Campton et al.: Genetic patchiness among Strombus gigas populations 



257 



or cohort that we sampled in three consecutive years 

 (1988-90). This population or cohort presumably re- 

 sulted from a large recruitment event during the sum- 

 mer and fall of 1987. We estimated that the 1987 ag- 

 gregation at Coffins Patch consisted of at least 250,000 

 animals covering an area of approximately 30 hectares 

 (Berg and Glazer, unpubl.). 



Although the Coffins Patch aggregation appeared to 

 be a single cohort, we detected a significant allele- 

 frequency variation among years (1988-90) at MDH-1*. 

 This difference was due to the absence of the MDH-1 * 

 (120) allele in the 1988 sample {n 100) versus the pres- 

 ence of eight *100/120 heterozygotes in both the 1989 

 {n 102) and 1990 {n 100) samples. The 1989 sample also 

 had one * 120/120 homozygote. Sampling error does not 

 adequately explain those results because the probability 

 of obtaining all *100/100 homozygotes in the 1988 

 sample was only (0.9552)^"*^ = 0.0001 (assuming the 

 true frequency of the *120 allele was 0.045 [mean of 

 1989 and 1990 samples] and random mating). Similar- 

 ly, differential mortality among genotypes does not 

 adequately explain those results unless heterozygotes 

 were initially very rare and the subsequent mortality 

 of *100/100 homozygotes was extremely high. 



Alternatively, recruitment to the Coffins Patch area 

 in 1987 may have been from more than one source 

 population. This could have resulted in an aggregation 

 that was not distributed randomly. Subsequent mixing 

 and/or possible immigration of juveniles (e.g., Stoner 

 1989) could thus explain changes in allele frequencies 

 between 1988 and 1989. None of these hypotheses can 

 be excluded with the available data. 



Regardless of actual mechanism, the presence of only 

 one highly abundant year-class at Coffins Patch over 

 a 3-year period indicates that recruitment to specific 

 localities in the Florida Keys can be highly variable and 

 unpredictable. This observation thus supports the in- 

 terpretation that genetic patchiness may simply reflect 

 stochastic events prior to settlement. 



DPEP-1 * We observed a consistent deficit of het- 

 erozygotes (with respect to Hardy- Weinberg expecta- 

 tions) at DPEP-1 * but not at other loci. Similar deficits 

 of heterozygotes have been reported often for marine 

 mollusks (reviewed by Gaffney et al. 1990). Such 

 deficits are frequently associated with positive correla- 

 tions between body size and individual heterozygosity. 

 We also observed a positive correlation between body 

 size and heterozygosity, but genotypic variation at 

 DPEP-1* did not contribute to that correlation. These 

 results will be described in detail elsewhere (Campton 

 et al. In press). 



PGM-2* One possible point of inconsistency be- 

 tween the study described here and that of Mitton et 



al. (1989) concerns data iov PGM-2*. Mitton et al. (1989) 

 presented only limited data for this latter locus (9 of 

 17 populations), but those investigators consistently 

 observed a high frequency (0.57-0.69) polymorphism 

 for a "slow" allele. In contrast, we found PGM-2* to 

 be fixed for a single allele. Only PGM-1* and PGM-2* 

 are expressed in digestive gland tissue, which was the 

 only tissue assayed by Mitton et al (1989). However, 

 we also scored PGM in foot muscle which clearly re- 

 vealed a third, more cathodal locus {PGM-3*). We also 

 observed three distinct loci in foot tissue of a second 

 conch species, S. costatus. 



At least three possibilities could thus account for the 

 apparent difference between our results and those of 

 Mitton et al. (1989) at PGM-2*: (1) our inability to 

 resolve the variant electromorph at PGM-2*, (2) the 

 partial expression of the PGM-3* locus in digestive 

 gland tissue (e.g., Allendorf et al. 1983) of individuals 

 sampled by Mitton et al. (1989), thus giving false 

 readings of heterozygotes at PGM-2*, or (3) the re- 

 ported allele-frequency difference between the two 

 groups of populations are indeed real. Of the three 

 possibilities, we believe explanations (1) and (2) are the 

 most likely because of the high consistency of our allele 

 frequencies with those of Mitton et al. (1989) at all 

 other loci. Consequently, we believe that this apparent 

 discrepancy at PGM-2* most likely reflects laboratory- 

 specific adaptations of basic electrophoretic pro- 

 cedures. In this context, we were able to resolve several 

 loci not resolved by Mitton et al. (1989) and vice- versa. 



Conclusions 



The major finding of our study was the existence of 

 spatial and temporal genetic patchiness among popula- 

 tions of queen conch in the Florida Keys and Bimini. 

 We suggest that such genetic patchiness most likely 

 results from presettlement stochastic events and pro- 

 cesses in the marine environment. Nevertheless, these 

 populations are all very similar genetically, presumably 

 reflecting high levels of gene flow due to larval drift. 

 These interpretations are consistent with the results 

 of Mitton et al. (1989) and also explain similar patterns 

 of "chaotic genetic patchiness" in other taxa of marine 

 invertebrates. 



Acknowledgments 



We thank R. Estling, A. Kirkley, and W. Schumacher 

 for their assistance in the laboratory. 



