Tunicates are invertebrates with a very unique feature: they are born with a notochord, or rudimentary vertebrae, just like vertebrate animals. The notochord is lost when the animal passes the larval stage, and adult tunicates resemble any other boneless invertebrate. Take a look at one, and you might be surprised that it's closer to us than to jellyfish! Tunicates are named for their protective outer membrane, called a tunic, which makes them look clear and gelatinous. Benthic tunicates are commonly known as sea squirts, and are named for the water they squirt when disturbed. Planktonic, or drifting, tunicates are called salps. The tunicates that form pyrosomes are closely related to salps. In a pyrosome, each individual tunicate is called a zooid. The zooids have their own digestive systems, including an incurrent and excurrent siphon for filter feeding. If any zooids are wounded or killed, the remaining zooids can generate more copies of themselves to keep the pyrosome alive. They move through shared jet propulsion, releasing water through the excurrent siphon to power them through the water. In a pyrosome, the entire colony shares the same locomotive and excretory systems.
Giant pyrosomes are found worldwide, but occur more commonly in tropical areas with high primary production. Their favorite food is phytoplankton, and they feed by taking big gulps of plankton-filled water through their incurrent siphons. They are active after dusk, when planktonic communities engage in nocturnal vertical migration. They are affected by ocean chemistry, currents, and temperature, and you are most likely to see one in warmer waters where the phytoplankton are plentiful. A giant pyrosome is quite a rare sight! Monster Tales: Sea SerpentsIf you're familiar with mythical creatures, you've likely heard a few sea serpent tales. Legends of water monsters are tales as old as time. From the Biblical Leviathan, to the mythical Midgard Serpent of Norse mythology, to the Japanese water dragon, just about every mythos has its own sea serpent tale. These stories were passed down and adapted through the centuries. Sea charts like Olaus Magnus' Carta Marina (1539) featured oceans full of water horses, reptilian creatures, and vicious giant sea snakes. The Historia Animalium, published by naturalist Conrad Gessner in 1558, had a section dedicated to marine “monsters” and included a mention of the sea serpent depicted by Magnus. Most likely, these monsters were early attempts to make sense of the strange animals spotted at sea. The 19th and 20th century “fossil revolution” led to the discovery of even more monstrous sea creatures from the prehistoric era, which matched up with many of the sea monster descriptions from old myths and legends. As interest in our oceans grew, so did sightings of strange sea creatures. Sightings of alleged sea serpents continue to this day. In truth, the ocean is chock full of sea serpents, from sea kraits to oarfish to water tornadoes. And of course, there's the giant pyrosome. The Giant Pyrosome: A Real Sea SerpentGiant pyrosomes are very elusive creatures that most people outside of the marine bio field would not be familiar with. They are usually seen by divers, and may be occassionally spotted feeding near the surface. They are large, otherworldly looking creatures with gelatinous body forms, and they resemble giant worms with no appendages or features. Even those familiar with sea squirts at the beach probably wouldn't think to associate a giant sea worm with them, and the sight of a giant pyrosome might be quite mysterious and unsettling. Before the “marine biology revolution” of the late 19th and early 20th centuries, no traveler on the sea would know what to make of a massive tunicate. We know now that giant pyrosomes are the farthest thing from “vicious sea snakes.” They are completely passive animals, and their lack of a central nervous system means that not only would they never attack a ship, but they wouldn't even realize it's there. However, what else might a 19th century sailor think upon catching sight of one of these strange creatures? A long day at sea and a mind full of the old sea serpent legends can make the imagination run wild. Even a modern day sailor unfamiliar with marine invertebrates might mistake a giant pyrosome for a scary sea serpent. Most “sea serpent” candidates as we know them are harmless animals with no interest in attacking humans—the oarfish, for example, is another passive filter feeder. If a ship collides with a large animal, the resulting damage may lead to tales of a “brutal attack.” In the case of the giant pyrosome, a collision with a ship is much more likely to harm the pyrosome and not the ship. However, the zooids would simply regenerate and continue on their way. A Pyrosome Story? The Churchill Sea Serpent of 1884In August of 1884, the crew of the steamboat Churchill caught sight of a “horrifying sea snake” off the coast of Port Natal (Durban), South Africa. The creature was described as “being covered with large seashells, and to have a big, hairy head.” From the ship, the head and the tail appeared 60 feet apart. The animal was spotted at the surface for only a moment before it drifted beneath the ship, and very little further information exists on the encounter. Most of the details of the sighting come from Bernard Heuvelmans' In the Wake of the Sea Serpents. Could this startling sight have been a giant pyrosome? Of course, pyrosomes do not have “heads” or “tails,” nor do they have hair or any other features. But how else might a 19th century sailor describe such a creature? The pyrosome below shows that tunics may appear “hairy” or “fuzzy.” When viewed from the surface or from on board a ship, a surfacing pyrosome might appear to have a head covered in fine white hairs. For someone who has only ever seen seashells and not tunics, the individual tunics of the zooids might appear shell-like, especially if the animal was showing off its bioluminescence at the time; the effect can be very colorful, like light dancing off of iridescent shells. (Source) The area once known as Port Natal is located on South Africa's East Coast, where the Indian and Southern Oceans meet. The area experiences an average sea surface temperature of about 25 °C (77 °F) with little seasonal variation. The area is also quite productive, experiencing highs in primary productivity from the Antarctic waters below. It's an ideal location for a foraging giant pyrosome. Though it may be possible for a colonial pyrosome to reach a length of 60 feet, it would be exceedingly rare. The average length for a giant pyrosome is about 25 feet, with 30 at the highest. However, estimating length, with, or distance while at sea can be very tricky. The mind's eye cannot always be trusted, especially after having been startled by a “sea serpent.” Was the Churchill sea serpent really a giant pyrosome? We may never know for sure. But it is certainly possible! With our increasing awareness of the diversity of our oceans, new meaning is given to old sea monster tales. Sea serpents come in many forms, but they may not be exactly as old voyagers and naturalists imagined. The giant pyrosome is just one example of the many wonderful “sea monsters” that roam our oceans. With its alien appearance, its beautiful bioluminescence, and its mesmirizing undulations as it drifts across the sea, it's truly a fantastic creature worthy of a riveting sea tale! Additional References
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Star D.M.Star D.M. is an aspiring "mad scientist" looking to know the unknown and see the unseen. She holds a B.A. in marine science with a concentration in wildlife biology, and has a particular interest in ornithology, invertebrates, and cryptozoological research. Her inspirations include Dr. Patricia Tannis and Professor Kokonoe Mercury from the Borderlands and BlazBlue series of videogames. Archives
February 2020
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