Humidity is one of the areas that can be difficult to explain. In the plant and reptile keeping hobby, there are many hydrometers marketed for monitoring the exact humidity levels in a setup. This is important for reptiles so that the moisture is absorbed into their skin and they can shed properly; it is important for plants because many tropical plants don’t do well with dry air even when well-watered. It’s not so important for isopods- they aren’t exposed to open air very often.
So, should humidity levels be monitored for isopods at all? While this is an option, monitoring humidity closely is just a tight dance with insanity because it doesn’t convey much information about the environment. Isopods spend the majority of their time under leaves and in the dirt- humidity gages are made to measure the moisture in the air. The moisture that most affects isopods is the levels in the top inch of the substrate and in the leaf cover. While there are soil moisture gages available, these tools are several inches in length, and not very well suited to the small containers that isopods are usually kept in (most Porcellio species only benefit from an inch of soil depth.)
What then, is the ideal moisture level for isopods? This varies greatly between species, and can be a complex concept. For most species, an ideal level of moisture is to have the soil damp to the touch, but not wet. If water drips from the soil when squeezed, there is too much moisture. The high water levels pool and collect around the soil, providing a perfect breeding ground for bacteria. When the moisture levels are this high, the isopods don’t have an opportunity to dry off and will have failed sheds. The combination of bacteria and poor shedding can cause a culture to completely to crash. Isopods that have succumbed to pooled, stagnant water will turn a vibrant orange in death, while isopods that died from failed sheds will have the remnants of shed on their bodies they were unable to get off.
Moisture levels being too low is another cause of death for isopods. Isopods have what are called “pleopodal lungs”, lung sacs that are located on the underside at the back of their body. Porcellio and Armadillidium typically have 2 pairs of lungs (for a total of 4 sacs), while several Cubaris and Trachelipus have 5 pairs (for a total of 10 sacs). These lung sacs evolved from gills, when these isopods still lived in the ocean depths. All lungs need to be wet in order to work correctly; it’s just that vertebrates keep their lungs internally so we’re not as consciously aware of this need. In an overly dry environment, the isopods will dry out quickly and die – this is called “dessication”. Some isopods are more resistant to excessive moisture, but all isopods need access to a constant source of moisture to prevent dessication. Isopods do get their lungs moistened while burrowing from soil, but also can be observed dipping their rear where their lungs are located on a moist source or while being misted in a twerking motion. While they do this, they are also drinking and absorbing water into their body.
A bone-dry setup in unsuitable for the majority of isopod species. If unsure about species specific needs, the best policy is to adopt a 50/50 policy (half wet and half dry), and adjust according to isopod behavior. Species that do need a setup that is mostly dry include P. magnificus, P. spatulatus, P. wagneri, P. spinicornis, T. rathkii, and T. arcuatus. Keeping mostly dry species can be a challenge, because with such low moisture levels the setup is at higher risk of drying out completely and desiccating the isopods. As a safety measure, a clump of moss can be placed in the corner that is wetted down during checks. This will help ensure that not only an area retains moisture, but that it keeps this moisture in a small area. Caution should be used when caring for other giant Porcellio species. It is frequently advertised that all Spanish and similar giant Porcellio species need a “bone dry” setup with minimal moisture but this is incorrect. Many species were collected from caves and cliffsides that have high moisture AND airflow, so instead need moderate moisture with lots of free flowing air. For cave species like P. bolivari and cliff species like P. expansus, the females and juveniles in particular need a generous amount of moisture in order to thrive.
Some isopods do need the majority of their environment to be moist. The good news is that, for these isopods, adopting a 50/50 policy won’t harm them – they will just stay out of the dry area and hang out in the wet area. Nearly all dwarves require a setup that is moist, excluding a few micro species from Africa. P. scaber, P. dilatatus, P. laevis, and the Porcellionides genus benefit from a setup that is moist throughout. While these larger species can thrive in a 50/50 setup and survive dry periods, their population booms dramatically when offered an abundant source of moisture.
How often isopods should be checked and watered varies on the culture size, and less on the species. When first being set up, cultures should be watered about twice a week to ensure that the soil absorbs water correctly without pooling or drying out. Small culture containers of 8-64oz may need to be watered weekly or biweekly to prevent desiccation, as small containers can only hold so much water. Larger containers, like the 25L containers we keep our cultures in, only need to be monitored every 2 weeks and can even go 2 months without drying out. It doesn’t hurt to check on a culture more frequently than needed if a keeper is unsure how the moisture levels are maintaining.
While it is a tricky concept, understanding the moisture needs is a very important aspect of isopod keeping. Starting out with more forgiving, less expensive species such as P. scaber and P. pruinosus is a relatively stress free way to master moisture levels while cultivating isopods. Remember, large amounts of moisture will kill slowly, while no moisture will kill very quickly, so it’s better to have a little too much than not enough.