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Biologyoftheworld’slargestinvertebrate,thecolossalsquid2 

(Mesonychoteuthishamiltoni):ashortreview3 

4 

5 

RuiRosa1*,VanessaM.Lopes1,MiguelGuerreiro1,KathrinBolstad2,JoséC.6 

Xavier3,47 

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1MARE –Marine Environmental Sciences Centre, LaboratórioMarítimo da Guia,11 

FaculdadedeCiênciasdaUniversidadedeLisboa,Portugal.12 

2 Institute forApplied EcologyNewZealand,AucklandUniversity of Technology,13 

Auckland,NewZealand.14 

3MARE–MarineandEnvironmentalSciencesCentre(MARE),DepartmentofLife15 

Sciences,UniversityofCoimbra,3004‐517Coimbra,Portugal.16 

4 British Antarctic Survey, Natural Environment Research Council, High Cross,17 

MadingleyRoad,CambridgeCB30ET,UnitedKingdom.18 

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*Correspondingauthor:rrosa@fc.ul.pt21 

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Abstract34 

Thepresentstudyaimstoreviewthecurrentbiologicalknowledgeonthelargest35 

(heaviest) living invertebrate, the colossal squid Mesonychoteuthis hamiltoni36 

(Robson,1925).Thissquid isknowntobeendemicoff theSouthernOcean(SO),37 

withacircumpolardistributionspreading fromtheAntarcticcontinentup to the38 

Sub‐AntarcticFront.Smalljuveniles(<40mmmantlelength)aremainlyfoundfrom39 

the surface to 500 m, and the late juvenile stages are assumed to undergo40 

ontogeneticdescenttodepthsreaching2000m.Thus,thisgiantspendsmostpartof41 

itslifeinthemeso‐andbathypelagicrealms,whereitcanreachatotallengthof642 

meters. The maximum weight recorded so far was 495 kg. Mesonychoteuthis43 

hamiltoni is presently reported from the diets of 17 different predator species,44 

