Despite the abundant diversity of worm species on Earth, many may struggle to name species beyond the common Earthworm (Lumbricina). In fact, many would be surprised by the extent and complexity of their classifications. Worms are present in several invertebrate phyla, with the most common being Platyhelminthes (flatworms), Nematoda (nematodes), and Annelida (segmented worms)^[1]Worm | animal | Britannica. https://www.britannica.com/animal/worm.. Phylum Annelida comprises three separate classes of worms: Polychaeta (marine worms), Oligochaeta (earthworms), and Hirudinea (leeches);  marine worms (polychaete), in particular, have developed remarkable physiological adaptations and strategies to survive within the ocean ecosystem^[2]Worms: Phyla Platyhelmintes, Nematoda, and Annelida | manoa.hawaii.edu/ExploringOurFluidEarth. … Continue reading. So, what are these adaptations, how are these adaptations affecting the ocean, and why do these worms warrant our attention?

What are Polychaetes?

The first step in understanding an organism’s importance in its ecosystem is understanding the organism itself. Polychaetes are marine annelid worms, often referred to as “bristle worms”. They are present in almost all aquatic environments, from shallow estuaries to abyssal zones and hydrothermal vent regions^[3] Osborn, D. K. Why I Love Polychaetes | Smithsonian Ocean. https://ocean.si.edu/ocean-life/invertebrates/why-i-love-polychaetes.. Their bodies are covered in fleshy protrusions called parapodia, which contain chitinous (i.e., made of chitin — a carbohydrate polymer that some organisms use for structure) bristles called chaetae. The worms use the parapodia for locomotion, feeding, and tube-building. Polychaetes are also classified by their lifestyle types as free-ranging, burrowing, or tube-dwelling^[4]Annelida: | Infoplease. https://www.infoplease.com/encyclopedia/ecology/animals/invertebrates/annelida/class-polychaeta.^[5]World Polychaeta Database. https://www.marinespecies.org/polychaeta/..

Errant Polychaetes remain active, crawling or swimming along the ocean floor using their parapodia, and although they may spend some time in burrows or under rocks, they do not stay in one location or settle. These worms are scavenging omnivores with well-developed eyes and sensory tentacles for hunting. One common species of errant Polychaetes are clamworms (Alitta succinea), which can grow over a foot in length and act as

Figure 1: Phyllodoce rosea ^[6]https://en.wikipedia.org/wiki/Polychaete

an essential contributor in the food chain for many bottom-dwelling organisms like skates and crabs^[7]Gardiner, S. L. Errant Polychaete Annelids from North Carolina. Journal of the Elisha Mitchell Scientific Society 91, 77–220 (1975)..  Due to their position low on the food chain, clamworms have developed a unique defense from predators: if the worm feels threatened, it can secrete a mucus cocoon around itself, which quickly hardens and acts as a protective shell around the body^[8]Stabili, L. et al. First Insight on the Mucus of the Annelid Myxicola infundibulum (Polychaeta, Sabellidae) as a Potential Prospect for Drug Discovery. Mar Drugs 17, 396 (2019).^[9]Common Clam Worms. SERC Education Docent and Volunteer Web Page https://serceducationvolunteers.wordpress.com/2013/08/30/common-clam-worms/ (2013).. 

The next group of Polychaetes is the burrowing worms, such as the lugworm (Arenicola marina), which dig u-shaped, mucus-lined burrows in the sediment or, in some cases, shells of other organisms such as oysters or abalone. These worms inhabit these burrows and use mucus and collected detritus to build the interior lining. Because these worms are relatively sedentary, they usually lack eyes and sensory tentacles and have few parapodia^[10]Zottoli, R. A. & Carriker, M. R. Burrow morphology, tube formation, and microarchitecture of shell dissolution by the spionid polychaete Polydora websteri. Mar. Biol. 27, 307–316 (1974).. The lack of sensory tentacles means that their hunting abilities are severely limited, so these Polychaetes feed by swallowing large amounts of sediment and filtering and digesting any organic materials^[11]Rouse, G. W. & Fauchald, K. Cladistics and polychaetes. Zoologica Scripta 26, 139–204 (1997).. Lugworms (Arenicola marina), a common species of Polychaete found in the British Isles, dig their borrows in the intertidal zone. These worms provide a crucial food source for sea birds and bait for many global fisheries. 

