EARNING YOUR STRIPES
One of the most common fish around can be a spawning and biotope delight, writes Chris Sergeant. It’s time to pay dues to the humble Zebra danio.
One of the most common fish around can be a spawning and biotope delight, writes Chris Sergeant. It’s time to pay dues to the humble Zebra danio.
WHEN IT comes to the Zebra danio, Danio rerio, the phrase ‘used and abused’ has never felt more appropriate. Cheap to purchase and hardy to keep, they were often marketed as the ideal beginner option, perfect for kickstarting a fish-in tank cycling process. But with hobby advancements, improved technologies and a greater understanding around fish welfare, and with those pre-dated methodologies seemingly behind us, where does this leave these little protagonists? Sure, Zebra danios are still represented through the odd tank or two in almost every store, but it feels like nobody really talks about them anymore, at least from a hobbyist perspective. After all, when was the last time you saw a biotope aquarium dedicated to these little guys?
By complete coincidence, that’s a point that was also raised in an episode of The Tint podcast, run by Scott Fellman from Tannin Aquatics. In it, Scott discussed this perception that we feel we have to create our aquariums for the most exotic, rare and sexy of fish, while the more common species get left by the wayside. This isn’t to say that Zebra danios aren’t aesthetically pleasing, they undoubtably are with their black/blue and white horizontal stripes (the Zebra name isn’t given to them lightly), while a closer look reveals flashes of yellow in the fins. Could it just be that these aquarium staples were ticked off in our early forays into fishkeeping — been there, kept that?
Medical heroes
One place Zebra danios haven’t been overlooked is in the laboratory. In biomedical research, they have been front and centre for years and they have some track record. These remarkable little fish are capable of regenerating not only damaged fin tissue, but also heart, kidney, retina, spinal cord, and telencephalon — the most highly developed part of the forebrain — cells too. From liver function to cardiovascular diseases, cancers to congenital hearing loss, they have played a key role. They’ve even been on the International Space Station as part of a study into muscle atrophy. Being a small shoaling species, straightforward to obtain and maintain, it’s easy to see how they came into favour. Their highly fecund
fish to disperse into these temporary tributaries, known as nalas, as well as marginal pools and rice paddies within which they breed.
While the substrate across the habitats are mixed — they are found over sites with silt, mud and sandy bottoms in the temporary pools, as well as finer gravel and small rocks in the main channels — there are some constants when it comes to their natural environments. The waters where they dwell are typically shallow with little flow, and have copious amounts of overhanging terrestrial vegetation, while aquatic plants provide further habitat complexity and cover.
In terms of water parameters, multiple Zebra danio habitats are diverse. They have been observed at elevations of 8–1576m, at temperatures between 12–39°C with pH levels of 5.9–9.8 and in salinities of between 0.01–0.8ppt. It’s no wonder they have been termed bombproof in certain aquatic circles. It’s worth noting that most Zebra danios sold within the UK are kept in temperate holding tanks at around 18°C, so transitioning to higher temperatures should be slow and gradual. Remember, surviving is not indicative of thriving, and so just because a fish can survive such parameter fluctuations or extremes, does not mean that should be subjected to them.
In the wild, Zebra danios are omnivorous, so as well as being prolific predators of mosquito larvae, along with other adult and larval
insects, they feed on algae, plant material and assorted detritus too. In an aquarium setting, they are totally unfussy and will readily accept most types of flake, granule and other dried foods, as well as a variety of live and frozen fare.
Biotope bounty
When it comes to biotopeauthentic tankmates, there are plenty to choose from. Sympatric species found alongside Zebra danios include the Rosy barb,
Pethia conchonius, Flying barb, Esomus danricus, Blue panchax,
Aplocheilus panchax, Scarlet badis, Dario dario, Moustached danio, Danio dangila, Bengal danio,
Devario devario, Honey Gourami,
Trichogaster chuna, and loach species such as Lepidocephalichthys annandalei and Lepidocephalichthys guntea to name a few. They also share their waterways with numerous predators, such as the freshwater needlefish, Xenentodon cancila, Bronze featherback, Notopterus notopterus, Tyre-track eel, Mastacembelus armatus, as well as multiple species of snakehead, Channa sp. and even large dragonfly nymphs. It goes without saying that with fish like these, not every biological interaction needs to be replicated within a biotope.
Making babies
Zebra danios can be prolific spawners, and while the tendency to try and raise every individual is understandable, unless you can support them all, you will need a plan. The first thing to do is acquire a mixed-sex group, taking care to avoid any ill or deformed individuals. They appreciate a shoal, so consider a group of 8-10 or more for a natural look. This will also help to keep any hierarchical disputes down to a minimum as each fish seeks to find its place within the shoal. While they will happily spawn within the main aquarium, it’s best to separate them into pairs for ease of egg collecting.
