一世 (issei):

From Old Reef  ----> Towards New Coral Reef

         _____           _________
        /      \        /         \
       | Old  |        |  New     |
       | Reef |--------|  Coral   |
       |      |   🦑🦑🦑  |  Reef   |
        \_____ /        \_________/
        (Home)       (First Generation Squids Reach - Issei)

The squids moving from the old reef to the new coral reef represent the first generation (issei) arriving at a new location.

一代 (ichidai):

Different Generations at the New Coral Reef

            _________
           /         \
          |  New     |
          |  Coral   |
          |  Reef    |
          |          |
         /|🦑🦑🦑      |\
        / |          |\      (Generation 1 - Ichidai)
       /  |----------| \
      /   |🦑🦑🦑🦑🦑    |  \
     /    |          |   \   (Generation 2 - Ichidai)
    /     |----------|    \
   /      |🦑🦑🦑🦑🦑🦑🦑  |     \ (Generation 3 - Ichidai)
  /       |          |      \
 |        |----------|       |
  \________\_______/_______/

The generations of squids living at the new coral reef represent ichidai - different generations or eras.