Bloom­ing jel­ly­fish

Drift­ing with the ocean cur­rents, these crea­tures are of­ten mis­un­der­stood. We talk to sci­en­tists at The Deep aquar­ium in Hull and un­cover why we shouldn’t fear them

World of Animals - - Contents - Words Suzy Rowe

We talk to jel­ly­fish sci­en­tists at The Deep aquar­ium in Hull and un­cover why we should mar­vel at – and not fear – these strange aquatic an­i­mals

With no brain, blood, heart or bones, a jel­ly­fish’s body is made up of 95 per cent wa­ter. So how do they func­tion? This jelly-like, slimy body is ac­tu­ally an ex­cel­lent con­duc­tor for ner­vous en­ergy. With many lack­ing even prim­i­tive eyes, they have a com­plex net of nerves through­out their bod­ies and can sense changes within their en­vi­ron­ment in or­der to nav­i­gate the seas.

From polyp to medusa, jel­ly­fish un­dergo a com­plex life­cy­cle. Start­ing life as an anemone­like stalked polyp, they at­tach them­selves to rocks, ex­tend­ing their ten­ta­cles to cap­ture plank­ton pass­ing in the cur­rent. When the con­di­tions are right these polyps re­pro­duce asex­u­ally, re­leas­ing tiny jel­ly­fish called ephyrae, which bud off sin­gu­larly or in long chains into the open ocean. At only two mil­lime­tres (0.07 inches) wide, they form part of the plank­tonic soup, grow­ing rapidly and tak­ing ad­van­tage of hav­ing prey within a ten­ta­cle’s reach.

Once they’ve de­vel­oped into an adult medusa, jel­ly­fish re­pro­duce sex­u­ally – so a jel­ly­fish is born ei­ther a male or fe­male. The male re­leases sperm into the wa­ter to fer­tilise the fe­male’s eggs. Once fer­tilised, they un­dergo the em­bry­onic de­vel­op­ment typ­i­cal of all an­i­mals and hatch as free-swim­ming plan­ula lar­vae. The oval-shaped lar­vae drift through the ocean for a cou­ple of days be­fore sink­ing to the seabed, where they set­tle and root them­selves to a rock be­fore trans­form­ing into a polyp ready to be­gin the cy­cle again.

With a con­tin­u­ous breed­ing cy­cle and hav­ing to rely mostly on the ocean cur­rents to move through the seas, it’s no won­der jel­ly­fish are able to form such in­fa­mous and spec­tac­u­lar blooms. How­ever, as hu­mans con­tinue to pop­u­late the world’s coastlines, our pres­ence has re­sulted in a rise in the oc­cur­rence of these blooms, of­ten with neg­a­tive re­sults. Hu­man con­tri­bu­tions to marine pol­lu­tion and ris­ing sea tem­per­a­tures are gen­er­at­ing an in­creas­ing in­flux of plank­ton blooms, and where there is more food, there are more jel­ly­fish. How­ever, jel­ly­fish have been form­ing blooms in the ocean for over 500 mil­lion years, so we must not lose sight of their ne­ces­sity within the ocean ecosys­tem.

Jel­ly­fish, hy­dro­zoans, siphonophore and ctenophores form an im­por­tant part of the ocean food chain, main­tain­ing an ev­er­fluc­tu­at­ing bal­ance by con­sum­ing al­gae and zoo­plank­ton. They are also prey, not only on the menu for larger jel­ly­fish species but a cru­cial meal for the sur­vival of many sea tur­tles, sun­fish and spade­fish. They also prove to be a tasty treat for some crabs and other large crus­taceans, and in some parts of the world, hu­mans too.

Many young fish and small crabs use the sting­ing ten­ta­cles of the jel­ly­fish to their ad­van­tage. Unaf­fected by their po­tent venom, they are able to hide among the flow­ing ap­pendages, pro­tect­ing them­selves from preda­tors un­til they are big enough to

“Marine pol­lu­tion and ris­ing sea tem­per­a­tures are gen­er­at­ing blooms”

de­fend them­selves. Rather self­ishly, they also take a share of the jel­ly­fish’s food, form­ing a com­men­sal re­la­tion­ship rather than a sym­bi­otic one, mean­ing that while one an­i­mal de­rives a ben­e­fit from the ar­range­ment (in this case the crea­tures hid­ing in the jel­ly­fish’s ten­ta­cles), the other is nei­ther helped nor hin­dered by it.

Sci­en­tists are work­ing to fur­ther un­der­stand both the ben­e­fits and im­pacts of jel­ly­fish blooms on our ocean ecosys­tems, but with so much ocean and miles of coast­line to study, more eyes are needed. Beach­go­ers, divers and fish­er­man – to name a few – can all help pro­vide the vi­tal data needed to suc­cess­fully map sight­ings and track the en­vi­ron­men­tal changes that may be trig­ger­ing them.

Jel­ly­fish are also the sta­ple diet of the leatherback tur­tle, which are threat­ened through­out their range. Sea­sonal vis­i­tors to the UK, re­search has re­vealed that these gi­ant rep­tiles feed on sev­eral species of UK jel­ly­fish. By com­par­ing the dis­tri­bu­tion of jel­ly­fish with fac­tors such as sea tem­per­a­ture, plank­ton pro­duc­tion and wa­ter cur­rents, sci­en­tists hope to gain a greater un­der­stand­ing of what in­flu­ences the sea­sonal dis­tri­bu­tion of jel­ly­fish in UK wa­ters and the im­por­tant re­la­tion­ships they form within the ocean ecosys­tem.

Re­port your jel­ly­fish sight­ings

www.mc­suk.org/sight­ings/

Spot­ted a jel­ly­fish? We want to know! Your re­ports help sci­en­tists mon­i­tor the state of jel­ly­fish and their bloom­ing events through­out the UK.

Jel­ly­fish hitch­hiker: A rock crab (Me­tacarci­nus gra­cilis) hitch­ing a ride on the oral arms of a Pa­cific sea net­tle(Crysaora fusce­sens)

Feed­ing time: Us­ing pow­er­fulsting­ing ne­ma­to­cysts, the jel­ly­fish paral­y­ses its prey be­forecon­sum­ing it

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