BBC Wildlife - UK 2020-07)

(theyhavea trickyhabitofdoingso),and

tellmycompanionstolookdownintothe

blue.Fora splitsecond,thereis nothing

excepta murkyplanktonsoup.Butsuddenly,

animmenseshadowbeginstoapproach

fromthegloom.A cavernousmouth,

almost1mwide,is headinginour

direction,thegillrakersstarkly

white– thesearecomb-like

structuresthatactasa

kindofsieve,trapping

zooplanktonasseawater

passesoverthegills.

Baskingsharksareknown

aspassivefilterfeeders,

meaningthattheyrely

onforwardmovementto

pushwaterintothatgaping

mouthandoutthroughthe

gills,whichcompletelyencircle

thehead.

Baskingsharksareoftenfound

congregatedat tidelines,usingtheforceof

anincomingoroutgoingtidetoshovelas

muchfoodinaspossible,withminimum

effort.Tomaintainsucha hugesize,these

animalsareactualfeedingmachines,capable

offilteringalmost1 millionlitresofseawater

perhour.Withsucha low-energyfood

source,theymoveslowlyanddeliberately

and,withouttheneedforcomplexhunting

strategies,havea brainofaboutjust10cm.

Theytrulyaregentlegiants,thinkingoflittle

butwherethenextbuffetis.Forthisreason,

withtherighttraining,careandrespectfor

theseanimals,wecangetcloseenoughto

observethemintheirnaturalenvironment.

Bigmystery

I amluckyenoughtospendmysummers

asa guideforBaskingSharkScotland,a

wildlifetourismoperationbasedonthewest

coast.Wehavea dualpurpose.Wetakevery

smallgroupstoencountersharksinthe

wild,toeducateandraiseawarenessabout

thespeciesandthewidermarine ecosystem.

Butwealsousethisopportunity to gather

vitaldata,suchassex,sizeandany markings

thatmayhelpustoidentifyindividuals

returningtothesamearea.One of the most

alluringthingsaboutbaskingsharks is that

weknowrelativelylittleaboutthem. Much of

theirlifeis shroudedinmystery.

Forexample,weknowthatfrom May to

Septemberbaskingsharksreturn to coastal

surfacewatersafterspendingwinter at

deeperdepthsoffshore.It is this sun-seeking

habitthatgivesthespeciesitscommon

nameandis drivenbyanabundance of

food,followinga chainofevents. In Spring,

oceanicandweathercyclescreate ideal

conditionsforphytoplankton– microscopic

organismsthatphotosynthesise, and thus

relyonsunlighttoproduceenergy – to

inhabitsurfacewatersandexplode in

vastnumbers.Shortlyafter,animal-based

zooplankton– whichincludeseverything

Basking shark biology

BASKING SHARKS

42 BBCWildlife July 2020

Top left: basking

shark meat for

sale at a French

fishmonger in

the 1940s. Above:

the shark’s large

fins can give away

its location.

Basking shark migrations

are a little like extended

family having a reunion at

their favourite restaurant.

BRAIN

Despitetheshark’slarge

size,itsbrainisactually

verysmall(about10cm).

To quote the Scottish

poet, Norman MacCaig,

“That room-sized monster

with a matchbox brain”.

However, there is a larger

nerval mass around the

brain dedicated to sense

of smell, highlighting the

importance of scent when

searching for zooplankton.

MOUTH

The basking shark is a passive filter

feeder. It swims with its mouth open,

using forward movement to funnel

water into the mouth and out past the

gills. Specially adapted bones, called

gill-rakers, catch tiny zooplankton and

filter them out from the seawater, like

a giant sieve. Every once in a while the

shark closes that enormous mouth and

takes one big gulp!