Atmospheric Water Absorption and the Water Budget of Terrestrial Isopods (Crustacea, Isopoda, Oniscidea)

11 Pages·1993·4.9 MB·English

by  J C Wright

Reference: Biol. Bull 184: 243-253. (April.
Atmospheric Water Absorption and the Water Budget
of Terrestrial Isopods (Crustacea, Isopoda, Oniscidea)
JONATHAN WRIGHT AND JOHN MACHIN
C.
Department ofZoology, University ofToronto. 25 HarbordStreet,
Toronto, Ontario, Canada M5S 1AI
Abstract. Studies ofterrestrial isopods (Crustacea, Iso- ismatid Thysanura (Noble-Nesbitt. 1970;Okasha, 1972);
poda. Oniscidea) have revealed acapacity foractivewater Blattodea (Edney. 1966; O'Donnell. 1977, 1981. 1982);
vapor absorption (WVA) in the taxonomic sections Cri- Siphonaptera(Knulle, 1967; Rudolph and Knulle. 1982);
nocheta and Diplocheta but not in Synocheta. Uptake Psocopteraand Mallophaga (Knulleand Spadafora, 1969;
thresholds in Crinocheta are modest by comparison with Rudolph, 1982a, b, 1983); tenebrionid larvae (Mellanby,
other vaporabsorbers, but standardized uptake fluxesare 1932; Ramsay, 1964; Grimstone et a/.. 1968; Machin,
amongthe highest recordedandare probably an adaptive 1975); and lepidopteran larvae (Chauvin and Vannier.
requirement to counter the high transpiratory losses. 1980). Othermajorexponentsofvaporabsorption include
Comparativedata for uptake fluxes, thresholds, and tran- certain acarine families (Lees, 1946; Rudolph and Knulle,
WVA
spiratory losses allows the compilation ofwater budgets 1979: Knulle and Rudolph, 1982). Recently, has
in hypothetical temperature and humidity regimes. Given beendemonstrated intheoniscidean isopods(Wrightand
a 12-h light-dark cycle, with saturated ambient activities Machin, 1990), the only crustaceans havingattained ma-
for diurnal WVA, all species could recover water losses jorecotypicdiversification on land. Adaptations forWVA
incurred during nocturnal foraging in an ambient water have yet to be demonstrated in myriapods and the non-
activityof0.75,andxericspeciescould forage inactivities acarine arachnids. There is one described case of vapor
below 0.30. Xeric trends based on these models agree absorption in a desert plant No/ana mollis, a succulent
closely with predictions from ecotypic surveys. In the lit- shrub from the Atacama (Mooney et ai, 1980).
toral Ligia oceanica (Diplocheta) haemolymph hyperos- Our understanding of the physiological processes in-
mosisandperiodicsubmergenceprovideadditional means volved in vapor absorption is fragmentary. Any mecha-
ofwater balance regulation. It is proposed that WVA in nism does, however, have basic prerequisites. A water-
Ligia provides an essentially solute-free water source to collecting surface must be brought into contact with the
counteract salt-loading in the splash-zone. The absence humid airand must contain fluids ofdepressed waterva-
WVA
of in synochetes, together with their cryptozoic por pressure, and hence depressed water activity (= rel-
habits, reflects an alternative terrestrial strategy to those ative humidity/100) such that water vapor moves ther-
ofother oniscideans. modynamically into the collecting fluid. The collected
fluid must then be moved internally, byimbibition orco-
transport with solutes. The minimum water activity (aw)
Introduction generated by the collecting fluid constitutes the minimum
Atmospheric water vapor absorption (WVA) is an ac- activity ofwater vapor, or ambient activity (Aaw), from
which absorption is possible, denned as the uptake
tive process enabling certain terrestrial organisms to ex-
ploit a physical state ofwaterwhich is often spatially and threshold (Machin, 1979a). The activity at which vapor
tfoermpWorVaAllyhamsobreeeanbduensdcarnitbetdhiannsleivqeuriadlsionusreccets.grAoucpasp:aclietpy- iatcobansl,oer1qp9ut6ii4lo)in;bbraainlueamtncagceatsiinvpiaitsnysiwovaretCelroEsAissest(ahifsutsreerpfoKesrnsruielbdlleteoiaannsdatmhWbehicaerrint-t-
activities above the CEA. Physical principles of vapor-
Received 27July 1992;accepted 25 January 1993. liquid transitions and energetic considerations are dis-
243
244 J. C. WRIGHT AND J. MACH1N
cussed by Wharton and Richards(1978), Machin (1979a). vapor condensation. Pleopodal ventilation presumably
and O'Donnell and Machin 1988). functions to circulate air over the uptake fluid and may
(
Vaporabsorption systemsbased on colligative lowering be involved in fluid transport and dissemination.
