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1999 Macmillan Magazines Ltd Stone, R.A., Lin, T., Desai, D. & CeharVision Res.35,1195Ð1202 (1995).6.Raviola, E. & Wiesel, T.N. N. Engl. J. Med. 312,1609Ð1615(1985).Larsen, J.Acphthalmol. (Copenh.)49,873Ð886(1971).Robinson, J., Bayliss, S.C. & Fielder, A.R. Vision Res.31,1837Ð1840 (1991).9.Hansen, R.M. & FoB. in Early Vevelopment,Normal and Abnormal (ed. Simons, K.) 130Ð142 (Oorniv.Press, New York, 1993).10.Daw, N.J. Arch. Ophthalmo502Ð505 (1998).the strain along the ith axis and Ce elas-tic moduli, cannot provide the informationded. With idenoting the c-axis direc-tion, the right-hand side is large and nega-tive3,4for La21xSrxCuO4, but its indicatedrelation to a quanthich presupposesthat all coorxcept those along the c-axis remain unchanged does not hold.Locqt et al.incorr 1.Locquet, J.-P. et al.Nature394,453Ð456 (1998).2.Genbeer, F. et al.Phys. Rev. B49,13137Ð13142 (1994).3.Nohara, M. et al. Phys. Rev. B52,570Ð580 (1995).4.Chakravarty, S., Sudb¿, A., Aern, P. W. & Strong, S. Science261,337Ð340 (1993).5.Tsveovet al.Nature395,360Ð362 (1998).perate. But oxygen solubility increases assalinity decreases, and replotting TS95/5ainst water-dissolved oxygen contentremoves this discrepancy and produces alinear relation: TS95/54142.6&0.252 O2( tific correspondenceNA|VOL 399|13 MAY 1999|www.nature.com Supercuctors under stressLocqt et al. mphipod length (mm)010203040506070809010025201510502520151050Proportion of species in size class (%) Figure 1Amphipod size spectra for the two sites8, top; Lake Baikal9, bottom) for whichthe maximum and minimum TSvby bars, were obtained. In total, data were obtained Polar gigantism dictated by oxygen availabilityThe tendency of some animals to be largerat higher lates (Ôpolar gigantismÕ) hasnot been explained, although it has oenbeen attributed to low temperat 1999 Macmillan Magazines Ltd 1999 Macmillan Magazines Ltd ods, and not by amphipods (S. Ruffo,pernal commioMum size does not vary sig-ly with environmental ogeearor-reion coefficient40.402, P40.195).Thus, MPS increases drly with oxy-gen, modal size increases less, and mini-mum size does not increase at all. Theoverall effect widens the size spectrum,ing that oxygen avilitsame selective pressor all species, butrr sepeIn ectotherms, metabolic rate increaseswith teere, increasing tissue mainte-e costs2h MPS deeased temperator marine sites, thelargest amphipods were not found in thecoldest sites (high Antarctic, 0 ¡C), but inLake Baikal (&6 ¡C). Temperatepen-dent tissue synthesis and catabolism trade-offs do not limit MPS, as the observedeases in MPS at freshwater sites, despitegrer osmoreory costs, would reqeeasibly large increases in resource acq-Ogen enters amphipod blood througha low-efficiency gill3and is transportedboth as dissolved oxygen and bound tohaemocyanins. Marine amphipod haemo-lymph contains 10Ð20 mg ml11haemo-cyanin4, which is low for crustaceans. Polarhipods are thought to be similar to theActic giant isopod Glyptonotus antarcti-, which also has relatively little haemo-cyanin5and carries 60Ð70% of itsculating oxygen in solution. The amountof oxygen carried in solution is thought tobe 30Ð40% in tropical amphipods and60Ð70% in Lake Baikal species, and differsbecause marine amphipod haemolymph isisotonic with seawater (950Ð1,150 mosM),whereas freshwater species are hypertonic(340Ð360 mosM) (I. Zerbst-Boroffka, per-nal communication). Amphipod haemo-anins may also be more important forregulation than oxygen transport6.The amount of oxygen dissolveu-red haemolymph in amphipods fromLake Baikal at 6 ¡C is similar to that ofmarine species at 0¡C, and species fromLake Baikal and the Antarctic should besimilar sizes. TS95/5is 54.4 mm for Baikalamphipods and 43.6 mm for Antarcticspecies. This suggests that sation leveof dissolved oxygen in the haemolymphincrease from tropical to polar environ-nts, and from marine to freshwater envi-ronts, with gill efficiency being theitical factor deterThe difference in partial pressurebeeen the eerdium and circulat-ing haemolymph determines oxygenuptake across the gills, according to FickÕslaw. For similar external partial pressureand efficiency, more oxygen enters theblood at