John A. Beardmore

Artemia : basic and applied biology

Preface. Editorial Note on Terminology. Acknowledgments. I. Artemia Morphology and Structure. II. Reproductive Biology of Artemia. III. Physiological and Biochemical Aspects of Artemia Ecology. IV. Zoogeography. V. Evolution and Speciation. VI. Applications of Artemia. Index.

Monosex male production in finfish as exemplified by tilapia: applications, problems, and prospects

Abstract 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 YY/GMT 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.

Environmental sex determination: the effect of temperature and salinity on sex ratio in Oreochromis niloticus L

Abstract This paper reports the effects of environmental conditions during the period of sex differentiation on the sex ratio of the Nile tilapia (Oreochromis niloticus). Different sex genotypes were exposed to varying temperatures (putative all-female, all-male and all-YY males) and salinities (putative all-female progeny only) for a minimum period of 21 days after first feeding and were on grown prior to sexing by gonad squash. The majority of the putative all-female progeny exposed to high temperature (36.54±0.39°C) produced significantly higher percentages of males compared to controls reared at ambient temperature (27.87±1.40°C). Similarly, at high temperature, some of the all-male and YY male progenies had significantly lower percentage of males compared to controls. Sex differentiation in YY males appears to be more labile than in normal XY males although this could possibly be attributable to different levels of inbreeding. Low temperature (25.78±0.24°C) and varying levels of salinity (11.30 to 26.65 ppt) did not significantly affect sex ratios. The apparent sensitivity of sex differentiation to some environmental factors is considered in the context of a predominantly monofactorial genetic sex determining mechanism. Implications for sex control technologies are discussed.

The ‘YY’ supermale in Oreochromis niloticus (L.) and its potential in aquaculture

Abstract A male Oreochromis niloticus obtained by gynogenesis was found to sire 100% male progenies consistently when crossed with normal females. Evidence suggests that this fish is a ‘YY’ supermale and that its mother was spontaneously sex reversd (XY). Hormone treatment of normal fry followed by progeny testing was used to generate sex-reversed male and female O. niloticus . Intercrosses among these sex-reversed fish, normal males and females, and the supermale result in progeny sex ratios which are consistent with a monofactorial system of sex determination in O. niloticus where males are XY and females XX. However, some exceptional results were obtained suggesting the existence of minor sex-ratio modifying factors. It is shown experimentally that YY males can be used to study sex reversal to female in this species without the need for progeny testing. A programme for the commercial production and maintenance of YY broodstock and its use for the generation of monosex male progenies in aquaculture is discussed.

Evolution and Speciation

The brine shrimp Artemia comprises a group of bisexual and parthenogenetic, morphologically similar, species very likely to have diverged from an ancestral form living in the Mediterranean area some 5.5 million years ago (Abreu-Grobois and Beardmore, 1982; Abreu-Grobois, 1987; Badaracco et al. 1987), though this estimate, based on allozymes, could be somewhat conservative as compared to that based on mitochondrial DNA (Perez et al. 1994). At that time the area was, according to geological indications, the only place in the world exhibiting for significant periods of time the very high salinity required for Artemia to thrive (Abreu-Grobois, 1987; Badaracco et al. 1987). The hypothesis of the Mediterranean as the centre of radiation for Artemia is also supported by the diversity of Artemia types currently found in the area, i.e. bisexuality and parthenogenesis on the one hand, together with diploidy and polyploidy on the other (see below).

Patterns of genetic subdivision in populations of a clonal cnidarian, Zoanthus coppingeri, from the Great Barrier Reef

Samples of an intertidal zoanthid, Zoanthus coppingeri, Haddon and Shackelton, 1891, were collected from three localities in the Great Barrier Reef region during 1992–1993, and subjected to allozyme electrophoretic analysis at seven polymorphic loci. The reduced ratio of observed to expected genotypic diversity indicated that populations were partly clonal, but they were not dominated by a few clones as occurs in some other cnidarians. Regular disturbance by wave action is postulated to prevent the formation of large stands of particular clones by clearing space and mixing genotypes over small scales. The sexual origin of clonal genotypes was confirmed by conformance to Hardy-Weinberg predictions of genotype frequencies at all but one locus. Values of the standardised genetic variance among populations, FST, were highly significant between localities and between replicate sites within localities separated by only 50 m. Strong genetic structure has not previously been described in a Great Barrier Reef invertebrate species, and is considered to be the consequence of stochastic changes in gene frequencies as a result of low levels of gene flow. High clonal longevity and low recruitment rates may maintain genetic differences over long periods. Similar effects may be seen in other Great Barrier Reef invertebrate species with comparable reproductive patterns.

