This species is oblong and flattened in shape, measuring up to 10 mm for males and 7 mm for females. Its most striking characteristics are its very long antennae (longer than its body) and long uropods (longer than the pleotelson), as well as the concave edges on the first four pereon segments. The male preoperculum is also a useful diagnostic. The species is a yellowish colour with red or brown mottling.
In the intertidal zone, J. maculosa can be found towards the lower tidal limit (typically under stones) on and among sponges, bryozoans, etc. and in Laminaria holdfasts. It can probably be found all around the coasts of Britain and Ireland, but is most common on southern and western shores.
Naylor, E. & A. Brandt. 2015. Intertidal Marine Isopods. Synopses of the British Fauna (New Series), No. 3. Field Studies Council, for The Linnean Society of London.
http://www.marinespecies.org/aphia.php?p=taxdetails&id=118732
http://www.aphotomarine.com/isopoda_janira_maculosa.html
