Friday, March 22, 2013

To Go Where No Crustacean Has Gone Before

I want to talk about a particularly interesting little critter I stumbled upon while doing some research on detritivores. Since this guys's story was new to me perhaps it is new to you.

Hemilepistus reaumuri. wikipedia
Ok, so a freakin' pillbug (aka woodlouse) what is so special about them? Yes we are all familiar with pillbugs, rolly-pollies, sow-bug whatever name you know them by they are found or introduced all over the world- usually in dark/damp places and feed mainly on dead vegetation. They are crustaceans,  Isopods, and represent the most successful radiation of terrestrial crustaceans- the suborder Oniscidea. Now that is all fine and dandy- but what I want to talk about is the guy pictured above- Hemilepistus reaumuri- which "inhabits the driest habitat conquered by any species of crustacean".

Now I grew up in southern California and these guys were definitely part of my childhood urban fauna growing up. The crustacean in question most certainly is Armadillidium vulgare native to Europe including the British Isles. It thrives in coastal areas, especially with calcareous soils- so it does alright here. Despite being invasive it is considered relatively benign on the ecology, unlike the situation of our crayfish in British waters. In addition to  A. vulgare, southern California also hosts at least two sowbugs (the ones that can't roll up); the European woodlouse Oniscus asellus; and the rough woodlouse, Porcellio scaber. Both require somewhat wetter
conditions than the rollies to respire. In the pic to the left you can see the difference between the two with European woodlouse having the colored specks while the rough woodlouse is more textured. Interestingly these isopods are doing so well in the irrigated sections of California that their European native predator- the woodlouse Spider Dysdera crocata- came along too and is doing just fine here eating 100% sowbug al dente which it pierces their exoskeleton with those big ol' fangs it has. And its a good thing that spider is around to eat 'em as many spiders find them distasteful as they excrete ammonia gas and have blue blood due to the copper based blood- haemocycanin- that they utilize. Anyways, despite their familiarity as urban landscape critters they are quite interesting
and worthy of admiration. That being said the isopod at question, Hemilepistus reaumuri, departs from these other guys both in respect to the harsher environment that it thrives in and the sociality that this species has evolved.

Native to steppes, semideserts, salt lake shores and real deserts of North Africa and the Mediterranean this isopod not only survives but thrives in an extreme environment. Central to its success is its habit of burrowing. New burrows are only made in early spring by a single woodlouse which guards the entrance assiduously against other conspecifics. Only after a prolonged courtship does the desert woodlouse accept a member of the opposite sex. Once bonded the crustaceans form an alliance that is about as magnanimous as any in the animal kingdom. They both guard the entrance to the burrow, taking turns for foraging as needed, and reject intruders. Partners are recognized based on scent. When the female gives birth (woodlouse eggs hatch on the underside of their body so they appear to "give birth") to 50-100 young they actually stay and hang around for quite some time being fed by the parents. All the offspring develop a specific "scent badge" which allows the parents to identify them as kin and reject (or even consume) the young of other woodlouse. In this way only the offspring of the monogamously mated pair actually benefit from the refuge of the burrow in an environment that would otherwise fatally desiccate these crustaceans. The parents, in addition to their offspring, form a sort of colony and individuals will embark on provisioning trips for food which is brought back to the burrow. Over time a faecal embankment builds up around the colony which demarcates the extent of their territory. The pair only raise one brood and an individual only lives about 15 months max.

So what we have here is an isopod, a crustacean, that is dependent on some moisture to keep its tracheal lungs operating- living at the extreme edge of its ability and adapting to this environment with an advanced sociability, burrow construction, food provisioning, pair bonding, and child rearing. Cool. Oh yeah, it is also very successful at this lifestyle and biomass studies indicate it is often the most abundant critter in its environment- supporting a myriad of predators including being the primary prey for a species of scorpion, Scorpio maurus.

Negev Desert, natural habitat for desert woodlouse. 

Of course such an interesting juxtaposition of an intensely social crustacean setting up shop in the desert has stimulated much scientific conjecture. Do harsh environments encourage parental care strategies? How might such a system have evolved? Does this provide a model for how other social invertebrates (social insects) may have evolved their life strategies?

Anyways, pill-bugs rule.


K. Eduard Linsenmair (1974). "Some adaptations of the desert woodlouse Hemilepistus reaumuri (Isopoda, Oniscoidea) to desert environment" (PDF). Verhandlungen der Gesellschaft für Ökologie 4: 183–185.


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