Well-fed females are plumper, larger, and if you look extremely closely, possess a tiny protruding ovipositor too. In contrast, conditioned males are sleeker and ‘torpedo shaped’ and often have a brighter yellow colour on their fins, particularly the anal and caudal fins, and a subtle red hue to their flanks. In the wild, breeding is brought on by the summer monsoon, but aquarium fish spawn all year round. Condition them with live or frozen feeds on top of their usual meals and then, around evening time, net a pair into a separate spawning container.
A simple hang-on breeding box is ideal, especially a design with water circulation throughout the container. In both wild and domesticated Zebra danios, the photoperiod is the most important factor influencing spawning, and being diurnal, courtship and spawning behaviour is most apparent in the 30 minutes directly after sunrise.
For Zebra danios kept in smaller aquaria, or one that cannot support the weight of a hang-on breeding box, any small container over 1 litre will do. Wedge a piece of mesh a centimetre up from the tank base, ensuring the fish cannot get underneath and then fill it with tank water. They will happily hoover up any exposed eggs, and will also eagerly engage in infanticide given a chance, so the mesh prevents this. Once pairs have been set-up, they’ll take it from there. At dawn, the male will manipulate the female towards a spawning site in the container, before aligning his genital pore alongside the female’s and then oscillating his body to stimulate the female to release the eggs. After which, the pair’s job is finished and they can be netted out to re-join their shoal.
Zebra danio eggs take two or three days to hatch, after which, the fry will list around on the bottom of the tank until they inflate their swim bladders around day four. At this point, they are still poor swimmers and so it’s worth initially dropping the water depth in the fry tank down to a centimetre or so to help them out, before very slowly increasing it over the next week or two.
After utilising the yolk sac, the fry are capable of independent feeding at day five. They have a relatively large gap size, enabling them to take live paramecium, rotifers and commercial fry diets with a particle size of <50microns, and after a couple of weeks, enriched Artemia nauplii can be added too.
If for whatever reason your pairings are not working, there are a couple of things you can try. Firstly, Zebra danio females can be picky, and have
been shown to allocate their reproductive resources according to the attractiveness of their mate. In the female’s eyes, size matters. The larger the male, the greater the number of eggs released by the female, and in the case of mating with multiple partners during a spawning event, the female apportions a greater percentage to the larger individual, so it might be worth picking your biggest, ‘showiest’ male for each pairing.
The other point to note is that substrate and water depth can influence spawning behaviour. When it comes to parental duties, they might not display an ounce of interest post egg release, but they do care where they lay them. Lab studies have shown a preference for a gravel substrate over either silt or a bare tank floor in both wild caught and domesticated zebra danios. The theory here is that the gravel provides refuge spaces within which the eggs drop, remaining hidden from predators but still well oxygenated. Eggs laid on a silt substrate are in danger of being either eaten, or being buried under the sediment. Water depth also plays a part in spawning and egg survival, and to quote the opening line in a Guardian article on Zebra danio research, ‘It begins with sex on the beach’. In the wild, adults regularly congregate in the shallows to breed, but in a lab setting with uniform tank depth, the ‘beach’ is achieved by either tilting a breeding pair’s enclosure, or by raising the base. It seems there’s no such thing as too shallow, and tanks set with a depth gradient of 0-4cm facilitate a range of mating behaviours. The sentiment by the adults is the same again.
By spawning in shallow waters, the deposited eggs are out of reach of many of their larger predators and the reduced water depth also aids fry survival by reducing the energy expended in swimming to the surface to inflate their swim bladders. Other tips to help encourage your fish to breed include the addition of plants, real or artificial, to the spawning tank, as domesticated individuals show a preference for vegetation, or by adding cooler water to the spawning tank. The ease at which they breed has seen a sharp drop in the numbers of wild-caught individuals, so much so that the vast majority, if not all Zebra danios on display, are likely to be captive bred. This in part has also contributed to the numerous
other strains on the market, with fish specifically bred for elongated finnage, spotted patterns, golden colouration or albino colour morphs. Glofish — GM danios spliced with coral and jellyfish genes so that they fluoresce under UV lighting — are banned in the UK and Europe but popular elsewhere.
Across all the variants, the wildcaught individuals catch the eye the most. The colours appear crisper and the golden hues that much more vivid. Sometimes it’s worth us accepting that nature knows best.
So, sure, Zebra danios might be common and the perennial underdog when compared to gaudy tankmates, but these are no ordinary fish. They should be revered, put on a pedestal, or failing that, the very least we can do is take them back to their roots and make them the focal point of their very own biotope aquarium. After all, who knows what else we might discover about these extraordinary little cyprinids.
The wild-type habitat of the Zebra danio is subject to seasonal change, which opens up a host of potential aquascaping options