Development of rotifer strains with useful traits for rearing fish larvae
Introduction
The euryhaline rotifer Brachionus plicatilis has been used as an indispensable source of initial live food for mass-rearing of marine fish larvae. B. plicatilis is a species complex, which includes different morphotypes; culturists describe them based on lorica size as L (large), S (small), and SS (super small) types, and biological traits were compared among morphotypes by Hagiwara et al. (1995b). Based on information on morphology (Fu et al., 1991a), allozyme pattern (Fu et al., 1991b) and karyotypes (Rumengan et al., 1991), Segers (1995) classified L-type as B. plicatilis and others as B. rotundiformis. Recent studies indicate that the so-called S- and SS-type rotifers can be classified as different species; the former as B. ibericus and the latter as B. rotundiformis (Ciros-Perez et al., 2001, Kotani et al., 2005). It should be mentioned, however, that despite the strong species boundary observed between B. plicatilis and others, a weak species boundary was observed between B. ibericus and B. rotundiformis based on the male mating behavior when challenged with females of different species (Kotani et al., 1997). Recent studies in molecular phylogeny using ITS1 and COI (see Section 6) indicate that B. plicatilis sp. complex may include at least 9 species (Gomez et al., 2002). Their results indicate that so-called L-, S- and SS-type rotifers include 4, 4 and 1 species, respectively. But their morphological differences among species have not been clarified and species names have not been given. It is important to continue and further confirm the molecular phylogenic results as the current analyses are based on only two partial DNA sequences. As the current results suggest that the taxonomy of B. plicatilis sp. complex is not yet clear, and in this paper, we use in this publication the terms of L-, S- and SS-type, that are commonly used terms among scientists and technologists in the area of aquaculture biology.
In order to provide cultured rotifers to fish larvae, it is important to produce sufficient large number of rotifers, not only as a cost effective food, but also for increasing survival, growth and viability of fish larvae, resulting in better quality and quantity of larvae (Hagiwara et al., 2001b). For this purpose, it is important to provide the appropriate size of rotifers to the larvae (Hagiwara et al., 2001a), in parallel to enhancing the stability of rotifer cultures to meet the demand of fish larvae consistently. Providing cultures with an appropriate size of rotifers facilitates size dependent selectivity of the feeding larvae and results in larvae with higher survival, growth and stability. For example, selection of smaller and larger sized rotifers is essential for rearing fishes with smaller and larger mouths (Oozeki et al., 1992, Fernandez-Diaz et al., 1994, Olsen et al., 2000, Tanaka et al., 2005). To conduct larval rearing of cold water fish species, it is strongly expected to have rotifer strains with higher tolerance of transfer to cold temperatures.
In this paper, we review recent progress of this area of research, especially those studies that were conducted after the last review publication by the same research group (Hagiwara et al., 2001b). We start with a description of importance of selection of appropriate size of rotifers for successful rearing fish larvae. Second, we discuss the degree of size variation among genetically different rotifer strains and how rotifer size can be artificially manipulated. Third, we review the difference in rotifer reproduction, i.e. population growth and resting egg formation among strains, as well as the resistance against environmental stress (such as an increase in ammonia concentration, protozoa contamination and the effect of increased viscosity). These topics highlight the significance of strain selection for conducting stable rotifer cultures. In this section, we also reviewed the benefit of application of chemical compounds for recovering the health status of rotifers. Fourth, we indicated the significance of cross-mating trials resulting in hybrid strains, which have useful characteristics in terms of size, reproduction and stress resistance. Finally, we describe the initial stage of our work on molecular tools for analyzing specific characteristics of rotifers, which could lead to improve our understanding on the mechanisms involved in the regulation of the rotifer life cycle, and their application in the future, for the development of ideal rotifer strains. In our research project, we employ rotifer strains from the culture collection maintained in our laboratory. These were originally collected from geologically different natural sites and aquaculture farms and cloned before they were added to the culture collection.
Section snippets
Importance of rotifer size for rearing fish larvae
Prey size is important in foraging behavior of fish larvae (Ivlev, 1965, Shirota, 1970). In order to compare the effect of feeding of rotifers with different size, yellowtail Seriola quinqueradiata, spotted halibut Verasper variegatus and Platycephalus sp. were fed from day 0 to day 15–20 posthatch (Hagiwara et al., 2001a). Fish larvae reared in 2000 l tank were transferred to 30 l tank, and rotifers were fed to larvae at 5–30 ind./ml. After 30 min, 20 larvae were sampled and their gut was
Variability and regulation of rotifer size
The lorica length of rotifers shows a rapid increase during the first 48 h after hatching. We generally observe 30–40% increase in size during this stage (Snell and Carrillo, 1984). The lorica length of 2 day old rotifers ranges from 170 to 320 μm in observation made using 70 genetically different rotifer clones, collected from different sites, cultured at 25 °C (Fig. 2, Fu et al., 1991a, Hagiwara et al., 1995b). The ratio of lorica length to width was found constant and independent of the
Reproduction and stress resistance
Temperature and salinity optimum for population growth vary among strains. When culture temperature was regulated at 25–30 °C, the highest population growth was observed at 30 °C for L-type rotifers, while it was at 35 °C for S- and SS-type rotifers (Hagiwara et al., 1995b). Sexual reproduction was induced only at 25 °C with L-type, but with S- and SS-types, it increased at higher temperature. The sexual and asexual reproduction was vigorous at lower salinities for all rotifer types (Hagiwara
Cross-mating for breeding useful rotifer strains
Monogonont rotifers including those belonging to the genus Brachionus show cyclical parthenogenesis and produce fertilized eggs during the mictic generation. The genotypes of some loci of the progeny rotifer from the fertilized eggs are generally different from the one of parental rotifer (Fu et al., 1993). It is an accepted dogma in biology, the meiosis results in reshuffling of the genes in the fertilized diploid progeny, so it is quite clear that traits of the parent generation may not
Application of molecular tools
Molecular biological techniques have been widely used for biochemical, biological and phylogenetical studies. Recently, phylogenetic analyses of DNA sequences from genomic (ribosomal internal transcribed spacer I, ITS1) and mitochondrial (cytochrome oxidase I, COI; 16S rRNA) genes were employed in taxonomical studies on the rotifer populations (Gomez et al., 2002, Derry et al., 2003). However, molecular studies on functional characteristics of rotifers are scarce, although these studies are
Acknowledgments
This research was supported in part by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C), 2004–2005, No. 16580153, Grant-in-Aid for Scientific Research (B), 2006–2008, No. 18380118, and the Nagasaki Prefecture Collaboration of Regional Entities for the Advancement of Technological Excellence, Japan Science and Technology.
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Present address: Department of Marine Biotechnology, Faculty of Life Science and Biotechnology, Fukuyama University, 452-10 Ohama, Innoshima, Hiroshima 722-2101, Japan.