue. The net ion fluxes ofacontrol group
mentsofpH andCeo(mmol 1~')foreach in vitrosample. oftencontinually immersed clamswerealsodetermined.
The calculation was carried out by rearrangement ofthe
Henderson-Hasselbalch equation as follows: Statistics
C
pK = pH - CO. Alldataareexpressedasmeans SEM. Forthechanges
app log CO2 Pco, in ECF ion and acid-base variables, a single classification
ANOVA
was performed with time ofsample as the clas-
The solubility coefficient ofCO2 (CO2) at 10Cwas cal- sificatory variable; emersion and reimmersion samples
bacsuelt0aw.te0eed6n9bypmKimnaotpeplrap(on1ldaTtopirHorn)"(frr1.=oAm0st.h0te1h;tearPbel>ewai0ns.C0n5a)om,ertrehloeantaiv(oe1n9rs8ah6gi)ep wcwaehnritech(bPotth<he 0ci.on0nc5tl)ru,odlesdev.qaulIefunetthiweaalAscNocnOotmrVpaAsatrsperdwoevwrieetdhpteoerafbcoehrsmeiexgnpdiefrii-n-
value ofpKapp (6.436 0.035) was used to calculate the imental value to detect specific significant differences (P
PCO, isopleths for the plot ofpH against apparent bicar- < 0.05). SYSTAT procedures were used for statistical
bbrooennmaaattieeni(cnogHnCcafeOtne3tarrpatpthiemodnmios(slsoeleIv"eF1di)gi.CsOt6)h2.atiTsphroeermtaoipovpneadro;efnitttheibsiCpcrCaero--, wainoaanslyftsleeussxt.eedTxhbpeyerliremealesantttis,oqnuasahrrieepspeblaeittnewedaeremnreea[gsrCueas]rseiasonndA.N[FoOHrCVtOAh3eawpnape]st
dominantly HCOj, but includes carbonate, carbamates, performed on the flux ofeach ion over time. The mean
andotherion pairs (see Heisler, 1986). Concentrations of fluxes were compared to the flux of the control group,
apparentbicarbonatewerecalculated from Ceo,according and the difference and its significance were determined
to the equation: by post-hoc Neuman-Keul's tests ( = 0.05) using the
HCO3app = Cco, - CO2 Pco, repeated measures ANOVA mean square errorterm and
an "n" of 10.
The HCO3app and pH data were used to estimate the //;
vitrobuffercapacity ofthe ECFasthechange in HCO,app Results
perunitchangein pH(Slykes). Apparent"invivo'buffering
capacity wascalculated from measured HCO^appand pH Allexperimental specimensofAnodontagrandissimp-
values over the first 96 h ofemersion. soniana survived 6 days ofaerial exposure at 10C and a
subsequent 24 h of reimmersion. While emersed, the
Net ionflux on reimmersion clams lost about 1.6% oftheir total body water per day
(Fig. 1). The weight loss was consistently significantly
Ten clams were aerially exposed for 6 days at 10C greaterthan pre-emersion controls by 48 h emersion. ECF
under conditions ofabout 100% relative humidity. The osmolalityincreasedfrom acontrol valueof42 1 mOsm
clamswerethen placed in individual bathscontaining 50 kg ' to 54 2 mOsm kg"' by 144 h emersion (Fig. 1).
ml APW. Once the clams had opened their valves and This constituted a 28% increase in solute whereas water
had commenced siphoning (5-15 min), bathing medium loss was only 13%. ECF osmolality was significantly re-
samples were taken; two additional samples were taken lated to emersion time:
292 R. A. BYRNE AND B. R. McMAHON
60 Emersion resulted in an ECF acidosis, with pH falling
significantly below pre-emersion values after the clams
55 had been in air for 7 h (Fig. 2). Although the acidosiswas
progressive, the rate ofpH decline slowed after 24 h and
OI 50 reachedaseemingly stablelevel near7.5;thereitremained
until the end ofthe emersion period. But the steady pH
r 45
is misleading, as [H+] continued to rise until 96 h after
40 the onset ofemersion; then it remained unchanged. ECF
pH returned to control values within 2 h ofresubmerg-
35 ence.
Both ECF [Ca] and [HCO,app] changed significantly
with thedurationofexposureandon returntowater(Fig.
