Merkouris et at: Genetic diversity in Chionoecetes bairdi and C opilio 
535 
esize gene flow through these northern seas even in 
the presence of sea ice pack. Alternatively, it is pos- 
sible that gene flow is restricted, but that the differen- 
tiating forces of drift, selection, and mutation have not 
yet produced significant detectable genetic divergence. 
Species differentiation, 
hybrids, and gene introgressiom 
The morphological criteria for differentiating C. 
bairdi and C. opilio have been well described 
(Rathbun, 1925; Garth, 1958; Karinen and Hoopes, 
1971). However, a few crabs in our study that met 
one or more morphological criteria were probably 
hybrid or recombinant individuals. Their composite 
genotypes were atypical of the parental species and 
were instead indicative of Fj hybrids or backcross 
matings. Species differentiation in allelic frequen- 
cies was observed between C. bairdi and C. opilio , 
most notably for AH-3*, IDHP-1* , and PROT-3 *. The 
unusual composite genotypes were most clearly dif- 
ferentiated by the PROT-3* 100 allele in C. bairdi 
collections, and conversely, by the PROT-3*88 allele 
in the C. opilio collections from the Bering Sea. The 
PROT-3* 100 allele contributed significantly to the 
among-regional differences in C. bairdi. This marker 
is also very useful for investigation of hybridization 
between these species. 
Gene flow and geographic barriers 
Our findings are consistent with previous electro- 
phoretic studies that infer that decapod crustaceans, 
especially large, mobile species, exhibit low levels of 
variation at allozyme markers (Nelson and Hedge- 
cock, 1980). The high dispersal potential of marine 
species is often used to explain the low levels of varia- 
tion detected among populations (Avise, 1994). Such 
distribution appears to be the case with Chionoecetes 
as well as with other marine organisms with similar 
geographic distributions in the North Pacific, Gulf 
of Alaska, and Bering Sea, such as Pacific halibut 
(Grant et al., 1984), Pacific herring (Grant and Ut- 
ter, 1984), Pacific cod (Gadus macrocephalus) (Grant 
et al., 1987), Pacific ocean perch ( Sebastes alutus) 
(Seeb and Gunderson, 1988), and red king crab 
(Paralithodes camtschaticus ) (Seeb et al., 1990a). 
Limits to the actual dispersal of marine species, 
despite high dispersal potential, may periodically or 
continuously limit gene flow in some directions 
(Avise, 1994 and references therein). The Alaska 
Peninsula-Aleutian chain appears to be a significant 
barrier to gene flow in some species, such as Pacific 
ocean perch (Seeb and Gunderson, 1988), Pacific 
herring (Grant and Utter, 1984), rock sole (Pleuro- 
nectes bilineatus) (Mulligan et al., 1995) and red king 
crab (Seeb et al., 1990a). 
The Alaska Peninsula-Aleutian Chain appears to 
limit gene flow among C. bairdi populations as evi- 
denced by the differentiation detected among regions 
above and below this geographic barrier. The lack of 
introgressed C. opilio alleles in the Gulf of Alaska 
and Southeast Alaska populations studied also sup- 
ports this view. Our findings suggest that the nu- 
merous rare alleles observed in Bering Sea, and not 
Atlantic, populations of Chionoecetes may be ancient 
alleles that have been maintained over time in the 
large populations that have inhabited these waters; 
however, larger sample sizes of Atlantic Ocean C. opilio 
are needed to confirm this hypothesis. Detection of low- 
frequency Bering Sea Chionoecetes alleles in North 
Atlantic C. opilio and the lower heterozygosity of North 
Atlantic C. opilio, coupled with the lack of congeners 
in the North Atlantic, allow speculation that the genus 
Chionoecetes may have arisen in the North Pacific. 
Acknowledgments 
The authors thank the following Alaska Department 
of Fish and Game personnel for population sample 
collections: Dean Beers, Cathy Bothelo, Bill 
Donaldson, Wayne Donaldson, Lee Hammarstrom, 
Ken Imamura, Dave Jackson, Al Kimker, Tim 
Koeneman, Al Spalinger, Dan Urban, and especially 
Donn Tracy, Ken Griffin, and Ranee Morrison. We 
thank the National Marine Fisheries Service person- 
nel for collections: Pete Cummiskey, Jan Haaga, John 
Karinen, Rich Macintosh, Eric Munk, and Brad 
Stevens, and we thank Bob Otto for his efforts in 
coordinating the Atlantic C. opilio collection. We also 
thank Dave Armstrong of the University of Wash- 
ington, and M. John Trembley of the Canadian De- 
partment of Fisheries and Oceans for their assistance. 
Penny Crane and three anonymous reviewers provided 
critical reviews. This research was funded in part by a 
cooperative agreement with the National Oceanic and 
Atmospheric Administration (award NA37FL0333). 
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