This is the very first of controversial posts, therefore I'm writing little introduction.
Many people who want to start aquarium visit pet shop and buy whatever personnel recommends. Some receive animals from others. That way mistakes happen - fishes collide, tank is too small, everything goes wrong and beginner is discouraged. They sell aquarium out, animals are returned to shop or to next victim.
Animals that are wrong choice have some common characteristics:
- Easy to breed. That can be reason why tank is overpopulated, algae grow or keeper must kill young. Inbreeding can happen too. Offspring can be ugly when fed wrong, many decorative morphs lose their special features.
- Big. Beginners often purchase young, little specimens that grows much larger later on. Those animals are suitable only for big tanks. Later tank is dirty, animals feel uncomfortable and keeper loses his reason.
- Invasive. You can't set them free, you must kill them. Freed ones can survive in wild and endanger local populations.
- Aggressive. No one enjoys hurt fishes when needs to point out and eliminate aggressive specimen.
I will omit problem of difficulty. Difficult fishes are pricey and not so popular in aquarium trade.
I would like to start with peculiar case of marble crayfish. I was unlucky enough to have two of them, luckily enough it did not breed.
Marble crayfish called marmokrebs too is a very interesting animal. It hasn't been found in nature. It breed by parthenogenesis. I mean, its population are only females, however somewhere I found mentioned, that in very bad conditions males can appear. It was spotted in German aquarium trade. Had it escaped Frankenstein's laboratory?
That interesting story is appealing. Crayfish looks harmless. 10cm long, nice with marble exoskeleton. It can be kept without problems - wide temperature from 5 to 30 Celsius degree. No particular water parameters recommended. Only two conditions - water should have rather strong flow and there must be cover, because crayfishes can go out of tank for a walk. And you CANNOT allow that. They need bit of sand to moult properly. They eat anything edible, walk a lot searching for food. They can be active all the day. It's hilarious when they try to climb constructions. They move rather slowly.
Unfortunately, marble crayfish has all of listed above "bad" characteristics. I can't imagine any other animal that suits so well in terms of being "bad choice".
For first marble crayfish breeds uncontrollably. Any single female produces fertilized eggs. That means, if you bought one crayfish you should expect tank full of them in the future. Only good thing is, they reduce population to comfortable maximum of 1 crayfish per 3 litres of water. There is no other method to control than regular depopulation.
Marble crayfishes are quite big. Obviously, crustaceans can be kept in relatively small tanks, but crayfishes eat everything and pollute water. They aren't as good at keeping tank clean as most of crayfishes and shrimps - actually opposite, they prefer fresh food than detritus (dirt).
The most important - those crayfishes have confirmed populations in Italy and Germany. They are invasive species and CANNOT be freed to wild. They can live and repress original populations. That's why you cannot let them out of tank. They will probably die out of dehydration, but who knows for sure.
And surprise for last. Shrimps and most of popular crayfishes are peaceful and useful part of community. They clean bottom, eat dirt. But it's a different story with marble crayfishes. They eat plants, I have had all eaten. They eat big slimes and even hunt for fishes. Little fishes are eaten whole, bigger have fins bitten. Supposedly marble crayfishes do not prey on shrimps.
If you really want to keep marble crayfish remember - nor plants and slimes will live. Little fishes are doomed too. Fishes with large fins (Betta, Angelfish) probably too. Bottom will be regularly dug. Crayfishes will reproduce. If you want to change you must kill crayfishes, because there is no other way.
In my personal rank marble crayfishes win for the title of the worst aquarium animal. However easy to keep, nice and interesting, they should never be recommended for beginners. They are actually suited only for determined and heartless keepers.
Other species of crayfishes despite similar pattern of behaviour are more difficult in breeding and aren't invasive.
My marble crayfishes are already dead. They did not breed. One lost one arm and was later destroyed by second and second died after two years. Both were old when were given to me by friend.
If I would had been forced to kill them I would boil or freeze them. Other methods are not recommended, because crustaceans have different organism than we have. Decapitation isn't really lethal, they die later from starving.
Marble crayfishes probably aren't toxic, but there is little meat to eat. I didn't find any informations about it and I didn't try. They are banned in some countries due to being invasive. Marble crayfish is probably subspecies of crayfish from Florida peninsula - Procambarus fallax. Wikipedia states that for sure, but it hasn't been proven as far as I know and that crayfish do not breed by parthenogenesis.
