The wild world of shipwrecks
This month, the US coastguard sank the Japanese vessel Ryou-Un Maru in the gulf of Alaska after she spent nearly a year adrift at sea. She joins three million other shipwrecks on the ocean floor. But are shipwrecks good or bad for marine life?
In 1881, the Kingston set sail from London. The captain and crew believed they were heading for Aden, but the ship never made it past the Red Sea.
End Quote Dr Keith Hiscock Marine Biological Association
It's a different sort of habitat”
On the 22 February that year, she smashed into Shag Rock near the entrance to the Gulf of Suez. The crew were rescued, but the ship sank to the ocean floor.
More than a century on, colourful coral covers the Kingston. The same sponges, tunicates and anemones that live on the adjacent reef are on the wreck. Around 38 species of stony corals and ten soft corals adorn the ship's surface. In many ways it has become part of the sea floor.
Scientists are now studying the ill-fated Kingston, and other wrecks like her, to gauge their impact on the underwater world they have joined. And different wrecks are throwing up different surprises. While some are literally repulsive to marine life, others are becoming home to new, unexpected communities of animals.
Yehuda Benayahu, professor of marine biology at the University of Tel Aviv has spent much of his career in the Red Sea, examining shipwrecks there.
On old boats such as the Kingston, the wooden parts decay but the steel does not, offering a good foundation for coral. An accidental shipwreck soon becomes an artificial reef, which scientists can analyse.
- Coral reefs are formed when a hard coral organism (a polyp) attaches itself to a rock, then divides into thousands of clones.
- Polyps can exist alone, or join up together to create a colony that acts as a single organism.
- Coral polyps are translucent, and get their bright colours from the algae they host.
- Corals use their stinging tentacles to catch prey including zooplankton and small fish.
"Our study in the Red Sea took advantage of the fact that there are seven to 10 different shipwrecks at the entrance to the Gulf of Suez, the date of the tragedy is known, all dated... and all of them are within the depth range of 30 metres and alongside natural reefs," he told the BBC.
Each ship provides a snapshot in the evolution of an artificial reef after a certain number of decades.
An artificial reef tends only to mimic an adjacent natural reef if there are sufficient structural similarities, his research suggests.
So most shipwrecks are new habitats with distinct communities living in them.
The first organisms to arrive are usually larvae.
"The marine larvae take advantage," he said. "They start a sort of succession, which means you get different organisms settling with the progression of time."
Older ships such as the Kingston attract soft corals from the Xeniidae family, research has revealed. Younger shipwrecks tend to attract stony corals from the Poritidae and Pocilloporidae families. It may take several years for these to mature but as they do, the food web expands.
Open water fish are attracted to the reef in search of food. Colourful reef fish such as purple tang arrive seeking food and shelter. Both attract predators such as lionfish.
Eight years ago, an old Royal Navy frigate HMS Scylla was gutted and scuttled off the coast of Cornwall, UK. The project, co-ordinated by the National Marine Aquarium, created an artificial reef in Whitsand Bay. From the beginning scientists observed the wreck monitoring how sea life interacted with it.
There were some big surprises, according to Dr Keith Hiscock, associate fellow at the Marine Biological Association in Plymouth.
"The first two years the colonisation was all over the place... masses of sea urchins, a species we only normally find under boulders on the sea shore, colonised it in vast numbers.
"Then in year two, wrasse arrived and ate the sea urchins," he said.
An English coral on a nearby reef took almost three years to travel 30 metres to the wreck, but once there spread prodigiously and unexpectedly quickly.
Pink seafans on the wreck are also growing surprisingly fast, said Dr Hiscock. They were thought to grow about one centimetre a year, but some have grown 40cm in just a few years, he explained.
Artificial reefs such as HMS Scylla do not replace natural reefs that have been lost to dredging, however.
Studies in the Red Sea and on HMS Scylla confirm that wrecks often create new and different sorts of habitat.
"Scylla does not emulate rock reefs. You can't say 'oh heck we've put so many scallop dredges through the rock reefs that we've wrecked them, let's put down another frigate as a replacement" said Dr Hiscock.
"No, it's a different sort of habitat."
Life on the sunken Scylla:
|Time after sinking (March 2004)||Marine life growth|
Small numbers of poor cod are observed after around 10 days.
Barnacles, tube worms and hydroids begin to colonise.
"Wandering species" such as spiny spider crabs and long-spined sea scorpion arrive.
Brown filamentous algae appear.
Barnacles, tube worms and hydroids become common.
Kelp, solitary sea squirts and anemones settle along with mussels, anemones and a mass of queen scallops.
Green urchins and common starfish settle in large numbers.
Calcareous sponges settle.
Common starfish start feeding on barnacles and mussels.
Corkwing wrasse are observed "holding territory".
Watching HMS Scylla from the moment it was sunk has also provided a reminder of the effects that some man-made chemicals can have on marine species.
The hull was painted with an anti-fouling paint containing tributyltin (TBT). The TBT paint on the wreck was not removed before the wreck was scuttled.
Though it has been more than 25 years since the paint was last applied, TBT coated areas on HMS Scylla's hull still remain almost entirely free from colonisation.
Professor Benayahu has come across it in the Red Sea. "For years they have been using a nasty paint labelled TBT," he said, "…which is considered one of the most toxic compounds ever introduced into the marine environment by human beings."
TBT is now banned under international agreements and is known to be extremely toxic to molluscs. In gastropods, it can cause the growth of sexual organs that do not match the creature's gender.
"That's a project waiting to happen," said Dr Hiscock. "To see what the levels are in those organisms that are surviving, and in the water adjacent to the TBT anti-fouling paint. To do a bit of a detective piece of research to try and work out what's going on there", said Dr Hiscock.
A reminder perhaps of what is still to be learnt about how the world above the waves affects marine life below.