The Wonderful World of Worms

Happy New Year! I hope everyone is starting the year with a new appreciation for the world around us and the science being conducted to understand it further. While the cruise is over, there are still some things I’d like to share with you all.

This post I want to introduce to you the abundance and diversity of worms at hydrothermal vents. In the deep-sea, worms rule. Filter feeding, recycling nutrients and sediment, grazing, parasitizing, and even ambushing prey; you can find worms in many roles and ecosystems with vents systems being no exception. I’ve already introduced a few of these worms, but first I’d like to delve further into the largest of them, Riftia pachyptila, the large red tube worm that dominates our study sites.

These worms are particularly interesting because they are chemosymbiotic, meaning they have a relationship with bacteria that use chemicals for energy, that live inside them. These bacteria consume the chemicals in hydrothermal vent fluid and create byproducts that the host worm can use. These worms have three main segments. The first is their large red plume. This allows the worm to take in the nutrients in the venting fluid, as well as their bacterial symbionts. This is the only part of the worm that sticks out of its tube, and it has mechanoreceptors that sense when it is touched, so the worm can retract to safety. Often, we see crabs and eelpouts eating the tubes and trying to nip the worms. The second segment is the vestimentum, a muscle that anchors the worm into its tube, and houses the animal’s heart, and reproductive organs. The third section is the trophosome, instead of having a stomach and gut like most animals, once their bacterial symbionts are acquired, they transition from a gut into having essentially a bag of bacteria, a powerhouse for turning sulfide into food. (I think it looks like a blood sausage.) The scientists on board received a private lesson from Dr. Stéphane Hourdez (CNRS) and Dr. Shawn Arellano (WWU) through a dissection of a Riftia we collected. Once we dissected it, we could see the white eggs located on the vestimentum, and Vanessa Jimenez (WWU) extracted them to look at them under the microscope. These worms are fascinating not just because they are uniquely adapted to hydrothermal vent habitats, but also uniquely adapted to the relationship with their specific bacterial symbionts.

There are two species at the East Pacific Rise; Alvinella pompejana, and Alvinella caudata. The most noticeable part of Alvinella are four petal-like gills that stick out of their tubes for gas exchange.  Unlike Riftia, they still have a functional gut, where they can consume bacteria that they grab with their feeding tentacles. Their symbiosis is a little different. Instead, the backs of these worms are covered in white hairs, which are actually bacteria, that eat a mucus the worms produce. It’s hypothesized that these bacteria could help insulate the worms against the hot temperatures. Alvinella are fascinating as one of the most thermally tolerant animals in the world, living on the edge of when the mitochondria in their cells begin to break down.

Along these sulfide spires, these worms are intermixed with other tube-dwelling worms called Paralvinella. As you can guess, both Paralvinella and Alvinella are named after the very Alvin submersible we’ve been using on this cruise! They look very similar to Alvinella, but to the trained eye (mostly Stéphane) there are noticeable differences. The major identifying difference is which segment their chaetae (crawling legs) begin.

Crawling amongst all these tube-dwelling worms are a type of polychaete worm called Polynoids, or scaleworms. These animals have scales covering their backside for protection from predation. They come in many colors, but the ones around here are a pink to purplish color. Dr. Stéphane Hourdez on board is particularly interested in these organisms, experimenting on the thermal tolerances of scaleworms, and other organisms collected opportunistically. The first challenge with this experiment is that he must keep these organisms at a pressure similar to their natural environment in order to ensure they are reacting properly to temperature changes without additional stress. To do this, he has a pressure chamber, that with using an HPLC (High-Performance Liquid Chromatography) pump, pressure can be set to 250 bar, equal to 2,500 meters deep. Once the pressure is set, he observes how the animals react to increasing temperatures. Since they live at these hotter than normal habitats, we’re curious about their thermal adaptations and the limitations.

There’s one more symbiotic polychaete that is commonly found at these sites, although very different from the previously introduced ones. These Polynoids, genus Branchipolynoe, are actually found inside another animal. The large mussels at the EPR called Bathymodiolus thermophilus, can often be found to host one of these large Polynoids. While the other symbiotic relationships both parties benefit, only the Polynoids are benefitting from this relationship. They receive a safe habitat to develop and reproduce and the mussels just have a roommate that pays no rent. Each mussel only has enough room for one large female though, while there could be multiple smaller males. These females are very fecund, meaning they have a lot of eggs. These eggs are also very large for a polychaete. With a quick cut on the underside, the eggs erupt out of the females. These worms are found in other deep-sea habitats as well, where we’ve found them in the Bathymodiolins at methane seeps in the Gulf of Mexico. I still remember how surprising it was to open a mussel and find a giant worm inside!

I hope this gives you a little more appreciation for worms. While these live in relatively extreme habitats, they are found all over the marine world, and can often be found if you just dig in the sand at your local beaches. They can be quite beautiful with iridescent scales reflecting rainbows back, or swirling feeding crowns of tubeworms. If anything, I hope you appreciate the diversity of these worms more. They come in all colors and sizes and can play many roles within an ecosystem. I want to leave you with a fantastic drawing that second mate Kenny Beaver of the R/V Atlantis drew about the Riftia tube worms we studied.