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Start with the center and the way the animal moves. A sea star’s arms usually merge into the body’s outline, while a brittle star has a distinct central disc with clearly attached, often slender arms. Sea stars commonly crawl using tube feet underneath; brittle stars commonly row or drag themselves with their arms. These clues help distinguish the groups, but none is a universal species-level test.
Check the center before counting arms
The clearest first clue is the boundary between the body and the arms. In a sea star, the arms generally radiate from a central axis and grade into the body, so the animal’s outline often looks continuous from its center outward. In a brittle star, the central disc is more sharply defined, and the arms appear to attach to that disc rather than blend into it.
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Smithsonian Ocean describes a brittle star as having a disc-shaped center and arms that are often serpentine. That contrast is useful even when an animal is not moving: look for a compact, clearly outlined disc surrounded by arms, rather than assuming that any five-armed animal is a sea star.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsTo make the comparison, focus on the junction between the center and each arm rather than just the overall silhouette. A sea star can still have recognizable arms, but they typically look like extensions of a continuous body. On a brittle star, the disc reads more like a separate central unit, with the arms visibly beginning at its edge. This is why the center is a more reliable first check than a quick glance at the number of points.
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What to look for in a photo
- Find the central body, then trace where each arm begins.
- Ask whether the center has a distinct disc outline or transitions gradually into the arms.
- Compare the arms: brittle-star arms are often long, narrow, and visibly separated from the disc; sea-star arms often look broader or less sharply separated from the body.
These are typical group-level patterns, not rules that describe every species. A still image also cannot show how the animal moves, so use the body outline and arm shape together rather than treating either as conclusive by itself.
Image quality affects how much you can conclude. If the center is hidden by another object, the animal is tightly curled, or the photograph is too distant to show the arm bases, do not infer a sharply defined disc from the apparent outline alone. Look for a clearer view if one is available. A close, top-down image can help with the central body, while an image showing the underside may reveal tube feet; neither angle guarantees a species-level identification.
Watch how it moves, if movement is visible
Locomotion provides a second strong clue. Sea stars commonly move by coordinating tube feet on their underside. Brittle stars commonly propel themselves by rowing or dragging with their long, flexible arms. Smithsonian Institution’s Q?rius brittle-star resource describes most brittle stars and basket stars as making rowing motions along a surface with those arms.
In practice, look for the source of the movement. If the animal progresses while its arms make sweeping or rowing motions, that supports a brittle-star identification. If it creeps using the underside rather than visibly rowing with its arms, that supports a sea-star identification. The distinction is especially useful when the body is partly obscured but the animal’s movement can be observed.
Movement is still a clue, not a guarantee. A brief clip may not show the animal’s usual motion, and the group-level descriptions do not mean every individual will move in an obvious or identical way. If no movement is available, return to the central disc and arm attachment.
When watching a clip, follow the body as well as the arms. A brittle star’s arm strokes may pull or push its body along, and Smithsonian’s resource notes that it may switch which arms are rowing. So do not expect a single fixed pair of arms to lead throughout the clip. The useful distinction is the broad pattern: flexible arms doing the propulsive work points toward a brittle star, while coordinated tube feet on the underside are the characteristic sea-star clue.
Also distinguish movement from posture. Arms spread across a surface in a still frame do not show whether the animal is rowing, and a sea star’s arms may appear to shift as it moves. If you have only a photograph, leave locomotion unassessed rather than guessing from the arrangement of the arms.
Compare the main clues
| Clue | Sea star | Brittle star |
|---|---|---|
| Center and arm attachment | Arms radiate from a central axis and usually blend into the body outline. | A distinct central disc is apparent; arms attach clearly to it. |
| Typical arm shape | Arms are often broader or less sharply separated from the center; the number varies. | Arms are often slender, flexible, and serpentine. Basket stars can have branching arms. |
| Common movement | Commonly crawls using tube feet on the underside. | Commonly rows or drags itself with its arms. |
| Taxonomic class | Asteroidea | Ophiuroidea |
| Madreporite | Smithsonian Ocean says it is often visible to the naked eye on the upper side. | The cited sources do not establish a complete group-wide comparison of its position. |
The table summarizes broad identification tendencies. It is not a key for identifying a particular species, and the madreporite should not be used as a stand-alone test. A visible feature may help when other clues are ambiguous, but the central disc, arm form, and movement together are more useful for this comparison.
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The madreporite is therefore best treated as a secondary observation, not a shortcut. Smithsonian Ocean says it is often visible to the naked eye on the upper side of sea stars, but the cited material does not provide a complete group-wide comparison of its position in brittle stars. If you see a possible madreporite, record it as one feature among several. If you do not see one, that absence does not resolve the identification.
