GENETIC COMPARISON OF 2 POPULATIONS OF THE DEEP-SEA VENT SHRIMP RIMICARIS EXOCULATA (DECAPODA, BRESILIIDAE) FROM THE MID-ATLANTIC RIDGE

Allozyme data are presented for populations of the bresiliid shrimp Ri micaris exoculata from two hydrothermal vent fields, Trans-Atlantic Ge otraverse (TAG) and Broken Spur, located along the Mid-Atlantic Ridge. These indicate that all morphotypes of R. exoculata examined, includi ng those previously interpreted as representing separate species, are con-specific. Conversely, genetic identity between a single specimen o f Chorocaris sp. and R. exoculata was high for intergeneric comparison s. Genetic variation in the populations of R. exoculata (H-0 = 0.034 t o 0.056) was in the lower range of that estimated for other vent organ isms, but similar to values obtained for other species of caridean shr imps in previous genetic studies. F-statistics were used to examine th e population structure of R. exoculata. Estimates of variance of allel e frequencies among populations (F-ST) between TAG and Broken Spur wer e very low (mean F-ST = 0.001), indicating no significant genetic diff erentiation between these populations although they were separated by similar or equal to 370 km. The number of migrants per generation was estimated from F-ST and by a private-alleles method, and indicates tha t migration between the two fields exceeds 100 individuals per generat ion. This may be because of efficient larval or adult migration or a c ombination of both. Estimates of the correlation between homologous al leles between individuals within local populations (F-IS) of R. exocul ata were high at two enzyme loci and indicate a heterozygote deficienc y which caused a significant deviation from genotype frequencies expec ted under Hardy-Weinberg conditions. This deficiency was caused by the occurrence of rare homozygous genotypes in small individuals. In larg e individuals, rare alleles decreased in frequency or disappeared comp letely. This is discussed in relation to previous genetic investigatio ns on other vent and non-vent organisms.