What Do Box Jellyfish Look Like? Safe Identification Guide

Box jellyfish have a squarish, transparent bell with tentacles attached near its four corners. The name describes a whole class, Cubozoa, so size, tentacle number and color vary by species.

Key identification features

• Box-shaped bell: viewed from above, the swimming bell has four sides rather than a simple round outline.
• Corner structures: muscular pads called pedalia extend from the lower corners and carry one or more tentacles.
• Transparency: many species are nearly colorless, making them difficult to see in water.
• Active swimming: cubozoans can pulse and steer more purposefully than the stereotype of a passively drifting jellyfish.
• Four sensory centers: each rhopalium carries a set of eyes and balance-related structures.

What does Chironex fleckeri look like?

The Australian Museum describes a large transparent bell up to about 30 cm across and tentacles that may extend to about 3 m. Adults can carry up to 60 tentacles in four clusters. A juvenile or a different species may look very different, so those measurements are not a universal box-jellyfish template.

Large and small species

Some cubozoans are palm-sized or smaller and may have a single tentacle at each corner. Several of the small species associated with Irukandji syndrome can be exceptionally difficult to spot. Appearance alone cannot tell a swimmer whether an animal is harmless.

Can you identify one safely?

Do not handle a jellyfish or detached tentacle for identification. Stinging cells may still fire after separation or death. Use photographs taken from a safe distance, note the location and follow local lifeguard or wildlife-authority advice.

A safe identification sequence

1. Stay out of reach of the animal and tentacles.
2. Record the exact coast, date and whether it was swimming or stranded.
3. Photograph the full bell and lower margin.
4. Look for four sides, corner pedalia and the number of tentacles per corner.
5. Compare only with species documented in that region.
6. Use an expert or responsible authority for confirmation.

Feature checklist

Feature	Supports Cubozoa	Limitation
Four-sided bell	Strong broad clue	Soft bells flex, and photographs distort shape
Pedalia at corners	Important cubozoan structure	Can be hidden by angle or damage
One or multiple tentacles per corner	Helps distinguish major groups	Broken tentacles make counts unreliable
Active directed swimming	Consistent with cubozoan behavior	Current can mask or exaggerate movement
Transparent body	Common	Shared by many unrelated gelatinous animals

Common look-alikes

Moon jellyfish

A round saucer-like bell often contains four horseshoe-shaped gonads. Those four internal shapes are not the corners of a cubozoan bell.

Portuguese man o’ war

A blue-purple gas float sits above the surface. It is a siphonophore colony, not a medusa.

Comb jellies

Rows of cilia can produce rainbow-like light. Comb jellies do not possess cnidarian nematocysts.

Why species identification is harder

Accepted names can depend on internal canal patterns, stomach structures, rhopaliar openings, preserved type material and DNA. A clear photograph may establish “possible cubozoan” while remaining insufficient for a species.

What makes an identification authentic

A high-quality record connects the image to date, coordinates, measurement method and the person or institution that reviewed it. Do not use an unrelated stock photograph as evidence for a local species.

Sources

• Australian Museum: identification and size of Chironex fleckeri
• NOAA National Ocean Service: box jellyfish overview
• Northern Territory Government: identifying features and risk

Editorially reviewed: 22 August 2026.

Verified photographs: compare structure, not danger

These photographs come from files with named identifications and explicit reuse records. They demonstrate diversity within Cubozoa; they do not provide a universal visual test for sting severity.

Photo-reading checklist

1. Provenance: is the photographer, location and date available?
2. Taxonomic basis: was the name supplied by a specialist or linked to a voucher?
3. Bell geometry: does more than one frame show the squared profile?
4. Pedalia and tentacles: can you see one versus branching, multi-tentacled corners?
5. Damage: could missing tentacles or collapsed tissue change the apparent form?

Never use a photo match to declare water safe. Follow local warnings, and photograph stranded animals only from a distance.

Continue with the interactive anatomy diagram or browse the species directory.