Ecology

Using sponges to census the ocean

They trap cells shed by other creatures, permitting DNA analysis

STUDYING THE health of ecosystems on dry land—a habitat that biologists share with the organisms they are looking at—is challenging enough. Studying marine ecosystems is far harder. Even discovering which species are there, let alone in what quantities, can be tricky. But an insight by Stefano Mariani of the University of Salford, in Britain, may make things easier. As he describes in Current Biology, he thinks that sponges growing on the sea floor may offer a short cut to such information.

One way of sampling marine life is to sample the seawater itself, looking for DNA shed by creatures as they go about their lives. This is known to work in principal, but in practice it means running huge amounts of water through filters, to extract things like shed skin and blood cells for analysis. That is awkward, meaning sampling tends to be patchy. And attempts to automate the process using drones are proving pricey.

Sponges, however, are natural filters. Indeed, they make their livings that way, sucking in thousands of litres of water a day and running it through intricate systems of channels that trap particles of organic matter, which they then digest. Taking samples from sponges, and checking the DNA therein, might thus, Dr Mariani reasoned, be an effective way to assess the diversity of species in the region surrounding a particular sponge.

To test this idea in principle he and his colleagues removed some pieces from sponges collected originally for other research purposes. Five of these had come from Antarctica and four from the Mediterranean. Analysis revealed, in the Antarctic samples, the presence of animals including chinstrap penguins, Weddell seals and Perknaster aurantiacus, a type of starfish. In the Mediterranean samples there were rock gobies, horse mackerel and sardines. The quality of the DNA was not always sufficient to make a precise identification all the way down to the level of “species”, but, overall, the researchers recognised genetic material from at least 31 different groups of animal—meaning that was the minimum number of species represented.

Clearly, this is but a fraction of the total likely to have been present in the sponges home areas, and the data also give little idea of the size of those populations. To do all that would mean looking at a lot more sponges. Equally clearly though, Dr Mariani’s method does work in principle. Refining it and making it more sensitive is now a matter of detail. Since sponges cost nothing to manufacture, deploy or maintain, and also regenerate when they have pieces removed from them, they may thus turn out to be an ideal way of monitoring what is going on, unseen, in Davy Jones’s biological locker.

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