The striped mullet Mugil cephalus L. is a circumtropical species whose extreme conservative morphology stands in contrast with the degree of genetic differentiation at a global scale. One hundred and fourteen mitochondrial control region DNA sequences were analysed from four localities in the Gulf of Mexico (Texas, Louisiana, Mississippi, Florida), one on the U.S.A. east coast (N. Carolina), and one in Hawaii, giving very high levels of molecular diversity (h=1·0 every haplotype was unique in all samples, =1·1-2·0% in Gulf-Atlantic and =3·1% in Hawaii). With no genetic evidence of dramatic population expansions, mismatch distributions were still very different in each ocean. Stable population levels in Hawaii have fostered the generation and persistence of very high molecular diversity, but Gulf-Atlantic samples suggest a shorter span of population stability. Ninety-five per cent of the molecular variance was allocated between ocean basins and virtually none among Gulf-Atlantic samples. A neighbourjoining reconstruction revealed Atlantic-and Pacific-specific lineages separated by more than 24% uncorrected sequence divergence (d=0·49 Tamura-Nei Gamma-corrected). The lack of phylogeographic structure among Gulf-Atlantic samples corroborated the AMOVA results and supported the existence of a single population with high levels of gene flow along the Gulf of Mexico and north-west Atlantic coasts. The genetic differentiation between oceans points to the absence of gene flow and an accelerated rate of mitochondrial evolution in the genus Mugil. 2000 The Fisheries Society of the British Isles

The mitochondrial DNA control regions of red snapper (Lutjanus campechanus) from the Gulf of Mexico (n = 140) and Atlantic coast of Florida (n = 35) were sequenced to generate a prestocking genetic baseline for planned stock enhancement. Intrasample haplotype and nucleotide diversities ranged from 0.94 to 1.00 and 1.8% to 2.5%, respectively. All population analyses were consistent with the hypothesis that red snapper constitute a single, panmictic population over the sampled range. A ubiquitous, predominant haplotype, shared by 23% of the specimens, appeared to be evolutionarily recent, in contrast to previous findings based on restriction fragment length polymorphism data. Tajima's D values were suggestive of a recent bottleneck. Mismatch distributions from Gulf samples were smooth and unimodal, characteristic of recent population expansion. However, the Atlantic sample exhibited a comparatively broader, possibly multimodal distribution, suggestive of a more stable population history. Additional control-region data may clarify potentially disparate demographic histories of Gulf and Atlantic snapper.

Infections of the virus Baculovirus penaei (BP) have historically impacted penaeid shrimp production in both hatcheries and ponds. BP causes cytopathological alterations and mortality in at least four species, including Penueus vannamei. This study established experimental infections with BP in laboratory‐reared P. vannamei. The most useful protocol involved BP infection in third substage protozoea (P3) induced by feeding virus‐contaminated material to rotifers and, in turn, feeding those rotifers to the shrimp larva. Infections were also established by delivering virus‐containing brine shrimp to mysis (M) and postlarval (PL) stages. When virus originating from infected adults and juveniles was fed to P3's, the shrimp exhibited patent infections with hypertrophic nuclei, polyhedra, free virions, and occluded virions five or six days after being fed the virus. In contrast, when the source of virus material was from bioassay larvae rather than from adults and juveniles, similar patent infections developed in P3's by one to two days. A significant mortality in the resulting M's and PL's was associated with the infections with short but not long prepatent periods. In experimentally infected shrimp, examination by light microscopy and transmission electron microscopy revealed extensive viral infection in many cells in the anterior midgut and as many as 80–90% of the proximal and medial hepatopancreatic tubular cells. Free and occluded virions capable of producing disease ruptured into the gut lumen soon after infections became patent. Tests conducted in 1 L Imhoff cones, 160 L spat‐cones, and aquaria all produced infections, usually with a prevalence of 100%. The system provides a useful method to detect and assay for infective agents, to amass infective material for research purposes, and to assess the biology of and host response to the virus under different conditions.