Quirks & Quarks

They're not speedy, but these seafloor sponges are on the move

Sponges are generally thought to be stationary, but evidence to the contrary has been observed on a submerged Arctic ridge

Sponges are generally thought to be stationary, but researchers have found evidence to the contrary

Sponges leave a trail of spicules, bits of themselves that break off, as they move across a submerged ridge in the high Arctic (AWI OFOBS team, PS101)

Sponges are considered to be stationary organisms. Researchers thought they planted themselves firmly and permanently on the sea floor and devoted their energy to filtering food out of the water.

But a new study by Autun Purser, a polar and marine researcher at the Alfred Wegener Institute in Bremerhaven, Germany, found evidence of sponges on the move on a submerged ice-covered ridge in the high Arctic.

In 2016, an underwater camera picked up two surprises on that ridge. The first was that the peaks were covered with sponges, some as big as 40 to 50 kilos in weight. The second surprise was that most of the sponges, showed evidence of having travelled along the seabed.

Sponges do not have muscles to initiate movement, but they can respond somewhat to changes in their environment, such as currents, by expanding and contracting. This mechanism was considered to limit their ability to change location, until now. 

The trail of spicules behind these sponges indicate that they have been on the move (AWI OFOBS team, PS101)

Tell-tail trails

Sponges support themselves on structures called spicules. Purser says it was remnants of these spicules that provided the evidence that the sponges were on the move. The camera picked up many trails formed by bits of the light brown spicules that are broken or scraped off as the sponges move. The trails were as high as 10 centimetres and many metres in length. 

At first Purser thought that the sponges were simply responding to gravity, moving downhill as you would expect on the peak of a ridge. However, there were as many trails that indicated an uphill journey. As there is very little current, that too was ruled out as a means of transport. 

An underwater remote camera picked up sponge trails on the Langseth Ridge in the high central Arctic (AWI OFOBS team, PS101)

Purser believes the sponges move because this is such a low nutrient environment, being stationary is simply not an option. He also thinks that by moving, sponges are separating themselves from the juveniles, optimizing their chance of survival for their offspring.

Written and produced by Mark Crawley.

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