comprising penguins, sea birds, fishes and marine mammals, and feeds on45 

myctophids,Patagoniantootfish,sleepersharkandothersquids.Isotopicanalysis46 

placesthecolossalsquidasoneofthetoppredatorsintheSO.Basedonphylogenetic47 

inferences,itisassumedthatthissquidisnotcapableofhigh‐speedpredator–prey48 

interactions,butitisratheranambushpredator.Theireyes,thelargestintheplanet,49 

seemedtohaveevolvedtodetectverylargepredators(e.g.spermwhales)rather50 

thantodetectmatesorpreysatlongdistances.Ontheotherhand,andlikemany51 

otherdeep‐seaanimals,itpossesseslight‐emittingorgansontheeyes,whichmay52 

provide ventral camouflage and counter‐illumination or some sort of deceptive53 

techniquetocapturetheirprey.AlthoughM.hamiltoniisquiteabundantintheSO,54 

its life existence remains one of the ocean’s great mysteries. This unique55 

invertebrategiantcontinuestoattractconsiderableattentionfrommediaandthe56 

publicingeneral,butitsstudyalsoconstitutesavaluablesourceofinsightintothe57 

biophysicalprinciplesbehindbody‐sizeevolution.58 

59 

Keywords:colossalsquid,Mesonychoteuthishamiltoni,cranchiids,SouthernOcean;60 

gigantism61 

62 

3  

Introduction63 

TheSouthernOceanconsistsofasystemofdeep‐seabasinsseparatedbythe64 

ScotiaRidge,theKerguelenplateau(Indiansection)andtheMacquarieRidge(South65 

ofNewZealand).WhilethesouthislimitedbytheAntarcticcontinent,thenorthern66 

part is surrounded by theAntarctic Polar Frontal Zone (APFZ), beginning at the67 

Antarcticconvergence/AntarcticPolarFront(APF).Themainsurfacecurrentisthe68 

AntarcticCircumpolarCurrent(ACC),whichrunseastwardsaroundthecontinent69 

(Orsietal.1995);Mooreetal.(1997);(Mooreetal.1999).Theseassociatedfronts70 

createanobstacletonorth‐south“meridional”circulation,andthediscontinuities71 

of temperature and salinity in the APFZ constitute an important barrier in72 

biogeographicalinteractions(Barnesetal.2006).Nonetheless,Antarcticlifeisrich73 

anddiversewithhighlevelsofendemism(Arntzetal.1994;ClarkeandJohnston74 

2003).75 

Withinthisuniqueregion,cephalopodsareknowntoplayakeyroleinthe76 

marinefoodwebs,becausetheyconstituteavitalalternatefoodsourcetoAntarctic77 

krillEuphausiasuperba(CollinsandRodhouse2006).Theyareimportantprey,and78 

thussustainlargepopulationsofpenguins,procellariformbirds,sealsandtoothed79 

whales(Clarke1980;Xavieretal.2002a;CherelandDuhamel2004;Chereletal.80 

2004;CherelandHobson2005;CherelandHobson2007;Chereletal.2007;Xavier81 

etal.2011).Yet,despitetheirimportantecologicalrole,thebasicknowledgeofthe82 

bio‐ecologyofSouthernOceancephalopodsisstillpoorlyknown(Xavieretal.1999;83 

Xavier et al. 2014; Alvito et al. 2015; Guerreiro et al. 2015), probably due to an84 

absenceofafisheryindustry.Thismayberelatedtothelownumberofexploitable85 

species(Xavieretal.2007),lackofbasicknowledgeofthespecies’biology,stocks’86 

4  

unpredictability,andtherelativeinaccessibilityofthesestocksforexplorationand87 

study(Rodhouseetal.1996;Rodhouse1998;CollinsandRodhouse2006).88 

Themajorityof the resident SouthernOcean cephalopod fauna comprises89 

endemic species of octopods (cirrate and incirrate), and oegopsid squids,which90 

differgreatlyfromthetaxafoundatlowerlatitudes.Thereisalsoatotalabsenceof91 

myopsid squids and cuttlefish, and normally abundant families are rare. Unique92 

Southern Ocean taxa include the squid families Psychroteuthidae and93 

Batoteuthidae, and the genera Psychroteuthis, Kondakovia, Alluroteuthis,94 

Slosarczykovia, Batoteuthis and Mesonychoteuthis. Moreover, several of these95 

cephalopods occupy similar niches to (and compete with) some pelagic top96 

predators,namelysharksandlargefishes(RodhouseandWhite1995;Rodhouseet97 

al.1996;CherelandDuhamel2004;CherelandHobson2005).Oneoftheseisthe98 

mostmassive living cephalopod ‐ the colossal squidMesonychoteuthis hamiltoni99 

(Robson1925)(Fig.1).ThissquidisknowntobeaSouthernOceanendemic,and100 

althoughitispreyedbymanytoppredators,itsbasicbiologyandecologyremain101 

oneof theocean’sgreatmysteries.This reviewaims to summarize thecurrently102 

scattered bio‐ecological knowledge of the elusive, largest living invertebrate; an103 

overviewofthespecies’taxonomyanddetailedtreatmentofthemorphologywillbe104 

treatedinaseparateforthcomingpublication(Bolstadetal.inprep).105 

106 

Polargigantismandsomekeymorphologicalfeaturesofthecolossalsquid107 

Foralongtimepolarscientistshaveobservedthatorganismsofsuchareas108 

canreachremarkablylargesizes.Polargigantismisnotabletoolforunderstanding109 

the biophysical principles and ecological theories (e.g. temperature‐size rule,110 

Bergmannrule,oxygenavailabilitycoupledwith lowmetabolicrateshypothesis)111 

5  

behindbodysizeevolution(Atkinson1994;ChapelleandPeck1999;Chapelleand112 

Peck 2004; Vermeij 2016). Although not consensual, some argue that this113 

phenomenon may be linked to the fact that polar giant taxa share common114 

evolutionary histories with deep‐sea taxa. In other words, polar gigantism is115 

associated with abyssal gigantism. Yet, it is worth noting that the evolutionary116 

historyof(benthic)SouthernOceanfaunaiscomplexandtherespective“invasions”117 

workedbothways(i.e.shelftodeepandviceversa)(Brandtetal.2007;Strugnellet118 

al.2011).OneextremecaseofpolargigantismintheSouthernOcean(whereother119 

examples of this phenomenon include foraminiferans, sponges, ctenophores,120 

isopods,copepods,amphipods,pycnogonids,pteropods,annelids,echinoderms)is121 

the colossal squid (Fig. 1), which was first described by Robson (1925) from122 

fragments from two squids obtained from sperm whale stomach contents.123 

Posteriorly,thedescriptionofitsbeakswasdonebyKlumovandYukhov(1975),124 

McSweeny (1970), and then reviewed by Clarke (1980) and Xavier and Cherel125 

(2009).126 

ThecolossalsquidbelongstothefamilyCranchiidae,or“glass”squids,which127 

are generally small‐to‐medium sized species; M. hamiltoni is several orders of128 

