Flying freshwater jellyfish

Abstract

On March 11, 1950, The Mail, a newspaper published in Adelaide, South Australia, ran the headline “Tiny African jellyfish in SA reservoir”. A jellyfish in freshwater? Who would not be interested by that? Admittedly, the paper did get some stuff wrong: this creature, Craspedacusta sowerbii (Figure 1), is not from the “Great Lakes of Tanganyika”, as the story implies. And the journalist’s scientific source, Mr Ifor Thomas of Adelaide University, is perhaps misquoted as saying it was first found outside Africa in London’s Kew Gardens in the 1880s, having arrived on African water lilies destined for the collection. That’s not quite right either (see below). But let’s be forgiving: this was a sighting of a freshwater jellyfish, an absolute novelty for most people! And just as fascinating, as Mr Thomas made clear, was how it could have got to this Australian reservoir. Well, it couldn’t have flown there…could it? T Bamford/B O’Brien, U Waikato In fact, there is much more than a “Down Under” conundrum here, for this little animal, usually seen as a ~25-mm diameter medusa, has popped up all over the world. With the exception of Antarctica, the literature now records it having appeared everywhere from China to Chile, usually in lakes and ponds, natural and man-made, sometimes entirely without connection to other bodies of water. Indeed, it was first scientifically documented back in 1880, after somehow penetrating the Victoria amazonica tank at the Royal Botanic Society’s facilities in Regent’s Park (London, UK). E Ray Lankester, who examined the strange new creature (and within four days saw his work published in the June 17th issue of Nature, and then repeated in Science on July 17th!), speculated that it had made its way there from the West Indies. However, he noted its provenance was very difficult to trace since no plants had recently been added to the lily house, and the water was drained off every year. It’s now thought that C sowerbii originates from the catchment area of the Yangtze River in China. There, blooms of medusae of both sexes frequently occur in ponds and pools, while in all other places these flourishes are nearly always composed of either males or females. “For example, in New Zealand we appear to have all males, but in Ireland so far all that have been observed are females”, explains Ian Duggan (University of Waikato; Hamilton, New Zealand). “The US and Sweden have both males and females, but in these locations blooms of either sex seem to occur in separate lakes or at different times. Where they have invaded it is rare to find both sexes blooming together.” Perhaps conditions outside China are unsuitable for both sexes to bloom simultaneously. But unisex jellyfish clouds may simply reflect the gender of the colony founder, which begins life as an immobile polyp stuck to a rock. It then reproduces asexually by budding off more polyps. From time to time these polyps may also asexually bud off a free-swimming form: the medusae or jellyfish that we typically notice. It’s the polyp form, however, that provides a little clue as to how the species might disperse. When environmental conditions get tough, they can turn into cysts called podocysts. These metabolically dormant, drought-resistant capsules might conceivably be spread by passively hitching rides on water plants (thus reaching the Royal Botanic Society and eventually contaminating the lily tank), or on boats, allowing them to invade new lakes and waterways. But how does C sowerbii get to isolated lakes where no boat goes? Via introduced fish? On angling gear? Maybe. But perhaps also by flying – with “Waterfowl Airways”. Many authors have suggested such aerial arrivals, but while waterbirds have been shown to carry the propagules of certain invertebrate species on their bodies, no direct evidence exists of C sowerbii podocysts ever getting off the ground. “As far as I am aware, no one has found it attached to the proverbial ‘duck’s feet’, but it does seem highly plausible that they are being moved by waterfowl”, says Duggan. “Our surveys in the North Island of New Zealand showed that the polyp stage of the species was widespread among ponds around the island, including in a number where the medusa had never been found. Not only were these ponds widespread, but some were fairly isolated, and without any kind of boat access, so dispersal associated with waterfowl seems plausible.” But if Duggan’s tiny polyps were found where no medusae had ever been seen – the latter appear only when the water gets above 20°C – maybe C sowerbii is far more widespread than we ever thought (Aquat Invasions 2012; 7: 271–75). Perhaps this small creature has for centuries been traveling the globe, silently conquering new pools and ponds in the form of resistant podocyst passengers, flying in on (or even within?) the bodies of waterbirds, only long after to be noticed as medusae. You might even say that these little freshwater jellyfish actually appear out of thin air. Adrian Burton