THE ANIMAL FILES

Blue sea dragon

Glaucus atlanticus

The blue side facing the sky is actually the sea slug’s underside.

Blue sea dragons are small ocean-going sea slugs that float upside down near the surface. Gas helps maintain buoyancy, and the contrasting surfaces face different viewing directions. Feeding on floating cnidarians links them to the same drifting surface community.[1]

Follow the evidence ↓
Blue sea dragon
Image: Sylke Rohrlach from Sydney · Commons · CC BY-SA 2.0
Orientation
Upside down at the surface[1]
Buoyancy
Gas held in the digestive system[1]
Food
Floating cnidarians and other surface organisms[1]

Upside down changes the view

A gas-filled sac helps the animal float, with the anatomical underside uppermost. Blue and blue-white colours face upward, while the true back is silver-grey and faces down. This countershading pattern must be described with both anatomy and position, otherwise “upper” can accidentally mean two opposite surfaces.[1]

Prey within the drifting community

The slug takes organisms including Portuguese man-of-war and by-the-wind sailors. Serrated radular teeth and jaws help grasp and remove prey tissue. Branched cerata can also make slow swimming movements toward food or mates. Drifting with water therefore coexists with some local movement.[1]

A borrowed defensive structure

Stinging capsules from cnidarian prey can be retained in specialised sacs at the cerata tips. This is kleptocnidy, using structures acquired from another organism. It is different from manufacturing those capsules from scratch. Ocean currents can also carry the slug ashore, so a beach record does not make it a normal sand-dwelling animal.[1]

A COMMON MIX-UP

Is the side facing upward always the animal’s anatomical back?

No. This slug floats upside down, exposing its underside to the sky.[1]

TRY A LITTLE REASONING

What is borrowed when the slug retains a prey’s stinging capsules?

Reveal the explanation

Pre-existing stinging capsules acquired from its prey, rather than newly grown teeth or spines.[1]

Sources & editorial notes

Adapted and translated from the cited Wikipedia revisions, with source-linked reasoning. Image credit and licence are listed separately above.

Adapted and translated from Wikipedia contributors; exact revisions are linked below. CC BY-SA 4.0

  1. Wikipedia contributors — Glaucus atlanticus (adapted and translated; exact revision)
  2. NCBI Taxonomy — Glaucus atlanticus (identity and game lineage)

Last fact check:

One discovery leads to another

Roman snailThe shell has a doorway—and sometimes a temporary seal.Helix pomatia ↗Pacific banana slugA banana slug need not be yellow.Ariolimax columbianus ↗Common periwinkleA snail on a rock may be grazing a film you can barely see.Littorina littorea ↗