One of the most fascinating organisms on earth can be found in your very own home. If you own an aquarium or have ever visited an aquarium, lake or ocean, the chances are you have observed a fish. Fish have a very unique anatomy and physiology so that they can thrive in underwater environments.
Like humans fish have a closed circulatory systems, but the way that blood flows throughout the body is different and perhaps even simpler. In a fish blood flows from the heart, to the gills, to the body and back. Gills are analogous to the lungs of humans but the way they take in oxygen is very different. Water flows over the gills and the gills absorb dissolved oxygen.
Fish sense their environment in a number of different ways that are unlike humans. Some fish can smell their environment through their skin in addition to using their nares. Additionally fish can sense electric fields and their environment using a structure called a lateral line. These structures are unique to fish and allow them to perceive their environment and live successfully.
Have you ever wondered how fish swim? Fish maintain their buoyancy through an anatomical structure called a swim bladder. Essentially this is like a built into float that can inflate and deflate to help achieve neutral buoyancy at different depths. Some fish have swim bladders, while others that dwell on the bottom of the sea floor, which do not move up and down the water column, do not have a swim bladder. Interestingly the swim bladder also helps the fish to produce sounds. These sounds can be warning and mating signals to other fish and the types of sounds are specific to particular fish species. To help fish swim they have a number of fins (dorsal, ventral, pelvic, anal and caudal) and these fins could be rayed or composed of adipose tissue. Some fish have fins that can retract to help reduce drag while swimming, and some fish only have a couple types of fins, which suit their body form and life style. Fish are incredibly diverse! One suborder, Percoida, contains over 3,000 species and 78 families!
The next time you see a fish, I am sure you will now appreciate how their form has made them suitable for a life underwater. They are truly fascinating creatures!
Dissection Images: http://australianmuseum.net.au/Dissection-of-a-Blue-Mackerel-Scomber-australasicus
Helfman et al. 2009. The diversity of fishes: Biology, evolution and ecology. Wiley-Blackwell, London. 736pp.