ofvapor-pressure require a compartment, isolated from This explanation for the absorption mechanism re-
the haemolymph, in which solutes can be accumulated quires modification at vaporactivitiesclosetothe uptake
by active transport. Hyperosmotic fluids have been iden- threshold. Undertheseconditions, pleon fluid osmolalities
tifiedinthecryptonephridial systemsoftenebrionid larvae will sometimes equate with water vapor pressures above
(Ramsay, 1964; Machin, 1979b; O'Donnell and Machin. those ofthe ambient air, and are thus incompatible with
1991; Machin and O'Donnell, 1991), creatingan activity uptake by direct condensation. It has been proposed
gradient between the rectal lumen and the peripheral (Wright and O'Donnell, 1992) that cyclical compression
Malpighian tubules. They have also recently been de- ofair within the PV serves to increase the vapor density
scribed inthesalivarysecretionsofticks(Sigaletal., 1991), and hence increasewateractivityabovethatoftheuptake
although conflicting data is presented by Gaede (1989). fluid. Intermittent compression ofair during pleopodal
Deeply folded apical and basal membranes with densely ventilations issuggested by visible pulsesofdisplaced fluid
packed mitochondria suggest ion transport compatible atthe pleopodal margins. Such 'pressure cycling' hassub-
with production/resorption ofhyperosmotic fluids in the sequently been confirmed by monitoringpressurechanges
recta of flea larvae (Bernotat-Dalielowski and Knulle, coincident with pleopodal ventilations (Wright and Ma-
1986) and thysanurans (Noirot and Noirot-Timothee, chin, 1993). Interventions ofpressure cyclingare also de-
1971). tectable gravimetrically as sharp deviations from the typ-
The hypopharyngeal bladders ofthe desert cockroach ical linear uptake kineticswhen net flux isplotted against
Aarneinsimva(Og'aDaopnpneelalr.to19e8x1p,lo1i9t8a2)d.ifTfheerenbtlaadbdseorrsptarieoncomveecrhe-d Avoalwv.edSuarcehrepllaottisveilnydismcaaltle:t3h.a6t5-t6he.38prkesPsaurfeorcsheavnegnecsoimn--
in hydrophilic hairs which are moistened by the cyclical mon, temperate Crinocheta. It provides a supplement to
secretion of a fluid comparable in ionic strength to the the usual colligative absorption mechanism, allowingan-
haemolymph. It isproposed that, byalteringthe iso-elec- imals to boost uptake in near-threshold activities. The
tric point ofthe cuticularchitin and/or protein, this fluid feasibilityofpressurecyclingasa meansofwaterrecovery
reduces water affinity and the hairs release water which was first noted by Maddrell (1971). It has subsequently
is subsequently imbibed. Condensation onto the bladder been suggested as a widespread means ofwater conser-
surfaces reverses the trend, decreasing the ionic concen- vation in insect tracheal systems, and to playarole in the
tration ofthe remaining frontal body fluid, and increasing vapor absorption mechanism oflepismatids and siphon-
the water affinity of the cutaneous hairs. The process is
apterans (Corbet, 1988).
trehluesasseedlfb-ysutshteainneixngtpaunldsethoefuhpatiraskeabfsluoirdb. Awastiemriluanrtimlecithi-s appTrhoexidmeastcerliybeadwu=pta0k.e90th(rWersihgohltdsafnodr Monaicshciind,ean1s990a)r,e
anism may operate in the Psocoptera and Mallophaga, modest bycomparison with many othervaporabsorbers.
based on alterations in the iso-electric point of protein
The pyroglyphid dust mite Dermatophagoides jarinac
polymers secreted onto the lingual sclerites (Rudolph,
Hughes, for example, has a critical equilibrium activity
1982b, 1983; O'Donnell and Machin, 1988).