The taxonomy row names the broad classes, not species. Asteroidea is the class that includes sea stars; Ophiuroidea includes brittle stars and basket stars. If your goal is simply to tell the two body plans apart, the class names are less useful than the visible anatomy and movement. They can, however, help keep the terminology straight when you consult a field guide.
Use arm count and color only as supporting clues
Five arms are common in both groups, so a count of five does not establish that an animal is a sea star. Sea stars may have six or seven arms, and some have many more. NOAA’s guide to organisms observed in, on, or near shipwrecks and habitats at Stellwagen Bank National Marine Sanctuary describes several multi-armed sea-star species as well as a brittle star with long, thin arms.
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Color is similarly unreliable as a group-level rule. The NOAA guide’s descriptions include multiple colors within and across sea-star species. A color that seems familiar may help someone compare an animal with a regional species guide, but color alone does not distinguish sea stars from brittle stars.
Arm count and color are most useful after the main structural clues. Note them alongside the animal’s location, the form of its central body, arm shape, and any observed movement. A regional guide can then help narrow down species, but a broad group comparison should not be mistaken for a species identification.
For example, a five-armed animal with a sharply defined disc and long, thin arms should not be called a sea star just because it has five arms. Conversely, a sea star with more than five arms is not ruled out by the count. This is the practical reason to rank clues: first examine the center and arm attachment, then use movement where available, and only then add count and color as context.
Understand what the groups share
Sea stars and brittle stars are both echinoderms, a group that also includes sea urchins, sand dollars, and sea cucumbers. Echinoderms have a water-vascular system. In sea stars, coordinated tube feet support movement; that underside activity is one reason tube feet are useful to watch when distinguishing a moving sea star from an arm-rowing brittle star.
The arms of brittle stars have a different described structural feature: Smithsonian’s Q?rius resource explains that they are supported by calcium-carbonate ossicles connected by ball-and-socket joints. Side plates can provide traction. Their flexible construction helps explain why rowing arms are a characteristic movement clue, while the visible attachment of the arms to a central disc helps distinguish their body form.
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The shared echinoderm identity explains why the animals can look broadly alike, but their taxonomic classes differ: sea stars belong to Asteroidea and brittle stars to Ophiuroidea. When a casual resemblance makes the name uncertain, classify first by the body plan and movement rather than by the broad star-like shape.
In other words, they are related groups with a familiar overall plan, not two names for the same animal. Both have a star-like arrangement of arms, and both belong among echinoderms, but the way the arms relate to the center and the role of the arms in movement offer useful contrasts. The arm joints and side plates described for brittle stars also provide anatomical context for their flexible, arm-driven movement; they are not features that need to be identified in an ordinary photograph to make a basic group-level distinction.
Do not use a broken arm to decide
A missing or damaged arm does not settle the identification. Brittle stars can lose arms as a defense and later regrow them. Smithsonian Ocean also describes regeneration in both groups. An animal with fewer arms than expected may therefore have lost one, and the count should not override the body outline or movement evidence.
When one arm is missing, examine the remaining arms and the center. The distinct disc and clearly attached, often slender arms remain the more relevant brittle-star clues; arms that merge into the body outline remain a typical sea-star clue. If the specimen is damaged enough that those features are difficult to assess, the evidence may support only a broad or tentative identification.
Do not use the number of intact arms to reconstruct a supposedly definitive original count either. The available clues support the possibility of loss and regrowth, not a conclusion about how many arms a particular individual began with. In a damaged specimen, record what is visible and separate that observation from your identification: for example, “disc distinct; one arm absent” is more careful than allowing the missing arm to decide the group.
A practical identification sequence
- Locate the center. Decide whether there is a sharply defined disc or a body outline that continues gradually into the arms.
- Inspect the arms. Note whether they are long, slender, flexible-looking, and clearly attached to a disc, or broader and less sharply separated from the center.
- Watch movement if available. Arm-rowing or arm-dragging supports a brittle-star identification; movement using coordinated underside tube feet supports a sea-star identification.
- Record supporting details. Note arm count, color, and whether a madreporite is visible, but do not use any of those details alone to decide the group.
- Use a regional key for a species name. Group-level clues can distinguish the general body plans, while species-level determination may require the animal’s geographic location and a regional guide.
This sequence is useful for a live observation, a photograph, or a specimen image. With a photograph, center shape and arm attachment are usually the available primary clues; with a moving animal, behavior adds another. If the clues conflict, report the identification cautiously rather than allowing arm count or color to decide it.
A useful field note can be short and structured: record where the animal was observed, whether the central disc is distinct, what the arms look like, whether movement was visible, and any supporting details such as count or color. This prevents a memorable but weak clue—such as an unusual color—from outweighing the features that better distinguish the groups. If you later consult a regional key, the location and observations give you a more useful starting point than a name guessed from one photograph.
