As an avid aquarist and fish enthusiast, I’m often asked how fish reproduce. The short answer is that most fish lay eggs while some give live birth. But the details of fish reproduction are remarkably diverse and fascinating! In this article we’ll dive deep into the reproductive habits of our underwater friends.
Eggs Galore: Oviparous Fish
The vast majority of fish species – over 97% – are oviparous, meaning they lay eggs that hatch outside the mother’s body. Just like chicken eggs that we eat for breakfast, fish eggs contain all the necessary nutrients for the embryo to develop and hatch.
Fish eggs come in a stunning array of shapes, sizes, and colors. Some species lay individual, separate eggs, while others lay eggs in connected strings or gelatinous clusters. The eggs can be demersal, sinking to the bottom, or buoyant, floating at the surface. Pelagic fish tend to have buoyant eggs that drift with ocean currents, while freshwater species often lay demersal eggs that stick to aquatic plants or rocks.
Most fish scatter their eggs and show no further care. But some species display remarkably sophisticated parental care behaviors. Cichlids and catfish, for example, hold the eggs in their mouth for protection. Male sticklebacks build underwater nests with plant bits and shelter the eggs until they hatch. Parental care boosts the odds of survival for at least some of the hatchlings.
Since most fish lay huge numbers of eggs – some species lay millions at a time! – even a low hatching success rate can perpetuate the species. The fertilization process is also a numbers game. Female fish release their eggs into the water, then male fish spray the eggs with sperm, or milt, to achieve fertilization. With so many eggs and sperm flooding the waters, enough embryos are fertilized to maintain the fish populations.
Live-Bearing Fish: Viviparous Wonders
While oviparous fish dominate the oceans, lakes, and rivers, nearly 150 species across 40 families give birth to live young. These viviparous fish have internal fertilization and the embryos develop inside the female’s body. She then gives birth to fully formed, miniature versions of herself that rapidly start swimming and feeding.
Live-bearing is common in sharks, rays, and commercially valuable fish like snook and grouper. It’s also prevalent in smaller species that live in the darkness of caves, deep waters, or dense vegetation. Since their offspring don’t have a pelagic egg stage, the young are less vulnerable to predators. Plus, giving birth to larger, well-developed fry improves their odds of survival.
Live-bearing fish fall into three categories
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Ovoviviparous: The embryo develops inside the egg while inside the female, who gives birth to larvae or juvenile fish.
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Viviparous: The embryo develops inside the female’s body, nourished by the yolk sac. She gives birth to advanced fry.
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Matrotrophic: The embryo derives nutrients from the female’s body, not just the yolk. She gives birth to an essentially fully developed fish.
From sharks to guppies, seahorses to swordtails, the reproductive diversity of fish never ceases to amaze. Next time you see a pregnant fish in your tank or at the aquarium, take a moment to appreciate the incredible biology that allows fishes to produce the next generation!
Frequency of Entities
Eggs: 14
Live-bearing/Viviparous: 6
Oviparous: 3
Fertilization: 3
Milt/Sperm: 2
Embryo: 2
Larvae/Fry: 3
Maternal Care: 2
Pelagic: 1
Demersal: 1
Freshwater: 1
Saltwater/Ocean: 2
Sharks: 2
Rays: 2
This is How These 15 Fish Lay Eggs
Do fish lay eggs on land?
However, there is a group of over 2800 species of fish that do not lay their eggs on land but release them into the water. Female fish release their eggs and wrap them in layers of gelatinous material for protection. The term for this reproductive process is called spawning. How Does Spawning Work?
How can one know that their ovaries are producing eggs?
The signs that suggest ovaries are producing eggs and there is ovulation is by checking the basal body temperature. There is a surge in the body temperature, just before ovulation. Other methods are by checking the pattern of cervical mucus. The cervical mucus will be very wet, slippery, extremely stretched, and raw egg white and consistent when the ovaries are producing eggs. Another method is by ovulation prediction kit which detects the LH hormone surge during ovulation.
How do fish lay eggs?
Egg-depositors lay larger eggs that stick to substrate, wood, rocks, or the tank base. Other species are egg buriers, which means females will bury the eggs in the substrate on the bottom of the tank. The male must dive into the substrate to fertilize the eggs. A few species of fish do not lay eggs but produce live babies, called fry.
Which fish lays eggs?
There are numerous examples of fish that fall into the category of egg-laying species. One well-known example is the salmon. Salmon are anadromous fish, meaning they migrate from the ocean to freshwater rivers to spawn. Female salmon create nests, called redds, by digging depressions in the riverbed and depositing their eggs.