Why are some guppy fry growing much larger and faster than others?

Question

I only have 5 fry in a 10 gallon tank with a pleco as the only tankmate. 1 of the fry is much much larger than the rest. Another one is also bigger but not as big as the first one. The other 3, at 2 months old, are probably small enough to get eaten by an adult guppy still. I don't know how big a cm or a mm is compared to an inch but the big one is less than 1 inch (maybe 3/4"?). The small ones are maybe about 1/4". Why the great disparity in growth? They all seem healthy and swim fine, etc. They all seem to get to eat when I feed them. I feed them several times a day and usually do partial water changes once a week but sometimes it's longer.

Also, how young can the fry get pregnant? I have not separated them- I don't care about that really- and the 2 biggest ones (females) have pretty dark gravid spots and seem kinda fat. I couldn't imagine them having babies already though. I know they can get inseminated very young... but how young can actually give birth? Thanks!

I have a whisper filter and it does have charcoal. Water quality is good. At least one of the small fish also shows a gravid spot though, so I'm not sure sex is the main factor in the size difference. Should I separate the larger 2 from the rest now? Or are they fine? Thanks!

Answer 1

Its unlikely that the sex is defined properly in any of the fish yet however, the simple fact is the large one gets more food, in all fish the strongest will compete more aggresively for food therefore getting the lion share each time the larger he gets the more he gets, the fish should now be moved on if you want the others to grow quicker.

AJ

Answer 2

Differential growth rates are caused by sex differences(the makes are naturally smaller). Also the smaller males mature at different rates too. It's likely that your two larger fish are females. As to the age of reproductivity it depends on several factors and is hard to pin down to a simple number.(You know,like 10 mm = 1cm). However,if none of the fish in the tank have developed a gonopodium,then no-one has committed parenthood. The differential of the growth rates of your fish does seem too great,though. Are you sure about the water quality? You don't mention your filter system,but if it doesn't use charcoal there may be a build-up of organic compounds that could stunt the growth of the fish.

Answer 3

(Too many details!)

Survival of the fittest.The stronger ones get more food.Genetics,they are not clones.Sex,males are smaller.The baby females may start getting pregnant after about 3-4 weeks old

Answer 4

One is probably male and the other is probably female (male being smaller) and/or the bigger one eats more