Jaera are ubiquitous intertidal isopods found all around the coasts of Britain and Ireland. You can easily find them by turning over stones, though they are small (typically 2-5mm) and may go unnoticed till you get your eye in for them. They can also be washed from seaweeds. Typically you find clusters of them slowly shuffling away in all direction from the newly exposed side of an overturned stone. They can easily be collected with a fine paint-brush, and if you wish to identify them to species you will need to do so as they cannot usually be identified in the field. A note of the exact microhabitat (including details of where in the intertidal zone they have been found and the presence of any freshwater run-off) should be made for each collection.
There are six Jaera species in Britain and Ireland. One of these, J. hopeana, is very rare, being known in the past from only one location in Devon and having a very specific habitat (it lives ectocommensally on the underside of the isopod Sphaeroma serratum). It should, however, be looked out for wherever its host occurs on southern and south-western coasts. The others species are common and are mostly widespread around the coasts of Britain and Ireland, though they are severely under-recorded. The different species can only be reliably identified by examining males under a microscope (though the shape of J. nordmanni can be distinctive), but if you have the necessary equipment, their identification is not especially difficult. This page offers some tips for doing so. You will need a copy of Naylor & Brandt (2015) Intertidal Marine Isopods or Naylor (1972) British Marine Isopods. The text and keys are the same in both editions. The colour and patterning of individual Jaera, like quite a few other intertidal isopods, varies considerably and is mostly not useful for identification (except perhaps for the rare J. hopeana, whilst the coloration of J. nordmanni can be suggestive).
In order to ensure you have males, you should collect at least ten or a dozen Jaera at a time, making sure not to ignore the smaller ones, which are often the males (except for J. nordmanni, where the males are usually bigger). In most species, the females are generally bigger and may be greenish due to carrying green eggs in their undersides. Different collections of Jaera, even from the same area of shore, should be kept separately as different species can occur close together (or indeed may be mixed together). You can store them in sea-water or kill them instantly in preserving fluid. The first method means that once back home you can examine them alive under the dissecting microscope and take close-up photographs of them under more studio-like conditions; photographing Jaera in the wild is difficult due to their size and the fact that they are normally under a thin film of shiny water (and rarely stay still for long). The disadvantage of this approach is that Jaera will savage each other if kept together in a tube, biting off antennae and sometimes legs, so that preserving them as you collect them ensures the specimens remain otherwise undamaged. In any case, you will need to preserve them to identify them to species. (Don't put Jaera into tubes of water with larger marine isopods, as these will munch them and you'll have nothing other than maybe some fragments to examine when you get back home!)
In order to ID to species, they should first be examined under a dissecting microscope (x20 - x40 magnification). This will enable you to sort the males out from the females. You will need to kill the specimens first; dropping them into 70% ethanol does so quickly. Males can be separated easily from females by examining their reproductive structures (on the undersides of their pleotelson). The male preopercula are distinctive, and are often slightly pigmented, making them easier to see.
At this stage you may already be able to make an identification. The shapes of the preopercula of J. nordmanni and the rare J. hopeana are diagnostic for those species. These can easily be checked under a compound microscope without dissection for further confirmation. If the male preoperculum is T-shaped (this can be seen easily at x20 - x40, and can be further checked under a compound microscope at x40 without dissection), the species belongs to the ‘albifrons’ group (four species). In order to determine the exact species, start by removing the 6th pereopod from one side (only) of the specimen with a pair of fine forceps (best done on a microscope slide in a drop of preserving fluid so that the tiny legs don’t have to be transferred). Removing legs from one side of the specimen only means that the other side is left intact for any future examination. Although this can seem a bit tricky with such small creatures, you'll soon get the hang of it. The 6th pereopod should be examined under a compound microscope (the key features can easily be seen at x40 magnification).
In the case of J. albifrons (s.s.) and J. ischiosetosa, the 6th pereopods are diagnostic and the identifying features are easily seen (a carpal bulge in J. albifrons and a dense cluster of short, curved setae on the ischium in J. ischiosetosa). (Note that it is usually possible to discern the carpal bulge on J. albifrons (s.s.) using only the dissecting microscope, sometimes even on live specimens, but you may want to check more closely.) If the 6th pereopod does not have either of these two features, one of pereopods 2-4 (preferably on the same side of the specimen) should also be examined in the same way. J. praehirsuta will have dense clusters of long, curved setae on the merus, carpus and propodus, whilst J. forsmani will only have few setae on the same leg segments, some of which are usually slightly curved. Furthermore, J. forsmani has a very prominent distal carpal spine on the 6th pereopod, whilst J. praehirsuta does not (though it does have a small spine in the same place). This combination of features will allow you to determine which of these two species you have.
If you have males (preferably more than a few in case you botch dissection or have a mixture of different species), identifying Jaera to species is relatively straight-forward (assuming you have access to dissecting and compound microscopes). Key ID features should be photographed to provide verification of your ID for recording purposes, and specimens should be retained (including detached legs).