The saga of zombie worms continues in the land of Oz

In May 2023, two new species of zombie worms were published in the Records of the Australian Museum. To understand the importance of this event, we need to go back in time and answer a simple question.

The question is, is there a life on the bottom of the ocean? And you would say it's a really strange question. However, believe it or not, the negative answer was given. And this type of answer to the question brings up the memory of Edward Forbes, a British naturalist who in 1841 participated in an expedition to Mediterranean, where samples were collected from the depths, like the deepest depths at the time. After this expedition, Edward Forbes developed this interesting theory, according to which the bottom of the ocean below, about 550 metres, is basically a lifeless area. And he named this the Azoic Zone. His theory at the time was widely accepted and seemed quite logical, given the enormous pressure, cold and eternal darkness of the deep-sea environment. Seriously, it took about 25 years for this Azoic hypothesis to fall from grace. So that's Edward Forbes.

And this was due to efforts of another gentleman, Charles Wavell Thompson. This is the guy on the right. This was a guy who led probably the most famous oceanographic expedition in the world, hoping to disprove this Azoic theory. The voyage of H.M.S. Challenger was the first expedition to examine the deep sea and answer these comprehensive questions about the ocean. Imagine, for the 19th century, the importance of this expedition was comparable to the importance of space programs of the 20th century. H.M.S. Challenger travelled over 100,000 kilometres for three years, collecting samples on the way. And in total, he made 362 stations, up to the depth of 5500 metres. After his return, Thompson published a travel report and the conclusion of this report was both short and historical: Life exists at all depths of the ocean. Later, scientific reports were compiled to a whopping 50 volume Challenger reports, and it took 19 years to complete. These reports opened the era of descriptive oceanography. And not surprisingly, numerous European deep-sea expeditions followed.

One we want to focus on was the expedition on board the Dutch H.M.S. Siboga. In 1898, the ship sailed from Amsterdam to the eastern Indies and began its one-year-long exploration of the Indonesian seas. The ship's complement included six scientists, including the couple Max and Anna Weber. Interesting, inclusion of Anna Weber was important because she was the first woman scientist to take part in a major oceanographic expedition. But that's not why we mention it. The reason why I find this expedition important is because they collected little thread-like worms, quite thin, about one millimetre in diameter and only 15 millimetres long, with single spiral tentacles. It happened that later, French zoologist Maurice Caullery in 1914 described this slender little thing, and called it Siboglinum, obviously after the ship Siboga. And he placed it in a like-new family, Siboglinidae. Caullery didn't refer Siboglinidae to any particular higher taxon, but he did compare it to deuterostomes, like hemichordates. Basically, animals that are closer to vertebrates like us than to annelids, segmented worms. Curiously, the animals didn't have any mouth or gut, so it was totally unclear how the animals’ feed. And some even suggested that they can basically absorb nutrients through the body's surface. A bit later, in 1933, Russian zoologist Pavel Uschakov found another gutless animal from the Okhotsk Sea. And this one was a bit larger, about 50 millimetres long. And unlike Siboglinum, it had like a beard of tentacles. So Uschakov decided to call it Lamellisabella zachsi, which basically means that it's a sabellid animal like this, like Sabella spallanzanii, a polychaete, an annelid. However, to confirm his assignment of animals, he sent it to Norwegian zoologist Johansson. And Johansson completely disagreed with Uschakov regarding the affinity of Lamellisabella. So, he said it's not a worm, it's not a polychaete, and he referred it to a new class he called Pogonophora. The class, he suggested, was close to phoronids, if anything else. Later, another Russian zoologist, Beklemishev, in a new textbook, erected a new phylum, Pogonophora, and again, so he raised class Pogonophora, to phylum and suggested that it's again close to hemichordates, deuterostomes.

