Terrestrial Isopods's Journal

The Pacific Coast of North Amer from Alaska to Baja CA have 2 native species of Ligia or 'rock louse'. Based on GBIF records and this atlas, the native Ligia pallaisi is supposed to range from Alaska to the San Francisco bay (with a disjunct Santa Cruz population). The native Ligia occidentalis is supposed to range from the California border south to ~ the tip of Baja in Mexico.

The above pic compares the two with Ligia occidentalis at the top. Ligia pallaisi and Ligia occidentalis can be distinguished by the width between the eyes (wide in the former, narrow in the latter) and the length of the 'uropods' (forked tail parts) which are shorter in the former and longer in the latter.

There's also the introduced Ligia exotica from Southern California south into the tropics with even longer uropods which are greater than 1/2 the body length

Odd then to find these two (here by @nanorca13 and here by @chlorophilia) observations of whats pretty clearly Ligia occidentalis in Canada based on uropod length. Here's one of @chlorophilia's photos:

This site does mention Ligia occidentalis in BC, but as I mentioned above, GBIF records and the recent major treatment on these guys don't have any records of these north of CA. So this counts as a discovery in my book. Nice work gang, keep the observations coming!

Our resident woodlice hunter @cedric_lee strikes again. This time he found an observation of a rare woodlice that lives communally with ants called Platyarthrus aiasensis.

This species is from the Mediterranean and was first discovered in the Americas in 1989. It has since been reported from CA and TX with just 2 observations total in GBIF proving that this is a rare find indeed!

In May @berkshirenaturalist posted an observation another member of the genus Platyarthrus hoffmannseggii in CT. Its also non-native to the US but has become established in the North East. Its smoother and flatter than Platyarthrus aiasensis.

This brings our woodlice species count for the US up to 33. Still a small fraction of the 113 reported species from the US meaning there's still alot to discover - get out there and find me some woodlice!

Also apologies for the US focus to this post. Lots of cool woodlice postings coming in from elsewhere, but I definitely lack the expertise to put most of the non-US species in context (I also lack my secret weapon Prof Wright from Pamona @jcwright - who has been invaluable helping confirm IDs). If you are or would like to become/recruit a woodlice 'expert' in your neck of the woods, join our effort!

Common Shiny Woodlouse (Oniscus asellus) is native to Northern Europe, but has become the most frequently encountered Terrestrial Isopod in much of NorthEastern North America.

It can be distinguished by the scalloped flat and jagged edges of its body segments, the 3 segments that make up the tip of its antennas and the often greenish-gold flecks on the back.

Photo: (c) AJC ajcann.wordpress.com, some rights reserved (CC BY-SA)

@metsa discovered the first ever published record* of Common Shiny Woodlouse from California in San Francisco in 2014. And this week @kestrel has discovered the second in the East Bay!

Its unclear whether this little invader will ever become as common in California as they have become in much of the NorthEast. But its certainly possible, and if they do, tracking their spread could be very interesting. So keep your eyes peeled for these little buggers if you happen to be exploring California!

• by 'published record' I mean records published to the Global Biodiversity Information Facility - for all I know there's a record buried in a museum or agency somewhere. But, hey, publish or perish right? There's also no mention of Oniscus asellus in either Jass and Kalausmeier, 2001 or Garthwaite and Lawson, 1992 which cover Terrestrial Isopods in all of California and the Bay Area respectively.

@cedric_lee came through with another great Woodlice find. This time he reported the first record ever of Common Striped Woodlouse (Philoscia muscorum) reported from California.

This European is common in the Eastern United States where it has been introduced. As a result, its not entirely unexpected from California. But until Cedric's record, has never been reported from the state.

congrats to @cedric_lee for finding the first
Brackenridgia heroldi on iNat!

This native, eyeless woodlouse is typically found under rocks in California during wet weather. It is uncommon everywhere but widely distributed across California. It was first described from specimens form the San Francisco Bay area. But on iNat, so far no one has reported it from there. Way to go Cedric!

60% of all the terrestrial isopod (Oniscidea) observations on iNat belong two two European species that have been introduced around the world. They are the Common Pillbug and one of several species of Woodlice (Porcellio scaber, P. laevis, and P. dilatatus). Because they are so commonly encountered and posted by amateurs there's a lot of mis-IDs resulting from confusion between these groups. Fear not! They are easy to tell apart!

The butt (posterior) end of terrestrial isopods have 2 projections (the 'uropods') sprouting from either side of their triangular shaped last body part (the 'telson'). If you have a Pillbug (aside from being able to roll up) the uropods are very short and don't protrude past the telson. The black arrow here points to one of the two short uropods:

If you have a Woodlouse (aside from NOT being able to roll up), the uropods are pointy and stick out past the telson. The black arrow here points to the telson:


Like these two groups, by far most of the terrestrial isopods in the US are a handful of introduced European species that are actually really easy to learn (and wait for future posts here). Another commonly seen group are the Sea Slaters which are always found along the shore in the US (except one weird mountain one in Hawaii). Pretty much every other native terrestrial isopod in the US belong to amazing but rarely seen critters you have to hunt for. I hope you'll join me in trying to track down and share as many as these crazy creatures as possible!