Littorina compressa Jeffreys, 1865 is name currently accepted on WoRMS
Created 23rd February 2018 by I.F.Smith
Littorina compressa has the most distinct shell of any species in the Littorina saxatilis sensu lato complex and can be reliably identified by its sculpture when over 6mm high (apart from in N. Norway to Murman Coast, not included below).
Key identification features
1*: Shell has broad, flat topped or gently curved, straplike, spiral ribs with narrow, deeply incised grooves . 1Lc https://flic.kr/p/q596xT .
2: Ground colour of shell usually yellow, often white, occasionally reddish. Dark spiral lines present on some shores, absent on others 7Lc https://flic.kr/p/S64ejG .
3: Spiral, dark lines, when present, are almost entirely confined to the grooves 1Lc https://flic.kr/p/q596xT . N.B. black lines alone are not diagnostic; there are also nigrolineate forms of L. saxatilis and L. arcana, occasionally with the lines in the grooves .
5: Often a shallow, medial, secondary groove on the ribs. 4Lc https://flic.kr/p/q5iHCp .
6*: Removal of shell from mature female reveals, through translucent mantle, an oviduct that never has shelled embryos.
7**: Penis has 3 to 8 large mamilliform glands (extremes 1 to 13) in a single row along ventral edge 21Lc https://flic.kr/p/R5ME2t . Filament (glandless tip) triangular, sometimes somewhat mucronate 18Lc https://flic.kr/p/Sg8j85 , less than 5% of total length of penis.
8: Sheltered and wave exposed shores. Not on saltings.
Littorina saxatilis sensu stricto (Olivi, 1792) black lined forms.
1: When present, tops of spiral ribs not flattened. Ribs usually narrow, often with raised keel or edge. Intervening furrows, of varying width, not usually narrowly incised. 34Lc https://flic.kr/p/R5MBZT
2: Ground colour of shell varies; white, yellow 5Lc https://flic.kr/p/q3dpnQ , brown, grey, reddish
3: Black or dark brown spiral lines, when present, not usually confined to grooves 35Lc https://flic.kr/p/S8CCZD , but in the grooves occasionally in Britain and frequently in Spain 34Lc https://flic.kr/p/R5MBZT .
4: Interior of lips very often tinted brownish, yellowish, pinkish or other colour, columella lip rarely pure white 35Lc https://flic.kr/p/S8CCZD .
6**: Removal of shell from adult female reveals prominent brood pouch containing shelled embryos 36Lc https://flic.kr/p/R5MBAM . N.B. thin lipped juvenile female L. saxatilis s.s may have vacant oviduct.
7*: Penis on unsedated live males usually has mucronate tip on broad rounded distal end. 37Lc https://flic.kr/p/S8CCJZ . But not all L. saxatilis penes have a mucronate tip, and sometimes L. compressa has a similar tip.
8: On virtually all shores with hard substrate near high water mark, also on saltings.
Littorina arcana Hannaford Ellis, 1978 dark lined forms.
2: Ground colour of shell varies; white, yellow, brown, grey, reddish.
4: Interior of lips whitish, very often tinted brownish, yellowish, pinkish or other colour.
6*: Removal of shell from mature female reveals through translucent mantle an oviduct that never contains shelled embryos. 40Lc https://flic.kr/p/Sg8eGm
7*: Penis on unsedated live males lacks mucronate tip on steadily tapering filament (glandless tip). 41Lc https://flic.kr/p/Sg8ez7
8: Moderately exposed to very exposed shores. Not on saltings.
Juvenile Littorina littorea Linnaeus, 1758 to about 9mm high.
1: Sharp narrow spiral ribs, adapical edge (towards apex) is steeper. 42Lc https://flic.kr/p/S645iA .
2: Ground colour light brown to blackish brown, darkening with growth. Dark spiral lines frequent on ribs, not usually in grooves 42Lc https://flic.kr/p/S645iA .
4*: Columellar lip white with vitreous lustre untinted by other colour. Outer lip thin and usually scalloped and banded brown/white; basal flare brown 42Lc https://flic.kr/p/S645iA .
8: Usually lower on shore than species in <i>L. saxatilis</i> s.l. complex, but some overlap, especially with <i>L. compressa</i>.
9: Transverse lines on adult tentacles 43Lc https://flic.kr/p/S645xy https://flic.kr/p/qacRYq and anterior of body are fragmentary on very small juveniles 44Lc https://flic.kr/p/S6457d , but no longitudinal dorsolateral lines on tentacles.