Wet Beast Wednesday: bone-eating worm
Happy (almost) Halloween from us at Wet Beast Wednesday. What better to celebrate the spooky holiday than with a spooky animal? These critters are so spooky they even scare skeletons. Sometimes called zombie worms or bone worms, bone-eating worms are scavengers that play an important role in deep seas. Try not to get too scared.
(Image: a cluster of bone-eating worms on a bone. Their plumes are visible, looking like red feathery structures emerging from a clump of brown sludge. End ID)
Bone-eating worms are members of the genus Osedax, with 26 species currently known. My favorite is Osedax mucofloris, which means "bone-eating snot flower". They are small tube worms, reaching between 2.5 and 7 cm (1 to 2.7 in) in length. The body is divided into three segments, the trunk, ovisac, and root. The trunk makes up the majority of the body and it topped by red plumes that act as gills. At the base of the trunk is the ovisac, where eggs are produced. Below that are the roots that bore through the bones the worms live on. This is done by secreting carbonic acid that is produced through anaerobic respiration. The roots also produce a mucus sheath whose purpose is not fully known. It may protect the body from the acid or may prevent the acid from dissolving the hole the worm lives in. As with other tubeworms, the worm generated a protective sheathe to live in. Normally, the plumes extend out of the sheathe to respirate, but when threatened, they will withdraw into the tube.
(Image: a bone worm removed from the bone. It is a long, translucent tube with reddish plumes on one and a lump of wavy roots on the other. End ID)
Bone-eating worms lack a mouth, anus, and digestive system. To obtain nutrients, they exist in a symbiotic relationship with bacteria. As the worms break down the bone, they release lipids and proteins that the bacteria consume to produce energy in the form of glycogen, which is transferred to the worm. The worm then uses the glycogen to power itself and feeds it to the bacteria to keep them alive. The worms also use collagen, which is the primary component of bone. Many of the symbiotic bacteria species need the collagen, which the worm provides by breaking down the bone. Curiously, many of the symbionts produce toxins that disrupt the membranes at the roots, leading to infection. The bacteria are also found surviving outside of symbiosis with the worms Because of this, it is debatable whether the relationship between the worm and its bacteria is mutualistic (both parties benefit) or commensal (one party benefits, the other neither benefits nor suffers).
(Image: a cleared view of bone worm plumes emerging from a bone. End ID)
Bone-eating worms are found worldwide in oceanic depths ranging from 10 to 4,200 meters (30 to 14,000 ft). They are most commonly found on the skeletons of whales, but will also colonize fish bones and even, in one experiment, cow bones. Whale bones seem to be preferred both because of their large size and the large quantity of lipids found within. Whale skeletons can often be seen covered with bone worms, giving them the appearance of red shag carpeting. As the worms break down bones, other animals can take better advantage of the nutrients within. The presence of bone worms at a whale fall has been shown to increase the biodiversity of the site. Bone-eating worms are ecosystem engineers, organisms that significantly alter their habitat. They have been doing this since before whales existed. Fossil sea turtle and plesiosaur bones have been found with signs of bone worm colonization.
(Image: a lone bone worm with its tube visible. Its plumes are whitish. End ID)
The bone-eating worms have one of the most dramatic cases of sexual dimorphism in the animal kingdom. All the worms you see when you look at a whale skeleton are females. The males are 20,000 times smaller and fully microscopic. They still resemble larvae, making them a case of neoteny, an adaptation where juvenile characteristics are retained into adulthood. Harems of males live inside the females' tubes and feed on the nutrients released by the bacteria. As the female generates eggs, the males fertilize them. The eggs hatch inside the female's tube and stay for a while to mature before being released into the water. The fact that the worms are so widely distributed indicated that the larvae can travel vast distances to find a new set of bones, but the means they use to do so is unclear. The extreme sexual dimorphism reduces competition between males and females and ensures the males will always have an available mate to pass on their genes. The species Osedax pirapus do not follow this form of dimorphism. Males are still smaller than females, but they actually look like worms and share the same lifestyle. This increases competition between males and females, but ensures that males can make far more sperm due to their greater size.
(Image: a collection of images of multiple species of bone-eating worms. Source. End ID)
