Jellyfish Kreisel Tanks: Flow Design, Moon Jelly Life Cycle and Husbandry

Why jellyfish need kreisel or pseudokreisel tanks, how the laminar flow and outflow screen work, the polyp-to-medusa cycle of Aurelia aurita, Artemia feeding and water quality.

Why ordinary aquaria fail for jellyfish

Medusae are planktonic: they drift rather than hold position, so their tank must keep them suspended and minimise contact with every surface. In a standard rectangular aquarium they settle into corners, press against glass and, most dangerously, get pulled against pump intakes and overflows. Small, thin ephyrae and metaephyrae are easily held against a drain screen by suction. According to a review of collection and culture techniques by researchers from the Monterey Bay Aquarium Research Institute and the Monterey Bay Aquarium, the standard pelagic tanks used today are all variations of the planktonkreisel designed by Greve (1968), later redesigned for shipboard use and for public display.

Kreisel flow principles

A kreisel is a horizontal cylinder: a circular main chamber with curved sides and bottom and a flat front and back, holding the animals away from plumbing with a bare minimum of interior hardware. Water enters from an inlet chamber and jets in a laminar sheet across the inner face of a fine-mesh screen that separates the main tank from the drain. Any animal drifting toward the outflow screen is pushed away by the incoming water, and the circular current keeps animals in the middle of the water column. Parallel layers of polycarbonate double-wall sheet placed between the inlet chamber and the main tank can force the inflow into a smooth laminar flow. A submerged lid lets animals be added or removed without risk of them being drawn into the drain.

Screen mesh must be smaller than the smallest medusae in the tank; 120–500 µm is commonly used. One modern culture kreisel uses interchangeable screens of 150 µm to 1 mm chosen by animal and prey size, for example 1 mm when feeding Artemia (about 400 µm).
The screen area should be as large as possible, so the drain suction at any single point stays too weak to trap medusae; screens are typically placed across a whole side of the tank, several centimetres from the drain.
Pseudokreisel: a standard tank converted by filling the bottom corners with silicone and solid plastic or vinyl, with the outflow separated from the tank by the inlet and screen; usually U-shaped or semicircular and without a lid.
Stretch (Langmuir) kreisel: a tank about twice as long as it is tall, with circular ends and inlets on both sides, producing two opposing gyres that downwell at the centre and upwell at the ends. It suits species that actively swim into a current.

Bubbles, gas and lighting

Air bubbles help circulation in cultures of small gelatinous animals, and an air line close to the screen in a grow-out tank creates a gentle upward current that keeps juvenile medusae off it. For larger adults (over about 3 cm), bubbles are harmful: they are ingested, collect in the gut and radial canals, make the animal float and disrupt swimming and feeding, and can work slowly through the mesoglea and lead to infection. Supersaturated incoming water may need a degassing tower. Strong display lights do not appear to bother many gelatinous species, which have limited vision.

Aurelia aurita life cycle

The moon jelly Aurelia aurita alternates between a free-swimming medusa, which reproduces sexually, and a benthic polyp (scyphistoma), which reproduces asexually. One husbandry study describes five stages: planula, polyp, strobila, ephyra and medusa. Adult medusae are typically 25–40 cm across and drift with currents. The species tolerates estuaries, bays and fjords, and its reproduction is shaped by temperature, food, salinity and light. A native microbiome on the polyps is needed for normal strobilation; polyps without it produce few or malformed ephyrae.

Inducing strobilation

Strobilation is the transverse segmentation of the polyp into stacked ephyra buds, and in nature it is mainly triggered by seasonal temperature change. Methods reported for scyphozoan polyps include a brief reduction of 5–10 °C followed by a return to normal temperature over a few days, as well as changes in feeding, light, pH or salinity. In one Aurelia aurita study, polyps acclimated at 20 °C were cooled to 10 °C at 1 °C per day; strobilation took about 52 days and ephyrae grew into medusae over the next 30 days. Feeding fell during strobilation and stage IV strobilae and new ephyrae ate little until about day 4 after release. Newly released medusae should be moved away from the polyps, which can injure or eat them.

Feeding

Artemia nauplii are the backbone of most culture diets for polyps and medusae. In the Aurelia aurita study above, polyps and medusae were fed daily on 48-hour-old Artemia nauplii enriched with a fatty-acid enrichment. Very small polyps and newly metamorphosed animals may struggle to capture prey of Artemia size and can be offered smaller prey such as rotifers. Other foods used for gelatinous zooplankton include krill, finely chopped squid or fish, wild plankton and, for jellyfish-eating species, live Aurelia ephyrae or medusae. Remove uneaten food and debris regularly, since it fouls screens.

Water quality

Water must be clean and finely filtered: small particles quickly clog outflow screens, and filtration to 20 µm is usually sufficient, with 3 µm for fouling-sensitive cultures. The Aurelia aurita study kept stock polyps at 23 °C, salinity 33 and pH 8.2, and monitored pH 7.7–7.9, nitrate 0–10 mg/L, nitrite 0–0.5 mg/L and ammonium 0–0.2 mg/L. A flow-through kreisel system for Aurelia spp. and related species ran at 13–25 °C and salinity 36–37. Clogged screens make animals more likely to stick to them, with possibly fatal results, so screens need regular scrubbing; siphoning into a temporary container lets any animal removed by mistake be recovered.

Related marine guides cover coldwater tidepool tanks, octopus husbandry and the macroalgae refugium.