Surveys of mitochondrial DNA (mtDNA) variation in the giant tiger prawn, Penaeus monodon, using restriction fragment length polymorphisms have provided the first clear evidence that the Indo-West Pacific region is a site of accumulation of genetic diversity rather than a site of origin of genetic diversity. No haplotyes were found in common between a group of five southeast African populations and a group of five Australian (including Western Australia) and three southeast Asian populations. The dominant haplotype was different in the Australian and southeast Asian population groups. Genetic diversity (pi) was greatest in Indonesia (pi averaged 0.05), less in the Philippines and Australia (pi averaged 0.01), and markedly less in the southeast African and the West Australian populations (pi averaged 0.003). The high diversity of the southeast Asian populations resulted from the occurrence in those populations of a set of haplotypes found only in southeast Asia but derived from the southeast African haplotypes. These genetic variants therefore evolved in the Indian Ocean and later migrated into the Indo-West Pacific region. Low genetic variation in the geographically marginal populations in southeast Africa and Western Australia is considered to be the result of bottlenecks, but mismatch distributions suggest that large population sizes have been maintained in Indonesian populations for long periods.

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The solitary ascidian Pyura stolon;tera occurs in dense beds on the low intertidal and shallow subtidal rocky shore along the entire Soulh African coastline. The organism is used as bait by fishermen and is also heavily exploited for food in certain areas. The crevices and interstices between individuals in dense beds of P. sto/onifera provide a safe and stable habitat for a wide variety of benthic macrainvertebrates. Sixty-four and 61 taxa representing 10 phyla, of associated organisms were recorded respectively in an intertidal and a subtidal P. stoJon;tera bed. Forty-two taxa were common to both, but communities of macroinvertebrates associated with intertidal and subtidal P. stolonifera beds were different. Numerically, polychaetes (30%) were the dominant group intertidally, and crustaceans (40'%) subtidally. Porifera formed 71o:o1.:, of the biomass of associated intertidal organisms, while subtidal biomass was dominated by the bivalves Striostrea margaritacea and Perna perna (87%). Mean dry biomass of macroinvertebrates was 366 g.m-2 (SO= 196) intertidally and 670 g.m-" (SO = 119) subtidally. These values are between four and eight times higher than those recorded on the southern Cape coast. Recolonization of cleared areas is slow, so considerable secondary production is lost when harvesting practices result in bare patches in P. stolonifera beds.

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