P. Borsa et Jah. Benzie, POPULATION-GENETICS OF TROCHUS-NILOTICUS AND TECTUS-COERULESCENS, TOPSHELLS WITH SHORT-LIVED LARVAE, Marine Biology, 125(3), 1996, pp. 531-541
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.