Radial symmetry is a characteristic of such animals. Transformation from a sessile to a locomotive existence favours a bilateral symmetry. The currents of water often abrade the inhabiting flora and fauna and varied modifications are encountered to withstand this abrasive action.
Thick scales, strong shells and many attachment devices like the holdfasts and suckers all are the results of this environmental stress. The ability to breathe air dissolved in water, at times even resorting to anaerobic existence, the modification of various senses to respond to stimuli characteristic of aquatic environments, the phenomenon of osmoregulation, and above all the phenomenon of external fertilization are other spectacular physiological adaptations to live in an exclusively aquatic medium.
Atmospheric Humidity and its Ecological Effects:
The amount of water vapours or moisture in the atmosphere is referred to as absolute humidity. However, of great significance is the relative humidity which is the ratio of the actual amount of water vapours in the atmosphere to the amount that can be held in the air at a particular temperature and pressure.
As the air warms up, the relative humidity drops (as long as the moisture content of the air is constant) because warm air can hold more moisture than cool air. As the relative humidity drops, the water pressure deficit—the difference between the partial pressure of water at saturation and the prevailing vapour pressure of the air —increases and increased evaporation takes place.
The relative humidity varies during a twenty-four hour period and from place to place, decoding on the topography. General relative humidity is lower by day and higher by night. The daily range of humidity is greater in the valley and less at high elevations.
Both excessive and deficient moisture can be detrimental to organisms. Usually moisture becomes a problem only in two environments, the marine and saltwater environment and the desert. The former are physiological deserts, in which the concentration of salts outside the organism can osmotically dehydrate it. In the latter, an absolute lack of moisture exists. Most organisms are adapted to moisture extremes in one way or another.
Most of the plants cannot make use of atmospheric humidity, however, several lichens, mosses, filmy ferns, and epiphytic orchids can absorb moisture directly from the air. Some beetles can also absorb moisture from the air through their body surface.
Other plants, except the submerged aquatic plants, obtain their water supply by absorption through roots. Animals obtain water and solutes by anyone of following methods: (i) By absorbing it through their skin from contact with some damp ground, (e.g., by osmosis in amphibians); (ii) By drinking; (iii) Directly from their food; or (iv) From water produced by metabolism.
They can lose water and solutes through urine, faeces, evaporation over the skin and respiration. If the animal is not to dehydrate, the input of water must equal to the losses.
Since water makes up a large proportion of the bodies of plants and animals (e.g., cytoplasm holds 70-90 per cent of water), it affects all life processes directly. In plants, the rate and magnitude of photosynthesis, respiration, and absorption of nutrients, growth and other metabolic processes are influenced by the amount of water available.
Low relative humidity increases water loss through transpiration and affects plant growth. Conversely, plants in regions with high moisture reduce transpiration. They survive the physiologically dry periods of winter by shedding leaves. The germination of seeds and establishment of seedlings are directly affected by water.
In lower plants, water is essential for fertilization and among higher plants, pollination and seed dispersal are effected through the agency of water in many cases. The moisture and temperature, acting together, determine in large measure the climate of a region and the distribution of plant and animal life (see Smith, 1977).
The growth and reproduction of animals are also effected by water factor. Many lower animals show preferences for certain moist levels. The distribution of several species of spiders in the forest was found to be directly related to humidity levels (Todd, 1949).
A number of nematodes, annelids, isopods, chilopods, diplopods, and insects do not survive at humilities below 100 per cent. In many insects and mites the reproduction, fecundity and speed of development is directly governed by soil moisture. If the air is too dry, the eggs of some locusts and other insects may become quiescent.
There is an optimum humidity at which nymphs develop fastest. Some insects lay more eggs at certain relative humidity’s than above or below that point. The mosquitoes cannot withstand high temperatures under low humidity levels, and the malaria parasite cannot be transmitted at a temperature below 15.5°C and mean relative humidity under 65 per cent (see Gopal and Bhardwaj, 1979). Many insects and nematodes can survive extreme desiccation. Ramamurty (1966) demonstrated that the colouration of animal (e.g., Labidhura ipania, Insecta) is influenced by humidity.
Too much water may be as determinative as too little. High water tables result in shallow-rooted ties that are easily toppled by the wind and are sensitive to draught and frost. Terrestrial plants subject to prolonged flooding, particularly during the growing season, will die from lack of oxygen about the roots.
Similarly, long periods of dry weather that result in soil drought reduce plant growth, causing dieback of plants or outright death. Drought-injured plants are susceptible to outbreak of insects and highly susceptible to fire.
Heavy rains and prolonged wet spells cause widespread death among mammals and birds from drowning, exposure and chilling. Excessive moisture and cloudy weather kill insect nymphs, inhibit insect pollination, and spread parasitic fungi, bacteria and viruses among both plants and animals.
The moisture falling on an area in liquid, vapours or frozen form is designated as precipitation. Precipitation depends upon season, wind, air, pressure, and temperature. Rainfall is the most common form of precipitation which has ecological effects on organisms.
Light drizzle is of the little importance as very little moisture penetrates the soil because much of it evaporates rapidly. Gentle steady rain is most effective because much of it penetrates the soil. However, torrential rains on land are most disastrous because they lead to flooding, soil erosion, and destruction of vegetation and of animals.
Hail, sleet and snow are forms of frozen rain. Hail is in the form of balls of frozen moisture. These may be of variable size and weight and may cause considerable damage to agriculture. Sleet and snow form destructive glaze on trees, shrubs and grass. The weight of snow causes destruction and damage to plants.