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A rippled seabed withprominent diatom matbetween the ripple crests.(Scale: approx. 10 cmbetween crests)

FRESHWATER BIODIVERSITY

Alien predator:freshwater jellyfish in New Zealand

Ian Boothroyd

Kay Etheredge

John Green

Many people will have seen jellyfish washed upon the beaches around the New Zealandcoastline following storms at sea. Some will beaware of the notorious “stingers” (box jellyfish– Chironex sp.) that make Northern Queenslandbeaches potentially lethal for swimming duringDecember and January. There are also theinfamous Portuguese man-of-war jellyfishcolonies, occasionally seen on our beaches, butbetter known for their treacherous presence inthe warm waters of the world. All of these aremarine creatures. It may come as a surprise,therefore, to know that many lakes in NewZealand are home to a freshwater jellyfish. Butbefore you think twice about a summer dip in alocal lake – don’t worry. These jellyfish areharmless to humans and large animals, and feedonly on small zooplankton species.

Life in two formsThe freshwater jellyfish, Craspedacusta sowerbyi,belongs to the Phylum Cnidaria, in theLimnomedusae, a group of hydrozoan jellyfish(see panel). Hydrozoans have small solitary orcolonial polyps and free-living medusae. Polypsare typically cylindrical or tube-like with mouthand tentacles at one end, usually directedupwards. The other end is attached to asubstrate. The medusa stage is the classicaljellyfish form, with an umbrella-like body. Theconcave or underside of the umbrella facesdownwards with the mouth at the centre andtentacles hanging down from the rim.

Where did the freshwater jellyfishcome from?Medusae of Craspedacusta sowerbyi were firstdiscovered in a tank at Kew Gardens, London,in 1880, where the huge Brazilian water lilyVictoria amazonia was kept. How and when themedusae were introduced into the tanks wasdifficult to trace since no new plants had beenadded. It was concluded that the jellyfish was atropical species, because of its presence in warmwaters, and because it was first found inassociation with tropical South American plants.

Recent evidence suggests that C. sowerbyioriginated from the Yang-tse-kiang region of

China. An Asian origin is now generally agreedby scientists and today C. sowerbyi has aworldwide distribution.

C. sowerbyi in New ZealandMedusae of C. sowerbyi were first recorded inNew Zealand in Lake Taupo and numerousRotorua lakes by V.H. Jolly during her surveyof zooplankton in the North Island in the 1950s.It was first described in the New Zealandliterature from Lake Maraetai (Waikato) in 1971,and there have been sporadic records andcollections from all over mainland NewZealand.

C. sowerbyi occurs in lakes, including largehydroelectric impounded lakes on the Waikato

Ian Boothroyd wasformerly based atNIWA Hamilton andnow works forKingett Mitchell Ltd,Auckland; JohnGreen is AssociateProfessor at theUniversity ofWaikato; and KayEtheredge lives at TePahu, Pirongia.

Jellyfish relationshipsJellyfish belong to the Phylum Cnidaria(previously known as Coelenterata). Thephylum is characterised by possessing stingingcells, called cnidocytes or nematocysts, andoften having life cycles with hydroid andmedusa stages (see text). There are fourclasses of Cnidaria.Anthozoa: the true corals and sea anemones(see Water & Atmosphere 5(1): 6-7); these haveno medusa stage.Scyphozoa: the true jellyfish; 8-pointsymmetry, and a jelly layer – the mesoglea –lying between the two basic cell layers in theconvex part of the medusa.Hydrozoa: includes hydras, some corals andmany small jellyfish (e.g., Craspedacusta);often colonial in the polyp stages, e.g., largecolonial Portuguese man-of-war, Physalia.Cubozoa: the box jellyfish, including Chironex– "stinger" jellyfish; 4-point symmetry, complexeyes and a characteristic type of polyp.

Jellyfish arefamiliar marineanimals, but theyare also found infreshwater. Onespecies is quitecommon in NewZealand.

