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5 How the four fish astronauts were selected

Fish generally swim in loop patterns when they are exposed to 
microgravity. In the experiments using parabolic flights of 
an airplane, we have found a special strain of Medaka fish 
that does not exhibit looping behavior at all under microgravity. 
The success of the IML-2 Medaka fish experiment owes much to the 
use of this strain. Among the population of this strain, we paid 
much effort to select the final four fish. For such selection, 
various kinds of tests were given to many of the candidate 
Medaka fish.

1. There may be fish tolerant to 
microgravity

   On the Earth, fish body itself can be in near 
microgravity (micro-G) conditions because buoyancy 
is balancing its weight in the water. So, everybody 
had thought that among vertebrates, fish should be 
the easiest one to accomplish reproductive mating 
in space. In days of Skylab (1973) small fish 
(Fundulus) were sent to space, where they 
continuously looped for 3 days until they showed a 
sign of beginning to adapt gradually to micro-G 
(R.J. von Baumgarten et al., Aviat. Space Environ. 
Med., 46: 902-906, 1975). Between such looping fish, 
nobody expected mating performance at all. Even fish 
begin to adapt to space environment after a few days 
of continuous looping, they have already been 
exhausted from looping and eating no food, and as its    
consequence having no vigor at all for mating. Since 
then, 'Fish do loop under micro-G.' has been believed, 
and space studies using fish have been mostly on their 
looping mechanism, regarding it as a model system for 
space motion-sickness in human.
   There are many strains in Medaka fish (Oryzias 
latipes). I had a time to ask to myself what a strain is. 
For example, when we try to establish a new (inbred) 
strain, we start with a pair of fish (a male and a female). 
Usually, such a pair to start with are chosen almost 
randomly from a heterogeneous population. Then, repeating 
sister-brother matings through many generations, if lucky 
to have surviving offspring still, we have established a 
strain. Of course, we may start with a pair of fish which 
have a certain trait or gene(s) of interest. Even in such 
a case, when selecting a startig pair, it is usual to give 
no special considerations on the rest of genes. Thus, an 
established fish strain is a population of fish having an 
identical genetic constitution, however, all genes of which 
came from one pair of fish chosen randomly from among a 
heterogeneous fish population.
   When considering human responses to micro-


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