John Chapman's picture

What's living in my aquarium

Observed: 30th November 2010 By: John Chapman
Monster in my Aquarium 2
Monster in my Aquarium 3
Monster in my Aquarium 1

I have a small indoor heated aquarium which fits within a picture frame and hangs on the wall of my living room. The tank is about 2.5 foot long by about 18 inches high by about 2 or 3 inches deep and is fully enclosed in a wooden frame. It contains guppies and a lot of weed and snails.
A couple of weeks ago I saw what I thought was a spider drowning, thrashing/swimming in the tank. I tried to fish it out but it buried itself in the gravel and I lost it. A few hours later I was amazed to see it sitting in the weed. Upon closer examination I could see it has six legs, clearly has a head, abdomen and thorax, has a laterally flattened body and it's abdomen was pulsating, presumably to facilitate breathing. It was then about 10mm long but is now about 15. No fish seem to be disappearing and I have a number of baby guppies which are smaller than it is. But I suspect it is eating snails.
This may be a dragonfly nymph and was probably introduced on the weed, possibly as an egg. But since it is a tropical tank I have no idea whether the weed is of native or foreign origin. Obviously it would be irresponsible to release a non native insect species into the wild – although at this time of year it would probably mean instant death even if it is native.
Do you all think this is a dragonfly? More difficult can anybody tell me if this is native to the UK?

Species interactions

No interactions present.


RoyW's picture

Perhaps Oriental Scarlet?

Much of the water weed now sold in aquatic centres apparently now comes from south-east Asia, and there are a number of cases of foreign dragonflies and damselflies being introduced with this weed.

Crocothemis servilia (Oriental Scarlet, or Scarlet Skimmer, or Crimson Darter, or Ruddy Marsh Skimmer - to give a few of its names!), is a widespread Asian species that has been found in British fish tanks before (eg. ).
The photographs don't show the larva in enough detail for me to even be sure of the family (especially as it could be a family that I am completely unfamiliar with!), but from what I can see C. servilia would be a possibility (although there are potentially quite a few other possible species as well).

Anything that it is able to catch will be potential prey (including guppies) and snails.