The demands of the sea on its inhabitants
Life in the sea places significantly different demands on organisms than life on land. First of all, of course, there is an unlimited supply of water. All organisms need water – they consist largely of water. Terrestrial organisms must therefore protect themselves against evaporation more or less effectively depending on the dryness of their habitat. Aquatic organisms can do without that protection, apart from those that live in the coastal strip, which temporarily falls dry.
Oxygen and carbon dioxide are available in water, but oxygen may only be present in small quantities or not at all in the deeper layers of the sea, which then makes life there impossible except for bacteria. The oxygen content of the water depends on how much the water is moved by waves, winds and tidal currents, as well as on the temperature: Warm seas are lower in oxygen because less oxygen is soluble in water at higher temperatures. Simpler marine organisms often absorb oxygen from the water through their entire surface, while many marine animals possess special respiratory organs called gills. These are very sensitive to dehydration and unsuitable for absorbing oxygen from the air, so that these animals usually quickly perish on land. Reptiles and mammals that live in the water, having lungs that can take in oxygen only from the air, must regularly surface to breathe. Carbon dioxide is available in the sea in a similar way to the air: the amounts of carbon dioxide in the air and in the water are balanced through the processes of carbonation and decarbonation.
Water (especially salt water) carries much better than air due to its higher density. This makes it much easier for marine organisms to float in the water than it is for land animals to fly, and there are countless species that have adapted to a life suspended in water. Floating organisms often have gas or oil bubbles that help them regulate their buoyancy. Organisms without such mechanisms sink slowly, but often slow down their sinking speed with parachute-like appendages. Because of the buoyancy aquatic plants (and sessile animals) need a much less developed supporting apparatus than land plants.
All important mineral salts and nutrients are soluble in water. Accordingly, marine plants need their roots only to anchor themselves in the substrate, while they absorb most of their nutrients directly from the water. The upper, light-rich layers of the water are home to large quantities of microscopic, mostly single-celled algae that perform photosynthesis, as well as numerous bacteria. These primary producers provide the basis of life for the animal organisms of the plankton. Due to the evenly distributed mineral salts and nutrients, they can live permanently floating in the water.
Animals cannot perform photosynthesis and therefore depend on consuming organic matter. Some microscopic species of zooplankton feed in the easiest possible way: they simply absorb organic substances released by algae and bacteria that are dissolved in the seawater. A large part of the marine animals feeds on plankton, i.e. the free-floating single-celled algae, the zooplankton and the eggs and larvae of most marine animals, which are produced in enormous quantities and also belong to the plankton. These species feed by filtering the seawater or by capturing plankton using tentacles or similar appendages. In contrast to the land, where herbivores far outnumber carnivores and where only a few species of carnivores eat other carnivores, the food chain in the sea is very long: a very great proportion of marine life lives as predators, and each organism is in turn eaten by a larger one. Numerous marine animals finally feed on detritus, i.e. dead organisms that sink to the bottom or are washed up on the beach. Relatively few species feed on the algae or sea grass that grow on the sea bottom.
Many of the marine animals that feed on plankton are sessile. They need neither locomotive nor complicated sensory organs to feed, and often have a very simple body structure. Only reproduction is made difficult by the sessile way of life: sessile organisms (animals and plants) must release their spores or their eggs and sperm into the water for fertilisation and dispersal. This requires very large quantities of reproductive cells. Many organisms coordinate their spawning precisely in order to increase the probability of fertilisation. The reproductive cells and larvae of sessile organisms make up a large part of the plankton and thus contribute significantly to the wealth of the sea.
Although sessile marine animals hardly need organs for movement or sensory perception, and only require minimally developed digestive organs due to their easily digestible food, they do need an effective protection from predators, as they cannot escape by moving. Accordingly, they have developed many different ways to protect themselves, in particular hard shells and exoskeletons, spines and poisonous or unpalatable substances. Accordingly, many predatory species have developed special abilities and adaptations that enable them to overcome the protective and defensive systems of their prey.
In addition to the sessile animals, many free-swimming species, some of which are very large, such as whales and fish species, also feed on plankton. They too do not need to specifically catch or search for their food, but simply swim around with their mouths open. In contrast, predatory fish species that feed on other free-swimming species require good sensory organs and a high mobility, as do their prey, whose survival depends on their ability to escape the predators. Many smaller fish swim in large shoals, searching for protection in high numbers.
Most algae and sea grass species are scarcely eaten due to their unpalatable constituents. Many alga species deposit calcium in their cell walls so that they become stiff or even hard as stone. Accordingly, there are relatively few herbivores in the sea (compared to conditions on land), and most plants can grow unhindered, apart from competition for space with their neighbours. The same applies to most sessile animal species. This leads for example to the formation of large algae forests and great coral reefs. However, there are also a number of animal species that have developed special adaptations to be able to feed on calcareous algae or corals.
Underwater rocks are often completely covered by algae and sessile animals. The various species, such as sponges, calcareous algae, bryozoans and tube worms, overgrow each other and form a complex, colourful mosaic that sometimes looks almost like a reef, providing hiding places for a myriad of smaller and larger organisms, creating protected niches and special habitats. Accordingly, the vertical rock faces in the sea are one of the richest habitats on earth. The conditions in the horizontal seabed are completely different, as sessile species can hardly exist here because they are quickly buried by the constant sedimentation of sand carried into the sea or by detritus and other sinking material.
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