magnitudelargerthansomeothercranchiidspecies,attainingamaximummantle129 

length(ML)of2.5metersandtotallength(TL)of6meters.Themaximumweight130 

recordedtodatewas495kg(Fig.1)(JerebandRoper2010).Apartfromthecoelom131 

(fluid‐filledbuoyancychamber),whilemostcranchiidshavetranslucentbodies(see132 

examplesinFig.2A,BandC),M.hamiltonihasmuchmoremuscledensity(Fig.2D133 

andE).Afurthernotabledifferencebetweenthecolossalsquidandothercranchiids134 

isthepresenceofswivellinghooksonitstentacleclubs(Voss1980;Vossetal.1992)135 

(Fig.2G).Thus,thecolossalsquidisaclear“outlier”inthecranchidgroup.136 

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137 

Earlystages138 

Recordsofeggmassesandhatchlingsareessentiallynon‐existent.Between139 

1962and1985,only33juvenileswerereported,allcaughtusingRMTandIKMT140 

nets (Fig. 3). Their mantle length (ML) ranged from 3 to 146mm, with a clear141 

relationshipbetweenjuvenilesizeanddepthofcapture.Infact,individualsbelow142 

ML40mmhavebeenmostly foundbetweenthesurfaceand500m(Fig.3) (see143 

more juvenile morphometric information in McSweeny 1970; Fillipova 1972;144 

Filippova 1979; Rodhouse and Clarke 1985; Filippova 1991; Filippova and145 

Pakhomov1994;LuandWilliams1994;AndersonandRodhouse2002;Filippova146 

2002).Nonetheless,themajorityofthespecimensknownsofarhavebeencollected147 

bytrawls(N=155;Fig.3)butbesidestheirlocationnootherrelevantdata(e.g.ML,148 

lifestage)areavailable.149 

150 

Age,growthandreproduction151 

Besidesthecharacteristiclowtemperatures,theSouthernOceanisalsowell152 

knownbythedrasticannualcyclesofproductivity.Theoscillationsfromabundance153 

to famine are thought to be associated with increased resistance to starvation,154 

reducedcompetitionandunusuallyslowgrowthrates(LindstedtandBoyce1985;155 

ArnettandGotelli2003;Clarke2003).Deep‐seaandpolarenvironmentsarealso156 

usuallylinkedwithlowmetabolicratesandlongerlongevity.Forinstance,Robison157 

et al. (2014) recently reported a female deep‐sea benthic octopus Graneledone158 

boreopacifica undertaking an astonishing 53‐month brooding period (by far the159 

longestegg‐broodingperiodeverreportedforanyanimal).Ifbroodingcomprises160 

aboutaquarterofthisoctopodlifespan,thisspeciesmaybeamongthelongest‐lived161 

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ofallcephalopods.Theirfindingsareunparalleledbythosefromthewell‐studied162 

shallow‐waterspeciesandhighlightsthenotionofhowlittleweknowaboutlifein163 

marineextremeenvironments(deep‐seaandpoles).Withinthiscontext,onemight164 

imaginethatM.hamiltonimaylivemuchlongerthantheirtemperateandtropical165 

counterparts, i.e. far longerthan12to18months.Themostcommonmethodfor166 

measuringgrowthincephalopods,andconsequentagedetermination,iscounting167 

growth increments on statoliths (Lipinski 1986; Rodhouse and Hatfield 1990);168 

however,M. hamiltoni material reported to date has not yet been sufficient to169 

validatetheperiodicityofstatolith‐ringformationforthisspecies.170 

The colossal squid takes longer to mature and reproduce than its more171 

northernrelatives,becomingmatureatlengthsofatleast1mandweightabove30172 

kg(JerebandRoper2010).Althoughincolderwaterscephalopodstendtoproduce173 

a low number of large eggs (Collins and Rodhouse 2006), the colossal squid’s174 

potentialfecundityhasbeenestimatedatamaximumof4.2millionoocytes(Jereb175 

andRoper2010),whichmakes itoneofthemost fecundcephalopods,especially176 

whencomparedtootherpolarcephalopodfauna(CollinsandRodhouse2006).177 

178 

Distribution179 

Part of the known distribution of the colossal squid is based on the rare180 

encounterswithliveorrecentlydeadspecimensaboard(mainly)toothfishfishing181 

(e.g. Fig. 1 D and E) vessels and the stomach contents of the squid’s predators.182 

Mesonychoteuthis hamiltoni has a circumpolar distribution, ranging from the183 

Antarcticcontinent(excludingrespectiveshelvesandKerguelenPlateau)uptothe184 

Sub‐AntarcticFront(SAF)(Fig.4).BasedonXavieretal.(2015b),thehighestvalues185 

ofthespecies’predictedhabitatsuitabilityarefound:i)betweentheWeddellSea186 

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(Atlanticsector)andthewestoftheKerguelenarchipelago(~60°E),andii)between187 