0.70 (Arlian and Wharton, 1974). Psocoptera and Mal-
Blocking experiments have established that vapor up-
take in oniscidean isopodsoccurs in the pleon with water lophaga display uptake thresholds below 0.50 (Rudolph,
1982a. b, 1983)and respective thresholdsof0.45 and0.43
possibly being absorbed across the pleopodal endopods
have been demonstrated for Thermobia (Lepismatidae)
(Wright and Machin, 1990; Wright and Machin, 1993).
Prior to absorption, a strongly hyperosmotic fluid con- (Noble-Nesbitt, 1969) and larvae of Lasiodenna (Ano-
sviesnttirngalmroasutml'y(oPfVN)a(+WraingdhtCTanids sOe'cDroentneedliln.to1t9h9e2)',pltehoe- obinidtahee)m(aKnjuolrleceallnudlaSrpaeldeacftorroal,yte1s97(0)N.a+U,pKta+k,eClflu)idasrebalsiemd-
cavity formed between the imbricate pleopodal exopods ited in colligative vapor pressure lowering by saturation:
and the pleon sternites. A few minuteslater, the pleopods NaCl saturates at an activity of 0.75 and KC1 at 0.85
beginametachronal ventilatory rhythmandthe fluidvol- (Winston and Bates, 1960). However, it hasbeenproposed
ume declines. There is a simultaneous drop in the fluid that larvae of the tenebrionid Onymacris plana exploit
osmolality such that its vapor pressure increases to levels supersaturation ofK.C1 to depress the threshold down to
only slightly below that ofthe ambient air (Wright and 0.81 (Machin and O'Donnell, 1991). Avoidance ofcrys-
O'Donnell, 1992). Since pleopodal ventilation is invari- tallization may depend on 'supersaturation proteins'
ablyassociated with WVA ingravimetric studies, and has which bind to crystal surfaces, presenting conformations
onlyotherwisebeen observedduringmaxillary urination, incompatible with crystal growth. Analogous 'antifreeze
the dilution of uptake fluid is attributed to the onset of proteins' are widely documented in freeze-resistant or-
WATER VAPOR ABSORPTION IN ISOPODS 245
ganismsandservetoblockthegrowthoficecrystalsduring Watervaporabsorption wasdeterminedgravimetrically
supercooling (Davies and Hew, 1990). by monitoring mass-changes ofisopods in different water
Uptake thresholds and absorption kinetics are clearly activities. Individual animals were contained within 10
the main factors deteWrmVinAing the physiological and eco- mm Xmm25 mm cylindrical cages constructed from 1 mm
logicalsignificanceof in differentgroups. Together, or 2 mesh aluminum gauze. They were weighed in
thesedetermine the humidity rangeoverwhich absorption a movingairstream ( 1 cm s~') in acontinuous-recording
is possible and the water deficits which can be recovered Sartorius 4410 digital microbalance (Sartorius GMBH,
inagiven time. Theecological significanceofequilibrium Gottingen) sensitive to 10 j*g. The balance and weighing
humidities in arthropods has been discussed by Knulle chambers were enclosed in temperature-regulated water
and Wharton (1964),WwiVthAparticularreference toacarines. jackets permitting accurate control of temperature and
The new finding of in oniscideans is particularly humidity ofthe air stream. These parameters were reg-
interesting in this regard since, unlike other documented ulatedby means ofa Hewlett-Packard 71Bcomputerand
vapor absorbers, oniscideans are typically mesic-hygric 3421A data-acquisition unit; details are given in Machin
arthropods, usually considered poorly adapted to xeric (1976). All experiments were conducted at 20.0C.
habitats(Cloudsley-Thompson, 1956:Edney, 1968). Here Determinations ofnet flux in different ambient activ-
wereport resultsofacomparativestudyofthresholdsand ities permitted the quantification of passive losses and
uptake kineticsin theOniscidea. Such information serves uptake fluxesduring WVA. Passive fluxeswere standard-
both to establish the range of vapor-absorbing abilities ized for surface area assuming the formula of Edney
acrossthesub-order, and toprovidea means forassessing (1977):
ecotypic consequences ofWVA.
