Reproduction
Fish have evolved many spawning techniques designed to give them an advantage over their competing neighbours. Some common strategies are to drop 100s of eggs, (but exhibit no parental care), or only have a few (and guard them as long as possible), or have them often (monthly) and delivered alive (livebearers). They stick the eggs to leaves, bury them in the mud, hide them in their mouths, hide them inside freshwater mussels or disguise them with the eggs of other fish.
At some point, something in your aquarium has babies, and you start by identifying who the parents could be and researching what you can do to keep the fry alive. One of the many reasons for keeping fish is the challenge of getting particular fish to spawn. There are still fish today which have never been spawned in an aquarium, and new fish are still being discovered in the wild everyday, some at a rate where they have stopped giving them names and simply number them (http://www.planetcatfish.com L series).
There are many ways of categorizing fish in regards to breeding. Livebearers or egglayers, bubble-nest builders, mouthbrooders, by spawning sites, water requirements, parental care, secondary sex characteristics, courting ritual, prespawning activity etc. These topics can span 100s of pages for only one species of fish, and http://www.fishbase.org currently has 28,400 species listed.
The simplest method I could think of was to categorization them loosely by parental care into 4 main sections: Livebearers, Characidae+ (ie: Tetras, Corys, Barbs etc), Anabantidae (ie: Gouramis) and then Cichlids. I further categorized the Cichlids into substrate spawners, mouthbrooders, and others (shell & leaf spawners). For each I'll list a few unique peculiarities, referencing the common tropical fish we have available to us everyday.
Details...
While all fish are born either male or female, the primary sex differences are usually only revealed by dissection (ovaries and testes), so for the purposes of discussion, the secondary sex differences are always external differences, in appearance (size, colouration, fin length etc) or behaviour. Juvenile fish typically have no sex differences discernable.
Topics covered are livebearers (ie:Guppy), characins+ (ie:Tetras and other egg scatterers), anabitidae (ie:Gouramis), substrate spawning cichlids (ie:Firemouths), mouthbrooding cichlids (ie:Peacocks) and other cichlids (ie:Shellies, Angels, Discus..). As always, the information on this site is intended as an introduction to the topic.
LIVEBEARERS
Includes the Guppy, Molly, Platy and Swordtail (the last two are capable of cross-breeding). Mature males have a long thin anal fin, called a gonopodium, which is used to direct the sperm packet at the female's oviduct, and then the sperm travel up to the ripening eggs. There the sperm might be mixed with that of other males, and the female can store sperm for subsequent live birth deliveries. Deliveries typically occur every 28 days, with stored sperm lasting approximately 3 to 4 months. The quantity and size of the newborns vary by the size of the female and species (in respective order of decreasing fry size: Molly, Swordtail, Platy, Guppy). Typical quantity is 10 to 40 babies. Females can take several hours to a day to drop the entire delivery. No parental care is shown. Babies will immediately take to shelter until making an appearance later on. The probability of the amount of fry which will be eaten by the parents (and other fish) depends on relative size of adults to fry, amount & proximity of shelter available, fish density and appetite. Babies are born relatively large, and are mobile & eating very quickly.
Other external sex characteristics are the sword tail on the male Swordtail, larger dorsal fin on the male Molly, and Guppy females are larger, male Guppies are more colourful and have longer flowing fins.
CHARACIDAE+ (ie: Tetras, Corys, Plecos, Danios, Sharks, Piranha, Barbs, Loaches, Goldfish etc)
Typically, egg scatterers or they adhere the eggs to a surface. They either exhibit no interest in the eggs after the spawning, or will hunt them down as food. They are usually prolific, and many eggs escape un-noticed. Newborn fry are typically small, and more difficult than usual for the average hobbyist to keep alive the first few critical days. Because of the immense quantity of fish in this category, many exception exist, but generally, these fish have no or poor sexual differences (sometimes the fullness of a female carrying eggs is the only difference).
ANABITIDAE (Gouramis, Paradise fish, Betta)
Males are typically larger and/or more colourful and/or have longer fins. A bubble nest is build and guarded by the males, who entices a female over. If the courtship proceeds favourably, the male wraps his body around the female squeezing out the eggs. With some species, the eggs are heavier than water and the male begins retrieving them to carry up to the bubblenest (ie: Betta). With other species, the eggs float (ie: Pearl gourami). The eggs of Kissing gouramis float, they do not build bubblenests, and generally ignore the fry. Chocolate gouramis also do not build bubblenests, and they are mouthbrooders. With typical male guarded bubblenesters, the female should be removed, as he usually has very little tolerance for any other fish in the vicinity. Once the fry are free-swimming, the male should be removed.
Anabitidae have a supplementary breathing apparatus known as a labyrinth organ which allows them to live in oxygen poor waters (essentially, to a limited extent, they breath air). The development of the labyrinth organ in the developing fry requires the air above the water (under the aquarium canopy) to be warm & humid.
CICHLIDS - SUBSTRATE SPAWNERS (ie: Brichardi, Kribensis, Oscar, Chromides, Texas cichlid etc)
The cichlids mentioned illustrate their world-wide distribution, respectively African Rift lakes, African riverine, South American riverine, Asian, and Central and North America are represented with only 5 fish. Substrate spawners first go through an elaborate courtship which establishes interest and compatibility (usually by mouth wrestling to test for weakness in strength or in character). If this is completed satisfactorily, then site selection begins. Sometimes the male has already selected and begun preparing the site, and sometimes it's acceptable to the female. In some cases the female drives site selection. Usually both participate in some aspect of site preparation. Site is typically any flat surface at or near the bottom of the aquarium, from the sides of a clay pot, to slate or even the glass bottom of an aquarium.
