The use of monosex fish is intrinsically desirable in a variety of fish species in a range of aquaculture production systems. The potential advantages sought from their use may include one or more of the following features: achievement of higher average growth rate, elimination of reproduction, reduction of sexual/territorial behaviour, reduction of variation in harvest size, and reduction of risk of environmental impact resulting from escapes of exotic species.Fish as a group have systems of sex determination which are of considerable biological interest and significance for studies in evolutionary biology. However, they are very variable, relatively poorly understood and give rise to much variation in sex ratio between, and within, species. Enough is known, however, to enable us to say that these systems are often employed in ways which sharply distinguish the fishes from groups such as mammals, birds and reptiles. As a consequence, manipulations of sexual phenotype designed to produce monosex populations are not straightforward and the results are not necessarily predictable. This paper reviews the techniques for production of monosex males, and considers in detail the case of the Y Y / G M T technology in the Nile tilapia, which is the only example of a genetic technology for the production of monosex males so far widely adopted by the aquaculture industry. The considerable benefits accruing from the use of GMT are described. An attempt at projecting future developments in this area of aquaculture is made. 0 0 4 4 -8 4 8 6 / 0 1 / $ -see front matter ©2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 -8 4 8 6 ( 0 1 ) 0 0 5 9 0 -7 284 JA. Beardmore et al / Aquaculture 197 (2001) 283-301

StLircIi gel electrophoi-esis was used to study \-ariation at I I loci in mussels sampled mainly from British co'istal sites. Two types of mussel were identified, 2\f>sti"u ediilis, tlie common mussel rind its soutliern relatit e .\fvti/iir ga//n/~~,/,,or~iricin/i~. Se\ era1 partially dingnortic loci were used to miip tlie distrihtition of tlie t\vo forms. .\fvti/riJ ediilis \\as present ;it all sites sampled in Britain and Ireland hut \\as at low frequency in S\V England; .\I. ga//ul,ioi'incia/ir \vas detected in S\V England, tlie south and west of Ireland. Scotland and S E England. hut was absent from soutli \Vales. the Irish sea co;ists of tValt-s and Ireland. and SE England. Ap;irt fi-om tlie occurrence of . i f . gn//upinz,inrin/is in NE England. this distribution conforms bvith tlie results of studies using morphological characters and parallels the distrihution of many other soutlicrn species in Britain. At thc microgeographical l e d , .If. c d i i / i~ was foriitd to prcfrr more slieltered a n d rstiiarine conditions than :\I. ,~a//opmr,iiiria/i~. .Analysis rising the hest diagnostic loci sliowed that tiybridizati(in is occurring between .if, rdiilis and .\I. gn/lopiur~in~-ia/ir at all localities where the) occiir sympatrically hut that tlie extent of hybridization \aries considerahl) hetween localities. 'The distribution of localities ha\ ing Iiigli proportions of hybrid individuals is hest interpreted hy assuming that 11) hrids ha\ e higher fitness than parental tyries at these localities. :\ stlid! \v.is made o f \ ariation within and between those localities where only .\f. ~diilir indi\ idualsTvei-e ohser\.ed. Little significant geographic \.ariation in dlele frequency was detected, hut significcrnt deficits of 1ieteroz)gotes compared with H;irdy-lVeinberg expectations n e r e seen for most loci. .\n:ilysis suggests that the \Valilund effect is not involxd and that the most likely cause of tlie deficit is low frequencies (if niill 'illeles. In .\I. tdii/iJ no differences in phenotypic \.ariance in shell height and width v w e ohserved hetween samples of multiply lietcrozygoiis and multiply liomozygoiis individuals ;ind no qriietic differences were found bet\vern juveniles and adults. Overall little evidence was found t h a t Iialnncing selectinn is responsible for m'rintrn;ince of the polymoi-phisms studied in .if. edulir. 1 lie ~iatterii ofgcogr;ipliic \.ariation in gene frequencies in d i f v t i /~i s in the British Isles is discussed in relation to variation in tlie south and north of F.rirope and S o r t h . h e r i c a . It is concluded that steep clines in gene li-eqiiencies in .\I. idrrlis ohser\-ed h! otlier \viirkers in tlie Bnltir and in Long 1sl;ind Sound cannot tie .ittribiited to the presencr of .\I. gn//ol"ui,incin/i.

