Riftia Pachyptila (Vent Worms)
Host Ari Daniel dives deep to discover a white worm as tall as your refrigerator that breathes through bright red feathery “lips.” This isn’t a creature from outer space. Meet Riftia, a tube worm that lives in deep-sea vents, and learn the surprising lessons this denizen of the abyss is teaching scientists about life on Earth.
Riftia pachyptila is a giant tube-dwelling annelid in the family Siboglinidae. Siboglinids are important members of deep-sea chemosynthetic communities, which include hydrothermal vents, cold seeps, whale falls, and reduced sediments. As adults, these worms lack a functional digestive system and rely on microbial endosymbionts for their energetic needs. (Hilário et al. 2011)
Riftia pachyptila was discovered on hydrothermal vents at the Galapagos Rift in 1977. It is now known to be a widely distributed inhabitant of vents along the East Pacific Rise and Galápagos Rift (Coykendall et al. 2011). Larvae are estimated to disperse more than 100 km over a 5-week period (Marsh et al. 2001). Studies of vent community succession have shown that Riftia pachyptila is among the first species to colonize a new vent once suitable conditions are established. Within two years its numbers can grow to several thousand adult individuals, but changes in vent flow or overgrowth by mytilid mussels can lead to its replacement as the dominant species. This process can take months to years, depending on the location. (Coykendall et al. 2011 and references therein)
Coykendall et al. (2011) studied genetic variation among Riftia pachyptila populations at one mitochondrial and three nuclear loci. They found low rates of genetic variation, especially in southern populations, which exhibit lower occupancy (i.e., percentage of active vents occupied) than do more northern populations. They suggested that the observed geographic pattern of genetic variation is likely explained at least in part by geographic variation in rates of local extinction and (re)colonization. In the Eastern Pacific in general, vent habitats are highly ephemeral, persisting for a few years to several decades before fluid conduits are blocked, magma supplies shift, or lava flows extirpate local communities. On the source side, earthquakes can open fluid conduits, re-activating old vents, and magmatic eruptions spawn new vents.
Hilário et al. (2011) wrote of Riftia pachyptila: “[Riftia pachyptila] became the poster-child of deep-sea discovery, the ‘lost world’ of unknown animal lineages that scientists on the Challenger deep-sea expedition 100 years previously had so wanted, but failed, to find. Arguably, this single species of worm launched the careers of a generation of deep-sea biologists.” At one time, Riftia pachyptila was placed in its own phylum, the Vestimentifera, although this status was short-lived as a result of new phylogenetic investigations (for review, see Pleijel et al. 2009 and Hilário et al. 2011).
Like other siboglinids, adult Riftia pachyptila lack a gut, mouth, anus, and conventional feeding ability and possesses bacterial symbionts. (Hilário et al. 2011). Adult Riftia pachyptila are nourished entirely by sulfur-oxidizing endosymbiotic bacteria (Coykendall et al. 2011). Although the larvae of Riftia pachytila are symbiont-free and possess a transient digestive system, these digestive structures are lost during development, resulting in adult animals that are nutritionally dependent on their bacterial symbionts. Thus, each generation of tubeworms must be newly colonized with appropriate symbionts. (Nussbaumer et al. 2006)
In the deep sea, aggregations of vestimentiferan tubeworms at hydrothermal vents and hydrocarbon seeps host diverse assemblages of smaller invertebrates. At deep sea hydrothermal vents in the eastern Pacific, Riftia pachyptila form large and dense aggregations in a spatially and temporally variable environment. The density and diversity of smaller invertebrates is higher in association with aggregations of Riftia pachyptila than on the surrounding basalt rock seafloor. (Govenar and Fisher 2007 and references therein)
You can become a TerraMar Ambassador to Riftia Pachyptila (Vent Worms) and learn more about the species here.
Ari: For 30 years, scientists have traveled to the very bottom of the ocean thanks to Alvin, an underwater vehicle.
