THE ANIMAL FILES

Spiny dogfish

Squalus acanthias

A shark’s spines are not its teeth.

The spiny dogfish has two defensive spines, one beside each dorsal fin. White spots, an asymmetric tail and the absence of an anal fin help distinguish its body plan. It feeds on fish and several kinds of invertebrates.[1]

Follow the evidence ↓
Spiny dogfish
Image: Doug Costa, NOAA/SBNMS · Commons · Public domain
Back
Two dorsal-fin spines[1]
Fin clue
No anal fin[1]
Food
Fish and several kinds of invertebrates[1]

Reading the whole outline

White spots mark the back, the tail is asymmetric and there is no anal fin. When comparing photographs, the relevant body region must be visible before a hidden fin can be distinguished from a genuinely absent one.[1]

Defense beside the fins

Two spines beside the dorsal fins have a defensive role, distinct from the jaws used in feeding. Their location helps distinguish them from teeth and from the fins themselves.[1]

A varied underwater menu

Fish, squid, crustaceans and other invertebrates occur in the diet. Dogfish can form aggregations, but that observation alone does not demonstrate coordinated group hunting.[1]

A COMMON MIX-UP

Do the spines mean it feeds by stabbing prey with its back?

No. The dorsal spines provide defence; the jaws handle food.[1]

TRY A LITTLE REASONING

Why distinguish an absent fin from a fin hidden in a photograph?

Reveal the explanation

A camera angle can conceal a structure. Anatomical absence requires a view or description that actually covers the relevant region.[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 — Squalus acanthias (adapted and translated; exact revision)
  2. NCBI Taxonomy — Squalus acanthias (identity and game lineage)

Last fact check:

One discovery leads to another

Greenland sharkA centuries-old shark’s age is an estimate built from eye tissue, not a counted birthday.Somniosus microcephalus ↗Frilled sharkAn eel-like outline belongs to a shark.Chlamydoselachus anguineus ↗Blue sharkBlue above, pale below: an ocean swimmer has more than one colour.Prionace glauca ↗