Surface area (cm2) = 12 Mass (grams)067
Materials and Methods Uptake fluxes were standardized for surface area as-
Oniscideans were collected from diverse localities in suming a net pleoventral absorbing area of0.05 cm2 for
England and Ontario and maintained in laboratory cul- a 100 mg animal (Wright and Machin, 1993). The net
tureswith deciduous litter. Moistened papertowels served surface area ofsuch an animal computes at 2.57 cm2, so
to regulate the culture humidities between 95% and 98% we may assume the pleovent=ral area to comprise a pro-
and provided a locally saturated microclimate. The fol- portional area of0.05/2.57 0.0195. Incorporating this
lowing species were selected for study: factor into Edney's equation gives:
Section Crinocheta Pleoventral area for WVA = 0.234 M-67
Oniscits asellus Linnaeus, 1758
Fluxes were also standardized for vapor pressure dif-
Pluloscia nnisconim (Scopoli. 1763) ference between ambient air and haemolymph. For pas-
Armadillidium v/gare(Latreille, 1804) sive fluxes, thisassumed a haemolymph osmolality of700
Elumapurpurascens Budde-Lund, 1885 mosmol 1~' for Crinocheta and Synocheta (Little, 1983;
Cylisticus convexus (De Geer, 1778) J. C. Wright, unpub. data)and 1 157 mosmol 1~' forLigia
Porcellio dilatatus Brandt, 1833
oceanica (Parry, 1953). Since water activity is approxi-
Porcellio laevis Latreille, 1804 mately equal to solvent mole fraction (as follows from
Porcellio scaber Latreille, 1804 Raoult'sLaw)and the molarconcentration ofliquidwater
Porcellio spinicornis Say, 1818 at 20.0C is 55.5 mol 1 ', the haemolymph wateractivity
Porcellionidespruinosus (Brandt, 1833) for Crinocheta and Synocheta is given as:
Trachelipus rathkei (Brandt. 1833)
55.5/[55.5 + 0.700] = 0.988
Section Synocheta
Androniscus dentiger Verhoeff, 1908 Multiplyingthisvalue by thesaturation vaporpressure
Haplophthalmus danicus Budde-Lund, 1880 ofwaterat 20.0C(2339 Pa)givesthe haemolymph vapor
Trichoniscuspusillus Brandt, 1833 pressure in Pascals.
For uptake fluxes, standardization was based on the
Section Diplocheta
Ligia oceanica (Linnaeus, 1767) absorption threshold activities for each species, and thus
assumed initial secretion of uptake fluid at this activity.
The littoral species Ligia oceanica was maintained in Justifications for this analysis are discussed in a separate
anaereatedtankofartificialseawater(Marine Enterprises paper(Wrightand Machin, 1993). Unlessotherwise men-
Inc., Baltimore, MD)and fedonEnteromorphaxp. Emer- tioned, the term 'standardized flux' indicates flux stan-
gent rocks allowed animals to move freely between air dardized forboth area and vapor pressure difference (i.e..
and water. Mg h~' cm"2 Pa ').
