The reproductive glands in a starfish are called gonads. Most adult sea stars have a pair of gonads inside each arm, so a typical five-armed starfish has about 10 gonads. Gonads produce eggs or sperm, which usually leave through gonopores for external fertilization in seawater.
Starfish gonads are internal organs associated with the arms, not visible external reproductive organs. The exact anatomy and reproductive behavior vary among sea-star species, so “typically” and “usually” are important when describing the group as a whole.
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Key takeaways
- A starfish’s reproductive glands are called gonads, and the gonads produce eggs or sperm.
- Most adult sea stars have a pair of gonads in each arm; a typical five-armed starfish therefore has about 10 gonads.
- NOAA’s 2022 status review explains that a five-armed starfish has 10 gonads when each arm contains a pair.
- Gonads are internal and should not be confused with the digestive pyloric caeca that also extend into the arms.
- Eggs and sperm usually leave through gonopores near the bases of the arms on the dorsal, or aboral, surface.
- Fertilization commonly occurs externally in seawater, although some sea-star species brood embryos or have unusual larval development.
What are the reproductive glands in a starfish called?
The reproductive glands in a starfish are called gonads. Gonads are the internal organs that produce gametes: eggs in females and sperm in males. “Reproductive glands” is the plain-language description, while gonads is the biological term used for these organs in sea stars.
Starfish are more accurately called sea stars by many marine biologists, but “starfish” remains a common name. The general anatomy described here applies to typical asteroid sea stars, while the exact arrangement can vary among species.
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Where are the gonads in a starfish?
Starfish gonads lie inside the arms, which are also called rays. In most asteroid species, each adult arm contains a pair of elongated gonads. The gonads may be branched or sac-like, and they can enlarge substantially while eggs or sperm are being produced.
In a five-armed starfish, two gonads per arm usually means approximately 10 gonads altogether. NOAA’s Endangered Species Act status review, published in 2022, gives the same anatomical calculation: a five-armed sea star generally has 10 gonads because each arm contains a pair.
The gonads are not normally visible as external organs. Their openings, called gonopores, are generally found on the dorsal or aboral surface near the bases of the arms. “Aboral” means the side opposite the mouth.
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Does each starfish arm have reproductive organs?
Usually, yes: each arm of a typical adult sea star contains a pair of gonads. That arrangement means the reproductive system is distributed through the rays rather than concentrated in one central body cavity.
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The pattern is typical rather than universal. Sea-star anatomy differs among species, so a diagram or specimen of one species should not be treated as an exact template for every starfish. The number of arms also varies; a sea star with more or fewer than five arms will not necessarily have the same total number of gonads as the familiar five-armed example.
How are gonads different from pyloric caeca?
Gonads and pyloric caeca are different organ systems even though both extend into a starfish’s arms. Gonads produce eggs or sperm, whereas pyloric caeca are digestive organs associated with processing and absorbing nutrients.
| Structure | Where it is found | Main function | Reproductive role |
|---|---|---|---|
| Gonads | Usually paired inside each arm | Produce eggs or sperm | Directly involved in reproduction |
| Pyloric caeca | Branches extending into the arms | Digestion and nutrient absorption | None |
| Gonopores | Usually near the arm bases on the dorsal or aboral surface | Release eggs or sperm | Provide the exit route for gametes |
Because the pyloric caeca and gonads can both appear in an anatomical model or dissection, confusing the two structures is an easy mistake. Their functions, however, are entirely different.
How do starfish release eggs and sperm?
Starfish release eggs and sperm through gonopores, which are small openings associated with the gonads. The gonopores are generally near the bases of the arms on the dorsal or aboral side, although their exact appearance and position differ among species.
Most sea stars have separate sexes. An individual is generally male or female, but males and females usually look alike from the outside. The gonads produce sperm or eggs, and spawning releases those gametes into the surrounding seawater.
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In the common broadcast-spawning pattern, sperm and eggs meet in seawater and fertilization occurs externally. The Alaska Department of Fish and Game’s Starfish wildlife notebook describes this general reproductive pattern for sea stars.
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No. External fertilization is common, but reproduction is not identical in every sea-star species. Some species brood embryos, and some have nonstandard larval development rather than following the usual broadcast-spawning pattern.
The safest general statement is that many starfish release eggs and sperm into seawater and commonly reproduce through external fertilization. The NIH-hosted review on using sea stars to study reproductive processes discusses sea-star spawning and development in a research context, while also illustrating why reproductive details should be qualified by species.
| Reproductive feature | Common pattern | Important qualification |
|---|---|---|
| Sex organization | Separate male and female individuals | Males and females are usually not distinguishable externally. |
| Gametes | Gonads produce sperm or eggs | The number and size of gonads can vary with species and reproductive condition. |
| Release route | Gametes leave through gonopores near the arm bases | The precise position and appearance of gonopores are not identical in every species. |
| Fertilization | Usually external in seawater | Some species brood embryos or have atypical development. |
| Larval development | Often involves free-living marine larvae | Some sea stars produce nonfeeding larvae, brood offspring, or follow other developmental patterns. |
How many eggs can a starfish produce?
The number of eggs depends on the species, the female’s size, reproductive condition, and the surrounding conditions. A 2010 NIH-hosted reproductive-protocol review notes that one female sea star can provide millions of eggs for laboratory reproductive studies. That observation should not be treated as a fixed number for every female starfish or every spawning event.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How can you study starfish reproductive anatomy?
A labeled model is usually the simplest way to see how the gonads sit within the arms without interpreting the anatomy of a preserved specimen. A supervised starfish dissection kit can provide a more direct classroom or laboratory exercise, but preserved-specimen kits are educational materials intended for structured instruction rather than casual home use.
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Educational catalogs describe sea-star dissection materials and models that can display reproductive, digestive, and water-vascular structures. A reusable model is generally better for repeated demonstrations; a dissection kit is more appropriate when a teacher or laboratory instructor is guiding an anatomy investigation. For broader context on echinoderm body plans, Invertebrate Zoology: A Tree of Life Approach provides a wider zoological reference.
Any examination of a preserved animal should follow the supplier’s handling guidance and local school or laboratory safety rules. A live sea star should not be collected or dissected for casual identification.
Frequently Asked Questions
What are the reproductive organs of a starfish called?
A starfish’s reproductive glands are called gonads. Most adult sea stars have a pair of gonads in each arm, so a typical five-armed starfish has about 10 gonads. The gonads produce eggs or sperm depending on the individual’s sex.
Where are the gonads in a starfish?
Starfish gonads are internal organs located inside the arms, or rays. Their gametes usually leave through gonopores near the bases of the arms on the dorsal, or aboral, surface.
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How do starfish release eggs and sperm?
Starfish usually reproduce by releasing eggs and sperm into seawater, where fertilization occurs externally. Some species instead brood embryos or have unusual larval development, so the broadcast-spawning pattern is not universal.
The Bottom Line
Starfish reproductive glands are gonads. A typical five-armed sea star has a pair of gonads in each arm—about 10 gonads total—and releases eggs or sperm through gonopores near the arm bases. Fertilization usually occurs externally in seawater, although reproductive details vary among species.
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