180°Eand120°WintheRoss/Amundsenareas.188 

189 

Vertical(ontogenetic)movements190 

Available information on colossal squid’s verticalmigrating habits is very191 

scarce.AsshowninFigure3,smalljuveniles(<40mmML)aremainlyfoundfrom192 

thesurfaceto500m.Thehigherproductivitynearthesurfacemayconfertheyoung193 

higherchancesoffeedingbetweenhatchingandtheirprobablemigrationtodeeper194 

waters.Atlatejuvenilestages,M.hamiltoni isassumedtoconductanontogenetic195 

descenttodepthsaround2000m(LuandWilliams1994).Thus,thecolossalsquid196 

likelyspendsthemajorityofitslife(latejuvenile,subadultandadultstages)inthe197 

meso‐andbathypelagiczonesoftheSouthernOcean.Nonetheless,itisworthnoting198 

that femalespecimenshavebeencapturedbyfishingvesselsatdepthsshallower199 

thantheabovementionedrange,suggestingthatfemalesmaymigratetoshallower200 

waterstospawn,asdoothercranchiids.201 

202 

Predatorandpreyinteractions203 

Theundigestedremainsofcolossalsquid(beaks,tentacularhooksorsucker204 

rings)havebeenfoundinagreatvarietyofpredators’stomachcontents.Presently,205 

M.hamiltoni isknownfromthedietsof17differentpredatorspecies,comprising206 

penguins,seabirds,fishesandmarinemammals(Table1).Thelargerpredatorsof207 

the colossal squids are the spermwhales (Clarke 1980) and the sleeper sharks208 

(CherelandDuhamel2004) (Fig.5),but in contrast to the former, the latter isa209 

bottom scavenger and ambush predator. One of the more remarkable predator210 

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observations has beenmade at the Kerguelen Islands, where 66% of the shark211 

stomachscontainedcolossalsquidremains(CherelandDuhamel2004).212 

ColossalsquidbeakshavealsobeenfoundquitefrequentlyinthePatagonian213 

toothfish (Dissostichus eleginoides) stomach contents (Fig. 5), and in turn, beak‐214 

shapedbitesandscratches fromclubhooks (matching thecolossal squid’s)have215 

alsobeenfoundalongthebodiesofcapturedPatagoniantoothfish(Remesloetal.216 

2015).Thismaysuggestreciprocalpredator‐preydynamics.Patagoniantoothfish217 

may be able to forage squids larger themselves because they attack dead or218 

moribundcolossalsquids.Remesloetal.(2015)alsoarguethat,duetoanatomical219 

constraints(namelytoothshape),thetoothfishmayonlyscavengetheleftoversof220 

otherpredators,includingthecolossalsquiditself.Cannibalismisalsoverycommon221 

amongcephalopods,includinggiantsquids(BolstadandO'Shea2004;Ibáñezand222 

Keyl2010).223 

Seabirds,namelyalbatrosses,arealsoknowtoscavengeM.hamiltoni(Fig.5).224 

Albatrossstomachsoftencontaintheremainsofcephalopodpreythatwouldhave225 

been several times heavier that their ownmean size (Croxall and Prince 1994;226 

Xavier et al. 2003b; Xavier and Croxall 2007; Alvito et al. 2015; Guerreiro et al.227 

2015).Infact, incertainperiods(e.g.endofinterbreeding/beginningofbreeding228 

period)scavengingplaysacrucialroleinthedietsofwandering(Diomedeaexulans),229 

black‐browed (Thalassarche melanophrys) and grey‐headed (Thalassarche230 

chrysostoma)albatrosses,asmorethan60%ofthecephalopodremainsrecovered231 

had potentially been scavenged (more than 95% of the total estimatedmass of232 

cephalopodsconsumed)(Alvitoetal.2015).233 

Regarding the squid’s own feeding ecology,M. hamiltoni specimens with234 

intactorundigestedstomachcontentsremainunknown.Moreover,dietarystudies235 

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incephalopodsaredifficulttoconductasgutcontentsareusuallyinanadvanced236 

state of digestion, making identification of prey items difficult to impossible.237 