246 J. C. WRIGHT AND J. MACH1N
Results Table I
Oniscideans display considerable variation in their Menu Wd.y.vc.s andinli'Kiimcnial water//n.vt'.v<>lOniscidca
abilitytorestrictpassivewaterlosses. Sincethereare mul-
tiple componentstowaterlossesin thisgroup (Lindqvist,
1971; Hadley and Quinlan, 1984; Wright and Machin, Species
unpub. data), cumulative mass losses over time do not
strictly reflect cutaneous transpiration. However, with
continuousgravimetric monitoringitispossibletoidentity
and exclude the irregular bouts of maxillary urination
(Wright el al., in prep.). Losses from the moist pleopodal
endopodscan be estimated usingblocking techniques and
account for less than 5% oftotal losses in all species in-
vestigated (Hadley and Quinlan, 1984; Wright and Ma-
chin, unpub. data). Whole-animal losses have also been
O
shown, using an A12 3 humidity sensor, to be in good
agreement with losses measured across isolated cuticle
(HadleyandQuinlan, 1984). Finally,carbon in respiratory
CO2isasourceofnetmass-loss. Assumingatypicalresting
metabolism of 200 n\ g~' h ' for Crinocheta (Wieser,
1984), and a respiratory quotient of 1, the net mass-loss
attributableto respiration is 3.7 ^gh~' fora representative
70 mg animal. This is below 5% of measured losses for
ambient activities below 0.97, and typically below 1% of
losses for ambient activities below 0.85. For lower respi-
ratoryquotients, aswith significant lipid metabolism, the
respiratory mass-losses are negligible. Analysis ofgravi-
metric losses can thus provide a close approximation of
passive cutaneous transpiration.
Mean standardized loss fluxes are listed in Table I for
the species studied. Pressure-standardized loss fluxes are
also expressed as proportional losses (percent hydrated
mass per hour per unit activity). Species fall broadly into
three physiological categories, corresponding to the dif-
ferent taxonomic sections. The Synocheta and Diplocheta
both reveal highercuticle fluxesthanCrinocheta butpro-
portional losses differ markedly owing to size variation.
Synocheta thus sustain approximately 50% mass-loss
equivalent to approximately 65% water-loss (lethal)
within 1 h in dryair. Ligiaoceanica(Diplocheta), in con-
trast, suffers only 8.5%. mass-loss in the same conditions.
TheCrinocheta reveal theloweststandardized fluxeswith
the least permeable species (Annudillidium vulgare, Por-
cellionidespruinosus) possessingcomparable waterproof-
ing to mesic-hygric insects (EdneyM, 1977). Proportional
mass-losses range froMm 11.63% h"1 aw~' (Pliiloscia
muscorum) to 2.88% h"' aw~' (A. vulgare).
A capacity for WVA was confirmed for all represen-
tatives of the Crinocheta and for Ligia oceanica. This
WVA
extends the number ofCrinocheta for which has
been demonstrated to 12, and provides the first evidence
WVA
of in the Diplocheta. Despite repeated studieswith
bothcontinuousandintermittentweighing,ithasnotbeen
possible to identify vapor absorption in the Synocheta.
WATER VAPOR ABSORPTION IN ISOPODS 247
Table II
Mean uptakethre.iholtl.'iandcriticalequilibrium activitiesfor
CrinochetaandDiplnclicta
Threshold SE CEA
A, vulgurc
0.5 0.55 0.6 0.65 0.7 075 0.8 085 0.9 0.95 1
Ambient
Activity
Figure 1. Example plot ofnet flux against ambient water activity
foravapor-absorbingoniscidean(A. vuls>are). Each pointrepresentsthe
meanfluxmeasuredovera20or30minperiod.Passivelosses(indicated
aspositivefluxes)decreaseapproximately linearly with increasingAa^,
ceasingwherethisequalsthe haemolymph aw(0.988). Deviations from
this curve attributable to active water vapour absorption (WVA) are
evident in activities above the uptake threshold, with uptake maxima
showing approximate linear kinetics with Aaw. Uptake fluxes can be
modulatedinanygivenAa,,tobalancepassivelossesorgeneratemaximal
uptakeasrequired.Theycanalsobeboostedinloweractivitiesbypressure
cycling,evidentintworecordingperiods,wherecompressionofairwithin
thepleoventralcavityisusedtoincreasetheactivitygradientforuptake
(Wright and Machin. submitted).
