X.—NURSERY CUSTOMS IN THE FISH-WORLD.
In our last article we drew a contrast between those creatures which thrust their young upon the world at the earliest possible moment, taking care only to lay their eggs in a favourable spot, and leaving them to hatch or be eaten as the case may be, and those which display the most tender care for their offspring at least until they are able to fight for themselves. In the first case, thousands of young have to be brought into the world at one time because of the enormous death-rate which this helplessness brings about; in the second, comparatively few, sometimes only one or two, young ones make up the family.
I propose now to tell you more of this fascinating question; to show you some perfectly amazing instances of the care and love for their young which some of these little mothers and fathers display, and these instances shall be taken from what we call the 'cold-blooded' fishes.
Let us begin with one of the commonest and best-known of our fishes, the stickleback of our ponds and ditches. The male stickleback, as many of you may know, builds a wonderful nest (see fig. 1), in which, when finished, he invites his chosen mate to lay her eggs. As soon as these precious treasures have been entrusted to his care, he makes himself their sole guardian, forcing currents of fresh water through the nest by the violent fanning motion of his breast-fins, and driving away all that come near. Strangely enough, he has to exercise the greatest care to keep out his mate, who would eat every single egg if she could but get the chance! Every now and then he holds an inspection, shakes up the material of the nest, drags out the eggs, and pushes them back into their places again just to make sure that they are safe, and to keep them bathed in clear water. But soon the little fish appear, and then his labours are increased a hundredfold. If they rise more than a certain height above the bottom of the stream, or stray beyond what he regards as a safe limit from the nest, they are immediately seized in his mouth, brought back, and gently puffed or jetted into the nest again. Day and night his watch is kept, till at last they grow too big to be controlled, and are allowed to stray whither they will.
Fig. 1 Three Spined Stickleback
Fig 2 Butter fish
Another of our native fishes, found in the Thames among other places, builds a nest for its young. This is the 'spotted goby' or 'pole-wing.' Here an old shell is made to do duty for a nursery. The shell is turned over so that its hollowed portion forms a roof; the mud from below is scooped away and a tunnel is then made leading away from the chamber so formed. When all is complete, the mother enters and deposits her eggs, and leaves her mate to mount guard over the nest till the young are hatched and make their way out.
In the 'butter-fishes' or 'gunnels' which are found round our coasts, the eggs are rolled into a ball, and jealously nursed by the parents, each in turn coiling its body round the mass, and so protecting it from injury (see fig. 2).