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Fish show a dazzling amount of variation in their reproductive biology.
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Aquarium fish mate, spawn and reproduce in a huge variety of ways. However, despite the vast range of specifics, fishes methods of reproduction fall into several broad categories. For example, the actual act of mating can be grouped into internal or external fertilization. Placement and care of the eggs breaks down into several further categories. Mating and spawning are a major part of a fish's life history, and can help you understand much about an aquarium fish.


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Types of Fertilization

Sharks and rays use internal fertilization, even those who lay eggs.
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All fish fit into one of two categories: those that practice internal fertilization and those that practice external fertilization. Mammals, like humans, use internal fertilization. Several vastly different groups of fishes mate this way, including guppies and sharks. Both egg-laying and live-bearing fishes can use internal fertilization. However, internal fertilization is not the norm for fish, and the majority of species use external fertilization. In this mating system, eggs and sperm are released separately and are fertilized outside of the body.



Guppies give live birth, frequently.
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In the aquarium hobby, fish who give live birth are called live-bearers. The ubiquitous guppy falls into this group. Live-bearers exclusively use internal fertilization. In these fish, modified fins are used to transfer sperm from the male to the female. These specialized fins have different names, including andropodium, gonopodium or claspers depending on the species. Guppies and their relatives have a trait called superfoetation, which allows them to produce multiple batches of offspring from a single mating. In most live-bearers, females store the eggs, providing them with protection but not additional nutrition. However, in goodeids, distant relatives of the guppy, the mother provides nutrition to her developing embryo through a placenta-like organ. Common live-bearers such as guppies can often spawn quickly and easily in the home aquarium.


Broadcast Spawners

Many coral reef organisms reproduce through broadcast spawning.
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Both fish and invertebrates may spawn through broadcast spawning. In this form of external fertilization, sperm and eggs are both released around the same time in approximately the same area and fertilization is left to chance. Broadcast spawning fish provide no parental care. The young must fend for themselves among the currents, eventually developing into adults. This helps scatter individuals over vast distances, helping the species survive.


Egg Scatterers

Goldfish breed through egg scattering.
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Egg scattering falls into the category of external fertilization. Males and females chase each other around and release sperm and eggs at the same time. This scatters fertilized eggs over gravel or plants. Most fish who spawn by this method have no parental care, and may eat their eggs. To combat this, aquarium hobbyists often use dense thickets of aquarium plants, spawning mops or a layer of marbles to help keep some of the eggs safe from their parents. This form of breeding is seen widely among the minnow family and similar families of fish including the characins/tetras.


Mouth-breeding Fish

Saltwater jawfish are mouth-brooders.
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In mouth-brooders, fish spawn and raise their young using their mouth. Eggs are released and fertilized, usually externally. However, what makes mouth-brooders distinct is that one of the parents keeps the fertilized eggs and eventually the young fish in his mouth to protect them during development. Either the father or the mother may hold the eggs depending on the species. This behavior is seen among some species of bettas, cichlids as well as other groups of fish. In bettas, mouth-brooding is most common in river-dwelling species than bettas who reproduce in still waters such as ponds or rice paddies.


Fish Who Nest

The common betta (B. splendens) builds bubble nests.
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Many fish use various structures to spawn, including structures made by the fish themselves. Some spawn on substrate or other existing structures such as caves or even snail shells. Cave-spawning and substrate-spawning are common in the cichlid family. Some fish take this a step further and create relatively elaborate nests. For example, some species of betta make nests out of bubbles and bits of plants. Nest- and structure-using fishes often exhibit various degrees of parental care. This type of spawning can come from both internal and external fertilization, depending on the species.