Ten years ago, isopod keeping meant grey woodlice in a tub. Now there are dozens of colour morphs across multiple species, with new ones appearing regularly. Some sell for pennies, others for serious money. But what actually is a "morph", how do the colours work, and what happens when you mix them?
What is a morph?
A morph is a distinct colour or pattern variant within a species. The underlying animal is genetically the same species, with the same care requirements and behaviour. The difference is purely cosmetic: pigmentation. A Dalmatian Porcellio scaber and a wild-type P. scaber are both P. scaber. They eat the same food, need the same humidity, and breed at the same rate. One just looks different.
Morphs arise from naturally occurring mutations that affect pigment production or distribution. In the wild, these mutations usually make the animal more visible to predators, so they get eaten before they can pass the gene on. In captivity, there are no predators, so keepers can selectively breed the mutants and establish stable morph lines.
How colour genetics work in isopods
The formal genetics of isopod pigmentation are not as thoroughly studied as, say, ball python morphs. But the hobby has worked out the practical inheritance patterns through years of trial and error. Most colour morphs behave as simple recessive traits. This means:
- An isopod needs two copies of the morph gene (one from each parent) to show the colour. This is the homozygous state.
- An isopod with one copy of the morph gene and one copy of the wild-type gene looks wild-type but carries the morph gene. This is heterozygous, or "het" in hobby shorthand.
- Breeding two hets together produces roughly 25% visible morph offspring, 50% hets, and 25% wild-type. The classic 1:2:1 Mendelian ratio.
This is why mixing a morph colony with wild-types is a bad idea. The first generation all look wild-type (because the morph gene is recessive). The morph appears to vanish. It is still there in the hets, but picking them out visually is impossible. You would need several generations of selective breeding to get the morph back, and in a colony of hundreds that is not practical.
Common morphs by species
Porcellio scaber
The most morph-rich species in the hobby. Available morphs include:
- Dalmatian: white base with dark spots. Spot density varies between individuals.
- Orange: uniform orange colour. One of the most widely available morphs.
- Calico: mottled orange, white, and dark patches. The pattern varies per individual, which is part of the appeal.
- Snow/White: reduced pigmentation, appearing mostly white.
- Lava: dark, almost black colouration.
- Orange Dalmatian: orange base with darker spots. A combination line.
Armadillidium vulgare
- Magic Potion: orange body with grey/blue patches. Highly variable pattern.
- T-positive albino: reduced dark pigmentation, giving a pale orange or yellow appearance.
- Various orange and pied lines in development.
Porcellio laevis
- Dairy Cow: white base with dark blotching. The most popular P. laevis morph by a wide margin.
- Orange: similar concept to P. scaber orange. Uniform orange pigment.
Armadillidium maculatum
The "Zebra" isopod is itself a distinct species, not a morph. Its striped pattern is the wild-type appearance. There are not yet many established morphs within A. maculatum, though some are in development.
Selective breeding basics
If you want to develop or maintain a morph line, the process is straightforward in principle:
- Start with a group of the morph. The more individuals the better, for genetic diversity.
- Keep them isolated from other morphs and wild-types of the same species.
- As the colony breeds, remove any offspring that do not show the desired colour or pattern. These are either hets or wild-type reversions. Move them to a separate culture or sell them as mixed stock.
- Over generations, the percentage of visible morph individuals increases as the colony becomes more homozygous for the morph gene.
In practice, most keepers buy an already-established morph line and just keep it isolated. The hard work of selecting and stabilising the morph has already been done by whoever first bred it.
Can you cross different species?
No. Different species of isopod cannot hybridise. Porcellio scaber and Porcellio laevis are separate species and will not interbreed, even if housed together. You can keep different species in the same enclosure without hybridisation risk, though one species usually outcompetes the other over time, so it is not generally recommended.
Inbreeding
Isopod colonies are essentially closed populations, especially morph lines. Inbreeding depression is a theoretical concern, and in vertebrate breeding it would be a serious one. In isopods, the large brood sizes and short generation times mean the practical effects are less pronounced. Colonies can run for years without obvious decline.
That said, if you notice reduced brood sizes, higher mancae mortality, or generally smaller adults in a long-running line, introducing a few unrelated individuals of the same morph from a different breeder is sensible. This is not always easy for rare morphs, which is one reason rare morphs command higher prices: the gene pool is small.
New morphs
New morphs appear when someone spots an oddly coloured individual in a colony and breeds from it. Sometimes these are genuinely new mutations. Sometimes they are combinations of existing genes expressing in a new way. The isopod hobby is still young enough that new morphs appear regularly, and a genuinely novel morph in a popular species can be worth a lot in its first few years before wider availability brings the price down.
If you find an unusual individual in your colony, separate it and try to breed from it. It might be a one-off. It might be the start of something interesting.