Mystery fish revealed!

The results are in. Did you guess right?
08 November 2019

Interview with 

Louise Fraser, Illumina; Tony Sapsford, Frog End Aquatics


The tail of a piranha.


After a week and a half, the results are in. Phil Sansom went back to Illumina’s Louise Fraser to see if she could identify our mystery fish. Did you guess right?

Louise - The results just came in yesterday.

Phil - Ready to guess?

Louise - Oh, absolutely! What we got was around 500 million DNA sequencing reads, and we compared them to a database that contains all the species that have ever been sequenced completely. And about 98% of the reads actually didn’t align to anything in the database. Most likely those would be bacterial samples…

Phil - But those aren’t new-to-science creatures, they just haven’t had their genome sequenced yet?

Louise - That’s right, they’re just things that aren’t currently in the database.

Phil - OK, 98% unknowns…

Louise - Of the remaining 2% they broadly fall into four different groups. 95% of the classified reads are bacterial; 5% are from animals, and 1% from plants; and then there’s a tiny fraction that come from fungi and archaea. The most common animal species in the water sample was actually human. But the second-most-common animal was a family of fish called cichlids. And so that’s what we think was in the tank.

Phil - Is that your final answer? Louise, I hate to tell you… it’s not a cichlid! Any other ideas?

Louise - Yeah, so there’s low level of DNA from a number of other animals, such as rice fishes, carp, and even piranhas.

Phil - OK, this is interesting, because it’s actually one of those.

Louise - OK…

Phil - Do you want to guess which one?

Louise - Gosh, I’d like to think it was the piranhas, maybe.

Phil - It is piranhas!

Louise - It is piranhas?

Phil - It was piranhas!

Louise - OK, wow. That was a brave person that took that sample.

Phil - Did you guess right? Our mystery fish is indeed a piranha! Now Louise got piranhas as one of the results, but it was far from top of the list. What’s going on - well, the water in the aquarium tanks isn’t separated from each other, it’s actually filtered through a number of them. So i guess we didn’t make Louise identify a piranha from a piranha tank - so much as identify a piranha from a whole aquarium filled with different fish.

Louise - It would be interesting to know if there are cichlids within the same aquarium, and whether we’re just picking that up from a different tank.

Phil - There are, in quite a few of them.

Louise - Right. That might make sense then. There was also some other interesting DNA sequences that we picked up. Some mouse, or rodent DNA in there...

Phil - You should have all the pieces to the puzzle by now.

Louise - Oh, is it part of the fish food?

Phil - Oh yeah.

Louise - OK, right, that makes sense.

Here’s the audio that was hidden earlier - Phil Sansom learning about piranhas from Tony Sapsford at Frog End Aquatics...

Tony - Do you want me to say what they are?

Phil - Yeah, please.

Tony - Okay, this is the piranha tank. They've got red underbellies, red tails, large eyes. I mean they are only babies have only just got their teeth, so they’re only about 4 inches long at the moment, maybe 5 inches.

Phil - You can barely see them.

Tony - You can barely see the teeth.

Phil - Oh my God, you're putting your hand in.

Tony - It's not gonna hurt me trust me.

Phil - Are you sure?

Tony - Yeah.

Phil - Are they not aggressive?

Tony - Only in like a pack, when they're feeding and stuff like that. Like if he had a sick one in there, they’d annihilate it.

Phil - Am I right to be scared of them, or should  I not fear them?

Tony - No. No, it's nothing to be scared of. Unless you're gonna jump into a lake in the Amazon with a whole pack of them, I don't think there’s anything to worry about. Most people who buy fish like that think ‘oh it's great. I’ve got to have a piranha’, but then they get bored of it very quickly because they don't do anything.

Phil - Do people buy them because they think they're being hard?

Tony - Yeah, basically that's it. ‘Oh I got one of them, it’s a piranha’. And all they do is get big and boring.

You can read the full results from the fish tank water here.


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