How Fish Are Adapted to Depth of Water.

Hawke's Bay Tribune, Volume III, Issue 361, 21 February 1914, Page 4 (Supplement)

How Fish Are Adapted to Depth of Water.

Certain fish living on river bottoms have the swimming bladder protected more or less by a cartilage or bony covering, and as M. Otto Thilo remarks, it ie curious to note that fish which live upon the sea bottom have no such protection. The covering may be only partial, as in the carp, ranging up to a complete covering ds is found in the Cobitis fossilis which lives almost entirely buried in the mud. There is a perfect adaptation between the structurenf the swimming bladder and the fish’s habits. When the bladder is full of air it tends to lengthen and compress other organs. In the Saroodaces odoi of the Kameroon region, the bladder contains but little air, so that there is scarcely any strengthening to be seen here. An extreme case is found in the carp, and as the swimming bladder is much filled out with air it is protected by a shield in front, preventing it from pressing on the heart and esophagus. Were it not so, the bladder would hinder the blood circulation and respiration. Thus the amount of protection keeps pace with the filling out of this organ. In some eases the bladder has a fine elastic inner membrane surrounded by a stouter one, thus resembling a rubber tyre.

While river-bottom fish have an ossified swimming, bladder, in sea bottom fish it disappears altogether in most cases. The natujre of the bottom may account for this difference. The air bladder Of a fish entering and leaving soft mud has time to adapt itself to the different pressures. In the case of the Plies, living in sea bottom in a hard and compact -sand, the fish makes a great effort to penetrate into the sand, and when it comes out suddenly its highly-compressed bladdqr would expand so much that it would be in danger of bursting. Hence, in this case the bladder disappears, owing to adaptation to

surroundings.