Box jellyfish reveal hidden reproductive biology

The world's most lethal jellyfish are renowned for the cells that launch their venom. Researchers have charted and investigated these cells down to the level of their molecular workings.

A separate issue - the way these animals reproduce - has scarcely featured on the scientific agenda. That is beginning to change.

A fresh study of preserved museum specimens has revealed that their reproductive biology is far more elaborate than previously recognised.

The venom-delivering cells were discovered in a place nobody had considered examining.

A venomous reputation

These animals are box jellyfish, a group of about 50 species named after their transparent, cube-shaped bells.

A small number are among the ocean's most dangerous animals, while the great majority remain poorly understood.

One species, Chironex fleckeri - the Australian box jelly, or sea wasp - has been linked to dozens of documented human deaths. Studies of its venom place it among the fastest-acting lethal venoms known.

Its smaller relatives can cause Irukandji syndrome, beginning with a mild sting before severe pain spreads throughout the body hours later.

Despite the attention paid to the damage they can cause, box jellyfish breeding has received almost no research.

Jimena García-Rodríguez, a marine biologist at Tohoku University in Japan, aimed to change that.

Working with an international team, she assembled preserved box jellyfish from museum collections worldwide, including the Smithsonian's enormous invertebrate holdings.

Evidence from museum collections

Collecting a healthy box jellyfish is both hazardous and unpredictable. Persuading one to reproduce in an aquarium is nearly impossible, while observing the process in the wild is no simpler.

Instead, García-Rodríguez used specimens that had already been stored in museum collections, including animals collected decades earlier.

Her team studied 35 specimens from 15 species.

The detailed work required reproductive tissue to be cut into slices thinner than a human hair, followed by microscope examination of individual sperm and egg cells.

Unexpected features within the eggs

Two unexpected findings emerged from the eggs. In several species, García-Rodríguez observed egg cells crammed so closely together that they lost their rounded shape, developing multi-sided edges where neighbouring cells pressed against them.

This had never previously been documented in box jellyfish. The dense arrangement probably results from rapidly enlarging eggs forcing one another into confined spaces.

The other finding is more difficult to account for. The eggs of five species contained tiny round structures that cannot yet be identified.

These spheres emerge early as an egg produces its yolk, before disappearing once the yolk becomes dominant.

They could represent an unknown component of yolk production, storage compartments, or artefacts caused by preservation chemicals. High-powered electron microscopes will be necessary to distinguish between these possibilities.

Stinging cells in box jellyfish reproductive tissue

Box jellyfish inject venom using nematocysts - microscopic stinging capsules that propel a thread tipped with venom.

Such capsules usually cluster on the tentacles, which is where they would be expected. Yet García-Rodríguez repeatedly found them in reproductive tissue. This was the study's most remarkable observation.

The stinging cells were present in species belonging to both main branches of the family. Even more significantly, apparent immature forms - the precursors that develop into stingers - were also found. Previously, their origin was unknown.

Their presence suggests that these cells may be made in the reproductive tissue itself rather than transported there from the tentacles: a second site for weaponry concealed within tissue used for reproduction.

In one species that engages in courtship, the stingers even vary between the sexes, apparently protecting eggs in females and securing sperm packages in males.

Different routes to mating

Box jellyfish employ notably varied reproductive strategies. Some release eggs and sperm into open water, leaving fertilisation to chance.

Others mate directly, with males passing sperm to females.

The best-documented courtship occurs in the tiny species Copula sivickisi. A male holds a female by her tentacles before transferring his sperm package, behaviour examined closely in one study.

Several days afterwards, she deposits a sticky string of embryos unlike anything known from other jellyfish.

In its close relative, Tripedalia cystophora, fertilised eggs even start to develop within the reproductive organ.

When these behaviours are placed on the family tree, the evidence indicates that releasing gametes into the water is the ancestral pattern, with direct mating arising later in one lineage.

Why box jellyfish reproduction matters

Overall, the research shows that box jellyfish reproduction is much more varied than that of other jellyfish.

Neither the mysterious spheres within eggs nor stinging cells that appear to develop inside reproductive tissue had been observed before.

The wider benefit is practical. Learning how these animals reproduce can help improve forecasts of jellyfish blooms - sudden swarms that shut beaches and obstruct fishing equipment.

Forecasting such blooms has proved difficult, in part because jellyfish have highly complex life cycles.

“While focusing on learning only about the deadly aspects of these creatures is tempting, studying how they create life is equally important,” said García-Rodríguez.

Her next plans involve examining how these reproductive characteristics respond to a warming, changing ocean.