GOING TO SCHOOL
Cyprinids are great ones for schooling; a survival mechanism that evolved as an anti-predator measure (many eyes watching are better than just two), and as a food-finding mechanism (better a small share in the crowd frequently than the occasional big bite by yourself). It is known that cyprinids can recognize each other as individuals, and prefer to school with familiar friends rather than strangers, just as humans prefer to hang out with the old gang. The bigger a school is, the better it is for predator defense and food hunting. A recent study on the minnow Phoxinus phoxinus showed that given a choice between a shoal of friends or a bigger one of strangers, they prefer their friends . However, this preference decreases as the relative size of the school of strangers increases. When the strangers are about twice the size of the friends group, an individual minnow presented with a choice between the two will go with the strangers.
So from a practical point of view for the aquarist, what does all this business about schooling mean? Basically, it suggests that cyprinids are happier in groups rather than solitary fish in a tank. The bigger the group, the better, and probably the more successful you would be at spawning them. This is not to say that you would not be able to spawn a solitary pair, but if you are having problems, it is wise to consider how the fish behave in natural conditions.
Cyprinids are understandably timid fish, as in habitat they are usually more eaten than eater. The schooling mechanism of social behaviour is a common defensive mechanism against predators, since more fish mean more eyes watching more angles of attack.
A group of chemicals known as alarm pheromones are used to alert nearby fish about risks. There is a problem of false alarms, since there are numerous possible stimuli in habitat which might provoke false alarms. Cyprinids therefore not only have to keep an eye out for predators but to decide if an object is worth getting excited about. There are two types of chemicals needed to react to predators. Firstly, the predator leaves an odour in the water, and secondly some experienced fish has to secrete alarm pheromone when it detects the odour. Cyprinids can quickly learn to associate the two chemicals, such that when they smell the predator in the water they will automatically go on full alert .
There is also visual detection of predators. A recent study on this aspect used the fathead minnow Pimephales promelas as a test indicator . The experiment showed that motion was used as a more reliable alarm rather than shape. Lots of fish are fish-shaped, but one moving in a purposeful manner with gaping mouth directly to another fish can only spell trouble. Random motion, with fin-flicking, will not likely trigger alarms.
Historically, most aquarists forget or don’t know about pheromones in the water. Humans are visually oriented, and we have one of the poorest abilities to detect scents in the animal kingdom. What may seem to be clear water in a peaceful aquarium could be a stress-inducing soup of pheromones to the cyprinids in a community tank with potential predators. To take a commonly seen example, consider a community tank with angelfish and danios. The angelfish are predators, poor ones perhaps, but predators nonetheless. Small cyprinids can out-swim their attackers, but are unable to get away from them. They thus live in a soup of alarm pheromones and predator odours that stresses them.
It is known that minnows prefer to forage at night to escape attention of predators, but as hunger increases, they are more active during the day . This suggests that if an aquarist is having trouble finding the cyprinids in the tank, skimping on the food will make them more conspicuous. There is a common practice among many aquarists based on this idea. Fish intended for showing at a club exhibition should be starved for a couple of days prior to the show. Hunger increases activity and foraging behaviour, and will also increase the likelihood of impressing the judges. A well-fed fish entered in a show is more likely to laze about with droopy fins not doing much of anything, leaving the judges to pass it over in favour of its neighbour right up front at the glass wagging its body in the hope of food.
Cyprinid courtship usually involves the male displaying bright colours and/or unusual body characteristics such as over-developed finnage. Colours depend on diet, and females proceed on the assumption that a male with bright colours has a better diet and is healthier than a drabber male. However, there is a limit to extravagant ornamentation for sexual selection. If too extreme, it will affect adjacent body tissues or organs . In the aquarium this may not matter so much, but in habitat, a long-finned goldfish would not be able to out-swim a predator. The egg-shaped breeds of goldfish with cramped guts and a waddling gait would quickly be picked off by predators in habitat. What does matter for aquarists is that extreme ornamentation might affect the fish’s physiology, such as bizarre body shapes that produce distorted internal organs with concomitant internal troubles. Among the domesticated cyprinids, the only such suspect fish are goldfish breeds. This is when you see reference to some breeds as being delicate and difficult to keep. The real point is that they manage to stay alive, not that they are difficult to keep.
Just as with other types of aquarium fish such as cichlids, the cyprinids have dominance hierarchies. It has been previously shown that dominant males should be removed from a tank after they’ve spawned a few times in order to allow other males to contribute something to the gene pool. It does matter, however, what condition the smaller, sub-dominant males are in. In Pimephales promelas, small males will advance their spawning when the dominant males are removed, and will produce as many offspring. Such offspring are smaller though, which in habitat is because the small males didn’t have time to get their young plumped up before end of season . In the aquarium, where seasons don’t exist, this effect means that the young will have to be fed heavier and longer to get them up to par. Normally this would not be an important effect in the fish room, but for those aquarists who raise fish in outdoor ponds it might mean smaller juveniles. To summarize from a practical point of view, if you want to get a fresh burst of spawning from your fish, remove the dominant male and be prepared to feed heavily the next batch of fry.
You do weekly partial water changes on your aquariums, right? The filter is also rinsed out in the change bucket at the same time, right? It is not enough to say the aquarium is in good condition because the water is clear. Many problems are invisible, as not all toxins are pigmented or otherwise indicate themselves. Cyprinids tend to be messy fish, and even minnows will raise water turbidity and turn the tank into a phosphate soup if regular water changes are not done .
1] Wisenden, B.D., and K.R. Harter (2001) Motion, not shape, facilitates association of predation risk with novel objects by fathead minnows (Pimephales promelas). ETHOLOGY 107:357-364
2] Emlen, D.J. (2001) Costs and the diversification of exaggerated animal structures. SCIENCE 291:1534-1536
3] Barber, I., and H.A. Wright (2001) How strong are familiarity preferences in shoaling fish? ANIMAL BEHAVIOUR 61:975-979
4] Metcalfe, N.B., and G.I. Steele (2001) Changing nutritional status causes a shift in the balance of nocturnal to diurnal activity in European minnows. FUNCTIONAL ECOLOGY 15:304-309
5] Danylchuk, A.J., and W.M. Tonn (2001) Effects of social structure on reproductive activity in male fathead minnows (Pimephales promelas). BEHAVIORAL ECOLOGY 12:482-489
6] Korpi, N.L., and B.D. Wisenden (2001) Learned recognition of novel predator odour by zebra danio, Danio rerio, following time-shifted presentation of alarm cue and predator odours. ENVIRONMENTAL BIOLOGY OF FISHES 61:205-211
7] Zimmer, K.D., M.A. Hanson, and M.G. Butler (2001) Effects of fathead minnow colonization and removal on a prairie wetland ecosystem. ECOSYSTEMS 4:346-357 ?