The female will lay strings of eggs, followed by the male who fertilizes them. The process is continued until 50 to 500 eggs are typically laid. After the eggs are deposited the parents will alternate in duties which include perimeter protection, fanning the eggs (using their fins to push fresh oxygenated water over the eggs), removing dead eggs to prevent fungus from attacking viable eggs (they take them in their mouth, dead eggs explode and are ejected out their gills, while good eggs are deposited back with the others). If there is a threat to the spawning perimeter (typically an area of 1-1/2 to 2 cu.ft. with the eggs in the centre), both parents will work in unison to scare off the intruder, either by posturing, or by repeated concerted attacks. It takes a very determined fish to be able to fight off both parents and get to the eggs, and the beating received would probably make it not worth their while.
Fry typically hatch in 3 days, and are transferred to a depression. They are completely helpless and will slowly absorb their yolksacs for about 3 days. During this period, the parents will maintain a perimeter guard and periodically move them into different locations. When intruders approach, the male may fake the babies to be elsewhere, while they are really at another location with the female. Sometimes the fry will be in 2 groups, with each parent guarding a group, but they are typically kept in a single location.
Once free-swimming, the fry will need to locate a source of food. Powdered fry food, boiled egg yolk and baby brine shrimp (frozen or hatched) are commonly used for this. Fry are relatively large at this point (smaller than livebearer fry but larger than characidae fry). Parents continue in their parental duties, if anything, becoming more frantic about guarding the perimeter (which widens around the newly swimming fry). Fry are kept into a tight schooling formation by vibrations sent by the parents. Tanks with excessive turbulence and 'noise' will dampen or nullify this effect. With filter turbulence decreased, the school of fry is put through many tight formations.
Approaching the tank at this stage will cause the parents to typically send the fry to cover (dropping flat against the gravel, so they appear to be a mass of jiggling jello). Dropping
some fry food on the surface will immediately be challenged and investigated by one of the parents (usually the larger male). Upon his approval, he will signal the female to bring the brood up to eat. Together they will cause the fry to move upwards in a narrow funnel which widens just below the water's surface.
The extent of the parental care provided, and the duration varies by species and by individuals. Brichardi raise and tolerate their fry almost indefinitely, but the growing fry are tasked with helping protect the next brood of fry, and so on. Most other cichlids will show varying amounts of tolerance, but in the close confines of an aquarium, the fry represent competition, and this is not usually tolerated indefinitely.
External sex characteristics vary from obvious (Kribensis), subtle (Brichardi) to near impossible (Oscars). During egglaying, the female's ovipositor (tube which delivers the eggs) is usually shorter and thicker than the males. Venting is sometimes a more reliable method to sex these fish (a process of examining their underside for spacing and size of openings), but they rarely cooperate or appreciate the process.
CICHLIDS - MOUTHBROODERS (ie: Frontosa, Rustys, Auratus, Zebras, some Geophagus etc)
Also having a world-wide distribution, mouthbrooders vary a bit in strategy. Generally, after the eggs have been deposited and fertilized, the female (usually) scoops them up. Egg spots are a sometimes predominant feature on male mouthbrooders. The female will mouth that area thinking they are eggs and more milt will be mouthed to help fertilize more of the eggs. Mouthbrooders sometimes share the activity, but more commonly, the duty falls to the female who fasts while holding eggs. After the eggs hatch, the female may periodically release the swimming fry in a safe location, to either get some food, or only to begin acclimating the fry to the dangers of the environment. Eventually (around 3 weeks), she will release them and refuse to allow them back into her mouth. The fry are on their own from this point. Mouthbrooders typically do not provide further parental care.
External sex differences can be egg spots, and a few other subtle features. Many of the mouthbrooders have very distinct pecking order where only the alpha male wears the 'colors'. With these, identification of the alpha male is easy, and the females become the ones holding eggs, and the sub-dominant males are being tormented. In the highly aggressive Rift Lake hierarchy, uncooperative females will also be tormented, possibly to death. With both substrate spawners and mouthbrooders, failed courtships may lead to the death of the weakest fish, due to the close confines of an aquarium. In nature, the weak are chased away to make room for potentially stronger mates. In an aquarium, escape is not possible, and an aggressor staying true to instincts might not stop harassing the weaker fish until it died. This effect depends on the tank size relative to the fish size, the particular species, and the individual personality.
CICHLIDS - OTHERS (ie: Discus, Angelfish, Shellies etc)
Shell dwellers are cichlids from Lake Tanganyika. Their solution to their highly competitive and dangerous environment is to utilize the shells of large snails to hide their newborn fry in. Discus and Angelfish prepare a vertical surface (leaf, slate, heater tube, filter intake etc) as their spawning site. Like substrate spawners, they meticulously prepare the site by cleaning it. Eggs are laid in long strings and fertilized. Newly hatched fry are transferred to a different surface instead of to a pit dug into the gravel. Free swimming fry stay in a cloud around one or both parents. Angelfish parents can be removed from the spawning tank as soon as the eggs have all been deposited. Their function is replaced by an airstone under the eggs and meth.blue in the water to prevent fungus. Discus parents cannot be removed as their fry initially feed of off the parent's slime coat. Both Angelfish and Discus will vary in their ability to bring fry to free-swimming independence. Typically they do something wrong almost every time, (one eats the eggs, or they eat the newly hatched fry, or they eat the newly swimming fry etc) but progressing a bit farther with each attempt.
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