Shell sizes and usual dominant colours of L. obtusata and L. fabalis at different wave exposures on rocky British shores with substantial tidal range, full marine salinity and fucoid algal growth.
For detailed explanation of exposure scale 1 to 8 see Ballantine (1961), link to pdf in references. “L. obtusata” in Ballantine is sensu lato; it sometimes extends into higher wave exposure than he realized.
Shell images all to scale; mm heights are approximate means of thirteen sites, derived from Goodwin & Fish (1977).
Sheltered shores, not estuarine, scale 8, 7 or 6, are the best for initial experience of differentiating the two species, providing the tidal range is sufficient to clearly separate and define a zone of Ascophyllum nodosum on the middle/upper shore and a zone of Fucus serratus on the lower shore (low spring tide required to expose). Olivacea L. obtusata at its largest will probably be on the Ascophyllum, and easily differentiated from citrina L. fabalis at its smallest with thick aperture walls on the Fucus serratus. Unbiased samples can be obtained by shaking plants into a bucket. Juveniles of both species may be yellow and have a similar shape with an anterior spout, but juveniles from Ascophyllum with adult L. obtusata will likely be that species and be as large as adult L. fabalis and far larger than tiny juveniles of L. fabalis. If findings are as described, the identifications will almost certainly be correct, but examining the penes will add to experience. Sites with phenotypes as described can be found on the narrow inner portion of the Menai Straits between the two bridges and probably extending to Bangor and Y Felinheli; aerial photograph at data.nbn.org.uk/imt/?mode=SPECIES&species=NHMSYS00210… .
L. fabalis does not eat Fucus serratus, but uses the flat fronds as suitable feeding platforms for its diet of micro epiphytes and detritus. It is sometimes absent in sheltered situations, despite the presence of F. serratus, if excess sediment coats the plants, e.g. possible cause of absence in upper reaches of Severn Estuary (Williams, 1994).
Semi exposed shores, Ballantine scale 4, will provide experience at the opposite end of the phenotype sequence. Ascophyllum, a favoured food of L. obtusata, will be absent or present as a few scattered stunted plants, and L. obtusata will likewise be absent. or present as small specimens on Fucus vesiculosus or other algae. Fucus serratus is still usually common at scale 4 and L. fabalis usually achieves its largest size here, sometimes equaling or exceeding any L. obtusata present. Differentiating the species can be very difficult on such shores because their sizes are often similar, the predominant colour form of both species is reticulata, and both tend towards larger apertures with thinner shells. Penes examination is very necessary. A shore of this sort in North Wales is shown in the aerial photograph at data.nbn.org.uk/imt/?mode=SPECIES&species=NHMSYS00210… Large, reticulate L. fabalis were common (penes checked). No L. obtusata were found because they were absent or not detectable on the shore among very similar L. fabalis.
An example of a dwarf form at a more exposed shore, Ballantine scale 3, is shown and discussed at image 51Dof 51 Differentiation L. obtusata/L. fabalis .
Full account at: 02 Differentiation L. obtusata/L. fabalis