Most sea stars (often called starfish) pass waste through a tiny anus on the upper surface of the central disc. But not all do: brittle stars and some sea-star groups lack an anus and release waste through the mouth after digestion.
Where is a starfish’s anus?
In sea stars that have one, the anus is a small opening on the aboral surface—the upper side of the central disc, opposite the mouth. The Institute of Marine Research notes that when an anus is present, it opens on the dorsal side (Institute of Marine Research).
The mouth is underneath, on the oral side. So a sea star with a complete gut takes food in below and expels fecal material above. The opening can be difficult to spot because it is tiny.
How does food become waste?
Some digestion happens outside the body
When prey is too large to swallow, many sea stars push their cardiac stomach out through the mouth and digest the prey externally. OpenStax describes the cardiac stomach as capable of being everted through the mouth during feeding (OpenStax). The sea star then draws the stomach back inside; the National Aquarium describes this retraction after digestion (National Aquarium).
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The arms help digest and absorb food
Food moves into the pyloric stomach and paired pyloric caeca, which extend into the arms. These organs supply many digestive enzymes, absorb nutrients, and store reserves. Because digestion and absorption are extensive, little material may remain to pass as feces.
Remaining material leaves through the gut
In a typical sea star with an anus, material that remains travels from the pyloric stomach through a short intestine and rectum to the opening on the upper central disc. In Asterias rubens, the Australian Museum says defecation results from contraction of the rectal caeca wall alongside relaxation of the anal sphincter; its account also describes digestion as virtually complete, with little fecal matter passed (Australian Museum).
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Do starfish poop out of their mouths?
Some do, but it is not the usual route for sea stars with an anus. Smithsonian Ocean explains that brittle stars and certain groups of sea stars lack an anus and vent food through the mouth once digestion is complete (Smithsonian Ocean). Brittle stars are a related echinoderm group, not true sea stars, which is one reason casual references to “starfish” can blur anatomical differences.
| Group or anatomy | Waste exit | What to expect |
|---|---|---|
| Sea star with an anus | Tiny opening on the upper central disc | Little fecal material may remain after extensive digestion and absorption. |
| Brittle stars and some sea-star groups without an anus | Through the mouth | They vent remaining material after digestion. |
Do sea stars have butts?
In the anatomical sense, many do: they have a small anus, though it is not a buttocks-like structure. The accurate answer depends on the group. Some sea stars have an anus on the upper central disc; others, along with brittle stars, lack one and use the mouth to vent waste.
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Best Value
- 【Soft Material】 Made of high quality short plush, filled in PP cotton ,soft and comfortable.
- 【Size】Approx 7.87*7.87 inches.
- 【Exquisite Design】AB design. Imitate the bulge of starfish surface with foil printing in side A. Solid colour plush in side B.
- 【Lovely Companion】Plam size,easy to carry on.perfect decoration and play toy for coastal beach house ,home ,car, bag,office.Also a good choice for pet toy.
- 【Idea Gift】 Small figure toy, first choice for birthday/back to school/thanksgiving/Christmas/Valentine's day gift.
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