The fathead minnow, Pimephales promelas, is commonly sold as a baitfish and, in the aquarium trade, as a feeder fish. They are cyprinids that spend their time dashing frantically about along the bottom of the tank or trying to burrow through the glass in a corner of the tank. Typical brainless cyprinid behaviour. However, there is one distinctive difference from the usual cliche of cyprinids that aquarists expect. These minnows are not egg-scatterers as are most cyprinids. They are parental caregivers who guard the nest cichlid-style. I found their spawning behaviour quite fascinating. They are certainly cheap to buy, and easy enough to breed that you can pick up a few BAP points while observing some interesting behaviour.
Pimephales promelas is widely distributed across North America. It is tolerant of a wide variety of conditions in water chemistry. It begins spawning at about 18C water temperature, so it does not need a heated aquarium. Spawning is done frequently in stages, rather than an annual or occasional occurrence. This species is omnivorous, and thus easy to feed. It is short-lived, not usually beyond two years of age . There are two varieties, the normal and the rosy-red (which is more of an orange-yellow). The latter has a lower growth rate than the normal .
This species is timid in the aquarium, and prefers to school. The fry that I have raised tend to travel in midwater, while adults stay closer to the bottom. Schooling is an anti-predator tactic used by many fish . A predator attacking a school of fish has a difficult time picking out one individual from all the others and tracking it in the confusion. More fish together also means more eyes to keep watch.
The spawning of Pimephales promelas is fairly complex. The first sign is when the male develops a wen on his forehead. If you don’t know what a wen is, you’ve seen them on fancy goldfish, which are also cyprinids. These are what look like tumorous growths on the head, often with little tufts of white cotton during their development. Hood growths on goldfish such as orandas are merely an artificial selection for increased size of wens. Tubercles or hard white pimples develop on the forehead and gill covers, a sign that the male is ready to breed.
The male fathead minnow stakes out an overhanging ledge or cave and begins defending this territory against all comers. Once a female has been lured in and courted, the eggs are laid on the underside of the ledge in a monolayer. The male guards the eggs until they hatch. He may entice several females into spawning, in which case the final hatch will be a genetic mixture of half-siblings . Fully-conditioned females can lay 200 to 700 eggs. The eggs hatch in 4 to 8 days, depending on water temperature .
As far as the practical details are concerned for the aquarist wanting to set up a tank to breed these fish, an overhanging slab of rock makes the best location for spawning. It is better if it is not more than about one centimetre over the substrate, just a big enough gap for the minnows to squeeze under. This gives room for the fish and allows them to rest on the substrate will standing guard. Since Pimephales promelas is a bottom-dweller, there is no point in making a high stack of caves as you might for cichlids, since the minnows will not venture up into the water column. This species can be spawned in caves made of PVC pipe sections , but while practical, is not very aesthetic.
SPAWNING IN THE AQUARIUM
I seldom paid much attention to the fathead minnows I used as feeders for my cichlids. However, one day I noticed a few small fry darting about in the minnow holding tank, so I decided to spawn them in a more formal setup. I set up a 45-litre tank with incandescent lights and an airstone but no heater. The airstone should provide turnover of the water column but at not too high a rate, as turbulence will interfere with feeding by the fry . The tank was on the main floor of my house (in the kitchen, actually, right by the sink) which, like most post-WW2 houses of its age, overheats in the summer and gets chilly in the winter. The water temperature varied with room temperature and ranged from 20oC to about 28oC. The latter temperature was reached after the fry were a couple of months old, by which time the summer heat had begun.
Several rocks were placed in the tank in such a fashion that they created overhanging ledges. The bottom third of the tank was filled with filamentous algae and Java moss. The airstone was set to provide a strong flow of water. There was no filtration but a 25% weekly water change was done using fresh tap water. The female was placed into the aquarium a few days ahead of the male. Because they had previously spawned, I knew they were a pair, but in any event it was easy to separate them. The female was plumper than the male. The male, when added to the tank with the female, developed a clear wen on his head.
The first spawning, as with subsequent spawns, took place on the underside of an overhanging ledge. The female laid the eggs but then immediately turned around and ate them. A week later, another batch of eggs was laid. The male guarded these eggs, swimming in a figure-eight pattern continuously under them. The female showed no interest in the eggs, so I removed her. The day after the eggs were laid, the eyes were visible. On the fourth day the eggs were gone. I was uncertain as to whether the male had eaten them or if the fry were simply well hidden. As it turned out, the fry were seen the next day, having apparently dispersed after hatching into the thick profusion of algae. The male was removed at this time.
The fry were transparent and 5 to 6 mm in length. One fry was a goer and grew quickly to about 1 cm total length, while the rest stayed in the same growth cohort. For most of the first three months the fry were exposed to continuous light, the canopy seldom being turned off. The algae and its associated infusoria provided food for the fry. When they were a little older, crushed and finely-powdered flake food was supplied twice a day. After three months, the orange colour had developed. the fry were slow growing and were averaging about 1 cm in total length at this time. They were somewhat clumsy at catching food, although this could be a matter of taste discrimination. Minnows generally eat food particles about two-thirds of their mouth size and go after moving targets . In still water, they pick out the active food particles, such as live food or sinking flakes.
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