3). ECF [HCO,app] increased rapidly to 8.8 0.6 mmol
g 1 ' during the first 24 h of exposure and then rose less
is rapidlyto 12.9 0.8 mmol 1~' bytheendoftheemersion
period. ECF [Ca] had almost doubled from 5.5 0.4
mmol-1 ' to 10.0 0.5 mmol-1 ' after 48 h emersion
and continued to rise, but at a slower rate, reaching 13.1
0.8 mmol T1 after 144 h in air. When the clams were
reimmersed, the ECF concentrations of both Ca and
HCO,app declined rapidly. [Ca] returned to pre-emersion
-20 1
20 40 60 80 100 120 140 160 180
Time 100
(h)
Figure 1. Timecourseofchanges in ECFosmolality (upper panel) -, 80
andbodywatercontent(lowerpanel)inAnodontagrandissimpsomana
during 144 h emersion and 24 h reimmersion in pondwater at 10C. ,60-
Symbolsare mean valuesforfiveclams; barsare standarderrorsofthe 5*40
mean. Asterisks indicate values significantly different from control (P Q.
<0.05). Thearrowsindicate the timeofreimmersion in pondwater. 20h
Osmolality = 41.4 (3.2) +
14-
0.082 (0.010) X emersion time (h)
m
-
(SE;R: = 0.63; df = 1, 38) eg 8
This translates to an increase in osmolality of approxi- 3 6
mately 2 mOsm kg ' d~'. Reimmersion resulted in return L 4
of water weight to control within 2 h. ECF osmolality 2
declined to control within 6 h ofresubmergence.
8 4
During the first hour ofemersion the clams displayed 8.2
a significant decline in ECF P ,; thereafter, a further, but 8.0
slowerdecrease occurred overthe course oftheemersion
period (Fig. 2). Values remained above 20 Torrthrough- 77.86 - 20 xO
outthetime that theanimalswereexposed. Duringreim-
7.4
mersion, the ECF was rapidly reoxygenated, but not to 72
pre-emersion controls values, even after 24 h of resub- 50 100 150 200
Time
mergence. (h)
ECFPco, rosesteadilyduringthefirst 24 hofemersion, Figure2. TimecourseofchangesinECFP ,(upper),Pco,(middle),
reaching 7.7 1.1 Torr (Fig. 2). Thereafter, PCO; contin- andpH(lower)inAnodontagrandissimpsonianaduring 144hemersion
uvaelduetosrsistea,bibluitzeadtanesalrowe1r3raTtoer,ra.ndOnaftreeri9m6mehrosfieonm,ersEiCoFn (faotnridmfei2v4e=ch0l)armiessi;mrembpaeerrasstiaeordnetsoitnathnpedoanlrdedfwtaeftrorerorrcslaatorfit1ty0.heCS.ymmeTbahnoe.lscAosantrteerroimlsekasconinndvdiaitlciuaoetnse
Pco, declined to pre-emersion valueswithin 6 h ofresub- valuessignificantlydifferentfromcontrol(P<0.05).Thearrowsindicate
mergence. thetime ofreimmersion in pondwater.
EMERSION AND REIMMERSION IN ANODONTA 293
16 total ionic strength but showed significant increases be-
14 tween 24and48 hemersion, andthendeclinedtocontrol
12
levelsforthebalanceoftheemersion periodandthrough-
10
8 out the reimmersion period.
6 Asthe protein concentration in clam ECF is low (0.24
4 0.03 g-r1 R. A. Byrne and B. R. McMahon, unpub.
;
2 data), the /// vitro non-bicarbonate buffering capacity of
14 the ECF was also low. The in vitro relationship between
12 pH and HCO3app was:
o
180 [HCO3app] = 13.64 (0.30) - 0.68 (0.20)*pH
E.
"Oro 64 (R2 = 0.75, df = 1,4).
2 Duringthefirst 96 hofemersion, therelationshipbetween
50 100 150 200 ECF pH and apparent bicarbonate described a steeper
16 Time (h) linear association. This in vivo relationship yielded the
14 following regression equation:
12
o
10
E 8
25
6
O
4
20-
2
6 8 10 12 14 16 15-
[Apparent Bicarbonate] mmol/L
10-
Figure3. The uppertwo panelsshowthetimecourseofchangesin
extracellular[HCOjapp](upper),and[Ca](middle)inAnodontagrandis 5 -
simpsomanaduring 144hemersionand24hreimmersioninpondwater
at 10C. The control condition (time = 0) is repeated to the left for
clarity.Symbolsaremean valuesforfiveclams;barsarestandarderrors
ofthemean.Asterisksindicatevaluessignificantlydifferentfromcontrol ^ 20
(P<0.05). Thearrowsindicatethetimeofreimmersion in pondwater.
