For example, a Shark is classified as a fish, while a Dolphin is considered a sea mammal, a member of the Whale family. The differences are fascinating. Nature is truly remarkable in its subtle differences in various animals, and fishes are no exception.

Reef Shark

A dolphin and a shark differ in several ways: For instance, dolphin relies upon breathing in oxygen more like a human than a fish. In other words, the animal must "come up for air," in order to survive. A shark does not. A shark breathes through its gills to obtain oxygen from water

Seaworld, San Diego, CA

A dolphin is a mammal made of skeletal bone. A shark's skeletal structure is not comprised of bone, but entirely comprised of hard cartilage.

Some fishes, like sharks, reproduce by internal fertilization. This means the male deposits sperm into the female fish, where the eggs are fertilized inside of the mother. These fishes are called livebearers, which means that they give birth live young, not egg hatchlings.

Most fishes reproduce by something known as, external fertilization. In other words, a female fish releases eggs into the water, where they are fertilized by sperm from a male. This process is called spawning.

Fishes have three important traits in common:

• 1. They live in water

• 2. They have fins

• 3. They use gills to get their oxygen from the water.
  
  

Salmon Eggs


Notice how the eyes prominently show through,
very early on in their development.

Fish eggs may hatch within several days of spawning or many weeks later, depending on the fish species, water temperature and other varying circumstances.

Alaskan Salmon Spawning

Many fishes develop bright colors during their breeding seasons. Usually it is the male that is more colorful. He uses his bright colors to attract females. Color also makes it easier for fishes to recognize members of their own species.

Beautiful, Colorful Marine Fish

Beautiful Koi Breeding Pond

Some salmon species spend part of their lives in the salt water of the Pacific Ocean, but they travel many miles into freshwater streams and rivers to spawn.

Salmon Swimming Upstream for Spawn

Along the journey, their bodies change to a brilliant red color. Females release eggs along the gravel bottom, where they are fertilized by males. Both males and females die after spawning. The young fish hatch and spend a year in the fresh water before traveling out to sea. Two to three years later, the salmon return to the river as breeding adults. They spawn, and the cycle continues.

Many fishes simply release their eggs and sperm into the water and pay no attention at all to the fertilized eggs, but other fishes build nests and guard the eggs and young.

Fishes usually spawn during a particular time of year. In freshwater ponds and lakes, late spring and early summer are excellent times to observe fishes making nests and laying eggs. It's also a good time to observe hatchlings (baby fishes) schooling together.

Cichlid Mother Guarding Eggs

Guppy Family

Juvenile freshwater fishes tend to resemble their parents, but most saltwater fishes have larval stages. These larval fish drift freely in the open sea and many are bizarre-looking, with odd shapes or long trailing fins.

Most fish parents spend little time caring for their offspring. But some will guard their young nestlings for a few days after the eggs hatch. In a few species the adults carry eggs on their bodies. Some fishes actually keep the eggs in their mouths until they hatch.

When young fishes hatch they may look very different from adult fish of their species. A newly hatched fish may be nearly transparent, but in a few days they develop color patterns.