subtitle for learnabout section of A SNAIL'S ODYSSEY
  Identification of littorines
  There are 5 indigenous species of Littorina winkles inhabiting the wave-splash areas of west-coast shores.  The largest is Littorina sitkana and the smallest is L. subrotundata, with 3 intermediate-sized species Littorina scutulata, L. plena, and L. keenae.
 

TOP2 ROWS: Photographs of all but L. keenae courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC, photographs of L. keenae courtesy Gary McDonald, Santa Cruz, California CALPHOTOS.

 

 

 

Littorina subrotundata

photograph of littorinid snails Littorina subrotundata, courtesy Linda Schroeder, Seattle, Washington & Pacific Northwest Shell Club


 
photograph of littorinid snails Littorina subrotundata, marsh ecotype, courtesy Linda Schroeder, Seattle, Washington & Pacific Northwest Shell Club
Littorina sitkana showing colour variations
photograph of littorinid snails Littorina subrotundata, marsh ecotype, courtesy Linda Schroeder, Seattle, Washington & Pacific Northwest Shell Club
Littorina scutulata
photograph of littorinid Littorina keenae courtesy Gary McDonald, Santa Cruz
Littorina keenae 2X
 
THIS ROW: Photographs courtesy James Watanabe, Hopkins Marine Station, Pacific Grove, California SeaNet. photograph of Littorina plena courtesy Ken Watanabe, Hopkins Marine Station, Pacific Grove, California
Littorina plena 2X
photograph of Littorina keenae, Hopkins Marine Station, Pacific Grove, California
Littorina keenae 2X
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Research study 1
 

drawings of littorine shells showing measurements required for morphological differentiation of speciesgraph showing principal components used in study of species differentiation of littorines, L. scutulata and L. plenaOf the 5 littorine species on the west coast, Littorina scutulata and L. plena are the hardest to distinguish.  As adults, scutulata is larger than plena.   For equal-sized specimens, and if a 16% chance of error is acceptable, then plena can be distinguished by the presence of an ivory-coloured band on the base of the main body whorl.  If 16% is not good enough, but a 4% chance of error is, then one can use a multivariate statistical approach using several shell dimensions of each species (see drawings on Left). 

The analyses produce 1st and 2nd prinicipal components indicated here as size and shape variation, respectively (see graph). Note the overlap of 4% where differentiation of the two species is obscured.  The author notes that a multivariate statistical approach has the advantage of low error rate, can be used on empty shells, measurements are easily made, and the calculations can be done on a hand calculator.  Murray 1982 Veliger 24: 233.

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Research study 2
 

drawings illustrating differences between 2 species of littorines in egg-capsule morphology and penis shape, Littorina scutulata and L. plenadrawings comparing radular cusps in 2 littorines Littorian scutulata and L. plenaStill not clear, but have some extra time?  Then check out egg-capsule and penis morphology of the 2 species, which should yield 0% error.  Female Littorina plena produce planktonic egg capsules with 2 outer rims of nearly equal diameter, while males possess an attenuated penis with an elongated tip (see drawings on Left). 

In comparison, female L. scutulata produce egg capsules with 2 rims of different diameters, and males possess a somewhat truncated penis. Additionally, the radulae of the 2 species differ significantly in the length-to-width ratios of the rachidian or midline cusp (data not shown here). Finally, and as a major part of their study, the authors show clear separation of the 2 species using electrophoretic techniques.  Mastro et al. 1982 Veliger 24: 239.

NOTE  the mean ratios for N=20 for each species are 0.87 for L. scutulata and 0.99 for L. plena

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Research study 3
 

drawings comparing various morphological features of difference between littorines Littorina scutulata and L. plenadrawings comparing morphology of banding on shells of littorines Littorina plena and L. scutulataA study of Littorina scutulata and L. plena using specimens from California, Oregon, and Washington reviews previously reported differences and adds some new ones (see figure on Left). The drawings show previously established differences in penis morphology and egg cases, but adds the following: a basal ridge is absent on scutulata, a pale white band at the base of the shell is less distinct in scutulata than plena and, finally, checks are larger on scutulata than on plena, by about 4-fold.

Additionally, where bandings on the shell occur in Oregon/Washington specimens, there are usually 2 on the main whorl in plena, but 3 on scutulata (see figure on right). The author notes that plena is more common in large bays where freshwater enters, while scutulata is more common on exposed outer shores.  Rugh 1997 Veliger 40: 350.

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Research study 4
 

drawings showing features that differ in the shells of littorinids L. plena and L. scutulatamap showing frequency of occurrent of littorinids Littorina scutulata and L. plena in collections from northern WashingtonA later study on these 2 species reiterates previous findings regarding pale band, ridge, and size of checks, and adds another difference in tentacle banding and spots. To reiterate, a pale band and ridge most noticeable on the most recent whorl are found more often in L. plena, and large checks are characteristic of Littorina scutulata, while small checks are features of L. plena. New features include interrupted transverse bands or bands and spots on tentacles of Littorina scutulata, in comparison with a broad lengthwise strip with or without bands on tentacles of L. plena.

Collections of littorinids from around northern Washington reveal proportionately more L. plena than L. scutulata and considerable overlap in distributions. The authors note that while L. scutulata tends to be found only in moderately exposed to sheltered areas of Puget Sound, L. plena is found from sheltered areas to photograph of a littorinid Littorina scutulata crawling, courtesy Linda Schroeder, Seattle, Washington & Pacific Northwest Shell Club wave-exposed headlands of the outer coast. Note that this contrasts with what is found in the previous Research Study 3. Hohenlohe & Boulding 2001 J Shellf Res  20: 453. Photograph courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

NOTE  for additional help in identifying west-coast littorines using morphological and allozyme variations, see Boulding et al. 1993 Veliger 36: 43.



 


This crawling Littorina scutulata
presents a good view of the
markings on the tentacles which, for this individual, actually
appear to combine patterns of both species L. scutulata and L. plena

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Research study 5
 

schematic map showing frequency of occurrence of 2 species of littorinid snails Littorina scutulata and L. plena at sites from British Columbia to CaliforniaThe morphological and ecological similarity, and common occurrences, of Littorina scutulata and L. plena along the west coast prompts the question of evolution by sympatric speciation.  Data on distributions from 41 sites from southern British Columbia to southern California show common occurrence in 71% of the sites.  Twenty-four percent of the sites have only plena. The author has compiled estimates from several studies using a variety of sequencing methods, including cytochrome-b sequencing, and arrives at a mean value for divergence of 8.4mya (range = 1.3-18.6mya).  Based on the extent of their overlap in distributions, the author actually favours allopatric speciation followed by range expansion.  The author notes that while there is not much difference in ecological preferences of the 2 species, there appears to be a slight tendency of plena to live higher on the shore and to favour areas with more freshwater influence than scutulata.  Hohenlohe 2003 Veliger 46: 211.

NOTE  the author remarks that this is the most parsimonious hypothesis at this time and is one favoured by other authors.  For more information on evolution of world species see Reid 1996 Systematics & evolution of Littorina.  The Ray Soc., Lond, 463 pp.

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