To build his nest, this little fish chooses a quiet corner, then gathers pieces of green and purple seaweed.  He takes the pieces in his mouth, pushing them about until the shape is to his liking.  Having got his nursery to the right size and shape, the little builder next fastens it together.  How can he do this?  What mortar can he find in the sea?  It is quite simple.  He uses threads, which come from his own body.  He swims round the nest, again and again; and, each time, a thread is spun, binding the clump of weed into a safe, tight nest for the eggs.  When the task is done there is a weed-nursery about the size of your fist.  Now all is ready for the eggs to be laid by the female Stickleback.  You would expect them to be kept in a hole amid the nest, would you not?  Instead of that, they are tucked a few here, a few there, in the weed.

Then the father Stickleback mounts guard.  Woe betide any small fish looking for a dinner of Stickleback eggs!  The gallant little sentry will rush at him, with spines as stiff as fixed bayonets, ready to do battle to the death.  When the young are hatched out he still keeps guard.  They are not allowed out of the nursery for some time.  The watchful parent forces them back if they try to wander out into the perils of the shore-pool.

[Illustration:  Photo:  A.F.  Dauncey.  SKATE’S EGG CASE]

Let us look at another nest-builder—­the Sand Goby, or Spotted Goby, He is common enough in the pools at low tide, but not easy to find.  You can look at him, yet not see him!  For he takes the same colour as the rocks and sands of his home.  Amid the glinting lights and shadows of his rock-pool, with a background of sand, rock, and weed, this little fish is nearly invisible.  Of course it is a dodge, and a useful one, to escape the eye of the enemy!

Perhaps you will not think the Spotted Goby so clever at nest-building as the Stickleback.  He likes to use a “ready-made” house, whereas the Stickleback finds his own “bricks and mortar.”  In the pools of the shore there is no lack of houses to let, the empty homes of shell-fish are there in plenty.  So the little Goby, when nesting time comes, hunts round for the empty shell of a Cockle lying with its hollow side to the sand.

This shell is to be used as the roof for the nursery.  The Goby’s next task is to make a hole beneath the shell.  He sets to work and, by scooping out the sand, makes a hole about as large as a marble.  To keep the sand from tumbling in, he smears the hole with slime, which soon binds hard like mortar.  Now the nursery is nearly ready; but a passage-way is made, passing under the edge of the shell, and then, to make things quite safe, the whole roof is covered with sand:  it then looks more like a bump in the sand than a fish-nursery.

The female Goby enters the nest, and leaves her eggs in it; and then the little father fish is left in charge.  He rests on the sand, near the entrance.  When the little ones appear, he seems to think he has done his duty.  So away, he swims, not staying, like the father Stickleback, to guard the youngsters.  Again we see that the father, and not the mother, is the builder and nurse.