Cephalopodsalsomaceratetheirpreysfinelybeforeingestion,usingtheirbeaksand238 

radula. It hasbeen suggested thatM.hamiltoni feedsonmyctophids, Patagonian239 

toothfish, sleeper sharks andother squids (Jereb andRoper2010;Roberts et al.240 

2011). Based on phylogenetic (metabolic) inferences and polar gigantism241 

constraints,RosaandSeibel(2010)arguedthatthecolossalsquidisprobablynota242 

predatorcapableofhigh‐speedpredator–prey interactions (seealsosection3.5).243 

Therefore, the very large but immobilized (or weakened) toothfish specimens244 

captured by longline would present easy, opportunistic prey for M. hamiltoni245 

(Yukhov2012;Remesloetal.2015).246 

Anumberofstudieshavealsousedadifferentapproachtodeterminethe247 

trophicpositionofthecolossalsquid,namelytheuseofisotopicanalysis(e.g.Cherel248 

andHobson2005).Nitrogenisotopesratio(δ15N)indicatetheconsumer’strophic249 

position,whereascarbonisotopesratio(δ13C)canprovideinformationonhabitat250 

occupation. Assessment of the nitrogen signatures have demonstrated that M.251 

hamiltoniisdefinitelyoneofthetoppredatorsintheSouthernOcean(Fig.6).252 

253 

Vision254 

Giant(Architeuthisdux)andcolossalsquidspossessthelargesteyesinthe255 

Worldoceans(Fig.7),andontheplanet(Walls1942;LandandNilsson2002).Since256 

bothofthesesquidspeciesarepelagicorganismsthatshare(orcompetefor)similar257 

ecologicalnicheswithother(vertebrate)toppredators,thetremendousdifferences258 

ineyesizemayindicateadifferentpurposefortheuseofeyes.Nilssonetal.(2012)259 

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suggestedthattheverylargeeyesmayhaveconferredanevolutionaryadvantagein260 

increased ability to detect large predators (especially spermwhales), rather for261 

utilityindetectingmatesorpreyatlongdistances.Theseauthorsmodelledlarge262 

squids’visualrangeandproposedthatthegianteyesenablethemtodetectsperm263 

whales as they trigger plankton bioluminescence while swimming through the264 

water column. In other words, as whales conduct hunting dives, they attain265 

considerable speed and agitate the deep waters where a great variety of266 

bioluminescentorganismsthrive.Thecolossalsquid’seyes,duetotheirlargeretina,267 

areabletodetectsuchsmallsourcesoflightfromadistance,althoughtheymaynot268 

necessarilyregisterthepredatorsbeforebeingdetectedthemselvesbythewhale’s269 

sonar(therangeofwhichcanexceed120m).Thesquidmay,however,bevisually270 

warnedinsufficienttimetoattemptanescape(Nilssonetal.2012).Nonetheless,it271 

isworthnotingthatSchmitzetal.(2013)alsoarguedthatM.hamiltoni’seyesare272 

withintheexpectedallometricrangeforsquids,andthusmaybeasimpleresultof273 

aphylogeneticallyconservedtrait.274 

275 

Metabolism276 

Rosa and Seibel (2010) calculated the metabolic rates and energy277 

requirements of the colossal squid through a depth‐temperature gradient278 

representative of the Southern Ocean (Fig. 8). This phylogenetic inference was279 

conducted based on other deep‐sea cranchiid data and followed the expected280 

allometricrelationshipbetweenbodysizeandmetabolism(Seibel2007;Seibeland281 

Drazen2007;Rosaetal.2009).Basedontheirfindings,theauthorsarguedthatthe282 

colossal squid “is not a voracious predator capable of high‐speed predator–prey283 

interactions.Itis,rather,anambushorsit‐and‐floatpredatorthatusesthehookson284 

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itsarmsandtentaclestoensnarepreythatunwittinglyapproach”.Thecolossalsquid285 

