The most poisonous of poison arrow frogs, the Golden Poison Frog (Phyllobates terribilis), has enough toxin on average to kill ten to twenty men or about ten thousand mice.

Feel free to add your trivia fact!

if you fry a murr egg the youke turns red and the outter turns blue !

This certainly makes me unlikely to go swimming in Lake Nicaragua. However, the shark known locally as the Lake Nicaragua shark (and the Ganges shark and the Zambezi shark and probably dozens of other names) is the bull shark (Carcharhinus leucas) which, as you may have guessed, can be encountered in lots of river systems.

Bulls have an ability to tolerate brackish and even fresh water, allowing them to reach inland as far as the US state of Indiana. This sometimes causes confusion between bulls and 'true' river sharks (genus Glyphis) which I think are pretty much confined to river systems. But river sharks are rarely seen and shy; bulls ain't necessarily so.

I guess bulls haven't colonized any other freshwater lakes, but it's not because they can't. It's a little freaky to think of one of the more aggressive and (potentially) dangerous species swimming 'round in lakes or in the Mississippi, however rarely it actually happens. I think I recall a bull being the suspect in the Matawan Creek attacks in New Jersey in 1916: the attacks were in muddy, murky creek water 20+ km from the ocean.

I have to respect this adaptability; its one of the hallmarks of the evolutionary staying power shown by sharks of all kinds. But that doesn't mean I have to like it. . .