Littorina mariae

Paula Lightfoot Thank you very much for your comments on my Onoba photos! I also have a question about these periwinkles - is the smaller, darker one Littorina mariae and the larger one on the right is L. obtusata?

Ian Smith Hi Paula Much depends on the exposure of the shore and shore zone/algae they are found on as there are very distinct ecotypes. The fullest info is in an expensive Ray Society monograph on Littorina by David Reid of the Natural History Museum (BM). A free pdf is at http://www.field-studies-council.org/fieldstudies/documents/vol7.3_202.pdf but it is mainly from a study on the sheltered Menai Strait and has been overtaken by Reid (L. mariae is now L. fabalis). I'm going to duck naming your two shells; maybe Jan will be braver:-)

Ian Smith second thoughts; both L.obtusata. One on left has typical flare downwards of aperture found on immatures. One on right has wide open unrestricted aperture of obtusata.

Dave Rolfe Left one also has developed aperture of an adult fabalis, I would say fabalis on the left, obtusata on the right. To me it looks too big and well developed to be obtusata.

Jan Light I agree with Ian, both these specimens are Littorina obtusata.

Dave Rolfe Let us agree to differ, without the actual mollusc there is no absolute way of telling. I have the same problem with the L. saxatilis complex even though I have the book!