Ventilation of Gills

Various mechanical devices are used to increase the flow of water over the gill's surface. There can be two means of increasing the flow of water over the gill surface:

1. By movement of gills as seen in small organisms. Some aquatic insect larvae use this method, but this is not a very efficient and practical method. The force needed to overcome resistance to movement is great and the energy needed to move the gill also increases correspondingly. The mechanical strength of the gill in most larger animals would also need to be increased.
2. By moving water over the gill. This is achieved by ciliary action as in gills of mussels and clams. Movement of water due to a mechanical pump is a device used by fishes and crabs. The locomotion of many aquatic animals .helps to circulate the water. Many pelagic fish, especially tuna, swim rapidly through the water and create a rapid flow over the gills. Among invertebrates the octopus or squid ventilates its gills by taking water in the mantle cavity and ejecting it out through the siphon. This jet propulsion also provides the locomotion.