Medusae of Craspedacusta sowerbyi with a range oftentacle sizes and four large gonads in the centre.(Medusa approximately 16 mm in diameter.)

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River, throughout the upper North Island, aswell as in Hawke’s Bay (Lake Tutira). C. sowerbyihas also been recorded as far south as LakeBrunner, on the west coast of the South Island,and in Lake Waiola near Dunedin. Other recordsinclude lakes Ruatuna, Mangakaware,Mangahia, Kainui and Rotomanuka (Waikato)and Nihotupu Resevoir (Auckland).

Male-only area!The life cycle of C. sowerbyi includes both apolyp and medusa stage. The polyp may besolitary or may produce small colonies of afew attached individuals by budding. Theactive medusae also bud off from the sides ofthe polyps. Their production is thought to berelated to water temperature, with medusaeforming only in summer when watertemperatures reach about 25°C. The medusastage of hydrozoans is the sexuallyreproducing stage. The sexes are separate.Four gonads hang down from the undersideof the umbrella. Medusae mature in a few daysand congregate as swarms in the surfacewaters. Eggs and sperm are released into thewater and fertilisation is external. Thefertilised egg develops into a small larva thateventually settles on the lake bottom anddevelops into a polyp.

However, it seems that sexual reproduction canbe a frustrating exercise for Craspedacusta. Onlymale jellyfish have ever been recorded in NewZealand, as well as in many other parts of theworld. This means that the jellyfish must relyon budding polyps and perhaps accidentaltransport by humans (attached to weed or boats)to extend their range.

Food preferencesC. sowerbyi eats common zooplankton. Theyseem to particularly select Ceriodaphnia, a

common crustacean. Other small animals seemto be eaten in proportion to their abundance.Very tiny zooplankton such as nauplii (babycrusctacea) and other rotifers may be too smalland are missed by the tentacles, or perhaps arecaught and digested too quickly to detect.

The preference for Ceriodaphnia issurprising since medusae of C. sowerbyiusually swim upward and then coastdown with their tentacles spread out likea net, which will catch anything thathappens to be in the way. The medusaepossibly aggregate in places wheredensities of Ceriodaphnia are higher.Another possibility is that Ceriodaphnia isattracted towards the medusae, perhaps bychemical stimuli.

Does Craspedacusta predationaffect lake communities?We investigated the effect of Craspedacustafeeding on zooplankton in Lake Kainui, a smallpeat lake in the Waikato. To get an idea of theproportion of prey animals eaten we examinedjellyfish gut contents, and also estimated howmany medusae were feeding.

We found that the medusae ate only smallproportions of their prey items – generally below0.5% of the prey population per day. The onlysubstantial effect was on Ceriodaphnia for whichthe daily value was 3.4%, assuming a 7-daylifespan. Even in relatively long-livedzooplankton, such as the crustaceans, Calamoecialucasi and cyclopoid copepods, Craspedacustapredation would eat only 3–4% of thepopulation. It seems unlikely therefore thatjellyfish predation was a significant part ofzooplankton mortality in Lake Kainui. However,our data were from only one sampling occasionand this conclusion needs more rigorous testingin other lakes.

A threat to New Zealandbiodiversity?Craspedacusta is found in lakes and waterbodiesthroughout New Zealand, and feeds on smallzooplankton. The medusae usually occur onlyin summer as lake water warms. Although theyare active predators, with a possible preferencefor specific prey, it seems unlikely that thisjellyfish is a threat to the freshwater fauna ofNew Zealand. However, its influence on thefeeding behaviour and food availability for othernative zooplankton feeders, such as small pelagicfish, is unknown. ■

Ceriodaphnia, themain prey item ofCraspedacustasowerbyi in LakeKainui, Waikato.

Craspedacustasowerbyi medusae

in characteristicfeeding mode withtheir nematocyst-bearing tentaclesoutspread in the

form of a net. (Photos by BarryO’Brien, Universityof Waikato)

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