Quick answer: Scarlet isopods (Ardentiella sp. 'Scarlet', still sold as Merulanella in older listings) are large Vietnamese climbers, around 18 to 20mm as adults, deep red splashed with yellow and black. Keep them cool, at 19 to 24°C with nothing added to warm them, in a tall enclosure vented through two facing walls, with humidity in the 60s to low 70s held as a slope from a damp end to a drier one. They climb glass from birth, they never dig, and warm still air is how colonies are lost. Ours are captive-bred here in the UK. This is not a first isopod, and it is not the first Ardentiella either.
I will say the same thing on this page that I say on the Scarlet listing itself: this is neither an easy isopod nor a cheap one. It is also, moult for moult, the most spectacular animal I keep. The red on a settled adult is not the flat orange-red of a Powder Orange or a Lava; it glows, and it gets deeper every time the animal sheds. If you have kept an Ember Bee or a Red Diablo through a full year and enjoyed the fiddling, this is the next one. If you have not, read this to the end and then read it again in six months.
What a Scarlet Actually Is
Scarlet is a selectively bred colour line, developed from Ardentiella sp. 'Tricolor' stock by isolating the reddest animals over successive generations. Where Tri Colour wears tidy bands of red, yellow and black, Scarlet is red first and everything else second: yellow and black break the red up in splotches rather than bands, no two animals are marked the same, and the segment edges can look faintly translucent against the light. Adults reach around 18 to 20mm, which puts them at the top of the size range for the genus alongside Ember Bee.
They are also among the most watchable isopods you can keep. Most species are nocturnal and spend their lives under something. Scarlets are out in daylight, on the bark, moving. If you want an isopod you will see, this is one of the few that obliges. It does raise the question of what an animal that lives in the open actually perceives, and we have written separately on whether isopods can see.
The trade reports the genus as Vietnamese, and Scarlet along with it. Nobody has formally described the animal: there is no type specimen and no published wild locality, which is why a careful listing writes the genus, then a nickname in quotation marks, and stops there. The reason two names float about is a paper from March 2025 in which Kästle and Regalado Fernández took the old genus Merulanella apart, kept it for three New Caledonian species, and raised Ardentiella around two species from Myanmar. The Vietnamese trade animals were not part of that study; the hobby moved them across on its own reading. It is a reasonable reading and we follow it, but it is worth knowing that the label is an inference. Our Ardentiella care guide covers the genus as a whole and everything in it applies to Scarlet unchanged.
Cool, Not Warm: The Mistake That Costs Colonies
Scarlets are heat-sensitive, and this is the single most important thing on the page. Our ideal range is 19 to 24°C. The genus tolerates up to about 26°C, but Scarlet is the demanding end of the genus, and it shows the strain sooner than its relatives. A colony that gets too warm does not sulk; it declines, and it does so quickly enough that by the time you notice the numbers are down, the cause has usually passed.
So there is no heat mat, no heat cable, no warm cupboard. A British room sits inside the range unaided for most of the year, and the threat in this country is a July heatwave and not a January night. Keep the tub out of sun, away from radiators, and if the room bakes in summer, move the tub somewhere cooler rather than trying to fix it in place. We used to recommend heating for this genus and were wrong to; the temperature guide has the wider picture.
Humid Air That Keeps Moving
Everything difficult about Ardentiella comes down to one demand. They want humid air and moving air at the same time, and nearly anything that raises humidity, whether you seal a vent, mist harder or pile the substrate deeper, cuts airflow in the same moment. Scarlet gives you less margin for getting that balance wrong than an Ember Bee does.
The answer is cross-ventilation rather than simply more ventilation. Fine mesh in one wall and more in the wall it faces, so that air travels across the width of the enclosure instead of stalling behind a single vent. I run large vents on my own tubs, two to four per tub, and an oscillating fan across the racking that cycles through different speeds, so no tub sits in the same still air for long. Ventilation is the one point every source on this genus agrees on. Whether stagnant air is what kills them, or whether it is the mould and moulting failures that follow, is a fair question and one we have been honest about in our mould guide: heavy ventilation is house practice rather than a proven fact, and it is house practice because colonies do better with it.
With the air sorted, aim for humidity in the 60s to low 70s, our ideal being 60 to 75 percent, and hold it as a gradient. One end gets sphagnum, moss and a structurally rich, damp hide; the far end stays drier, and the animals choose. The drier end has to be truly drier. A tub running 75 to 85 percent everywhere is one condition, not a choice, and it is exactly the sealed, stuffy box this species does badly in. Published figures for the genus run from 60 to 90 percent, which should tell you to treat any single number as a starting point. The mechanics are in our ventilation and humidity guide.
They Go Up, and They Get Out
Scarlets are climbers with arboreal leanings. Sheer glass and polished plastic slow them not at all, the mancae manage it as well as the adults, and a newborn will use a gap you cannot see. Before anything goes in, check the lid fastenings, the joins and the rim of every vent, and assume anything not sealed is an exit. Fine mesh on the vents is a requirement rather than a preference.
