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Life in Freshwater

Adaptations to Living in Ponds

Plant Adaptations

The high density of water provides the support that plants need and so, especially sub-merged species, have a minimum of dense fibrous material running through the stems. This means that the macrophyte will be soft and fleshy with the water giving the support. To enable them to be buoyant, leaves like those in lilies have large areas of air tissue within them and so that the plant can have gaseous exchange the leaves have stomata on the upper epidermis rather than the lower surface.

Roots like any part of a plant needs oxygen for respiration and with them buried in the mud where there is anoxic conditions (both toxic and deoxygenated) they are likely to die. To enable them to respire air will be drawn in through the leaves and transported to the roots through air tissue (aerenchyma). The oxygen not only supplies the root with what it needs but also diffuses out into the sediment. This will kill the anaerobic bacteria and reduce the anoxia so that the sediment becomes less problematic for the plant. In so doing the sediment condition improves for other plants and is an aspect of succession. See hydrosere.

Leaves of many water macrophytes show a variation in shape as they grow. Often the first leaves are narrow. These occur under water where currents would favour more streamlining. As they emerge from the water so leaves may be broader. In some plants like the water crowfoot there may be two distinct leaf shapes on the same plant.

Invertebrate Adaptations

We see above how plant support is provided by the water. This is also the case with some groups of animals, for example with very primitive groups like sponges and cnidarians. They cannot exist on land due to the lack of a skeleton. Buoyancy is a common problem for invertebrates and some vertebrates. Suspended in still water an animal will easily sink. To maintain themselves in a fixed position air bubbles may be used either on the outside, e.g. diving beetles, or inside. An example of the latter is the phantom midge that has two air sacs (swollen extensions of the breathing tubes) at either end of the body. Fish (a vertebrate) has the swim bladder.

Those animals living on the surface film use the surface tension to hold them in place so they do not sink, e.g. pond skater.

Swimming animals are usually streamlined and to swim they need a method of propulsion, e.g. dense, hairy legs. The Greater Waterboatman has long hind legs covered in dense hairs to create the oars, a similar feature found in other bugs and water beetles.

Obtaining oxygen is essential and this they do by either diffusion of dissolved gas from the water or by rising to the surface and collecting air. Dissolved oxygen is not abundant, especially in a sun-warmed pond. Insects like some of the water bugs and adult beetles will float to the surface where air bubbles are trapped under the wings. The spiracles of the breathing system are located here so that air can be absorbed. In the beetles this bubble acts like a physical gill with gaseous exchange occurring, oxygen will diffuse into it and carbon dioxide out. This is rarely enough to sustain the insect and so it will have to go to the surface for it to be renewed. The water scorpion is a bug that has a siphon/tube that it uses to draw down air. Even some species living in the benthos may need to reach the surface as it is so dexoxygenated in the mud. The rat-tailed maggot is the larva of a hover fly and it has a telescopic siphon that can change length so that it can have a snorkel for air even though it is at the bottom. Other fly larvae like the mosquito has a larva and pupa stage able to swim to the surface for air. Chironomid larvae are numerous living both in the pelagic region and it the benthic detritus. It obtains its oxygen from the water. In this low oxygen area it depends on the pigment haemaglobin to absorb what oxygen is available storing it for periods when oxygen is lacking, e.g. at night.




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