Sex life of a tubeworm studied in the deep oceans

Researchers from Southampton have come up with startling new findings after studying the reproductive habits of creatures that live thousands of feet below sea level and in complete darkness.

Professor Paul Tyler and graduate student Ana Hilario, from the Southampton Oceanography Centre and colleagues from the University of Oregon have discovered that the females of five species of deep-sea tubeworms fertilize their eggs internally with the help of a hook-shaped 'sperm bank' in their genital tract.

The scientists' findings, described in the February 2005 issue of The Biological Bulletin, contradict the widely held hypothesis that these tubeworms breed by dispersing their eggs and sperm freely into the water - an external breeding method called broadcast spawning common in marine invertebrates, such as corals.

Scientists are interested in the reproductive and dispersal biology of tubeworms because of their ecological importance for the continued existence of deep-sea vents and cold seeps, geological phenomena on the seafloor that emit sulphur and other chemicals and offer almost impossible living conditions.

"The deep sea continues to surprise us with the adaptations of its fauna to the prevailing conditions. At vents, particularly, the turbulence and corrosive chemicals in the water column require special adaptations to ensure successful reproduction," said Professor Tyler.

To study the tubeworms, the biologists travelled in deep-sea submersibles to the worms' homes, lightless seafloor neighbourhoods at depths ranging from about 2,000 to 8,000 feet.

The researchers collected five tubeworm species-Riftia pachyptila, Ridgeia piscesae, and Tevnia jerichonana from Pacific hydrothermal vents, and Lamellibrachia luymesi and Seepiophila jonesi from cold seeps in the Gulf of Mexico, then studied the female reproductive tracts in a laboratory. In all five species, the scientists discovered the presence of a hook-shaped sperm storage region called the spermatheca, where the female stashes sperm and where her eggs are eventually fertilized in vitro before she spawns them.

"Our hypothesis is that the sperm either swim into the spermatheca or that the female somehow gathers the sperm by some unknown mechanism," said Professor Tyler.

The researchers also studied tubeworm eggs after the females had spawned them and concluded that fertilized eggs do not begin to grow into embryos until the female tubeworm releases them. The study also documented the fertilization rates of the eggs.

The creatures, properly called Vestimentiferan tubeworms, are Earth's own aliens.

Although they do not have eyes, mouths, stomachs or intestines, these creatures are adapted to life in deep-sea hydrothermal vents and cold seeps. Bacteria housed in the tubeworms convert energy-rich chemicals emitted from the vents and seeps into food, a process called chemosynthesis. Tubeworms include some of the longest living and fastest growing marine invertebrates.