sites with low temperature andsalinity because the absolute oxygen con-centration is higher. The saturation leveland absolute oxygen concentration in theblood are also higher, allowing increasedsize because a greater oxygen mass willease the possible path length of the cir-ory system.We obtained strong relations betweeneernal oxygen coeration and length,not body mass. However, amphipods, likeseveral groups exhibw-temperategantism (such as pycnogonids andnemerteans), have restricted circulatorysyems with few lateral branches. In othergroups, stronger relations with body masswould be expected. Whether the MPS isreached will depend on several factors. Inenvironments with many species, selectionpres wilive nicitatiore oes will be reprnted.Ogen supply may also have led toinsect gigantism in the Carboniferous peri-od, because atmospheric oxygen was30Ð35% (ref. 7). The demise of these insecwn oxygen content fell indicates thatlarge species may be susceptible to suchchange. Giant amphipods may therefore beng the first species to disappear if glob-al teere increased or global o-gen levels decline. Being close to the crMPS limit may be seen as a specializationthat makes giant species more prone toetn over gegical tthier Chapelle*, Lloyd S. Peck *Institut Royal des Scieelles de Bruxelles,Rue Vautier 29, B-1000 Bruxelles, Belgium British Antarctic Survey, High Cross, Madingoad, Cambridge CB3 0ET, UKe-mail: l.peck@bas.ac.uk 1.Atkinson, D. & SlyTrends Ecol. Evo235Ð239(1997).2.Ivleva, I. V. Int. Rev. Ges. Hydrobio1Ð47 (1980).3.Wv, H.P. & Waterman, T. H. in The PhylogfCrustacVd. Waterman, T. H.) 35Ð100 (Academic, NewYork, 1960).4.Spicer, J. I. Mar. Bevhy123Ð136 (1993).5.Whiteley, N. M., Tayr., Clarke, A. & El H. PolarBio10Ð15 (1997).6.Hagerman, L., Sandberg, E. & Vismann, B. Mar. Bio209Ð212 (1997).7.Graham, J. B., Dudley, R., Aguilar, N.M. & Gans, C. Nature375,117Ð120 (1995).8.Ledoyer, M. Faune de Madagascar. Crustaces AmphipoGiearis, 1982Ð1986).9.Bazikalova, A. Y. Laikal Amphipods. Proc. Baikal. LimnoStcad. Sci. USSR11,1Ð440 (1945). tific correspondenceATURE|VOL 399|13 MAY 1999|www.nature.com ean annual surface temperature (°C)Water oxygen content (mmol kgÐ1)302520151050Ð5150200250300350400450TS95/5 (mm)TS95/5 (mm)60504030201006040200Lake BaikalCaspian SeaBlack SeaCaspian SeaBlack SeaLake Baikalab Figure 2Effects of temperature and oxygen availabil-ity on amphipod MPS. Data are for nine marine(orange circles) and three reduced-salinity sites(blue circles). a,TSainst mean annualwater tempererted scale). b,TSainst calculated dissolved oxygen content at sat-ation (mmol per kg), based on surface watermean temperature and salinity. Not every habitat willexperience permanent high oxygen saturation, butthe 100% value represents optimal conditions forattaining large size. No role for colour in symmetry perceptionBeral colour symmetry, such as that evi-dent in a Siberian tigerÕs face (Fig. 1a), isrelevant to many animals1,2, includinghumans3,4. We examined the role of colourin symmetry perception by asking observersto detect colour symmetry in regular gridsof coloured squares (a colour-symmetricale has regions of the same colour locat-ed equidistantly from a vertical axis). Ourresults suggest, unexpectedly, that themechanisms of symmetry perception arerently colour-blind: although observerscan verify colour symmetry, they do so oyby shifting attention from one colour to thext and assessing the symmetry of regionsof that colour.Orvers were shown displays thateither exhibeomplete colour symmetryertical midline (Fig. 1b), or con-d one pair of squares that were mis-matched in colour. Subjects pressed abuon to indicate whether the pattern wascolour-symmetrical, and response timesand errors were assessed. The displays wereconstructed with either two colours (crim-n and scarlet) oolours (green, yel-w, blue and red). Colours were chosensuch that every pair of colours in the four-colour displays was more easily discr-ed than the pair used in the two-colouryThe most obvious hypothesis aboutcolour symmetry perception, called colourmatching, involves the colours of corre-sponding points or regions on differentes of an axis being compared at the sametime. Colour symmetry is registered when