Most fish fertilize their eggs externally. During spawning, the female releases eggs and the male releases sperm (milt) into the water, where sperm and eggs meet. Internal fertilization is also normal in all sharks and rays and in some bony fish, including guppies and mosquitofish.
Where fertilization occurs is not the same as where the embryo develops. An internally fertilized fish may lay eggs, retain them until they hatch, or give birth to live young.
How external fertilization works in most fish
In external fertilization, sperm and egg unite outside the female’s body, usually in water. A spawning female releases eggs while one or more males release milt over them. Fertilization can happen within seconds, but success depends on timing, water movement, sperm concentration and whether predators or debris interfere.
Salmon and many freshwater fish use this pattern. Species that shed both gametes into the water are often called broadcast spawners. A female may release many eggs because a large proportion can be eaten, swept away, fail to develop or never encounter viable sperm.
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What happens after spawning
External fertilization does not determine the level of parental care. Depending on the species, adults may leave the eggs, guard a nest, fan the eggs to improve oxygen flow, or attach them to plants, rocks or another substrate. Burbot, for example, is documented as an external broadcast spawner.
How internal fertilization works
With internal fertilization, sperm is transferred into the female and unites with the egg inside her reproductive tract. All sharks and rays use internal fertilization. Some bony fish do as well, including guppies and mosquitofish.
Structures that transfer sperm
- Gonopodium: Male mosquitofish and related live-bearing bony fish use a modified anal fin to deliver sperm.
- Claspers: Male sharks and rays use paired appendages at the pelvic fins to transfer sperm into the female.
Internal fertilization can protect sperm and early embryos from dilution and some external hazards, but it is not automatically “better.” Each reproductive system reflects the species’ habitat, anatomy, energy budget and survival strategy.
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Fertilization and embryo development are separate questions
Ask two different questions: Where does sperm meet the egg? and Where does the embryo develop? The combinations below occur in real fish species.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minute| Reproductive pattern | Where sperm meets egg | What happens to the fertilized egg or embryo | Main food source |
|---|---|---|---|
| External fertilization with egg-laying | Outside the female, usually in water | Eggs are laid and develop outside the parent | Yolk in the egg |
| Internal fertilization with egg-laying | Inside the female | Fertilized eggs are later laid; embryos develop in those eggs | Primarily yolk |
| Ovoviviparity (often called aplacental viviparity) | Inside the female | Fertilized eggs are retained; embryos hatch inside or shortly before release | Mostly yolk |
| Viviparity | Inside the female | Embryos remain inside and young are born live | Yolk plus some maternal nutrition, varying by species |
Oviparous fish
Oviparous fish lay eggs outside the body. The eggs may have been fertilized externally before release or internally and then laid later. In either case, the embryo develops in the egg and uses its yolk reserve.
Ovoviviparous fish
In ovoviviparity, the female retains fertilized eggs. Embryos are nourished mainly by yolk rather than through a placenta, and they hatch inside the parent or just before the young are released. Some sources group this condition under aplacental viviparity, so the underlying nutrition and retention pattern matters more than the label.
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Viviparous fish
Viviparous fish retain embryos and provide additional maternal nutrition to some degree. The young are born live. Sharks show several forms of internal development: some species lay eggs, while others retain embryos and give birth.
What is inside a fish egg?
Yolk is the basic food reserve for a fish embryo. After fertilization, the developing embryo uses yolk for energy and building materials until it can feed independently. In some internally developing sharks, embryos also receive nutrition from maternal tissues or, in certain species, other eggs. Thus, “inside the mother” does not always mean the embryo is fed in the same way as a mammal fetus.
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Yes. A female can release eggs that never become fertilized if no compatible sperm reaches them, the timing is wrong, the sperm is not viable, or the species requires a particular spawning cue. An unfertilized egg may still look like a normal egg at first, but it does not begin normal embryonic development and usually deteriorates.
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In aquarium settings, eggs can therefore be present without fry appearing. Removing dead, opaque or fungus-covered eggs may reduce spread to nearby viable eggs, but the correct practice depends on the species and tank conditions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Do fish fertilize eggs after they are laid?
In species with external fertilization, sperm is commonly released over eggs as they are being released or immediately afterward, so the eggs are fertilized in the surrounding water rather than later by a separate delayed process. Once an egg’s outer layer hardens and the fertilization window closes, sperm generally cannot fertilize it.
In internally fertilizing species, sperm reaches the egg before the egg is laid or before the embryo is released. A rare species may release eggs soon after internal insemination, so the timing is not an absolute rule for every fish; the species’ reproductive anatomy determines the sequence.
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External versus internal fertilization: the practical trade-offs
| Comparison | External fertilization | Internal fertilization |
|---|---|---|
| Where sperm meets egg | Outside the female, generally in water | Inside the female’s reproductive tract |
| Typical egg or embryo location | Eggs usually develop outside the body | Eggs may be laid, retained until hatching, or developed to live birth |
| Embryo food | Usually yolk in an external egg | Yolk, with additional maternal nutrition in some species |
| Specialized structures | Often no intromittent organ is needed for spawning | Examples include gonopodia in mosquitofish and claspers in sharks and rays |
| Common ecological exposure | Eggs and sperm face dilution, currents, predators and changing water conditions | Sperm and early embryos receive more physical protection inside the female |
| Typical reproductive output | Often many eggs, with survival spread across a large number of offspring | Often fewer, more developed young, although species vary widely |
| Parental care | May be absent, or involve nest defense, fanning or egg attachment | Protection is provided during retention; care after birth or release varies |
These are comparison axes, not a ladder from “primitive” to “advanced.” External spawning can be highly successful when a habitat supports large numbers of eggs, while internal development can be advantageous when protecting a smaller number of embryos is more effective.
Examples you can recognize
Salmon and many freshwater fish
The female deposits eggs and the male releases milt over them. Fertilization takes place in the water, and the eggs then develop outside the parents.
Guppies and mosquitofish
These bony fish use internal fertilization. Females retain developing young and commonly give birth to live offspring, illustrating why fertilization location and birth mode must be described separately.
Sharks and rays
All sharks and rays fertilize internally using claspers. Some species are egg-layers, whereas others retain embryos and release live young. Their different development modes occur despite the shared internal fertilization step.
Quick Recap
Key takeaways
- External fertilization is the usual pattern in fish: eggs and sperm meet in water during spawning.
- Internal fertilization is used by all sharks and rays and by some bony fish such as guppies and mosquitofish.
- An internally fertilized fish may lay eggs, retain yolk-fed embryos, or provide additional maternal nutrition and give birth live.
- Yolk feeds most fish embryos; some species add maternal or egg-derived nutrition.
- Unfertilized eggs can be released, and externally fertilized eggs are fertilized during spawning rather than at an arbitrary later time.
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