Philoscia affinis

CRUSTACEA > MALACOSTRACA > ISOPODA > Oniscidea > Crinocheta > Philosciidae

Although this variant species was known to occur in western Europe, it wasn't officially 'discovered' in Britain until 2017 when a specimen, that was initially assumed to be the common striped woodlouse Philoscia muscorum, was recognised as Philoscia affinis from a sample of species that were being examined during a survey in West Sussex. That discovery and realisation subsequently led to various stored collections of Philoscia muscorum being checked and reassessed, which produced a number of individuals that had been incorrectly labelled. At the same time, identification features of the 'new species' were circulated to BMIG recorders and it wasn't long before more specimens were being found.

As a result, Philoscia affinis was placed on the British list where it remains as an officially recognised and established species, but one that is still regarded as nationally scarce as there are so few confirmed records almost certainly down to the simple fact that it continues to be overlooked.

Checking the iRecord database today, nearly ten years on from when this woodlouse was first noted, shows little more than 200 confirmed records which are mostly from north western regions, and with only a few scattered records from elsewhere. There are none from my local area, but that certainly doesn't mean that they can't be found as can be seen from the photos featured below, which are an unexpected discovery of my own that started with a small immature individual deemed 'very likely affinis' by experts from the BMIG. So, on the basis that there had to be more, I went back to the site a couple of weeks later and managed to find around a dozen or so in the leaf litter, but they were only around 6-7mm long and despite being nicely marked I assumed that they were juveniles as Philoscia muscorum are much bigger by comparison as they grow to 11mm. However, that's not necessarily the case with Philoscis affinis as I was subsequently advised that they are adults, albeit small adults, which is not unusual for this species.

Unfortunately, despite confirmation that these specimens are indeed Philoscia affinis and not Philoscia muscorum, positive identification for recording purposes can only be given if an adult male has been examined to check for a specific diagnostic feature being a tiny triangular projection at the base of the merus on the final leg (pereiopod 7).

A reasonably safe assessment though can simply be made by looking at a potential candidate, firstly checking the head pigmentation of larger juveniles and adults, and then pigmentation of the body. In Philoscia muscorum the head is noticeably darker than the body, often but not always black, but always uniformly pigmented and usually with a yellow mark at the back, whereas the head of Philoscia affinis is typically brown speckled with paler yellow mottling and never uniformly pigmented. Understandably this feature is best observed in adults as immature specimens of Philoscia muscorum may have undeveloped head markings that are similar. The body markings of both species are variable, but typically Philoscia affinis lacks the conspicuous orange-brown and creamy white band around the edge of the epimera as usually present in Philoscia muscorum, instead having whitish oval spots towards the lower front edge of each epimeron.

New species added, Sept.26 (v.1)