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Deep in the ocean there are two animals living in unexpected harmony. A small species of scale worm chooses to make its home inside a carnivorous sponge.
The sponge could eat the worm, but it doesn't - and scientists don't yet know why.
The carnivorous sponge Chondrocladia robertballardi should make an easy meal of the tiny scale worm Neopolynoe africana, as it would with other polychaetes. But instead it allows this species to burrow into its body.
Living inside the sponge, the worm is provided with protection from other deep-sea-dwelling predators. But the sponge may benefit too, possibly from prey that the worm attracts.
In the video above Dr Ana Riesgo Gil and Dr Sergio Taboada Moreno, researchers at the Museum, discuss the odd relationship of these two deep-sea animals.
Sponges are sessile animals within the phylum Porifera. There are over 9,000 known species of sponge living in all parts of Earth's oceans, but estimates suggest that there could be as many as 25,000 species left to discover.
Sponges, the most basal of all multicellular animals, are crucial to the health of the oceans. They are used by some scientists for a variety of medicinal uses, and they're also of interest for bioengineering.
'Sponges are really important for biogeochemical cycles,' says Ana. 'They provide food for the entire ecosystem to be able to function.
'They can sterilise water because they take in bacteria and they absorb a lot of silica.'
The sponges that scientists currently know least about those found in the deep sea. Experts including Ana and Sergio are endeavouring to expand our knowledge of them.
Carnivorous sponges are highly specialised, with only 165 species known to science. They no longer filter food from water, feeding instead on aquatic invertebrates such as polychaete worms and crustaceans.
Ana explains, 'There is a sponge that allows a polychaete to live in a furrow that it creates. This type of sponge is actually supposed to be eating the worm, so we're trying to work out why it doesn't.'
A number of techniques have been used to try and understand the animals' alliance.
'We're using molecular techniques and we DNA-test the sponge and the polychaete,' says Sergio. 'We also do isotopic analysis to see what the diets of the sponge and worm are, because we think the worm might be stealing the sponge's prey.
'We're using micro-CT to look at the structure of the tunnel that the polychaete creates. We're also using SEM [scanning electron microscope] to look at the adaptations of the polychaete to be able to live in the sponge.'
The scientists hypothesise that the worm may use be using bioluminescence to attract prey. The food could then be consumed by both the worm and the carnivorous sponge.
There are other species of deep-sea sponge that are known to engage in symbiotic relationships with other animals. One of the best-known examples is the Venus' flower basket sponge (Euplectella aspergillum). This sponge has a latticed appearance and often contains at least two crustaceans.
The deep sea is vast, covering around 65% of Earth's surface. For two organisms of the same species, finding each other could be a challenge, especially for small crustaceans. But pairs of male and female shrimp-like crustaceans, as larvae, can be washed into the sponge.
As they grow, they become too large to leave and are trapped inside the sponge. But their lifetime of confinement offers them protection from predators. Their tiny offspring are small enough to be washed out of the sponge, however, allowing them to restart the lifecycle elsewhere in the deep sea.