Porcellio laevis, Swift Woodlouse, Smooth Slater
Classification
Class Maxillopoda (non-monophyletic group of Crustaceans), Order Isopoda (Woodlice and friends), Family Porcellionidae. The species was described by Latreille in 1804, along with the genus Porcellio.
Morphs
Wildtype, Dairy Cow, Milkback, White, Orange, Caramel, “How Now, Brown Cow,” Black, and California Mix (a locality genetic mix).
Appearance
A large woodlouse, morphs of the Swift Woodlouse can exceed 3/4 of an inch. As members of the family Porcellionidae, their exoskeleton flares out over their legs from the belly in a structure called an epimeron and two long, flat structures called uropods are visible at the back end of the isopod. Males have slightly longer uropods, although this varies with age of the individual as well. The Dairy Cow or Panda morph is patterned with dark gray and off white, reminiscent of the iconic pattern of black and white milk cows. Many morphs exist and continue to be developed. Dairy Cow Isopods are pale and patternless at first, gaining coloring as they mature. This phenotype is called Dalmatian and can be found in other isopod species. Oddly enough, in Porcellio laevis Dalmatian renders them unable to breed with other morphs of the species. Currently orange, white, Milkback (light central coloring fading into dark gray along edge), and Wildtype (gray) are commonly available. A black morph, which presents as very dark, is slowly becoming available, and Caramel, a golden honey color, is sometimes available.
Life Cycle
Porcellio laevis does not have a specified breeding season, and will produce year round in good conditions. They may live as long as a year in captivity and begin breeding while still quite small. Males and females are sexually dimorphic, with mature females being larger on average than mature males . Males also have modified pleopods (Nair 1975), though this can be difficult to spot. Easier to see is the presence of a marsupium, found only in mature females, which is used to carry the developing young. Eggs develop in this pouch and the female then gives live birth to a brood of many offspring (mancae). Babies appear essentially identical to adults, just in miniature scale, and will spend most of their time foraging subsurface.
Habitat
The Smooth Slater was recorded in Britain as early as the 1400’s and has since gained access to nearly every continent. Their actual origin may be northern Africa, where they are found even at high altitudes (Harding 2016). Today, they have cosmopolitan distribution, meaning they can be found worldwide. All isopods are crustaceans, and as such require moisture to survive. Moisture therefore dictates where they live. Swift Woodlice accordingly can be found in damp areas, under or in decaying logs and leaf litter, in greenhouses and garden beds, and beneath moist mulch.
Housing
Swifts can be kept in shoebox enclosures with no issues. While they are not truly social, they do not defend and fight over territories like some other isopod species (looking at you, Porcellio hoffmansegii) , so many individuals can be kept together. If the population becomes crowded and a food source is not made constantly available, some cannibalism will probably occur. Plastic containers are fine, as they are not adept at climbing smooth surfaces. Substrate should be provided and is less important as a food source than in other species, as Dairy Cows and other morphs are not picky eaters. It is of course still crucial to keep damp so that they can continue to breathe. Acceptable substrates include organic potting soil, coconut coir, dirt from outdoors (ideally sterilized to avoid bringing gnats, mites, or flies into your home), sawdust-soil mixtures, and flake soil. Any substrate should hold moisture and be deep enough for mancae to burrow, ideally two inches or greater. As the population grows, hiding spots like bark slabs, toilet paper rolls, egg cartons, wadded paper towels, and dead leaves will be very popular. In some substrates, such as coconut coir, you may be able to observe adults creating shallow burrows to peer out at their neighbors from. If flake soil is used or a rotten piece of hardwood is added, it will also be eaten by the isopods while they live in it, like a playground made of candy canes. The substrate should be kept damp, but not soggy, and not allowed to dry completely. Remember, isopods are crustaceans and need moisture to breathe! Most keepers like to add a portion of peat moss to one end of their isopod enclosures and spray it anytime it starts to feel dry. Doing this creates a gradient of moisture so that the isopods can regulate by moving from dry to wet and back again as needed. A relative humidity of 80% is about right to keep the air breathable, but not drown the ‘pods (W.H. 1941). Room temperature (70-75f) is ideal for Swift Woodlice, although they can tolerate higher and lower temperatures, making their range something like 60-80f. Isopods originally became popular because of their usefulness as a cleanup crew in vertebrate enclosures, such as dart frogs and geckos. However, if that is your goal Porcellio laevis may not be the right species for a janitor crew because of their large size, which might make them an enticing snack, and their propensity to swarm any possible protein source, which might make small vertebrate inhabitants an enticing snack.
