Living on Land and in Water: Some Freshwater Fish Cross the Boundary

Aquatic habitats are among the most varied anywhere on the planet. Conditions can be extreme and changeable over short periods of time.The fish that live in these environments must adapt to succeed.This remarkable ability means that fish are able to exploit habitats that are beyond the capabilities of other species. It allows these pioneers to prosper, away from the competition of others.

What are the hardships of living in an estuary?

Life in the world's estuaries is one of constant change. Every six hours, as the tides ebb and flow, the water chemistry undergoes a complete transformation from salt to freshwater and vice versa. The physiological demands on creatures in this habitat are extreme, but for those that are able to adapt there are rich food resources to exploit such as are found in mangrove swamps. Few species are able to cope with the stresses of living in estuaries, meaning that those who can have fewer competitors.

Can fish survive in temporary pools?

The short rainy season experienced in parts of equatorial Africa produces temporary pools that dry up to no more than cracked mud and dust once the rains recede. Yet even under these conditions, some fish still manage to eke out a living. Many species of killifish hatch, grow and reproduce during an incredibly short period of time. Turquoise killifish (Nothobranchius furzeri) typically complete their life cycle in just ten weeks.

Shortly after the arrival of the rains, the eggs hatch, producing tiny larvae that feed on algal cells and other newly hatched aquatic creatures. Growth is extremely rapid, the fish attaining full size after only a few weeks. During this time they mature sexually and begin to reproduce before the pools disappear under the hot sun. But as the adults die, the eggs remain in a state of suspended animation. Unlike the eggs of other fish species, killifish eggs are protected by a shell consisting of several layers, and remain viable until the rains return months later.

But not all fish that live in such conditions die once the waters vanish. Lungfish burrow into the soft mud and estivate — a state of being akin to the hibernation of many mammals that live in temperate climates. The body's metabolic processes slow to a minimum and the animal awaits better conditions, which in the case of the lungfish, means the return of the rains.

Can fish leave the water?

Some fish species take a different approach to this seasonal loss of habitat. When temporary pools disappear or become too harsh to support life, some species leave the pools to cross land in search of new habitats. Some, such as the mangrove killifish (Kryptolebias marmoratus) move regularly between tiny water bodies and are able to survive extended periods out of water in damp leaf litter. Others, such as the climbing perch and Oarias catfish, are able to travel considerable distances in their search for a new home. Mudskippers, which live in estuarine conditions across Australasia, are equally happy in water or on the shoreline, where the males construct nests out of the water. In fact these unusual fish spend more time out of water than in, climbing over roots and dashing across mudflats.