Another Russian zoologist, Ivanov, actually worked on these animals for his entire career. He compared Lamellisabella and Sigoblinum and referred them both to Pogonophorans. In the coming decade, he described over 70 species and published a major monograph on Pogonophorans. So here is the original monograph and translation into English. By then, all Pogonophorans were consistently treated as deuterostomes. Again, animals closer to vertebrates than to worms. However, soon the events took an unexpected turn. In 1964, a Norwegian zoologist, Webb, discovered something unusual. He discovered that all previous Pogonophora were actually based on incomplete specimens. So apparently, there is a segmented tiny posterior end of an animal that was torn from the end of the tube. And later, this segmented part was called opisthosoma. So, yet another Norwegian zoologist in 1970 suggested that it's probably like Pogonophorans are not deuterostomes. Maybe they are worms after all. So here is what this posterior end looks like.

Well, so far, the animals were quite small. However, in 1969, already mentioned Webb described an animal from the deep waters of California that resembled previously described Pogonophorans, but also different in many respects. The tubes of these animals were exceptionally thick, and the diameter was about one centimetre. And the animal had a complex crown and operculum to close the tube. Web named this new animal Lamellibranchia barhami, and he referred to a new taxon, Vestimentifera nested within the phylum Pogonophoran. This is a large-bodied relative of previously smaller Pogonophorans.

Now, let's step back. We remember that main conclusion of the Challenger expedition was that life exists at all depths of the ocean. However, prior to 1977, biologists believed that without the energy of sunlight to support the food chain, organisms in the deep sea depended only on debris that falls from the water surfaces. Scarce food means that the organisms were few and far between. This all changed in 1977. But one of the most important biological discoveries of the 20th century was actually done by geologists. Geologists looked for vents. They hypothesized that the cold water would fall through the crack of the ocean and then get heated under the crust here, and then it would rise to the sea floor from the vent. So to test this hypothesis, geologists searched for temperature anomalies at the Galapagos Rift, and after temperature-sensitive equipment returned some small temperature changes, cameras were sent to the site and returned this picture of heaps of clam shells. Well, the next step was to send a manned submersible Alvin to the supposed vent location located at about 2.4 kilometres deep. What scientists saw from that submersible was completely unexpected and changed our understanding of life on the planet. They found what was later called the Rose Garden, an impressive community of huge clams, white crabs, and giant tubeworms with red tentacles like this. And it was nearly like from the near-boiling waters of hydrothermal vents. Although the geological expedition was hardly equipped to collect biological samples, they still managed to collect the samples of these animals, and they sent the worm samples to Dr. Meredith Jones, at the time head of the Division of Worms at the Smithsonian National History Museum.

Jones and his fellow researchers made several exciting discoveries. First worm was the new species, (not surprising) and they named it Riftia pachytptila in honour of the rift that served as home of the worm. Second, the worm was actually huge, it was like 1.5 metre long and more than 2 centimetres wide, exceptionally large compared to any other deep-sea worms, which typically measure less than 3 centimetres long. More puzzling, Riftia didn't have any mouth or digestive tract. What turned out is that animals actually had symbiotic bacteria living in a special organ they call trophosome. Worms used the blood haemoglobin to pick up hydrogen sulfide from the hot bed, and then bacteria would break down the hydrogen sulfide to sulphur, thus releasing energy. This process is similar to what happens in photosynthesis, except that plants break down the water instead of sulphur. So, this is how chemosynthesis was discovered, and it became clear that life on the planet doesn't necessarily depend on the energy of sunlight.

Here you can see Dr. Meredith Jones holding Riftia in 1981. In 1985, Jones erected another phylum. He basically suggested that Vestimentifera should be a phylum, again with new six species. However, later, Jones and his colleagues discovered that larvae of Vestimentifera do have a mouth, digestive tract, and segmentation as larvae, but they lose it as larvae develop. Meanwhile, soon after, in 1983, yet another type of chemosynthesic communities was discovered in the eastern Gulf of Mexico. And again, these communities were dominated by tubeworms, mussels, and gorgonian corals. The worms, like this, were closely related to already mentioned Lamellibranchia and huge tubeworms of hydrothermal beds.

