Could I ask for confirmation or correction of this identification, if there is enough detail in the attached photographs and these notes.
Hayward and Ryland handbook key used, with reference to illustrations and key in Jacobs B.J.M. Revision Palaearctic Sphaeroma.
Preparation of mouthparts not made. Some difficulty discerning, but maxilliped appeared lobed.
Found on vegetated ungrazed salt marsh at mouth of Dee estuary, N. Wales, Britain. Salinity of sea that floods marsh at hw spring tides about 30p.p.thousand, but specimen was active in fresh water from the tap.
7 pairs pereopods, flat lateral uropods, less than 5 segments on pleon, so SPHAEROTOMIDAE.
no notch in posterior of pleotelson.
External edge of exopod not visibly serrated (but specimen 4mm, so juvenile).
Dorsal surface of pleotelson covered in small colourless tubercles that shriveled when dry. The tubercles did not coincide with the coral pink dots and black flecks that were clearly visible within the thickness of the translucent integument. The tubercles are just visible on the outline of the pleotelson in the lateral view photograph.
Jacobs' illustration shows the pleotelson subapically concave and slightly upcurved on L. rugicauda, but mine has a regularly convex pleotelson, like that of Sphaeroma serratum, but I dismissed serratum because of the tubercles on the pleotelson.
Does L. rugicauda develop the concavity with maturity, or have I got the det. wrong?
I have found more specimens of this common brackish water species, and, as I speculated, the pleotelson is subapically concave on more mature specimens. Below is a picture of a 6mm specimen curled into a pill (It's related to woodlice, some of which also curl into pills). I've marked the concavity "c". You can also clearly see the tubercles on the pleotelson.
It looks rather different from my earlier pictures because it unfortunately climbed out of the water and died, so it is dried out and has lost colour.
I have had one comment passed at second hand from a worker in Vladivostok who doubts the identification because she says the pleotelson is smooth on L. rugicauda. This contradicts Hayward & Ryland, Jacobs and its trivial name "rugi cauda" which means rough or rugged tail. However, Jacobs mentions variation in such features as tubercles between different geographical populations of Sphaeroma spp., so it is possible that the pleotelson is smooth in NW Pacific populations of L. rugicauda (if there are any). So I'll stick with my id unless someone can direct me to a convincing alternative.