waslongthoughttobeanintrepidpredator,duetothe“kraken”‐relatedmythsand286 

storiesassociatedwithitslargesize,butafterthisstudy,ithasbeenpostulatedthat287 

thecolossalsquidperhapsdoesnotdeservesuchareputation.Thequestionthatwe288 

nowraiseis:doesanorganismneedtobeanactivepursuitpredatortobefearsome?289 

Forinstance,whitesharks,leopards,crocodiles,largemonkfishandpythonsareall290 

ambushpredators(manywithlowenergydemands),anditistheircapabilitytohide291 

andstrikewithoutwarningthatmakestheminherentlyaggressive.292 

293 

Bioluminescence294 

Likethegreatmajorityofdeepseainhabitants,M.hamiltonipossesseslight‐emitting295 

organs(photophores),intheformoftwoelongatestructuresontheventralsurface296 

ofeacheye(Fig.9)(Herringetal.2002).Thesmallerislocatedanteriorlywhilethe297 

largeriscrescent‐shapedandlocatedposteriorlyaroundtheoutsiderimoftheiris298 

(Voss1980).M.hamiltoniphotocytescontaincrystalloidswhoseprofilesappearas299 

rectangularblocksorelongateneedles,andthereflectorplateletsareunusual,being300 

associated with a microtubular array that confers their edges a comb‐like301 

appearance (Fig. 9). Interestingly,withinCranchiidae,members of the subfamily302 

Cranchiinaeobliteratethesilhouetteoftheeyeballbyilluminatingnumeroussmall303 

ventrallydirectedphotophores,whereasthetaoniines(includingM.hamiltoni)use304 

fewer,moreelaborate,elongatereflectorstospreadthelightfromthephotophores305 

overtheentireprojectedventralsurfaceoftheeyeball(Herringetal.2002).Likeall306 

taoniines, the two photophores of the M. hamiltoni have a mirror‐image307 

arrangementinwhichtheilluminatedsurfaceofeachlight‐emittingorganextends308 

towards the other, across the main ventral area of the eyeball. The subocular309 

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photophores of M. hamiltoni may serve as ventral camouflage and counter‐310 

illumination, to blend in with downwelling light when seen from below (Young311 

1975;Young1977;Youngetal.1980).Yet,thisseemstobenotusefulinlargesquid312 

individuals since their main predators are not visually‐orientated hunters.313 

Photophoresmayalsobeusedasadeceptivetechnique–whenilluminated,other314 

organismsmaydisregardthecolossalsquidasathreat,allowingittoapproachits315 

prey.Alternatively,thephotophoresmaybeusedas“flashlights”tomoreaccurately316 

predictthedistanceneededtostretchitshookedtentaclesandsuccessfullycapture317 

unwittingprey.318 

319 

Futuredirections320 

Furthereffortsshouldbemadetounderstandmoreaboutthelifeofthisgiant.Ithas321 

beenassumedthatM.hamiltoniisquiteabundantintheSouthernOcean(biomass322 

ofaround90milliontonnes)andtherehavebeensomereportsonitsinterestto323 

fisheries(JerebandRoper2010).Everysample,fragmentorsightinghasprovedto324 

beahighlyvaluablesourceofinformationandhelpedscientiststounveilmoreand325 

moreaboutM.hamiltoni.Baitedcamerasandremotelyoperatedvehiclescouldbe326 

deployedinthesearchoflivefootage,ashashappenedrecentlyforotherdeepsea327 

“giant”squids,e.g.Architeuthisdux(KuboderaandMori2005)andTaningiadanae328 

(Kuboderaetal.2006).M.hamiltoniandotherAntarcticorganismshasbeenwidely329 

usedforeducationandoutreachactivitiesworldwide(Xavieretal.2015a).Besides330 

the fact that these unique invertebrate giants continue to attract considerable331 

attentionfrommediaandthepublicingeneral,itisimportanttohighlight,inthe332 

words of Vermeij (2016), that: “Gigantism is (… ) a functionally distinct and333 

ecologicallyimportantconditionthatisbothenabledbyresourcesandcompelledby334 

14  

naturalselection.Thedistributionofmaximumsizeintimeandspacecantherefore335 

informourunderstandingofmajorpatternsinthehistoryoflife.”336 

337 

Acknowledgements338 

ThisstudybenefitedfromthestrategicprogramofMARE,financedbyFCT(MARE‐339 

UID/MAR/04292/2013). JX is supported by the Investigator FCT program340 

(IF/00616/2013)andbytheFoundationforScienceandTechnology(Portugal)and341 

isassociatedtoSCARAnT‐ERA,SCAREGBAMM,ICED,BAS‐CEPHprograms.RRis342 

supportedbytheInvestigatorFCTprogram(IF/01373/2013).343 

344 

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Xavier JC, Wood AG, Rodhouse PG, Croxall JP (2007) Interannual variations in cephalopod 583 consumption by albatrosses at South Georgia: implications for future commercial 584 exploitation of cephalopods. Marine and Freshwater Research 58:1136‐1143 doi 585 10.1071/mf06237 586 

Young RE (1975) Transitory eye shapes and the vertical distribution of two midwater squids. 587 Pacific Science 29:243‐255  588 

Young RE (1977) Ventral bioluminescence countershading in midwater cephalopods. In: Nixon 589 M, Messenger J (eds) Biology of cephalopods Symposia of the Zoological Society of 590 London Nº38 Academic Press, London, pp 161‐190 591 

Young RE, Kampa EM, Maynard SD, Mencher FM, Roper CFE (1980) Counterillumination and 592 the upper depth limits of midwater animals. Deep‐Sea Research Part A 27:671‐691  593 