Well, let's fast forward one decade. This is when molecular data become instrumental in our studies of life. And in 1993, first molecular data were published indicating that Vestimentifera are, in fact, closer to polychaetes than to vertebrates and to chemichordates. Independently, based on analysis of morphological data set along with all polychaetes, Rouse and Fauchald concluded that Vestimentifera and Pogonophorans are polychaetes closely to families of Sabellidae and Owenidae, and they suggested to resurrect nearly forgotten family Siboglinidae within Polychaeta. And again, molecular data a bit later, 2004, supported this conclusion based on morphology. While Pogonophorans and Vestimentifera are unusual gutless animals, they are nevertheless a group of polychaetes close to Owenidae, animals like this.

The next thing that happened actually brings us close to zombie worms. And the most important event in our saga of zombie worms took place in February 2002 when MBARI (Monterey Bay Aquarium Research Institute) biologist Robert Vrijenhock was using a remotely operated vehicle in the Monterey Canyon and he found this skeleton of dead whale. These were just bones and a bit of soft tissue and a wonderful group of organisms living on and around the carcass. These numerous pink animals are scavenging sea cucumbers. But what about this red shag rug of animals? To answer this question, Vrijenhock and his colleagues collected a sample of soft tissue, so here it is. And the DNA analysis showed that the bones belonged to juvenile grey whale. But what about the worms? What about the red carpeting? To answer this question, they sent the sample to a colleague, Greg Rouse, who at the time was working in the South Australian Museum. And Greg replied that there are good news and bad news. He said that the bad news is that I don't think these are annelids. They don't resemble any worms I've ever seen. But the good news, we found something new and exciting.

Nevertheless, they turned out to be worms, although yes, admittedly unusual ones. Like the vent worms, worms of hydrothermal vents, these whale-borne worms had bright plumes of tentacles and these plumes acted as gills. And also, they lacked a mouth and a digestive tract. What was unusual is these green root-like structures used to digest fats and proteins. And these green roots were filled with symbiotic bacteria. So, this already sounds kind of familiar, right? We've heard something like this before. Looking at the worms in microscopes, scientists discovered that all of them were females. No males were found until another call from Greg Rouse, who said, you know what, Rob, it's worse than you think. There are males, but they are microscopic. They are dwarfs. And sure enough, living within the tubes of females, they found about 30-200 microscopic worms, only about one millimetre long. So, the worst part was that the males were actually larvae, so filled with yolk from an egg and basically just very simple bags of sperm. These animals don't feed. The male lives his entire life on the yolk that was provisioned by the eggs.

In 2004, as you imagine, the paper was published in Science, Journal of Science, by Rouse and colleagues, and that described these worms as a new genus, Osedax, which in Latin means “bone devourer”. Because of its unusual habitat, they were also called zombie worms. Well, genetic studies of Osedax supported the suspicions that they are polychaetes and their close relatives were Vestimentiferans, like the animals found in the communities of hydrothermal vents. As we remember, they also lack a mouth and digestive tract, and they feed using help of symbiotic bacteria. Thus, the new amazing worms found their most amazing relatives. Well, the next thing that happened was actually due to a random event, as many things happened during this saga. In October 2004, the dead blue whale, a large one, was found on a beach not far from the Monterey City Centre. Scientists used this opportunity to set up an experiment. They sank Frank, as they lovingly called the whale, to about 1,000 metres, and sure enough, they found new Osedax living on these whale bones within one year.