Thelowerpanelshowstherelationshipbetween [Ca]and [HCO,app]
intheextracellularfluidofboth emersedandreimmersed specimensof "5 15
Anodonlagrandissimpsoniana. Notethe 1:1 stoichiometry betweenthese 1
10
twoion species. O
5
values by 24 h resubmergence whereas, after the same
periodofreimmersion, HCO,appremainedelevatedover
controls. 0.8
The pattern of increase and decline in ECF Ca and
HCO3app was strikingly similar. Indeed, there was a 1:1 o 0.6
stoichiometric relationshipbetween ECFlevelsofCaand
,,0.4
HCO^app (Fig. 3) measured throughout the emersion and
reimmersion periods. The regression equation relating
[Ca] to [HCO,app] was:
0-0
[Ca] = -0.6 + 0.99*[HCO3app] 50Time 100 150 200
(h)
(R: = 0.72: df = 1,48)
Figure4. Timecourseofchangesinextracellular[Na](upperpanel),
The two other major ECF ions, Na and Cl, did not [Cl] (middle panel), and [K] (lower panel) in Anodonla grandis ximp-
crheainmgmeerssiigonnifipcearnitoldyst(hFirgo.ug4)h.oTutheeistlhigehrttdheecreemaesresiinoncono-r sS1oy0nmCib.aonlTashdeaurrceionnmgterao1ln44cvohanldeuiemtseirofsnoiro(ftniivmaeencdl=a2m04s);hisbrraeerpsiemaamrteeerdssttiaoonntdhaierndlepfeotrnrfdoorwrsactloaefrritthaye.t
centrations of both these ions on reimmersion was not mean. Asterisks indicate values significantly different from control (P
significant. ECF[K] constituted asmall proportion ofthe <0.05). Thearrows indicatethetimeofreimmersion in pondwater.
294 R. A. BYRNE AND B. R. McMAHON
EMERSION AND REIMMERSION IN ANODONTA 295
16 (Crenshaw, 1972), confirmingthatshell dissolution isthe
cause ofincreased calcium. Acyclical change in ECFcal-
"o cium, presumably associated with HCO3app production,
E was noted by Akberali el al. (1977). When the clam had
closed valves and became hypoxic, calcium levels rose;
during bouts of valve opening when ventilation would
recommence, calcium levelsdeclined. Additionally, Booth
el al, (1984) did not find any change in either ECF pH
60- or[Ca] inMytUmedulisduringshort-termemersiondur-
ing which metabolism remained predominantly aerobic.
4 However, under conditions of extended (6 days) aerial
7 7 2 7 6 exposure,Pco intheECFofMytilusedulisandModiohts
,
PH modiolus rose along with a decrease in pH (Jokumsen
andFyhn, 1982).
Figure 6. Diagram summarizing changes in extracellular pH, ap- The very limited non-bicarbonate buffersystem isbal-
p1a4r4enhtebmiecrasriboonnataendan2d4PhCor,eiimnmAenrosdioonntiangproannddiwsatseirmpaston1i0aCn.aCdluorsiendg anced bythe large storeofreadily mobilizable shell buffer.
symbolsjoinedbysolidlinesarevaluesforemersedclamswhilestippled Thisisespeciallyimportant forunionidclams, whichhave
symbolsconnectedbydashedlinearethoseofreimmersedclams. Values evolved a very dilute body fluid as an adaptation to life
aremeansandverticalandhorizontalbarsareSEM'sforHCO,appand in freshwater. The low ionic strength ofboth the extra-
pH.respectively.Thearrowsindicatethetimecoursewhilethenumbers cellularandintracellularcompartmentsreducestheenergy
alongsidethesymbolsindicatethelengthoftime, in hours,emersed(e)
or reimmersed (r). The solid straight line represents the in vitro non- requiredtomaintain homeostaticioniclevelsbyreducing
bicarbonatebutler line forclam EOFat 10C. the diffusive gradient. Nevertheless, emersion resulted in
a large increase in extracellular solute. ECFosmolality of
Corbiculafluminea during three days' aerial exposure at
CaCO + H+ Ca2+ 25Cincreased twofold(Byrneelal, 1989), andasimilar
3 increase was noted forthe unionid, Ligumiasubrostrata,
In a completely closed system, the levels of HCOr and during a 5-day emersion period (Dietz, 1974). However,
Cawould rise as longas protonswere being produced. In death from aerial exposure does not seem to result solely
the emersed, incompletely closed clam, the levels of from a simple lethal increase in solute, as C. fluminea
HCO3 thatareaccumulatedaredeterminedbytheability succumbed toaerial exposureafterlosingtotal bodywater
ofthe clam to offload CO? to the environment. The at- rangingfrom 30to 80% (Byrne elal. 1988). Rather, death
tainment ofthe new equilibrium between CO;., HCOr, seems related to the inability of the clams to maintain
and Ca:+ is achieved after a period in air, during which acid-basebalanceortoavoidtheproductionoftoxicend-
ECF Pco rises to the "equilibrium" level. In Anodonta products (Byrne el al. 199la, b).
