Because of the dark spiral grooves this form of L. saxatilis s.s. is sometimes mistaken in Iberia, where it is frequent, for L. compressa (e.g. www.asturnatura.com/especie/littorina-compressa.html ). However, investigation by D.G. Reid (1996) and examination of brood pouches by A. Richter (in litt.) confirm that they are L. saxatilis s.s.. Similar misidentifications may be the origin of many “Littorina nigrolineata Gray” records on the south coast of England further east than W. Cornwall.

L. compressa, L. saxatilis and L. arcana can all sometimes have nigrolineate forms with black or dark brown lines in the grooves between the spiral ribs. They are distinguished in Britain, Ireland and Brittany thus:

When present, the spiral ribs on L. saxatilis and L.arcana are usually separated by wide furrows. The tops of the ribs are keeled or tilted causing one edge to be raised; they are not strap-like and level. Black or dark brown spiral lines, when present, can be in the grooves and/ or on the ribs. L. saxatilis s.s. is ovoviviparous, so developing embryos in a large brood pouch are easily seen when a mature female is removed from her shell.

Littorina compressa has broad, flat-topped, strap-like ribs with narrow, deeply incised grooves with steep edges (but ribs on juveniles below 7mm height can have tilted tops). The spiral black lines, when present, are almost entirely confined to the grooves. There is often a shallow, medial, secondary groove on the ribs. L. compressa lays spawn (oviparous), so developing embryos are never seen in a brood pouch on females removed from their shells.

In Iberia, occasionally the grooves are narrower than usual; plate 106 fig O in Reid (1996) shows one. Anatomical examination is desirable for positive identification of such specimens. In the opinion of Reid, this is the form of Littorina saxatilis sensu stricto to which G.E. Gray, in The zoology of Captain Beechey’s voyage 1839, gave the name Littorina nigrolineata (also illustrated on plate 106, fig P). This name was for long after applied to the species now called Littorina compressa Jeffreys, 1865 on WoRMS www.marinespecies.org/aphia.php?p=taxdetails&id=140260

Reference: Reid, D.G. 1996. Systematics and evolution of Littorina. London, Ray Society.

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