POPULATION-GENETICS OF TROCHUS-NILOTICUS AND TECTUS-COERULESCENS, TOPSHELLS WITH SHORT-LIVED LARVAE

Trochus niloticus L. and Tectus coerulescens Lmk., two coral reef troc hid gastropods that have similar life-histories including a lecithotro phic larval stage, were sampled from reefs in the northern, central an d southern sections of the Great Barrier Reef (GBR) in 1991. Significa nt sex-ratio biases were noted, and these varied among reefs, apparent ly with latitude, Demographic data suggested that highly discontinuous and localized recruitment occurs. Surveys of allozyme frequencies at 12 loci revealed no significant genetic differences among populations of Trochus niloticus at any geographical scale. High gene flow between zones of the GBR was inferred, with the number of migrants per genera tion (N-m) of the order of 100. Directional selection was thought to o ccur at one locus (GDH). Genetic variability in T. niloticus (H = 0.0 69 to 0.110, and only three loci polymorphic) was low compared with ot her trochids. This was thought to be due to smaller effective populati on size, resulting from an unbalanced sex-ratio, aggregative spatial d istribution of adults, high variance in reproductive success, and/or t he occurrence of population extinctions and recolonisations. In contra st, Tectus coerulescens exhibited significant genetic differences betw een zones, indicating a much lower rate of migration between populatio ns (N-m similar to 1 to 10), and displayed high genetic diversity (H = 0.225 to 0.279). A trend for increasing genetic diversity from the no rthern to southern GBR was found in both species. It is not clear whet her the occurrence of two contrasted population genetic structures in species with apparently similar life-histories is due to ecological or historical factors.