Tube worms associated with hydrothermal vents or cold seeps! These species are polychaete worms in the taxonomic family Siboglinidae. They are sessile and can grow up to 10 feet from where they are fixed to the substrate, often sticking up into the water column. Each individual occupies a solid tube and extends its vibrant, red respiratory organ (or plume) out of the opening. In addition to their strange appearance, they have what could be considered be a very odd taste in habitats. The deep sea hydrothermal vents and cold seeps which they are associated with are characterised by the release of chemical rich fluids from under the sea floor. Some of these chemicals, such as hydrogen sulfide, are toxic and harmful to most life.

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Riftia pachyptila tube worms inhabiting a hydrothermal vent site along the Galapagos Rift: NOAA Okeanos Explorer Program, Galapagos Rift Expedition 2011, Flickr NOAA Photo Library.

Why live around hydrothermal vents and cold seeps?

The spouting of chemical rich fluids is the very reason that rich biological communities, including tube worm species, inhabit these environments. In fact, tube worms have been found to form dense aggregations around the vents and seeps, growing towards the flow of substances. Why is this? Because of chemolithotrophic bacteria! These organimsms create energy by oxidising hydrogen sulfide or methane produced by vents and seeps through a process called chemosynthesis. Many organisms simply consume these bacteria, but tube worms form a more mutualistic relationship with them. This involves the transfer of energy from the bacteria to the tube worm, a process which appears to be very efficient. These worms do not have a digestive system, as this symbiotic relationship is their lone source of energy. Despite this, tubes of the giant tube worm, Riftia pachyptila have been observed to grow at rates of over 85 cm per year, making them one of the fastest growing marine invertebrates.

A Beautiful Friendship

Tube worms host the bacteria in a safe environment within their bodies, in an organ called the trophosome. In return, the bacteria provide the worm with organic matter, by excreting the products of chemosynthesis. In order for the bacteria to chemosynthesise, the worm must provide them with oxygen and sulfide or methane. The giant tube worm absorbs oxygen and sulphide from the sea water using its plume. It maximises the absorption of sulfide by positioning its plume in the strongest outflow of hydrothermal vent fluid. Tube worm species associated with cold seeps extend root like structures down into to substrate, which absorb dissolved hydrogen sulphide from the sediment. The absorbed substances diffuse into the circulatory system of the worm and bind with specialised respiratory pigments, so they can be transported to the bacteria in the trophosome.

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Cross section diagram of Riftia pachyptila. Credit: Woods Hole Oceanographic Institution

The secret to Success in the fast and the slow lane?

The incredibly high growth rate of the giant tube worm (the result of a steady supply of energy from its symbionts) means that it is perfectly adapted to a life associated with hydrothermal vents. These environments are usually relatively short lived, as vents only last for a few decades. Therefore organisms associated with them must grow quickly in order to reach maturity and successfully reproduce before their habitat is lost and they perish.

Cold seeps are far more stable and long lasting. Tube worm species associated with cold seeps, are some of the longest lived animals on Earth. A study of the growth rate of Lamellibrachia luymesi on the Upper Louisiana Slope in the Gulf of Mexico revealed that this species can live for more than 250 years! The image below describes the methods used to obtain this information. Could the long lives of these species be a product of the energetic benefits of their endosymbiotic relationships?

Lamellibrachia luymesi on the Upper Louisiana Slope in the Gulf of Mexico. Their tubes were stained blue in June 2002 and this image was taken in August 2003. Therefore, the white area of each tube above the blue stain is the growth of each animal over 14 months. This data allowed for the determination of the age of each animal. Credit: NOAA
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