I. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Improvement of rotifer as the new food item in larviculture†
K. Tsuneizumi,∗1 M. Yamada,∗1 K. Ichinose,∗1 H. Ichida,∗1 and T. Abe∗1
In larviculture, rotifers (Brachionus plicatilis sensestrict) are generally used as the initial food source.1) The
lorica length of rotifers is distributed in the range of 170–
320 µm in individuals carrying amictic eggs. The next
food item Artemia nauplii has a body length of 400–1,000 µm. Although the proper management of feed
size and density associated with ﬁsh larval growth isneeded,2) there is a size gap between rotifers and Artemia
nauplii. No feed items to bridge the size gap of 320–
400 µm have been developed in the food scheme of ﬁshlarvae.
To improve the mass mortality and abnormal development of ﬁsh larvae during the period of growth corresponding to the size gap, this study applied carbon
and argon heavy-ion-beam irradiation in mutation breeding3,4) to select rotifer mutants with larger lorica sizes.
In our previous study, the Notojima strain, known
as the largest rotifer strain in Japan, was irradiatedwith carbon (C) and argon (Ar) ion beams at diﬀerent
doses. We developed a screening method with the lorica
length as an indicator and established 56 large mutants(Table 1).5) In this study, we measured the population
growth rate as the parameter of good bait. The population growth rate of each large mutant line was observedwith 5-mL cultures. For each line, every ﬁve individuals were inoculated into 6 wells containing 5 mL offresh culture medium (18 practical salinity units) withChlorella. The total number of rotifers in each well wascounted after 5 days. The population growth rate of eachwell was calculated with the following equation: population growth rate = ln (total population/5 individuals)/5Fig. 1. Photograph of rotifer lines with proliferative diﬀerence and the control. (a) Control rotifer. (b1-3) Largeand proliferative mutants with higher population growth
rates than the control. Values are mean ± standard errors, and images of rotifer lines are of individuals withthe average lorica lengths. Scale bars represent 200 µm.
†∗1Condensed from the article in Biosci. Biotech. and Biochem.
(DOI: 10.1093/bbb/zbaa094) (2021)
RIKEN Nishina Center

Table 1. Frequency of large and rapid-proliferative mutants

days). The average values of each large mutant line werecalculated using data obtained with six replications andstandardized using the control group.
The following 3 mutant lines showed a signiﬁcantly
higher population growth rate than the control: TYA41(Steel’s multiple comparison tests, P < 0.05), TYC78,and TYC176 (P < 0.01) (Fig. 1). The large mutant
lines showed an equal or higher growth rate than thecontrol are categorized as the rapid proliferation in Table 1. The frequency of appearance was calculated using
the mutant lines showing active proliferation after irradiation: mutant frequency (%) = (number of rapid proliferation)/(number of active proliferation) × 100. The
highest frequency was observed at 100 and 200 Gy with
the C-ion beam. For TYC78 produced at 200 Gy, we
decided that a C-ion beam of 200 Gy was the optimal
irradiation condition to establish a large rotifer. Similarly, the highest frequency was observed at 50 Gy with
the Ar-ion beam, and TYA41 was selected from the same
condition. This was also determined to be the optimum
irradiation dose. Thus, these three mutants are suitable
to solve the feed-size gap between rotifers and Artemia
nauplii and should improve the marine ﬁsh larviculture
system.
Food shortages due to population growth and increased consumption are a major global concern, andcountries around the world are searching for ways to increase food production. The enlarged rotifers obtainedin this study could potentially provide a stable supply oflarger rotifers at low cost, enhancing aquaculture.
References

1) S. Mills et al., Hydrobiologia 796, 39 (2017).
2) T. Kotani et al., J. World Aquac. Soc. 40, 383 (2009).
3) T. Abe et al., in Plant Mutation Breeding and Biotechnology,edited by Q. Y. Shu et al. (CABI, Oxfordshire, 2012), p. 99.
4) A. Tanaka et al., J. Radiat. Res. 51, 223 (2010).
5) K. Tsuneizumi et al., RIKEN Accel. Prog. Rep. 53, 213 (2020).

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