Uptake thresholds and critical equilibrium activities for
Crinocheta and Diplocheta are listed in Table II. Species
for which insufficient uptake data were available are ex-
cluded. Thresholds vary from 0.866 for Annadillidium
vitlgare to 0.927 for Oniscus asellits. Product-moment
correlation analysis revealsa significant positive relation-
ship between uptake thresholds and standardized loss
<
fluxes(PWV0.A01;Fig. 2). Specieswithloweruptakethresh-
olds for thus display lower integumental perme-
abilities, both constituting xeric adaptations. If the Cri-
nocheta are ranked in ascending order ofthresholds and
CEAs, thesequenceagreescloselywith xerictrends based
on the most comprehensive ecotypic surveys (Harding
and Sutton, 1985; Sutton et al., 1984).
Means and standard errors for vapor-pressure-stan-
dardized, aswell as vapor-pressure and area-standardized,
uptake fluxes forCrinocheta and Diplocheta are listed in
Table III. This table also lists net uptake fluxes expressed
aspercentageofhydrated mass. Correspondingvaluesare
given forother vapor absorbers, derived from data in the
literature. The most striking observation is the superior
efficacy ofthe oniscidean uptake mechanism, with stan-
dardized uptake fluxesan orderofmagnitude higherthan
in other vapor absorbers. The Oniscidea are also among
the most efficient absorbers when net uptake is expressed
248 J. C. WRIGHT AND J. MACHIN
Table III
Inward water//HAV.V measuredduring II I I <// andother vaporabsorbers
WATER VAPOR ABSORPTION IN ISOPODS 249
aging to the minimum ambient activities required for Table IV
diurnal replenishment ofthe resultant water losses. Such Lowerlethalanihicnt actn'iiei /<>/ nocturnalforagingin Oniscidea
water budget curves are illustrated in Figure 4 for the at20 C
species studied, assuming a uniform diel temperature of
20C. The nocturnal water debt could not exceed lethal
dehydration levels. Datagathered fora range ofCrinoch-
=
eta (5 species, cumulative n 14) indicate a lethal de-
hydration level of34.7% hydrated mass 2.4% SE. The
low variance and absence of outliers suggests this as a
useful approximate value for the section (ca. 50% water-
loss). Mean nocturnalactivitiesinwhich thestudyspecies
would sustain 34.7% desiccation in 12 h are listed in Table
IV. Formost species, thesearebelowthe minimum mean
foraging aw exploitable, given access to saturated air for
diurnal WVA; that is to say, the maximum water recov-
WVA
ered by could not exceed 34.7% hydrated mass in
12 h. The magnitude of water deficits incurred during
nocturnal foraging is thus limited by the capacity for re-
plenishment by WVA, ratherthan short-term (nocturnal)
desiccation tolerance. The major exceptions are P. spin-
icornis and T. rathkei in which the desiccation-limited
foraging aw exceeds the WVA-limited aw.
The most striking revelation ofthis analysis is the low
foragingactivitieswhich manyspeciescouldexploitgiven
even modest activities(0.94, 0.96) fordiurnal WVA. With
0.2
0.93 0.94 0.95 0.96 0.97 0.98 0.99
DiurnalAa,,(forWVA)
Figure4. Plotsofminimum mean ambientactivities in which Cri-
nocheta could forage nocturnally given different ambient activities for
diurnalWVA.Theanalysisassumesa 12-hlight-darkcycle. Speciesini-
tialsare indicated besidetheirrespective waterbudgetcurves. Foreach
diurnalAaw.themaximumattainablewater-gainiscalculated,knowing
thresholds, uptakefluxes,andsimultaneouslossfluxesforeachspecies.
Thenocturnalactivityinwhich acorrespondingwater-deficitwould be
sustainedisthendeterminedfromthestandardizedlossfluxes. Foraging
activities which would result in lethal desiccation within a 12-h period
are indicated by hatched bars. All species could theoretically forage in
activities below 0.75 given saturated air for diurnal WVA, and xeric
species(A. vitlgare, P spinicornis, Ps. pniinosiis) could forage in mean
activitiesbelow0.50.
