THE ANIMAL FILES

Green sea urchin

Strongylocentrotus droebachiensis

A spiny adult begins as a swimmer.

Adult green sea urchins graze on algae along the seabed. Eggs and sperm are released into the water, where swimming larvae develop before settling and changing into bottom-living juveniles.[1]

Follow the evidence ↓
Green sea urchin
Image: Creator Eric A. Lazo-Wasem Gall L (2019). Invertebrate Zoology Division, Yale Peabody Museum. Yale University Peabody Museum. Occurrence dataset https://doi.org/10.15468/0lkr3w accessed via GBIF.org on 2019-01-09. https://www.gbif.org/occurrence/350726575 · Commons · CC0
Adult form
A slightly flattened, spiny globe[1]
Food
Algae, with recorded preferences[1]
Young stage
A free-swimming larva before settlement[1]

Which side faces the rock?

The mouth-bearing side rests toward the substrate, while the opposite surface faces away. Rows of tube-foot openings make the adult’s five-part organization visible.[1]

A grazer within a community

Algae form part of the recorded diet, with differences in preference among available kinds. Other animals also use the urchin’s surface: some snails attach eggs near spine bases. A single individual can therefore be both a consumer and a small habitat.[1]

Settling changes the lifestyle

Eggs and sperm are released into the water, and development produces a swimming larva. Metamorphosis leads to a bottom-living juvenile with the adult organization emerging. The transition connects water-column dispersal with life on a surface.[1]

A COMMON MIX-UP

Is a young urchin always a tiny copy of the spiny adult?

No. A swimming larva precedes the bottom-living form. Development includes a substantial reorganization rather than size increase alone.[1]

TRY A LITTLE REASONING

Why can a seabed animal still depend on conditions in open water?

Reveal the explanation

Fertilization and larval development occur in the water column before settlement. Different stages expose the same species to different parts of the habitat.[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 — Strongylocentrotus droebachiensis (adapted and translated; exact revision)
  2. NCBI Taxonomy — Strongylocentrotus droebachiensis (identity and game lineage)

Last fact check:

One discovery leads to another

Deep-water redfish (sea cucumber)The name says “redfish”; the animal is a sea cucumber.Actinopyga echinites ↗Common starfishA small opening can turn into a meal.Asterias rubens ↗Brook lampreyTeeth appear just when eating ends.Lampetra planeri ↗