The species of worm living on the skeleton of a small gray whale in a deep-sea trench off the coast of California is the first known bone-eating marine worm. Once a female bone-eating worm burrows into a whale’s bone, it will never leave. The male worms do not burrow; instead, they live inside the females. Bacteria at the ‘roots’ of the female worms assist them in digesting the whale’s fat. A study on these two new worm species was published in the journal Science by the American Association for the Advancement of Science.
This worm species is the latest discovery in the branch of biology focused on life that arises from the remains of sunken whales. These whale carcasses sink to the ocean floor and become mysterious, colorful oases, according to Robert Vrijenhoek, a researcher at the Monterey Bay Aquarium Research Institute in Moss Landing, California, who authored the article in Science.
Bone-eating worms may play a crucial role in nutrient cycling. In terms of food mass, a dead whale is equivalent to thousands of years of “marine snow,” with carbon molecules that sustain life slowly falling to the ocean floor.
Worms with ‘roots’ and ‘tentacles’
The largest female tube worms discovered by scientists measure about the length of a finger and are as thick as a pencil. They possess an outer tube, a robust inner body, an ovary containing eggs, and ample space for males. A large egg sac burrows into the bone, surrounded by a tissue filled with bacteria, growing deep into the dead bone like roots of a plant.
These roots greatly assist bacteria in penetrating the bone from the front or from the surface, according to research author Shana Goffredi from the Monterey Bay Aquarium Research Institute.
The bacteria in the roots help digest the fat in the bone and transfer nutrients to the worm. How the bacteria and the worm communicate and exchange food remains a mystery.
Above the worm are thread-like structures that are red or white-tipped, known as “tentacles.” The hemoglobin in the tentacles gives them their red color, facilitating oxygen absorption for both the worm and its symbiotic bacteria.
This worm species is distantly related to larger tube worms found at hydrothermal vents and cold seeps. Hydrothermal worms also rely on symbiotic bacteria to extract nutrients from their environment.
Discovery of a golden worm species
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Vrijenhoek |
Scientists discovered the whale carcass while searching for clams in Monterey Bay. When the radar screen on a submersible lit up, Vrijenhoek thought they had collided with a 55-gallon drum that someone had kicked off a boat. Instead of debris, their remotely operated vehicle transmitted images of brilliant red clusters of worms surrounding the jaw, skull, and ribs of a small gray whale carcass. Tissues, fins, and many organs of the whale were still attached to the skeleton. The whale carcass likely fell to the canyon floor months before scientists discovered it. The depth of the trench protected the tissues and organs from predatory sharks and starving hagfish.
When scientists aboard the submersible used a robotic arm to pull a piece of whale bone, Vrijenhoek thought it was a known type of worm.
However, upon seeing the slimy tubes attached to the whale bone, he realized he had discovered a golden worm species. In 1995, Vrijenhoek and a team of scientists led by whale carcass expert Craig Smith from the University of Hawaii had found similar tubes but not worms. Without any worms to dissect or DNA to analyze, they referred to the unseen creatures living in the tube as “green slime worms.”
Now, he had the tube and the worm. Within a week of discovering them, scientists concluded that their DNA did not match any known worm DNA. Further analysis classified these worms into two new species, both belonging to the bone-eating genus Osedax.
The smaller worm, Osedax frankpressi, features white and red bristles. The larger worm, Osedax rubiplumus, has entirely red bristles.
The dwarf male worms
Although scientists established a new genus for these worms, meaning “bone-eaters,” only the female worms consume the whale’s bones. The tiny male worms, even when fully grown, do not eat whale bones. Instead, these dwarf male worms feed on the yolk droplets inside – the fatty remnants of the eggs they developed from.
Mature males do not change much in appearance from their larval stage, except for some bristles at the front and hooks at the back. In this species, males and females are as different as two extremes. “If you see a male worm, you would think it’s a larva, yet it can produce sperm,” Vrijenhoek said.
This phenomenon of “sexual dimorphism” is particularly surprising because male and female worms in closely related marine worm species tend to be similar in size, according to research author Greg Rouse from the South Australian Museum and the University of Adelaide in southern Australia.
Rouse discovered the tiny males inside the females after colleagues sent him a piece of whale bone with red-bristled worms surrounding it.
Sperm cells from the male’s packets compete to fertilize the eggs. They move towards the egg region at the head of the female. The largest female studied by scientists contained up to 111 males.
Among the many tiny worms adrift in the vast ocean, only a few can latch onto a dead whale. Scientists hypothesize that the found worm larvae that locate whale bones will develop into females. Those that attach to females will develop into males. Worms that fail to find carcasses or females typically die.