Cavanaugh: Yes, Alvin is, I always say it’s the submarine, not the chipmunk.
Ari: Colleen Cavanaugh is a biology professor at Harvard University, and she’s spied on the seafloor from inside Alvin a number of times.
Cavanaugh: It’s a so-called manned, which I call it a womanned submersible. It has a pilot and carries two scientific observers.
Ari: And it’s tiny. The space where you actually sit, there’s barely enough room to stretch your arms out to the sides. Tiny portholes let you look out. Not that there’s that much to see.
Cavanaugh: Most of the deep sea just looks like sediment or black rock, you know, very, very few organisms.
Ari: But the story’s different at places called hydrothermal vents. Basically, they’re hot springs on the ocean floor. It’s where two plates at the bottom of the ocean are being pushed apart as magma comes up. The magma’s hot and acidic and has all kinds of chemicals in it like sulfur. And, says Cavanaugh, it’s stunning when you’re in Alvin and you approach a hydrothermal vent.
Cavanaugh: And you start way out, you start to see a crab here, and a crab there, and then, you know, maybe a fish goes by. And as you get closer and closer, there is more and more life. When you actually come up to them, it’s an oasis. It’s biomass that rivals a rainforest.
Ari: When these hydrothermal vents were discovered, they completely blew open the view of the deep ocean as an empty, lifeless desert. There was tons of life down there. Thickets of it. One of the creatures in this deep-sea rainforest is something called Riftia.
Cavanaugh: A Riftia is… well, they’re so called giant tubeworms. They get up to 2 meters in length.
Ari: So imagine a long, white tube as tall as your refrigerator and as wide as a roll of quarters.
Cavanaugh: And the part that you see that’s so astonishing is that they have these brilliant red plumes.
Ari: Like a pair of wide, ruby lips at the end of this white tube.
Cavanaugh: And this is effectively their gill, or their lung. It’s the equivalent of our lung. The red that you see is actually due to hemoglobin. So just like we have hemoglobin that carries our oxygen, so do these worms. And this hemoglobin is unique in that it binds both oxygen, like us, but it also binds sulfide in a separate spot.
Ari: Strange enough yet? A worm at the bottom of the ocean as tall as your fridge that’s got hemoglobin just like we do. And here’s the really weird part about Riftia.
Cavanaugh: As an adult, it’s completely mouthless and gutless.
Ari: That’s right. Riftia worms have no mouth, no anus, no gut, and they stay put. So how do these things eat? No one knew. Until Cavanaugh had a kind of epiphany almost 30 years ago.
Cavanaugh: It was a moment, but it took a while to prove.
Ari: She was sitting through this talk during graduate school that wasn’t what you’d call riveting…
Cavanaugh: So unbelievably, I was still awake, when I jumped up and said, it’s perfectly clear. They’re effectively like plants but using sulfur instead of sunlight. I mean, they must have sulfur-oxidizing chemosynthetic bacteria.
Ari: Cavanaugh’s revolutionary idea was that a special kind of bacteria lives inside the Riftia worms and actually feeds them. The bacteria use the energy from all the sulfur billowing about the hydrothermal vents to make food – carbohydrates and proteins – for themselves and for the worms. But this is a two way street. The Rifita use their hemoglobin to bring the sulfur right to the bacteria’s doorstep. And the worms also give the bacteria a place to live.
Cavanaugh: Yes, so, they’re providing a home, a, in quotes, a warm, cozy environment. Neither the animals nor the bacteria could survive without the other.
Ari: And it turns out that this discovery opened scientists’ eyes to the fact that this kind of symbiosis, the kind involving inorganic chemicals like sulfur, is happening everywhere. Not just at the deep sea vents. But also in shallow coastal places, on the tops of mountains, and maybe even on other planets. All these critters, helping each other make a go of it in the universe. And all we had to do was take a lesson from a worm at the bottom of the ocean.
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