250 J. C. WRIGHT AND J. MAC HIN
in part, by the large size (and hence low surface area: able surface area/volume ratios, these result in propor-
volume ratio) and extreme osmotic tolerance. An ability tional loss fluxes of approximately 50% hydrated mass
towithstand haemolymph Inperosmosis upto2000mos- h ' aw~', equating with lethal desiccation times of ap-
mol kg ' (Parry, 1953) allows Ligia occunica to attain proximately 7 h in 90% RH, and within 1 h in humidities
thermodynamic equilibrium with an ambient water ac- below 30% RH. Nano-osmometric studies (J. C. Wright,
tivity of0.965. Provided these adaptations can maintain unpub. data) indicate haemolymph osmolalities for
sub-lethal hydration levelsthrough the 12-hourlytidalcy- Trichoniscus pusillus to be in the order of 680 mosmol
cle, water replenishment isthen available. The surprising kg '. Even on theassumption that dehydration isaccom-
WVA
discovery of in this species provides yet another panied byosmotic tolerance ratherthan haemolymph os-
means by which it can regulate water balance. Ligia fre- moregulation. this would only induce hyperosmosis to
quently inhabits splash-zone and supra-littoral habitats 1.36 osmol kg"1 at lethal desiccation. This represents a
(Harding and Sutton. 1985) where water supplies are re- water activity of0.976. which may therefore be taken to
stricted to spray or pools flushed only by spring tides. As represent the minimum activity (=97.6%' RH) in which
such, it may face severe salt-loading and dangers of hy- T. pusillus, and probablyotherSynocheta, could maintain
perosmosis. Vaporabsorption in littoral and supra-littoral long-term survival. The characteristically mural/lapidary
habitats may serve primarily to provide a salt-free water habitats ofAndroniscus dentigcr (Harding and Sutton.
source for osmoregulatory purposes rather than providing 1985) suggest unexpectedly water-stressed diurnal micro-
a source ofwater per se. This suggests a possible evolu- climates for a synochete and would repay further inves-
tionary scenario, with the pleon initially serving in active tigation.
saltsecretion in salt-loaded littoral species. Hyperosmotic Only the Crinocheta display substantial independence
NaCl in pleon fluidswould constituteapreadaptation for from liquid water or high ambient activities and thereby
colligative WVA, allowing animals to compensate for colonize mesic-xeric biotopes. Their 'physiological strat-
evaporativeconcentrationofbodyfluidsaswellasdietary egy' involves several adaptations to reduce water losses
salt-loading. Thiswould then conferobvious preadaptive (lowered cuticle permeability, reduction or loss of the
benefits for the colonization ofterrestrial habitats where maxillary/WS excretory avenue, substitution of respira-
liquidwatersuppliesareabsent orinfrequent. Such asce- tory pleopodal endopods with exopodal lungs) and the
nario is compatible with the known capacity for certain abilityto replenish water-debtsabovetheCEA usingvapor
Ligiaspp. to hyporegulate(Wilson. 1970),demonstrations absorption. Like Ligia spp., Crinocheta can therefore al-
ofhigh ATPase activity and ion-transport activity by the ternatebetween periodsofsustaineddesiccation and bouts
pleopodal endopods ofoniscideans (Dr. C. W. Holliday, ofwater recovery. It is well established that activity pat-
pers.comm.; Wright ela/., unpub. data),andseveral lines terns ofCrinocheta are intermittent and primarily noc-
ofevidence suggestinga marineancestry fortheOniscidea turnal (Cloudsley-Thompson, 1952; Brereton, 1957; Den
and the ancestral status ofthe Diplocheta with respect to Boer, 1961; Paris, 1963, 1965). We predict, from the high
Crinocheta (Vandel, 1943, 1965; Edney. 1968; Little, efficacy of their vapor absorption, that the Crinocheta
1983; Hoese, 1981, 1982, 1984). It would be interesting routinely sustain large water debts during nocturnal for-
WVA
to obtain data for and osmoregulation in more ter- aging. This supposition requires confirmation from field
restrial Diplocheta such as Ligidium hypnorum (Cuvier) studiesbutgains preliminary support from thesignificant
where close convergence with the Crinocheta might be activity ofseveral species in depressed humidities (Brer-
WVA
expected. The occurrence of in both Diplocheta eton. 1957: Den Boer. 1961; Paris, 1963, 1965). Such be-
and Crinocheta, but notin Synocheta, concurswith Van- haviorhasbeen suggested to subserve in thetranspiratory
del's (1960, 1965) classification, uniting the former sec- loss of excess water accumulated during the day (Den
tionsin the Ligian series, separate fromtheTrichoniscian Boer, 1961). Den Boer measured substantial diurnal water
(containing Synocheta) and Tylian series. However, fur- gains in P. scaber in aspen woods, though he assumed
WVA
ther studies are required to determine whether is these were attributable to passive, not active absorption.
