Gelatinous, yes, but a pyrosome is neither squishy nor a member of the phylum inhabited by jellies (Cnideria). A far more structurally sophisticated invertebrate, the pinkish-purplish, rigid tube is classified in the phylum Tunicata. The word “colonial” aptly describes the tubular unit comprising hundreds or even thousands of outward-facing individuals, each scant millimeters long, and joined at the hip (so to speak) by a gelatinous tunic. The tube’s tapered end is closed; the wider end is open and houses a diaphragm for moving water. Look at the tube’s rough-textured exterior, and you can easily determine the number and location of every individual because each projection represents one oral intake siphon. The siphon’s beating hairlike projections (cilia) pull in water while sieving out plankton delicacies, then the filtered water is expelled into the inside of the colony where it goes out the open end.
Cilia are central to a pyrosome’s existence and not only because they make food gathering possible. The beating hairs keep the water flowing so the members are continually supplied with fresh oxygen for respiration. And cilia allow locomotive action by jet propulsion.
Despite the importance of cilia to a colony, the scientific name, Pyrosoma, has nothing to do with water flow. Instead, it reflects the pyrosome’s ability to bioluminesce: the Greek word pyros for fire and soma for body. When disturbed, pyrosomes emit a dazzling blue-green light, which plays out in waves courtesy of photoreceptor cells inherent to each individual. One member is triggered to flash, which triggers their neighbors to flash, which triggers their neighbors to flash. The result is a wave of light rolling across the tube, kind of like fans performing an impromptu wave at the stadium, except a pyrosome’s wave is perfectly timed and stunning to watch.
Sexually speaking, pyrosomes enjoy a mix of reproductive phases. As hermaphrodites, each colony member produces both eggs and sperm, with the latter fertilizing the former to generate an embryo. The end result is conjoined quadruplets. Once a quadruplet is released, they continue growing their colony by budding, a method of asexual reproduction.
Though I’ve only them seen going solo or in small numbers, pyrosomes may be found in enormous numbers, making it possible to study their daily vertical migration. Come nightfall, they travel to great depths, then migrate toward the surface for daylight. Depending on colony size, migrations averaged about 1,300 feet, less for the small ones and more for the big ones. In terms of predators, pyrosomes don’t look particularly nutritious but, worldwide, they are a significant food source for many fish and several turtle species. More recent data reveals albatross species and sea lions observed chowing down on pyrosomes. In another study, researchers in a submersible documented thousands of dead P. atlanticum on the seafloor abutting an oil pipeline, suggesting pyrosomes that die and sink quickly to the bottom of the deep ocean may represent a major food resource for both deep sea microbes and larger bottom-dwelling organisms. And among invertebrates, copepods, amphipods and tiny shrimps have been found inside pyrosoma colonies, presumably feeding. That these critters support the rest of the ocean’s food chain is reason enough to care about pyrosomes.
— Judith Lea Garfield, biologist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. www.judith.garfield.org. Questions, comments or suggestions? E-mail firstname.lastname@example.org