Left: T. virginea covered by algae; identified by white faecal pellets with hemispherical ends, and pale feeding pits in encrusting alga. © D. Kipling.
Right: Juvenile and adult T. virginea. Adult apically and juvenile have brownish marks resembling T. testudinalis. Juvenile identified by inwardly oriented repugnatory glands and non-green mantle © A. Wake.
A particular problem with documenting the recent decline northwards of T. testudinalis has been the very frequent mistaken recording of T. virginea as it. Two of the causes are:
a) Specimens of T. virginea can be mistaken for it if the exterior markings are concealed by epizooic growths, as above left..
and
b) The markings on early juvenile T. virginea, such as tiny specimens from shell-grit samples, are often brownish and mistaken for those of Testudinalia testudinalis, as above right.
APPENDIX re range advance/retreat: flic.kr/p/2jW74ig