an ancestral or derived condition within the Diplocheta. Thispossibilitycan bedismissed since, foran isopod with
The Synocheta employ a cryptozoic or 'endogean' a hydrated haemolymph water activity of0.988, the ac-
strategy,seldom foragingintheopen(HardingandSutton, tivity gradient for integumental watergain in a saturated
1985; Sutton el a/., 1984). These species follow high am- diurnal microclimate (Aaw = 1) is only 0.012 (28.0 Pa at
bient activities by vertical migration, avoiding desiccation 20C) and would only induce a minuscule inward water
behaviorally (Sutton et a/.. 1984). Physiological adapta- flux of30-80 fig h"1 (ca. 0.1% body mass h"1) for calcu-
tionsto desiccatingconditions are poorly developed. Cu- lated integumental permeabilities of the study species.
ticle permeabilities ofthe three species monitored in the Waterlogged habitats, on the other hand, present a serious
present studyaresimilartoorsomewhat higherthan those dangertoisopodsowingtothehydrophilic ventral cuticle
ofLigia occunica. Combined with the animals' unfavor- and permeable pleopodal endopods. Drowning has been
WATER VAPOR ABSORPTION IN ISOPODS 251
showntoconstituteamajorcauseofmortalityingrassland minor significance during foraging outings, particularly
populationsofA. vulgare(Paris, 1963). Even largerspecies ifthese occupy restricted periods during the night (Den
regularlydiewithin an hourwhen immersed, presumably Boer. 1961). However, orienting kineses to temperature
from anoxia or from ion-loss and hypo-osmosis. The and humidity, aswell asthigmokinesis, will serveacritical
characteristic behavior of emerging onto the surface of role in the selection ofdiurnal microhabitats compatible
soil and litter during precipitation is equally apparent in with WVA. Although uptake fluxes are likely to increase
WVA
other permeable soil invertebrates (pulmonates. oligo- with temperature, beingan energy-dependent pro-
chaetes) and is generally regarded as an evasive response cess(Machin, 1979a), andpassivetranspiration inP. laevis
to waterlogging. has been shown to increase linearly with ambient tem-
WVA
Interspecific differences in thresholds, uptake perature over a biological range (Hadley and Quinlan,
fluxes, and integumental permeabilities reflect adaptive 1984), thereiscurrently insufficient dataavailabletoallow
responses to differing xeric stresses. The strong positive predictionsoftheeffect oftemperature on the overall wa-
correlation between uptake threshold and standardized ter budget. Excessive water vapor pressure deficits asso-
loss flux reflects parallel adaptive trends in vaporabsorp- ciatedwith diurnal temperatures mayconstitutethemajor
tion and permeability reduction. Low thresholds and high selection pressure restricting Crinocheta to a predomi-
uptake fluxes will both increase water recovery in sub- nantly nocturnal habit.
saturated diurnal habitats. The ability ofC. convexus, P.
spiniciirnixand T. rathkeito recover maximum tolerable Acknowledgments
water deficits in sub-saturated diurnal activities (0.991,
0.975, and 0.983, respectively) over a 12-h period may This study was made possible through the financial as-
reflect their frequently mesic-xeric diurnal habitats (Van sistance of the Leverhulme Trust. U.K. (JCW) and
Name, 1936: Harding and Sutton, 1985). However, the NSERC (JM). We extend ourthanks to Gloria Lampert,
significant positivecorrelationsbetween standardized up- Mike O'Donnell, and Berry Smith forhelpful discussions
take fluxand threshold, and betweenstandardized uptake at various stages ofthis work.
fluxandstandardized lossflux, both suggest compensatory
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