Feeding
When it comes to chowtime, these guys are not picky. At all. They’re the kids at school who eat all their friends’ gross lunch leftovers, but then they also eat the lunch sack. I refer to mine as piranhas. Calcium is important, but not needed in high amounts. It can be provided in the form of crushed eggshells or whole antacids. Veggies are accepted. Fruit is appreciated. Decaying wood and leaves are valued. But what these guys really love is protein. They are the protein bros of the isopod world. Fish food, dog food, unseasoned cooked meat, shrimp tails (yep, exoskeleton and all), dead or dying crickets and roaches, boiled egg… basically your Porcellio laevis bin is your new sink disposal. Just remember to take out anything uneaten when mold starts to show, not because they won’t eat it (they will), but because moldy, fungus-ey meat products are bad for us weak humans and gross to our romantic partners. Feed enough that it takes a few hours for the food to disappear, but not enough that scraps are sitting in the enclosure for three days. If not enough food is provided, they will turn their forks and knives on each other.
Breeding
Breeding is as easy as having a male and a female within reach of each other. Smooth Slaters seem to breed whenever the conditions are right, which is most of the time since they are less picky than many other isopod species. A male will court a female by tapping her sides from behind with his antennae. This may occur for some time. If the female is amenable, the male will partially curl around her at a diagonal angle to maneuver his copulatory organs beneath the female. Once mating has finished, the two will go their separate ways or the male may follow the female around in a partial piggyback. This behavior is called mate guarding and is a way for males to ensure that other males do not mate with the female, increasing chances that he will successfully father a clutch of offspring. Smooth Slaters are incredibly prolific, in part because they do not need to reach adult sizes before they can begin reproducing. They can begin mating while still only half their eventual size, although their brood sizes will contain fewer offspring. Females can produce multiple broods from a single mating because they can store sperm for some time. Eggs are produced continuously and develop in a pouch structure called a marsupium, because of its similarity to the pouches of marsupial mammals like the kangaroo. This pouch is the key to terrestrial isopod success because it is filled with fluid, allowing them to avoid having to lay their eggs in the water like their aquatic ancestors. When the fertilized eggs have developed into babies, called mancae, (singular, manca) they are born alive. The larger the mother, the more mancae she can produce at a given time, so older females tend to have larger broods. Swift Woodlice females give birth to an average of 21 mancae per brood and 66 over their lifetime (Warburg, Calahorra, and Amar 2001). It takes about seven weeks for a female to produce a new brood, allowing her to potentially give birth as many as seven times in her life. It is a strange fact that the Dairy Cow morph has not yet been documented successfully breeding with any other morph of Porcellio laevis, despite many efforts to combine Dalmatian genetics with other phenotypes, such as the Orange morph. It is believed by some hobbyists that the Dalmatian gene makes the reproductive organs physically incompatible with the rest of the species and there is even some conjecture that they may not be the same species at all. Until someone (maybe you?) with a microscope and some patience decides to tackle this question, this remains one of the great mysteries of isopod husbandry.
Preservation
Isopods should not be pinned. They can be preserved in ethanol well or with some work entombed in clear resin. They dry poorly and desiccate with little prompting. There are few hobbyist isopod collectors, so little information is available to those who may wish to document species diversity, although museums appear to primarily keep specimens in alcohol.
Sources
Harding, P. T. (2016). Is Porcellio laevis (Latreille) declining in Britain and Ireland. Bulletin of the British Myriapod and Isopod Group, 29, 23-27.
M. R. Warburg, Yael Calahorra, Keren-Or Amar, Non-Seasonal Breeding in a Porcellionid Isopod, Journal of Crustacean Biology, Volume 21, Issue 2, 1 April 2001, Pages 375–383, https://doi.org/10.1163/20021975-99990137
Nair, G.A. Life cycle of Porcellio laevis (Latreille) (Isopoda, porcellionidae). Proc. Indian Acad. Sci. 84, 165–172 (1976). https://doi.org/10.1007/BF03045537
W. H. (1941, October). Observations on the life‐histories of some terrestrial isopods. In Proceedings of the Zoological Society of London (Vol. 111, No. 1‐2, pp. 79-149). Oxford, UK: Blackwell Publishing Ltd.
