Marsh and Ditch Habitats: How Freshwater Fish Survive In These Environments

Many of the fish that find their way into aquariums originate from harsh, inhospitable habitats. Some of the conditions found in aquatic habitats in Southeast Asia are among the most difficult. Constrained into receding marshes and ditches for most of the year, the onset of the rains expands their environment across shallow paddy fields and low-lying fields. Such habitats are often choked with vegetation and low in oxygen, making life a constant struggle against these hostile conditions.

What is life like in the long dry season?

For much of the year, during the long dry season, fish may be trapped in isolated areas of habitat in ditches and ponds of stagnant water. Often, populations of fish become concentrated by the receding waters, so competition for space, for food, and for the limited oxygen is intense. On top of this, the large numbers of fish attract the attentions of predators, such as wading birds. Plants also invade the water with their roots
with their leaves. Open water habitat is therefore at a premium and light is restricted.

What adaptations do fish show to these habitats?

Despite the tough conditions, fish not only survive but seem to thrive.The secret of the fishe's success lies in their adaptations. Perhaps, the most dramatic of these is the ability of some species, including the Anabantoids — the gouramis and their relatives — to breathe atmospheric oxygen using the remarkable labyrinth organ.This adaptation allows the fish to live in some of the most difficult conditions, occupying habitats where the lack of dissolved oxygen excludes others. Size also matters in these conditions; a large body is a disadvantage in the tangled, airless conditions, so the fish that live in them are usually small.

Maneuvering in the thick vegetation and the bottom mud to find the prey that live there requires a flexible body, and no fish exemplifies this more than the kuhli loach, which lives in these waters.

What happens when the rain arrives?

The influx of water that comes with the rains expands the fishe's habitat across a wide range of low-lying land, including the fields of rice that thrive under these conditions. The shallow waters are rich in food and after the daily struggle during the dry season, fish can now grow and breed. Although the rains expand the habitat the water remains extremely turbid — visibility may never increase above a few inches — so just as in the blackwater regions of the Amazon, fish must rely on their other senses. Smell and taste are especially important for these fish. The barbels of loaches, for example are covered in sensory cells to detect small prey such as, snails and worms, in the soft mud, Croaking gouramis communicate extensively with sound, while other gouramis use specially adapted pectoral fins to feel their way around their world, in much the same way as insects use their antennae.