Because they live on wood rather than in soil, they get more from height than from floor space. Stand cork bark upright, give them branching routes off it, and add mossy wood to clamber over. A tall enclosure gets worked from floor to lid, and far more of the colony stays in view than any flat hide will show you. I keep most of my colonies in 11 litre tubs, and for this genus the height of the tub matters more than its footprint.
Substrate, Litter and Calcium
Substrate matters less for a climber than it does for a burrowing Cubaris, but it still has to feed them. Five to eight centimetres of organic topsoil, flake soil and crumbling white-rotted hardwood, with sphagnum worked through, is plenty. Coco coir on its own is the wrong choice: it holds water and offers nothing to an animal that eats what it walks on. Our substrate guide has a working mix.
Over that goes a deep layer of hardwood leaf litter and white rotten wood, which between them make up most of what a Scarlet eats. Something chalky stays in the enclosure permanently, whether cuttlebone, crushed oyster shell or limestone chips. A 20mm isopod moults hard and gets through calcium faster than you expect, and the calcium guide explains why it is not optional.
Feeding
Leaf litter and decaying white wood are the diet, topped up as they vanish rather than fed to a timetable. On top of that base, offer soft vegetables in small amounts, courgette, sweetcorn and sweet potato all go down well, a little protein now and then as fish flake or dried shrimp, and lichen-covered bark or twigs, which this genus grazes readily. I feed a wide variety across the whole range of foods we sell, because a varied diet is the thing I have found does most for breeding, and the Scarlet is no exception.
Remove any fresh food still sitting there a day later. At this humidity leftovers turn to mould fast, and mould brings mites shortly after. A springtail culture seeded from day one stays ahead of that better than spot cleaning ever does, and on a colony this valuable it is cheap insurance.
Breeding
For all the fuss over environment, Scarlets breed readily once they are settled. The difficulty is getting a group to that stable, established point, not the breeding itself. Hold the temperature, the airflow and the moisture steady, keep food and calcium available, and resist the urge to dig about to check on progress, which is the fastest way to slow it down. Give a new group a couple of months before you expect anything.
Broods are small by the standards of a Dairy Cow or a Zebra, and the young take some weeks to colour up: mancae start pale and the red arrives over successive moults, so do not judge a line by its babies. I start my own colonies at twenty or more animals where I can, because a bigger group establishes faster and gives a better genetic spread. With a species at this price most people begin smaller, and that is fine; it simply takes longer, and the line will still get there.
One thing worth doing from the outset is keeping Scarlet on its own. The Ardentiella morphs are close enough to interbreed, and a Scarlet crossed into Tri Colour or Lava gives you muddled colour within a generation and a line nobody can name. One morph, one tub.
Is This the Right Isopod for You?
Not if it is your first, and not if it is your first Ardentiella. Scarlet is strictly a species for keepers who have already succeeded with something demanding, whether a cave Cubaris such as the Rubber Ducky or, better, a more forgiving member of this genus. Ember Bee is the usual door in: it wants the same enclosure and the same air, and it forgives the mistakes you will make while you learn what airflow is doing in a box. Come to the Scarlet once you can hold conditions steady for a few months without thinking about it.
It rewards the collector chasing exceptional colour and the keeper who wants isopods they will actually see. It is emphatically not for anyone who cannot hold a room cool in summer, or who has an enclosure with a gap in it. We list it among the rare and exotic isopods worth keeping, and the reason it is on that list is the same reason it is not on the beginner one.
Frequently Asked Questions
How big do Scarlet isopods get?
Around 18 to 20mm as adults, which is the top of the range for the genus. They reach it slowly, and the colour keeps deepening after the size has stopped.
Do Scarlet isopods need a heat mat?
No, and fitting one is a common way to lose them. Our ideal is 19 to 24°C, which a normal British room provides for most of the year. Heat is the hazard with this species, not cold.
Why are Scarlet isopods so expensive?
Slow breeding, small broods, and a colour line that took years of selection to fix. Every one we sell was bred here rather than imported, which costs more and matters more: wild-collected animals in this genus are markedly harder to settle, and the 2025 authors note that parts of the group are under real pressure from collection for the pet trade.
Can Scarlet isopods climb glass?
Yes, easily, and so can the young from the day they leave the pouch. The lid has to seal everywhere apart from the vents themselves.
Are Scarlet and Merulanella Scarlet the same thing?
Yes. Merulanella is the older name and both are still in circulation. The 2025 revision restricted Merulanella to three New Caledonian species and set up Ardentiella for two from Myanmar, and the hobby shifted its Vietnamese morphs across on that basis.
What is the most common way people lose them?
A warm, sealed, damp box. Too much heat and too little airflow, usually together, and usually in summer. Vent two facing walls and keep the tub cool before you blame the food or the animals.
Where to Start
If the ventilation has not put you off, our Ardentiella range has the rest of the genus, and the Scarlet listing carries current stock. Every colony we send is captive-bred here in the UK, travels in a ventilated deli tub on damp sphagnum inside a double-walled box, is dispatched Monday to Thursday so nothing sits in the post over a weekend, and comes with our live arrival guarantee.
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