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Environmental Management

Begoña Vendrell Simón

Biologist specialized in marine ecology

The forgotten ‘underwater forests’

APRIL 6TH, 2011

The seabed of the channel between Mallorca and Menorca is dominated
by what is known as maërl (small blocks of coralline algae not attached to
the seabed that move with the current)

The sea bed is inhabited by diverse communities of organisms. For example, there are sea grass prairies in shallow areas and sandy stretches that seem, at first glance, devoid of life, while in the gloomy depths there are benthic communities dominated by sessile animals like sponges, incapable of movement.

Along the Mediterranean’s rocky shoreline we find coralline communities, structures consisting primarily of carbonated algae and animals like filter feeders (feeding on organic particles floating in the water), among many others. In fact, we could say that coral reefs have a structure similar to that of a forest, where the structural function of trees is carried out by both algae and animal species (sea fans and sponges, for example) that provide habitats for other sponges, ascidians (sea squirts), bryozoans (sea mosses), and polychaetes (sea worms) as well as other inhabitants of these communities, including epibionts (barnacles, algae, etc.) along with mollusks, crustaceans, pycnogonids (sea anthropods) and fish. This ecosystem is rich, complex and diverse; it is an ecological wonderland.

In the reef, the structural function of trees is carried out by algae and animal species But the coral reef is not the only kind of underwater forest. In some depths where little or no light can reach, there have developed large communities of organisms dominated by animals such as corals and sponges. For instance, along the rocky walls of submarine canyons like the one off the coast of the Cap de Creus in Catalonia, there are cold coral communities. And on the seabed of the continental shelf of the high Antarctic there are communities dominated by large extensions of siliceous sponges. These are two more examples of the ocean’s animal forests, where the sessile (non-moving) fauna form a three-dimensional structure very similar to terrestrial forests formed by trees.

Differences and similarities with terrestrial forests

The fundamental difference between marine and terrestrial forests lies in the mechanisms they use to obtain food. On land, plant organisms produce their own food, while marine forests are made of heterotrophic animals, meaning the receive nutrients through digestion. These underwater sessile animals feed either by actively capturing or passively filtering out particulate organic matter and the remains of organisms that settle to the bottom of the seabed or are transported there by currents.

The ecological success of this trophic strategy is evident if we consider the number of species that have developed it, and that it requires very low levels of energy to obtain food. In fact, in these underwater forests sessile suspension feeders often cluster together to act as common filters: the dense structure facilitates the capture of particles by slowing down the speed of the water flowing between them.

Many of these animals can reproduce asexually, as often happens with plants It is interesting to observe how the similarities between these marine animals and terrestrial plants do not end with the fact that both provide habitats for many species. Among other things, many of these animals reproduce asexually, as often happens with plants, forming new individuals by budding.

Under threat

The presence of these underwater animals forests is important because they are rich areas of biological diversity. Their existence depends, firstly, on floating particulate matter (and therefore the presence of currents that carry them) and, secondly, on their protection from human disturbances. In fact, the two biggest dangers to these communities are large injections of sediment and sludge, which can collapse the filtering structures of suspension feeders, and human activities such as trawling, which damages or even destroys these habitats that give refuge to many juvenile fish of species with commercial value.

New technologies are increasingly allowing further studies of these communities whose ecological role appears to be vital to the entire marine ecosystem. In the International Year of Forests, it is time to make a call to promote the conservation of these unique underwater forests.

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