Yukhov VL (2012) Antarctic deep‐water squid (АDS) Mesonychoteuthis hamiltoni, Robson, 594 1925. Distribution and borders of the geographic range. Main results of complex 595 research in the Azov and Black Seas Basin and the World Ocean. Kerch: YugNIRO 596 50:241‐248. Russian  597 

 598 

599 

20  

Table1–Listofknownpredatorsofthecolossalsquid(Mesonychoteuthishamiltoni)600 

Predatorgroupsandcommonnames

Species Geographicallocation Reference

Penguins

Kingpengin Aptenodytespatagonicus Crozet Ridoux(1994)Adéliepenguin Pygoscelisadeliae ShirleyIslands,EastAntarctica Kentetal.(1998)Emperorpenguin Aptenodytesforsteri Mawson Robertsonetal.(1994) Seabirds Southernfulmar Fulmarusglacialoides SouthBrazil FonsecaandPetry(2007)Wanderingalbatross Diomediaexulans Marion,Macquarie,PrinceEdward,

CrozetIs.,SouthGeorgia,Antipodes(ClarkeandPrince1981;ImberandBerruti1981;Rodhouseetal.1987;CooperandBrown1990);Cooperetal.(1992);(Imber1992;Ridoux1994;CherelandKlages1998;Xavieretal.2003a;Xavieretal.2003b)

Black‐browedalbatross Thalassarchemelanophrys SouthGeorgia Xavieretal.(2003a)Grey‐headedalbatross Thalassarchechrysostoma DiegoRamirez;SouthGeorgia, (CherelandKlages1998;Xavieretal.2003a;Arataetal.

2004;Alvitoetal.2015)Light‐mantledalbatross Phoebetriapalpebrata Crozet,PrinceEdwardIsland,

MarionIsland;MacquarieandHeardislands

(ImberandBerruti1981;CooperandBrown1990;Ridoux1994;CherelandKlages1998;Greenetal.1998)

White‐chinnedpetrel Procellariaaequinoctialis SouthAfrica(BenguelaCurrent);MarionIs.

LipinskiandJackson(1989)

Sootyalbatross Phoebetriafusca MarionandCrozetIslands (ImberandBerruti1981;CooperandBrown1990;Ridoux1994;CherelandKlages1998)

Fishes Sleepershark Somniosuscf.Microcephalus Kerguelen CherelandDuhamel(2004)Patagoniantoothfish Dissostichuseleginoides Crozet,SouthGeorgia (Xavieretal.2002b;Chereletal.2004)Antarctictoothfish Dissostichusmawsoni LazarevSea,SouthSandwich

Islands(PetrovandTatarnikov2011;Robertsetal.2011)

21  

601 

602 

603 

604 

605 

606 

607 

608 

609 

610 

611 

612 

613 

614 

615 

616 

617 

618 

619 

Marinemammals Southernelephantseal Miroungaleonina SouthGeorgia Rodhouseetal.(1992)Spermwhale Physetermacrocephalus Antarctica,SouthAmerica,South

Georgia,PeruandChile,Tasmansea

(Korabelnikov1959;Clarkeetal.1976;Clarke1980;ClarkeandMacLeod1982;Fiscusetal.1989)

Southernbottlenosewhale Hyperoodonplanifrons SouthAmerica ClarkeandGoodall(1994)Long‐finnedpilotwhale Globicephalamelaena SouthAmerica ClarkeandGoodall(1994)

