CLUSTERING EXPERIMENT: BNU, UPMC, Dec 2013


Animal: Euglena gracilis

It is well known that there are clustering phenomena 
for many micro-organisms. The protist Euglena is.
about 50 micrometer in length, it can swim fairly
fast (about 2 or 3 times its length in one second) thanks
to a thin flagellum (not visible on the pictures).  

For our experiments,
the great advantage of micro-organisms is that the
observations involve large numbers, from thousands
to millions. 
The drawback is that one cannot see the
individuals except with a microscope. As a microscope has a 
fairly small observation field, it will not be easy
to monitor the global clustering process.

It should be noted that whereas clustering of ants or bees
occurs without any external influence, 
the kind of cluster shown here occurs under
the influence of light. In addition, the time constant is much
shorter. For ants or bees clustering 
requires about one hour, against only 5 to 10 minutes
for euglenas.

At least, this fast aggregation process
shows that there is no
strong repulsive force between the euglenas even at short range.
Thus, it is quite possible that in proper circumstances
they may cluster even without any external stimulus. As
far as we know, such a process has not been clearly observed so far.
Euglenas kept in a bottle exhibit a decreasing 
concentration gradient from the bottom to the surface.
This gradient is easily observed through the color change from
dark green to light green.
It is true that this gradient 
may be attributed to the effect of gravity.
Hovewer, for other micro-organisms (e.g. paramecium) the
density is fairly uniform which suggests that the
gravity explanation may not be really satisfactory.