As a kid, I think one of the most memorable sights in my first aquarium, complete with blue gravel and plastic plants, was my group of 8 Zebra Danios (Danio rerio) racing at high speed around the tank in a furious fashion, as if they had to get somewhere in a big freaking hurry...only to reverse course, and do it all again. I've never forgotten how much I liked the Zebras- or almost every other Danio species kept in the aquarium.
Yeah, this fish has been known to science- and the aquairum hobby- for a very long time. It was first described by Francis Hamilton, a surgeon with the British East India Company, stationed in West Bengal in the early 19th Century. He published "An Account of the Fishes Found in the River Ganges and its Branches" in 1822, in which he described this fish and 9 other Danio species.
The more I researched this fish beyond the usual aquarium hobby stuff, the more remarkable stuff I found. Like, there are no less than 13 scientifically recognized wild strains of this fish!
And there's more cool stuff you can find in the scientific literature:
For example, ichthyologists feel that this fish, "...appears to be primarily an annual species in the wild, the spawning season starting just before the onset of the monsoon.." And, "Spawning is induced by temperature and commences at the onset of the monsoon season. Food availability also acts as cue for breeding." (Source - Fishbase)
And about this "annual" thing. Sure, in the aquairum, they can live 4-5 years. However, in the wild, length- frequency analysis by researchers demonstrated two distinct "age classes" during the summer months, 0 to one year, and 1 year...indicating that the main period of rapid growth in these guys takes place during the monsoon months (June-September), a period of high temperatures (up to 34 °C) and food availability. Spinal curvature- a sign of old age in captive fishes, was not found in wild-caught specimens, leading researchers that the fishes expire well before this malady can occur.
SImple, boring, "beginner's fish" my ASS! The Zebra Danio is is increasingly being used by researchers as a model for studying genetic effects in vertebrate development. Zebra Danios are able to regenerate their heart, nervous tissues, retina, hearing tissues, and fins!
This fish has got a lot going on!
And the amazing thing about this fish is that it's probably THE most bulletproof species you can keep. In fact, I recall reading somewhere that it's "tolerated temperature range" based on wild type localities is from 76.2 – 101.5°F (24.6 – 38.6°C). I mean, if THAT isn't a broad range, nothing is! And it tolerates water with a pH from 6.0-8.0.
Yeah, these guys are hardly what you'd call "fussy" fish!
Its diet, based on gut content analysis, consists primarily of zooplankton and insects. The usual stuff, right? Well, it also revealed that the fishes consume filamentous algae, terrestrial insects (including small spiders) detritus, sand, and mud! No wonder this fish is considered easy...it literally eats anything! It feeds at the surface and in the substrate.
And you know me- once I hear that kind of stuff, I get these weird ideas like, "What if we mimic the conditions of the natural habitat of the fish? Would they do better? IS there an advantage somewhere?"
I think like this for so many fishes, as if to shun the fact that 90% of what we keep in the aquarium these days has never seen a stream, pond, or river...😆
It's just..I don't know..irresistible to me to think about this kind of stuff! Taking the most common of common aquarium fishes and giving them "throwback" conditions; seeing if it somehow "awakens" something locked into their genetic code over eons...something...
I mean, it's kind of silly, I suppose...there are so many other things to do in the hobby...yet I can't help but wonder if we can learn something from replicating some aspects of their long-forgotten wild habitats...
And, in regards to the Zebra Danio, what's interesting to me is the habitats in which these fish are found. One of the habitats is known as a "beel", which, according to Wikipedia, is "...a lake-like wetland with static water (as opposed to moving water in rivers and canals.)"
In one study of the locations in which these were found, out of 26 reported occurrences, 14 were in ponds, 3 were in ditches connected to rice paddies, or in the rice paddies themselves, and 9 were canals or small rivulets adjacent to larger rivers.
Typically, these fishes are found in Northern India, and this area is subjected to seasonal rainfall between the months of June and September due to the summer Monsoon, and the water levels and characteristics vary considerably at different times of the year. They are often found in inundated rice paddies and marginal pools/ditches adjacent to them, with silty, kind of turbid water with very little movement. During the dry times of the year, they spend their time in calm, shaded areas of streams, with rocky or gravel-strewn substrates.
This is interesting, because it reminds me a bit of the Amazon igarape, although instead of rain forest, you've got rice paddies...
An excerpt from a paper I found online about this species describes the unique habitats in which they are found:
"(The Zebra Danio) appears to be a floodplain rather than a true riverine species.They are most commonly encountered in shallow ponds and standing water bodies, often connected to rice cultivation. This association with rice cultivation may relate to the use of fertilisers which may promote the growth of zooplankton, a major component of the zebrafish diet (Spence et al.) Spence et al. (2006) found no zebrafish either in rivers or temporary creeks that opened during the monsoon season.