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For a static image, the sequence stops naturally after the structural checks and supporting notes. For a moving animal, add the movement observation without treating one short action as conclusive. For a badly obscured or damaged specimen, it is reasonable to stop at “likely sea star” or “likely brittle star” rather than claim certainty. The method is designed to sort broad groups first; it does not replace the more detailed characters required for a species-level identification.
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Be cautious with regional species guides
NOAA’s Stellwagen Bank National Marine Sanctuary guide is specific to animals found in, on, or near that sanctuary’s shipwrecks and habitats; it is not a universal identification guide for all coastlines. NOAA also labels it a work in progress and notes that scientific and common names and descriptions may change as more information is collected. Its examples can illustrate variation, but should not be applied as though they cover every region.
For a species-level answer, geography matters: use a guide relevant to where the animal was observed. The broad body-plan distinction in this article can help determine whether to start with sea stars or brittle stars, but a photograph and a general description may not contain enough detail to identify a species confidently.
Use a regional guide in the right order: first decide which broad group the body plan most resembles, then compare the animal with species known from the relevant area. A guide built around one sanctuary’s shipwrecks and habitats can be useful there, but its listed species and descriptions should not be assumed to represent another coast. NOAA cautions that its Stellwagen guide is still a work in progress; its names and descriptions may change as more information is collected. Treat a guide as a tool for narrowing an identification, not as proof that an animal from elsewhere must fit one of its entries.
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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11That geographic caution matters especially when color or arm count seems to match an illustration. Those features can vary, and a regional example is not a universal rule. If the location is unknown or the available image lacks detail, the sound answer may remain at the group level.
How many kinds are there?
Smithsonian Ocean’s educational overview, published in April 2022, gives broad counts of close to 2,000 sea-star species and over 2,000 brittle-star species. These are overview figures, not counts for a particular region or a claim about a current census. The scale of that diversity is another reason a simple clue should be treated as a way to distinguish groups, not as a complete species key.
The numbers also explain why broad descriptions use words such as “usually,” “often,” and “commonly.” They summarize patterns across large groups, not an identical appearance or behavior shared by every species. The basic distinction remains practical: sea-star arms usually blend into the body, while brittle-star arms attach to a distinct disc; sea stars commonly use tube feet, while brittle stars commonly use arm motions. For finer identifications, local range and a suitable regional key matter.
Frequently Asked Questions
What is the difference between a sea star and a brittle star?
A sea star’s arms usually blend into the body outline around a central axis. A brittle star has a more sharply defined central disc with clearly attached, often slender arms. Sea stars commonly move using tube feet underneath, while brittle stars commonly row or drag themselves with their arms.
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Start with the center: a distinct disc and arms that look attached to it point toward a brittle star; arms that grade into the body outline point toward a sea star. If you can observe movement, arm-rowing supports brittle star, while underside tube-foot movement supports sea star. “Starfish” is another common name for sea stars, though “sea star” is often used.
Do both sea stars and brittle stars have five arms?
Five arms are common, but that count does not distinguish the groups. Sea stars can have six or seven arms, and some have many more. Use the central disc, arm attachment, and movement as the main clues.
Can color tell me whether an animal is a sea star or brittle star?
No reliable group-level rule follows from color alone. NOAA’s Stellwagen Bank guide describes multiple colors within and across sea-star species. Treat color as supporting information for a regional species comparison, not as the deciding clue.
Can a broken arm change the identification?
A missing arm does not settle the identification. Brittle stars can lose arms as a defense and later regrow them, and Smithsonian Ocean describes regeneration in both groups. Examine the center and remaining arms instead.
What if I only have a photograph?
Use the central-disc outline, how the arms attach, and their shape. A still image cannot show locomotion, so do not infer rowing or tube-foot movement from the posture alone. Arm count, color, and a visible madreporite are secondary clues.
Can I identify the exact species from these clues?
Usually not from the broad group clues alone. Species-level identification may require the animal’s geographic location and a regional key. A local guide should be relevant to where the animal was observed; NOAA’s Stellwagen Bank guide, for example, covers animals associated with that sanctuary and is labeled a work in progress.
Are sea stars and brittle stars related?
Yes. Both are echinoderms, along with sea urchins, sand dollars, and sea cucumbers. Sea stars belong to class Asteroidea, while brittle stars belong to class Ophiuroidea.
The Bottom Line
For a quick distinction, inspect the center first: a sharply defined disc with slender, clearly attached arms points toward a brittle star, while arms that blend into the body outline point toward a sea star. If movement is visible, brittle stars commonly row or drag with their arms, whereas sea stars commonly use tube feet underneath. Treat arm count, color, and any single feature as supporting clues, and use a relevant regional guide for species-level identification.
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