The initial assumption was that the Osedax, zombie worms, were whale specialists. However, in 2007, Osedax was found near Manus Basin hydrothermal vents near Papua New Guinea, and it was found on bones of terrestrial quadrupeds. The robust pelvis and leg bones clearly show didn't belong to a cetacean. So the bone samples were not taken, but this gave scientists some new ideas. So what they did is they basically deployed the “cow bone tree” adjacent to whale bones found in Monterey Bay. And within a couple of months, colonisation by Osedax occurred in cow bones. So it's in time similar to the time it takes to colonise exposed whale bones. Morphological and molecular examination confirmed the presence of six out of eight species known from Monterey Bay. So this finding challenged the notion that these worms were whale bone specialists.

Bone worms turned out to be very common and actually widely distributed. By 2009, 12 species were found on whale bones sank between 2004 and 2008 off Monterey Bay alone. After the initial discovery, new species started to grow like mushrooms after the rain, and not only in California. Soon after discovery in California, a team of British and Swedish scientists sank a whale carcass to about 120 metres in the Northern Sea and sure they discovered a new species on the bone and they called it Osedax mucofloris like this one. And again, later the species were discovered in the Southern ocean. In May 2010, a cage with fish bones was left near a whale skeleton in Monterey. And again, as you imagine, Osedax was found after five months. So this finding clearly supported the idea that these animals are generalists, not specialists. And also it gave support to the idea that Osedax evolved millions of years before the Cretaceous period, well before the origin of marine mammals.

Well, 2013 comes and what we find about these strange animals? Well, they actually can secrete and emit acid to break whale skeletons. So the acid presumably allows the animals to release and absorb collagen and lipids that are trapped in the bone. And the model is remarkably similar to how mammals repair and remodel bone. But Osedax secrete acid to resolve foreign bone and access nutrients in the bone. Well, once again, once we thought we already knew everything about this bizarre creature, a new species of worm was discovered in 2015. And unlike in the previously discovered species, males of these animals were the same size of females. And because the males extend their bodies 10 times the usual state to find females to mate with, the new species was named Osedax priapus after the mythological god of fertility. Since 2002, species of Osedax have been found in various locations from the Pacific, North Atlantic and Southern oceans. All in depths from up to 3000 metres. In 2016, the animal was first found in warm shallow waters off the Mediterranean, living off minke whale bones planted at 53 metres. Important that the water temperature was from like 12 to 22 degrees centigrade, which is much warmer than the usual minus 1 to +15 of other shallow water Osedax habitats. At the time, no Osedax bone worms had yet been found in Australian waters.

Again, coming close to present time and again, we found that Osedax is a very common and unusual animal. So 14 new species were found again from Monterey Bay alone. And five of them had dwarf males while others had normal sized males. And they also for the first time were found on bird and marine turtle bones. The new rate, high rate of new species discovery clearly indicates that there is still a lot of undiscovered diversity. So these, the most recent ones, in 2019 and 2020 were found off coast of Brazil and then in the Arctic during colonising experiment. At the time, 27 species were published up to now and clearly still expected to find more and more.

Meanwhile, in Australia in 2017, we had this amazing voyage onboard RV Investigator along the East coast of Australia, Sampling the Abyss. And we found this small skull of a pilot whale with some vertebrae. And then sure thing, we found Osedax living in these bones. So the plan was to publish this specimen and the entire community actually living on the skull of whale. So I'm happy to report that this has been published on the 17th of May and we discovered and described two new species of Osedax or Aussiedax as they were lovely called on board.

So to summarize, so what happened with zombie worms? Obviously, the saga is to be continued. But what we know for now is zombie worms are related to giant worms of hydrothermal vent communities. They are found pretty much everywhere in the ocean from 50 to about 4,000 metres deep. They eat bone; to do that, they secret acids and they may or may not have harems of microscopic males – females that is. But other than that, they're just normal worms. So I would finish with a statement that actually the name “zombie worms” is a misnomer because the animals don't feed on brains as the zombie are supposed to feed. Instead, they feed on bones. But who knows what a new discovery would bring us? Maybe they will find that actually zombie worms do feed on brains. Watch your space! Thank you.