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Abstract. Zoanthid taxonomy is currently in a state of chaos, with many described species very few of which can be reliably identified. As part of a genetically based, objec-

The brine shrimp Artemia is a micro-crustacean, well adapted to the harsh conditions that severely hypersaline environments impose on survival and reproduction. Adaptation to these conditions has taken place at different functional levels or domains, from the individual (molecular-cellular-physiological) to the population level. Such conditions are experienced by very few equivalent macro-planktonic organisms; thus, Artemia can be considered a model animal extremophile offering a unique suite of adaptations that are the focus of this review. The most obvious is a highly efficient osmoregulation system to withstand up to 10 times the salt concentration of ordinary seawater. Under extremely critical environmental conditions, for example when seasonal lakes dry-out, Artemia takes refuge by producing a highly resistant encysted gastrula embryo (cyst) capable of severe dehydration enabling an escape from population extinction. Cysts can be viewed as gene banks that store a genetic memory of historical population conditions. Their occurrence is due to the evolved ability of females to “perceive” forthcoming unstable environmental conditions expressed by their ability to switch reproductive mode, producing either cysts (oviparity) when environmental conditions become deleterious or free-swimming nauplii (ovoviviparity) that are able to maintain the population under suitable conditions. At the population level the trend is for conspecific populations to be fragmented into locally adapted populations, whereas species are restricted to salty lakes in particular regions (regional endemism). The Artemia model depicts adaptation as a complex response to critical life conditions, integrating and refining past and present experiences at all levels of organization. Although we consider an invertebrate restricted to a unique environment, the processes to be discussed are of general biological interest. Finally, we highlight the benefits of understanding the stress response of Artemia for the well-being of human populations.

Electrophoretic studies of proteins remain a primary source of insight into genetic diversity in many species including the Atlantic salmon Salmo salar, one of the most culturally and economically important fish species of the North Atlantic region. Since 1966, >350 scientific papers on protein variation have been published encompassing 25 000þ salmon from over 400 locations in >200 river systems across the species' distribution. Variation has been detected at 30% of the 110 protein loci screened, though most studies examine <40. The method has been applied largely to the investigation of population structure and differentiation, but work has also led to the systematic revision of the genus Salmo and remains the primary source of insight into hybridization in the wild with brown trout Salmo trutta. Spatial patterns of differentiation show temporal stability, both within and among river systems, and strongly support structuring of the species into river and tributary specific populations and the designation of European and North American populations as distinct sub-species. They also show widespread regional differentiation within both continents, beyond the marked subcontinental differences between Baltic Sea and Atlantic Ocean populations in Europe. Most of the differentiation probably reflects gene flow and founder events associated with colonization following the retreat of the glaciers from much of the species' modern range. However, variation at MEP-2* shows strong correlations with environmental temperature, both within and among rivers, and associations with phenotypic performance. This suggests selection is acting on the locus and provides †Author to whom correspondence should be addressed. Tel.: þ44 (0) 796 472060; fax: þ44 (0) 1796 473523; # 2005 Crown copyright compelling evidence for the local adaptation of populations. Protein studies have led to more population centred management of the species and have been exploited in the discrimination of regional stocks in mixed stock analysis in high seas fisheries, particularly in the Baltic Sea, and as markers for the assessment of stocking success. They have also advanced insight into how the genetic character of populations can be changed in cultivation and the potential impact of salmon aquaculture and stocking on wild populations. The method has been largely superseded by DNA based analyses, but the results remain highly relevant to Atlantic salmon management and conservation and are an irreplaceable data set for studying genetic stability of populations over time. # 2005 Crown copyright

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