Figure 2: Burrowing Polychaete ^[12]https://biocyclopedia.com/index/general_zoology/class_polychaeta.php

The final group of marine worms are tube-dwelling Polychaetes. These worms construct calcareous tubes to act as shelter and hunting mechanisms and enable the worm to remain sedentary, attached to a rock or shell^[13]Kočí, T., Goedert, J. L. & Buckeridge, J. S. Eocene tube-dwelling annelids (Polychaeta: Sedentaria) from the Black Hills, western Washington State: the first record of Neodexiospira from North … Continue reading.

Figure 3: European Fan Worm (Sabella spallanzanii) ^[14]https://commons.wikimedia.org/wiki/File:CSIRO_ScienceImage_958_Sabella_spallanzanii_European_fanworm.jpg

Tube-dwelling Polychaetes are unique in that they lack parapodia because they do not swim. Still, they possess unique, fan-like anterior structures for feeding, allowing them to filter out plankton and pieces of detritus drifting down the oceanic zones^[15]Dill, L. M. & Fraser, A. H. G. The worm re-turns: hiding behavior of a tube-dwelling marine polychaete, Serpula vermicularis. Behavioral Ecology 8, 186–193 (1997)..  Tube-worms can then pull their fans back into the tube and feed on the collected plankton. Polychaetes construct their calcium carbonate (CaCO3) tubes out of sediment, shell fragments, and algae with inner layers that they continually reinforce to protect the structure of the tube^[16]Merz, R. A. Textures and traction: how tube-dwelling polychaetes get a leg up. Invertebr Biol 134, 61–77 (2015).^[17]Natural History Collections: Polychaeta. http://www.nhc.ed.ac.uk/index.php?page=24.25.312.329.359.. In recent years, the structural soundness of polychaete tubes has become threatened, primarily due to increased ocean acidification from climate change^[18]De Marchi, L. et al. The influence of simulated global ocean acidification on the toxic effects of carbon nanoparticles on polychaetes. Science of The Total Environment 666, 1178–1187 (2019).. Ocean acidification occurs as carbon dioxide levels in the atmosphere increase, which leads to more CO2 being absorbed by the ocean. As the CO2 is absorbed and dissolved, it lowers the pH of the water, making it more acidic. The increased acidity of the water leads to the weakening of calcium carbonate structures, such as polychaete tubes, inhibiting growth and forcing the worms to expend more energy on tube fortification rather than other biological processes, such as reproduction^[19]US Department of Commerce, N. O. and A. A. What is Ocean Acidification? https://oceanservice.noaa.gov/facts/acidification.html.^[20]Feely, R. A. & Doney, S. C. Ocean Acidification: The Other CO2 Problem. ASLO Web Lectures 3, 1–59 (2011).. Overall, ocean acidification weakens tube-dwelling Polychaetes and results in declining reproductive rates and population abundance.

Polychaetes and their Ecosystem Impacts

Although each polychaete group has specific adaptations for its survival, the tube-dwelling, calcareous Polychaetes’ adaptations are especially noteworthy in that they support the survival and success of many organisms within the ecosystem. These Polychaetes are considered “ecosystem engineers”, which are species responsible for creating habitat or ecosystem functions through their actions^[21]Martins, A. D. & Barros, F. Ecological Functions of Polychaetes Along Estuarine Gradients. Frontiers in Marine Science 9, (2022).. Some of the most well-known ecosystem engineers are beavers who build dams that result in abundant freshwater lakes. Calcareous Polychaetes earn their engineer status from their construction of tubes, which not only provide the necessary habitat for the worms but also alter water flow, increase the accumulation of organic matter in reefs and biotic diversity, and modify the distribution of infaunal (i.e., organisms living within bottom sediments)  and epifaunal (i.e., organisms living attached to the surface of the bottom sediments) organisms. Furthermore, tube-dwelling Polychaetes are filter feeders and thus impact the physical environment in the ecosystem, such as the water clarity. Although this is not considered an ecosystem engineering role, it nonetheless substantially affects the system’s overall function^[22]Bruschetti, M. Role of Reef-Building, Ecosystem Engineering Polychaetes in Shallow Water Ecosystems. Diversity 11, 168 (2019).. The changes in nutrient load and organic matter within reefs can support or deteriorate the ecosystem’s ability to host different species. Consequently, the nutrient cycling performed by calcareous Polychaetes regulates the abundance of crucial marine nutrients required for specific habitats to flourish. Based on their multitude of ecosystem impacts, Polychaetes demonstrate the importance of large-scale conservation efforts. We need to learn to extend protection to species of all sizes and abundance, as there is still much to learn about ocean systems and the organisms that are critical in their development and function.

Emily Padden

Research and Writing Intern

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