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Onoba aculeus

(Gould, 1841)

Paula Lightfoot Hi, are these Onoba semicostata? Found in Corallina sp in a rockpool at Scarborough today. The tallest one is 2.5mm long.

Ian Smith I think you are right Paula. O. aculeus is very similar but on it costae occur only on the two top "adapicial" whorls or not at all. The costae on yours aren't very clear, but the middle one certainly has costae on the 4th adapical whorl. Jan Light is the expert on these, maybe she'll post.

Ian Smith PS "costae" are ridges at right angles to the direction of coiling. In this case they cross the many spiral striae that follow the coils round.

Simon Taylor It can be very difficult going through large mixed grit samples of Onoba as there are always some that fall between being obviously one species or the other. I don't know if there are any clues that can be gained from live specimens...

Ian Smith Candidates for DNA sequencing to ascertain if really 2 spp. I think.

Simon Taylor Sounds like a good idea. There's a lovely anatomical drawing of this species in Fretter & Graham's Ray Soc. volume (under the name Cingula semicostata - p.112)

Paula Lightfoot Now I see that both species (O aculeus and O semicostata) were recorded by Jan Light at the same site in Scarborough in 1992, along with 37 other species. http://data.nbn.org.uk/imt/?baselayer=Hybrid&bbox=-0.42125702483723143,54.29592885136656,-0.39460659607004145,54.30376729918417&mode=SINGLE_DATASET&dataset=GA000321

Jan Light Hallo, coming late to this string as our internet was down in France during the storms and this disappeared off the posts that were showing up as current. The three shells grouped together are Onoba aculeus. I think the specimen on black background is O. semicostata. Costae are useful diagnostic although if weakly developed on aculeus this can be a red herring. Key character to work with is relative diameter of protoconch as incision of suture can be variable on the two species.

Jan Light Taking another look at the Onoba above, it has been suggested that the shell on black is the same as the middle specimen of the group of three. Is that correct Paula, it is not easy to see as the right hand side of the shell on black actually grades into its background. To my mind the interior of the aperture looks rather opaque and chalky whereas the interior of the aperture of the middle of three is translucent. Some clarification welcome here please?

Paula Lightfoot Thank you very much for looking at these, yes they are the same three shells (sorry I should have made that clear in the first post!). I tried different backgrounds and lighting while taking photos as I wasn't sure what would work best - it makes quite a difference and I can see now that the black background is not as good as the shell does blur into the background.

Jan Light Thanks for clarifying this. I think the shot of the 3 specimens on the light background works best in terms of ease of recognition. They are all Onoba aculeus. Would you like a separate of a short paper on Onoba which I co-authored in 1998? It contains SEM photos illustrating shells and protoconchs.

Paula Lightfoot Thank you for confirming, I have amended the record. I would be very interested in the paper. I was also wondering about all the 1992 records from Yorkshire in the Conch Soc marine dataset, whether these came from an organised field trip and if so is there a report from it?

Simon Taylor Jan showed me the reference this weekend and it has rather revolutionised my differentiation of the two species. I always felt the costae were not a wholly reliable characteristic because of the intergrades mentioned previously.

Jan Light I'll check the 1992 records for a report. Message me an address to post.

Message posted on British Marine Mollusca on 10 Mar 2013
Animalia (Kingdom)
  Mollusca (Phylum)
    Gastropoda (Class)
      Caenogastropoda (Subclass)
        Littorinimorpha (Order)
          Rissooidea (Superfamily)
            Rissoidae (Family)
              Rissoinae (Subfamily)
                Onoba (Genus)
                  Onoba aculeus (Species)
Associated Species