Where zebrafish are found in streams and rivers, these typically have a low flow regime and zebrafish tend to be found at the margins (McClure et al. 2006). Observations of their vertical distribution indicated that they occupy the whole of the water column and occur as frequently in open water as amongst aquatic vegetation (Spence et al. 2006a)."
So, yeah, this whole "fast-moving stream" idea we in the hobby have about these fishes seems far less common than good old-fashioned rice paddies and shallow, more still bodies of water! The Zebra seems to inhabit a world of marginal plants, turbidity, silty substrates, and lots of food.
And, I've been playing with rice seeds, silted substrates, and turbid water lately! Hmmm...
So, my simple thought is...this fish seems to hang out in what we as hobbyists would think of us "less desirable" conditions for much of the year- the silty rice paddies and their adjacent ditches...And only spends the dry season in the more permanent, less turbid streams. Why would this be? Is there some advantage? Like food, better substrates for breeding, protection? Why the turbid water? What does it bring to the fishes?
Would there be an advantage to keeping a fish like the Zebra in different conditions different times of the year, as in nature? Or simply in a tank representing one of the two habitats it's found in. Would you WANT or NEED to? I mean, the fish has been a captive-bred staple of the hobby for almost a century...but I can't help but wonder why these fishes live the way they do in the wild. What advantages do these habitats hold for the fish?
Would you get different behaviors, colors, health, spawning out of the fish by doing this "seasonal transition"..? Using a very fine sand substrate, maybe mixed in with some mud or something similar to replicate the rice paddies, with pump returns very gently angled at the bottom to simulate turbidity?
This just might have been the most ever discussed in a hobby context about the obscure habitats and characteristics of one of the most pervasive fishes in the aquarium hobby.
Again, why, you ask?
My answer? I just think it could be kind of cool. Weird, but cool.
Am I the only one who imagines weird stuff like this? Maybe?
On second thought- don't answer that!
I know, the fish is bred by the billion in fish farms all over the world, as are many much sexier, domesticated strains of its relatives...but wouldn't it be interesting to see what happens when you "repatriate" these "common" fishes to an uncommon execution of their natural habitats?
I think it would be. In fact, I'm certain that it would be! It's a fave hobby tangent of mine. I think that we'll do a periodic series, where we look at the wild habitats of the hobby's most common fishes..."Off The Grid."
Yes, the whole purpose of this piece is to encourage you to go "off the grid" with common fishes. Research them not in the hobby literature, and delve into some scholarly research about them instead. The insights you gather may not always be remarkable, but you might uncover just enough about the so-called "common fish" swimming at the LFS that you just might want to tackle them with a whole new approach!
Yes, your friends will think that your nuts. But that's the fun part of this, right?
What fish will YOU go "off the grid" with?
Stay curious. Stay resourceful. Stay brave. Stay educated...
And Stay Wet.
Really interesting post Scott, thank you. ]
I think like you. Research, then go off grid so to speak, with developing approximate conditions to those the original fish would experience. I’m currently doing it again after moving an aquarium and restocking it.
Way back in the 80’s I kept Zebra Danios in a 30 gallon long, heavily planted aquarium, plants ordered from a catalogue. Filter was an undergravel with 2 uplifts, powered by air pump, substrate was small pea gravel, flow was typically slow and the plants thrived. Other fish were a small Whiptail Catfish, pair of Pearl Gouramies and 3 or 4 Corydoras paleatus.
The Zebra Danios bred regularly and fry survived despite the Corys and Pearl Gouramies without my intervention. So much so that the number of Danios went from 10 to 20, then 40 ! At which point I started giving them to the local aquarium shop to stop the tank getting over crowded.
It’s interesting that in the intervening decades, opinion has changed in the hobby about these fish needing lots of flow, when mine lived long lives, showed fantastic colours, and enthusiastic, successful breeding in the conditions in that aquarium. I haven’t seen Zebra Danios that looked so good since, nor had an aquarium that was so easy to keep. If they invented a totally silent air pump I’d still be using an undergravel filter LOL
Sounds like a fun project! One of my all time fave fishes! Enjoy!
Hey just came across your article about zebra danios. I think like you, replicate the natural environment to see if anything changes.
I have just taken up breeding zebra and leopard danios in New Zealand. Where neither fish is being imported any more. I love them, such fun fish.
I’m working to prefect spawning them on large scale (with few fish, if that makes sense), raising the fry is easy enough.
I plan on summer tubbing them this summer, along with mountain minnows. You have made me think more about plants and depth of the water… So many things to try