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Inhabitants of the grains of sand

Biologists discover 13 species of invertebrates that live in marine sediments

20 years ago, marine biologist Judith Winston, a researcher from the Virginia Museum of Natural History, in the United States, discovered dozens of species of invertebrate animals living on the surface and in the inside of grains of sand taken from the bottom of the sea close to the coast of Florida. She thought that they existed only there, on the south coast of the United States, and that she would never find them again. But, in November 2002, taking part in an expedition for gathering specimens, with biologists from São Paulo, Judith asked for the sediment taken from the floor of the ocean in the environs of São Sebastião, on the north coast of São Paulo, to be separated for her. And there they were. There were even some new species, different from those identified in Florida, and many other interesting species, all with less than 1 millimeter. Only the external skeleton, loose or incrusted in the sand on the beach, can be found, albeit dead.

“It was never imagined that there was such a rich fauna living incrusted on grains of sand and shell fragments”, rejoices Alvaro Esteves Migotto, a researcher from the Marine Biology Center, linked to the University of São Paulo (USP).  Together with Judith Winston, Migotto signs a recently published study in the Invertebrate Biology magazine, reporting the discoveries of the São Sebastião coast, one of the points of study of a broad survey about the marine diversity of the São Paulo coast. From there, at depths that varied from 9 to 45 meters, there emerged 13 species of invertebrates that inhabited the surface and the pores of shell fragments, gravel and the thicker grains of sand (the diameter of one grain of sand, normally made from quartz, can vary from 0.05 millimeter – that kind of sand that massages the feet on the beach and escapes rapidly from the hands – to 2.0 millimeters).

The most abundant and diversified organisms were bryozoa, tiny invertebrates that form colonies, sprawling or upright, in the form of branches, with thousands of individuals. There were also hydrozoa, cnidarians and worms known as polychaetes. Of this total, four species were not encountered in any other marine environment and seem to be exclusive to these spherical worlds made of quartz; whereas in the study from Florida, specific to this group of animals, 33 species were recorded, of which 9 exclusive to grains of sand. “There must be many more”, says Migotto. The oceans, it should be remembered, cover three quarters of the surface of the planet.

According to him, the fact that these animals have been found in abundance at two spots thousands of kilometers away suggests that they may live in other places too, along the continental platforms. They may also have a greater biological importance than could be supposed, participating in the food chains, sometimes as predators and sometimes as food itself. Even the sediments from the bottom of the sea acquire a value, for being able to transport the invertebrate communities from one side to the other of the ocean floor, facilitating the conquest of new territories.

Young reproducers
Biologists that study benthos – the animals that live under or on the bottom of the sea – usually filter the sand or mud, separate the specimens that live loose in this sediment, and discard this material. They do not imagine that other benthic organisms can live and reproduce on or even inside the grains of quartz or the shell fragments. “Perhaps the sediments from the bottom of the sea may cease being seen like a desert for many sessile organisms (that live fixed to a surface)”, Migotto comments. “Many invertebrates find in the grains of sand a suitable space for fixing themselves, living and reproducing. They are not just restricted to rocks or relatively large sized fragments of rocks and shells, as used to be thought.”
The inhabitants of the sands of the bottom of the sea seem to be quicker in the attempt to perpetuate the species than their equivalents that live on rocks. The larvae of the bryozoa fix themselves to the surface of the grains of sand and reproduce in an asexual way, to start with. They form colonies that begin to reproduce in a sexual way when they are still young and shelter few individuals, unlike the bryozoa that live on rocks or algae. “As grains of sand are an extremely unstable environment and their surface is small”, says Migotto, “these organisms do not have room to grow much”.