22  

620 

Figurecaptions621 

622 

Figure1–Lengthandweight(logarithmicscale)ofthemarineinvertebrate(grey623 

bars) and vertebrate giants (blue bars), with a special emphasis on the624 

cephalopods:giantoctopus(Enteroctopusdofleini),giantsquid(Architeuthis625 

dux) and colossal squidMesonychoteuthis hamiltoni (red bar). Cephalopod626 

datafromJerebandRoper(2010)andJerebetal.(2014).Othergroups’data627 

fromMcClainetal.(2015)andreferenceswithin.628 

629 

Figure 2 – Morphological dissimilarities between the colossal squid630 

(Mesonychoteuthishamiltoni)andothercranchiidspecies.PanelsA,BandC631 

showthetranslucentbodiesofdeep‐seaglasssquidsTeuthoweniapellucida632 

and Taonius borealis (copyright: MBARI). Panels D and E show the much633 

densermusculatureofthecolossalsquid(photocredits:APSanAspringcrew634 

ofSanfordCompany).PanelsF,G,Hhighlightthebeaksize,thepresenceof635 

rotating hooks on its tentacles, and eye lens size of the colossal squid636 

(copyright:MuseumofNewZealandTePapaTongarewa).637 

638 

Figure3–Numberofcolossalsquid(Mesonychoteuthishamiltoni)specimenscaught639 

in nets (RMT‐ Rectangular midwater trawl, IKMT – Isaacs‐Kidd Midwater640 

trawl, and other trawls) within the Southern Ocean, and the relationship641 

betweenjuvenilemantlelength(mm)anddepthofcapture.642 

643 

Figure 4 – Map of the locations of all known colossal squid (Mesonychoteuthis644 

hamiltoni) occurrences. “Squid” symbol represents locations obtained from645 

nets and the other symbols represent predator diet studies (penguins,646 

seabirds,fishandsharks)[modifiedfromXavieretal.(2015b)].647 

648 

Figure5–Predictedmantle length(mm)of thecolossalsquid(Mesonychoteuthis649 

hamiltoni)foundinthestomachsremains(namelybeaks)ofitspredators.The650 

estimatedvaluesarebasedonbeaks’morphometrics(namely lowerrostral651 

length)andtherespectiveallometricequation(seeXavierandCherel2009).652 

23  

Abbreviations: A.p. – Aptenodytes patagonicus; Dio.e. – Diomedia exulans;653 

Diss.e. ‐ Dissostichus eleginoides; G.m. ‐ Globicephala melaena; H.p. ‐654 

Hyperoodonplanifrons;P.f.–Phoebetriafusca;P.m.–Physetermacrocephalus;655 

P.p. ‐ Phoebetria palpebrata; S.m. – Somniosus cf Microcephalus; T.c. ‐656 

Thalassarche chrysostoma. The references for beak data (for the different657 

predatorspecies)canbefoundinTable1.658 

659 

Figure6–δ15Nvalues(trophiclevel)ofsquid(greybars)andothermarineanimals660 

collected in the Southern Ocean. * ‐ corrected (beak) values. Data from661 

Guerreiroetal.(2015),Stowasseretal.(2012)andreferenceswithin.662 

663 

Figure7–Biggesteyes(mm)intheocean.DatafromWalls(1942),LandandNilsson664 

(2002)andNilssonetal.(2012).665 

666 

Figure8–Metabolicecologyofthecolossalsquid(Mesonychoteuthishamiltoni)in667 

the Southern Ocean. Panel A: Effect of size in the routine mass‐specific668 

metabolic rate of deep‐sea cranchiid squids (blue circles, from a 0.017g669 

Liocranchia valdiviae to 500 kg M. hamiltoni) and of other cephalopod670 

counterparts, including loliginids(green line), coastalommastrephids (dark671 

red line), coastal benthic octopods (red line), pelagic gelatinous octopods672 

(lightblue line)andvampiresquid(purple line).Valueof20kggiantsquid673 

(Architeuthisspp.;solidtriangle)wasbasedontheactivityofcitratesynthase674 

measuredinmantlemuscle(Seibeletal.2000).Rateswerestandardizedto675 

1.5ºC assuming a Q10 of 2. Data from Rosa and Seibel (2010) and Seibel676 

(2007).PanelsBandCrepresentdepth‐relatedchangesintemperatureinthe677 

circum‐AntarcticSouthernOcean.Dashedlinesrepresentsub‐Antarcticareas678 

(50.58S0.58E;50.58S90.58E;50.58S179.58W;50.58S90.58W)andsolidlines679 

represent Antarctic areas (60.58S0.58E; 60.58S90.58E; 60.58S179.58W;680 

60.58S90.58W);(B)projectedenergyconsumption(kcalday‐1;basedon4.7681 

kcal/lO2)asafunctionofdepth(temperaturedependenceof2)inthecircum‐682 

AntarcticSouthernOceanregion.Thicksolidlinerepresentsthemeanvalues683 

obtainedfromthedifferenttemperatureprofiles.Thindashedlinerepresents684 

24  

the95%confidence interval for themeans (modified fromRosaandSeibel685 

2010).686 

687 

Figure9–Photophoresofthecolossalsquid(Mesonychoteuthishamiltoni).PanelA688 

showsthelightorganattherearoftheeyeball,whilePanelBshowstheeyelid689 

beingpulledbacktorevealpartofthelightorgan(attherearoftheeyeball)690 

(copyright: Museum of New Zealand Te Papa Tongarewa). Panels C and D691 

show the colossal squid’s photocytes containing paracrystalline aggregates692 

(needle‐likeorrectanglestructures)andPanelEshowsthereflectorplatelets693 

with edges of comb‐like appearance (modified from Herring et al. 2002;694 

copyright:JohnWiley&Sons,Inc.).695 

696