A adults, whelks crawl little and, with no free-living larvae, gene flow is consequently low. Thus, snails in widely spaced communities may have intrinsically different population characteristics, i.e., the potential is high for population differences over their geographical range.

Different colour morphs of whelks Nucella lamellosa
collected from a beach in the Strait of Georgia, British
Columbia. The presence of so many varieties in an area of
only a few square meters is not usual for this species 1X

A study of genetic heterozygosity in the whelk Nucella lamellosa in northern Washington and southern British Columbia employs 2 different geographic scales. This first is along 100m of beach near Bellingham, Washington; the second, along 1000km of shoreline extending from San Juan Islands, Washington to entral Oregon.  2286 specimens are collected from 27 intertidal locations and analysed for 2 polymorphic allozymes Pep-2 and Pgm used as population markers.  At one extreme of the geographic scales used the authors find significant allele-frequency differences among 12 breeding populations, each comprising about 120 individuals, sampled on a single low tide along a continuous 100m boulder beach.  Overall, their results, both small- and large-scale, show the largest population subdivision yet reported for a marine gastropod, thus supporting the postulate that gastropods with limited larval and adult dispersals may have genetically fragmented populations.  Grant & Utter 1988 Malacologia 28: 275.

Kelletia kelletii is a buccinid whelk inhabiting west-coast shores from California to Baja California whose range recently has been expanding northwards, past its historic limit at Point Conception to sites as far north as Monterey Bay (see map).  The extent to which this expansion is related to climate change and other factors is examined by researchers from UC Santa Barbara and UC Los Angeles.  The authors compare historic and contemporary distribution patterns in relation to seawater temperature and water-circulation data, and find several points of interest.  First, contemporary populations have lower densities and less regular size-frequency distributions than historical ones,  suggesting a decrease in successful recruitment.  Second, the transition point between historical and contemporary populations corresponds with the confluence point of 2 major ocean currents at Point Conception  (see histogram), but warming trends beginning in the 1970s may have favoured the range expansion northwards.  The data suggest that changing temperature and oceanographic barriers have been major influences in both historical and contemporary patterns in population structure and northern range expansion in the species.  Zacherl et al. 2003 J Biogeogr 30: 913.

NOTE  determined from examination of published records of prehistoric shell middens

NOTE  the collison of currents creates convergent flow, thus presenting a potential barrier to transport of larvae north of the Point

Despite having no free-swimming larva for dispersion, Nucella lamellosa inhabits a range on the west coast of North America at least 1000km north of the southern limit of glacier cover during the last glacial period 14-20,000yr ago.  The explanation for this is either survival in northern refugia during the period of glaciation or post-glacial colonisation by juveniles and/or adults.  As for the latter, crawling would have taken much longer than 20,000yrs, although rafting on algae is a possibility.  Studies on haplotype¹ differences among individuals from several populations of N. lamellosa along the west coast from Oregon to Alaska indicate that the answer is most likely continuous habitation of refugia² during the glacial period. 

Haplotype frequencies at different locations along the coast are shown for Nucella lamellosa on the map. Twenty-five haplotypes are present at 10 sites along the coast, but only one of these, haplotype 6, is common to most sites.  Twenty of the haplotypes are specific to certain sites, with most occurring in Alaska.  Note the disjunction³ in haplotype types and distributions between southern Alaska and northern British Columbia. Although commonly thought of as a continuous sheet of ice during the last glacial period, it is more likely that the majority of coastal glaciers existed as discrete structures separated by areas of uncovered land.  The inter-digitation of ice and land may have disrupted species’ ranges, leading to genetic differentiation.  Marko 2004 Molecular Ecol 13: 597.

NOTE¹  lit. “single type” G., referring to a set of genes in each chromosome which tend to be transmitted as a unit to the next generation.  Haplotypes are defined here from mtDNA nucleotide-sequence data from the first subunit of cytochrome c oxidase.

NOTE²  at least one such refugium has been identified as existing through the last glaciation period on the east side of Graham Island in Haida Gwai, British Columbia, indicated by a blue dot on the map. Warner et al. 1982 Science 218: 675. 

NOTE³   similar disjunctions are described for other west-coast benthic marine invertebrates in the ODYSSEY, including sea cucumbers: LEARN ABOUT SEA CUCUMBERS: GENETIC DRIFT   None of these other invertebrates has a free-living planktonic larval phase