,
grandissimpsonianaat 10C,thisprocesstakes96 h. The Extracellular sodium and chloride remain highly reg-
continued production ofCa and HCO3 after this time ulated both duringemersion and duringthereimmersion
suggests that the buffering ofprotons continues with the period. A similar situation was reported for C. fluminea
furtherdissolution ofCaCO3, butthatanequilibrium has during aerial exposure and subsequent resubmergence
been established with CO2 being released. (Byrne el al. 1989). Reports ofchanges in ECF contents
The effectiveness ofthe shell buffersystem, presumably ofthese ions in other bivalves undergoing emersion gen-
the major buffer system in operation, can be seen by the erally showagradual increase in ECFion concentrations
10 fold increase in bufferingcapacity ofthe "in vivo" sys- related to the duration ofaerial exposure. Ligumia sub-
tem over the in vitro determinations on isolated ECF. rostrata had an increased level of extracellular sodium
Mobilization ofCaCO3 from shell hasbeen implicated in andchlorideproportionaltothelevelofdesiccation stress
acid-base control in a number ofbivalve species. During upon aerial exposure (Dietz, 1974). Similarly, chloride
aerial exposure, calcium levels in the body fluids ofMya levels increased passively as body fluids became more
arenaria rose (Collip, 1920). Similarly, when the marine concentrated during emersion in Mytilus edulisand Mo-
bivalve Mercenaria tnercenaria was emersed, CO: and diolus modiolus (Jokumsen and Fyhn, 1982). In severe
calcium levels in the ECF increased (Dugal, 1939). The hypoxic exposure, however, ECF sodium levels in Scro-
source of calcium is from the shell as was reported by biculariaplana remained constant overtime (Akberali el
Crenshawand Neff(1969) using45Calabellingtechniques. al. 1977). The constant extracellular fluid [Na] and [Cl]
Later, itwasshown thatcalcium appearsintheextrapallial in emersed Anodonta grandis simpsoniana suggests that
fluid before entering the extracellular compartment theseionsaremovingintosomeothercompartment, e.g.,
296 R. A. BYRNE AND B. R. MrMAHON
intracellular space. This possibility is further evinced by be general adaptations of freshwater bivalve species.
thelossesoftheseionson reimmersion, eventhoughECF Therefore, the extended tolerance ofaerial exposure dis-
levels are not significantly altered. It has been postulated played by many unionid species may not be due to be-
that, in Corbiculafluminea, these ionsare shunted to the havioral or physiological adaptations particularly unique
intracellular space to conserve cell volume (Byrne el a/., to the group, but may simply reflect a wide latitude of
1989), and a similar situation may be occurring in A. capacity adaptation.
grandis simpsoniana. In any case, the tight regulation of
these ions suggests that maintainingconstant extracellular Acknowledgments
concentrations ofsodium and chloride is a fundamental Wewish tothank Dr. T. H. Dietzand twoanonymous
requirement and may be associated with maintenance of reviewers for comments on the manuscript. This study
electrochemical balance.
was supported by a University ofCalgary Post-Doctoral
isrOenmorveeimdmoevresrioan2,4thheouarccpuemruiolda,teadndexEtrCaFcelclaullcairucmallcevieulms FelBlRowMs.hip to RAB and NSERC operating grant A5762
to
return to control values. The loss is also evident by the
high negative netfluxesforthisionduringtheinitialstages
Literature Cited
ofreimmersion. Other unionid clams recapture extracel-
lularcalcium inconcretions made upprimarilyofcalcium Akberali, H. B.,K.R.M.Marriott,andE.R.Trueman. 1977. Calcium
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