Not all fish can do that but for a few remarkable fish swimming upside-down means everything is great.

A species of deep-sea angler fish swims its whole life upside-down. Intrigued by this behavior scientist continued to research and found 8 other species of angler fish displaying the same behavior.

Some catfish species spend time upside-down to feed on surfaces of submerged logs, and roots. Tomb walls in Egypt have carvings of upside-down catfish dating back 4000 years. Catfish pendants were popular amulets to ward off drowning in Egypt.

In the Maldives we do not see catfish or deep-sea angler fish, but we have also observed fish swimming upside-down.

For example, the long nose butterfly fish and the soldier fish. They must be precision swimmers to be able to orient their bodies in different angles to be able to forage for food. They follow the roof of a cave or an archway or to swim in cracks and crannies of the reef.

The location of their air bladders or swim bladders makes it tough for fish to control that buoyance and swim upside-down. They roll easily and must overcome this by flapping their fins.

All fish are great swimmers which is an ability that doesn’t come from practice but from anatomy. Their swim bladder allows them to control their buoyancy without having to continuously swim and expend energy. For example, a fish gulps in air and inflates the bladder to stay afloat and when they want to sink, they deflate it.

When fish die this swim bladder accumulates lots of gas, fills up the body cavity, changes the center of buoyancy and the dead fish ends up floating belly up.

Fish can’t tell by gravity which way is up or down. Instead, they tell by brightness. Since Ocean water is dense, light does not penetrate very far. Bright water indicates up. Dark water indicates down. When the sun is out fish swim upright. When the fish goes into a cavern or overhang the shadow caused by the overhead obstruction means the water above is darker than the water below, so the fish orients itself belly up.

Some catfish even have evolved to have darker belies and lighter back. When swimming upside down, the darker underside makes it harder to be seen against dark water. That coloration is remarkable because it is the opposite of most fish. This is known as counter shading.

Next time you are on a dive watch out for fish swimming upside-down. Specially look in overhangs.