Growth performance trials of genetically male tilapia (GMT) derived from YY-males in Oreochromis niloticus L.: On station comparisons with mixed sex and sex reversed male populations

Genetic manipulation of predominantly monofactorial sex determining mechanisms in the Nile tilapia Oreochromis niloticus is proposed as a technology for the mass production of all-male progeny for culture. Existing alternative methods for producing all-male tilapia, principally direct hormonal masculinization, have produced inconsistent results and are largely the preserve of scientists and developed country aquaculturists. Successful adoption of these techniques in developing countries is seriously limited by the number and complexity of factors involved in their effectiveness. Novel YY-male genotypes have been shown to be viable and to produce all- or nearly all-male (XY) progeny in crosses with normal XX females. This paper presents the results from a series of on-station growth performance trials of these genetically male tilapia (GMT) in three environments. Two comprehensive and replicated trials in extensively managed earthen ponds, compared the performance of GMT with sex reversed male (SRT) and normal mixed sex tilapia (MST). Three further, smaller experiments, compared the growth of GMT with that of MST populations in rice-cum-fish culture and under intensive culture in closed recirculating systems. These comparisons were made within the ‘Egypt-UCS’ strain of O. niloticus. Results indicate that culture of GMT populations can improve marketable yields under all environments. In the first experiment in ponds, GMT had significantly (P = 0.003) higher yields than both MST (+ 58.8%) and SRT (+ 31.03%). The second experiment, following the same design, showed a similar trend, although differences were smaller and not significant. GMT produced yields 11.9% greater than MST in rice-cum-fish culture and 33.6–47.0% greater in recirculating systems. There was no recruitment in GMT pond populations whereas recruits were present in MST and SRT populations. Coefficient of variance for individual weight in ponds was significantly (P < 0.05) lower for GMT than for MST for both pond experiments. GMT produced from novel YY-males in this strain has considerable potential for increasing yields from tilapia culture.

Genetic and morphometric differentiation in Old World bisexual species of Artemia (the brine shrimp)

We studied bisexual populations of Eastern Old World (EOW) brine shrimp (Artemia) from China (A. sinica and Artemia sp.), Kazakhstan (Artemia sp.) and Iran (A. urmiana), together with one population of Western Old World (WOW) A. tunisiana (Italy) and a New World (NW) population of A. franciscana (U.S.A.). Allozyme electrophoresis, discriminant analyses of morphometric characters, and laboratory tests for reproductive isolation were performed. Brine shrimp show a wide range of levels of both genetic variation and differentiation, based on 20 loci (mean He = 0.058–0.108; Neis D = 0.005–1.501). Mean genetic distance values are of an order usually associated with specific separation. Barriers to gene flow between EOW and WOW populations are not absolute as they are between Old World and New World populations. Among EOW populations no significant reproductive isolation was found, and high levels of genetic differentiation exist (mean FST = 0.616) despite minimal apparent reproductive isolation. However, discrimination based on morphometric characters within the EOW group clearly separates A. urmiana from the other populations. The use of male morphometric characters has proven at least as informative in correctly assigning individuals to the appropriate group as the more traditionally used female characters. There is no evident overall congruence between genetic, geographical and morphometric distances between the EOW populations studied.

High genetic variability and patchiness in a common Great Barrier Reef zoanthid (Palythoa caesia)

Allozyme electrophoretic analysis of seven polymorphic enzyme loci suggested that 1261 samples of Palythoa collected along 1765 km of the Great Barrier Reef during 1992–1993 were members of a single taxon, identified as Palythoa caesia Dana, 1846, with high external morphological variability and possibly encompassing several previously described species. Populations were slightly genetically differentiated (standardized genetic variance, FST=0.010, p<0.05), but there was no evidence of isolation by distance, or of the particular genetic distinction of geographic sets of reefs such as the Swains, as has been observed in other invertebrates. Differentiation was thought to be the result of random selection acting on patches of larvae, not the consequences of long-term reproductive isolation of populations.

Genetic evidence for cryptic speciation in the marine bryozoan Alcyonidium gelatinosum

The common subtidal marine bryozoan Alcyonidium gelatinosum (L.) is one of the most confusing species of a taxonomically very difficult genus. It shows massive variation for almost all morphological characters. A number of sympatric colonies collected off the coast of Lundy (Bristol Channel) could be clearly divided on morphological grounds into two quite distinct types. Using starch gel electrophoresis the products of a range of genetic enzyme loci were compared between the two morphotypes and also between these and other Bristol Channel populations of A. gelatinosum. Minimal differences were observed between one Lundy morphotype and the other allopatric populations, which it was consequently concluded were conspecific. The second morphotype showed considerable genetic differentiation [genetic identity (Nei, 1972) =0.417] from the other Lundy population, with no common alleles at some loci. The probability of obtaining the observed results by chance in a single population is significantly low (P<10−14). It is therefore concluded that the two morphotypes of A. gelatinosum from Lundy are not conspecific.