Zoo-Aquarium Berlin – a centre for jellyfish breeding

Nevertheless, all this technology does not make a high-tech aquarium, but, much more importantly, the animal keeper’s instinctive and sensitive feeling for the animals. Aquarium installations alone can provide assistance only, at best the basis for successful upkeep and breeding, for example here at ZOO-AQUARIUM BERLIN, the first in the world to rebreed various coral fish after developing large marine plankton cultures.

These plankton cultures and new water movement and filtering installations today help us to keep jellyfish as well, which together with the corals and anemones belong to the phylum Cnidaria. Virtually everyone knows jellyfish from holidays by the sea as unpleasant stingers or, washed ashore, as unappetising blobs of goo. In reality though jellyfish are highly aesthetic and filigree beings. When they slide through the water with pulsating movements of their umbrellas they almost look like creatures from another planet. Practically nobody can fail to be fascinated by their aesthetic qualities. And there is no better place to observe jellyfish than an aquarium. Nevertheless, up until a few years ago, they were presented at most for a brief time at aquariums on the coast: their upkeep and breeding were too complex.

This is because most species of jellyfish do not stay afloat through their pulsating movements, but slowly sink to the bottom where they die after a time. Also their reproductive biology is highly complex.

Cubozoan medusae, on the other hand, not only reproduce by strobilation or budding, but the whole polyp metamorphoses into a medusa. Accordingly, only one medusa is produced from each polyp. Immediately following their strobilation these mini jellyfish, just like the adults, must be kept afloat by a slight, turbulence-free movement of water. When this is the case and there is adequate nourishment available the medusae need only a few months to reach sexual maturity. The first polyps are then in evidence soon after.

Polyp culture is more successful and more reliable than sexual reproduction. For jellyfish to produce offspring, only a number of polyps need to reproduce by strobilation, and here is where the animal keeper must find out the key factors for this mode of reproduction. In most cases there are rapid fluctuations in temperature, greater water movement, changes to the iodine and salt content, and frequently higher light intensity.

In Germany the Löbbecke Aquarium in Düsseldorf for the first time could keep moon jellyfish over the long term in a so-called planktonkreisel. Monterrey Bay Aquarium of California modified the planktonkreisel, designed originally for small plankton organisms only, and was the first aquarium to present a world sensational jellyfish show in 1992. Such large planktonkreisels could not be installed at ZOO-AQUARIUM BERLIN, so moon jellyfish have been kept here in quite normal, though modified rectangular concrete tanks since 1988. Installed behind perforated acrylic panels twenty centimetres from the rear and side panels are the filter and pump cables. The pumps’ vertical, valve-regulated discharge tubes are installed in and parallel to a section of acrylic panels and exhibit a row of holes over their whole length. This prevents turbulence in the discharge area, but assures slow, constant circulation of the whole water column without the turbulence and air bubbles dangerous to jellyfish.

The polyps of many species must be kept at temperatures about 5–10 °C colder than the medusae. Only polyps in good condition are selected for strobilation, when the water temperature is raised over several stages by up to 10 °C for two to four days. Then, after some of the water has been changed, its iodine and salt content is modified and its temperature returned to the initial value or even 5 °C lower for one to two days. Afterwards strobilation normally begins.

In the case of jellyfish with symbiotic algae (e.g. Mastigias papua, Cotylorhiza tuberculata, or Phyllorhiza punctata) light is also an important factor affecting strobilation. So that polyps and ephyra larvae cannot become entangled or compete for food, they must be separated from each other. Depending on their size and species ephyra larvae initially feed only on rotifers and even brine shrimp larvae. Although ephyra larvae initially need a relatively strong movement of water to breed, the small medusae must not be hindered from swimming, and their food must never be washed from their tentacles.

At the outset problems were encountered with the strobilation of rhizostome jellyfish (Rhizostoma pulmo) and the brown sea nettle (Chrysaora fuscescens), which underwent strobilation, but failed to stop. Consequently the few polyps diminished in size until they were finally unable to survive.

Often the moon jellyfish (Aurelia aurita) is seen as the “beginner’s jellyfish” for keeping. These animals need a slight, turbulence-free, horizontal circulation of water. The polyps are large enough and can be fed on brine shrimp larvae, as later the adult medusae. Yet experience in Berlin has shown that the tropical Australian spotted jellyfish (Phyllorhiza punctata) is almost easier to keep, but does need a lot of intense light for its symbiotic algae.

The third jellyfish type is the sea nettle (Chrysaora melanaster) with its long tentacles. This initially needs a slight vertical circulation of water, but grows quickly and is an active aquarium swimmer through the pulsating movements of its umbrella. Its young medusae feed on brine shrimp larvae, but they soon take larger prey, using their long tentacles to take it not only out of the water, but also from the bottom or the surface of the water. Feeding these on a varied diet, but above all on jellyfish basically assures fast growth. Sea nettles feed not only on jellyfish, but also on opossum shrimps, brown shrimps, fish, and even dry feed. When these species feed on jellyfish it is incidentally interesting that the sea nettle and fried egg jellyfish (Phacellophora camtschatica), as feeders specialising almost exclusively in jellyfish, never feed on their own species, not even parts of them. They probably recognise their own species through chemoreceptors.