Mysterious Underwater Creature Revealed to be a Group of Parasitic Worms
In April 2018, underwater photographer Ryo Minemizu captured photos of a peculiar creature while diving off the shores of Okinawa, Japan. The creature, resembling a jellyfish with a yolk-like center and wispy tentacles, left researchers and social media users baffled. However, a recent study published in the journal Current Biology has identified the mysterious swimmer as not one individual animal but a cluster of 1,020 parasitic worms.
Minemizu collected a sample of the creature and preserved it in formaldehyde for further study. Upon examination under a microscope, scientists discovered that the jellyfish-like organism consisted of two types of cercariae, the larval forms of parasitic worms called trematodes. Trematodes undergo complex life cycles, typically requiring two different hosts to complete their life cycle.
The life cycle of trematodes begins with eggs released in the feces of birds, mammals, or fish. These eggs hatch into salami-shaped larvae, which swim through water to find their first host, often a snail. Once inside the snail, the larvae clone themselves into cercariae, which are then released into the water in search of a secondary host such as a bird, fish, or mammal. The goal of the cercariae is to be eaten by the final host, where they reproduce and release fertilized eggs through feces.
In the case of the Okinawan creature, scientists identified two types of cercariae: larger tentacle-like “sailors” and smaller “passengers.” The specimen contained over 1,000 passengers arranged in a hemisphere, with the larger sailors anchored to one side and waving their tails. The synchronized movement of the sailor tails allowed the entire worm cluster to move, pulsing or jumping when in unison and smoothly jetting around when asynchronous. The specimen had a yellowish color inside the sailor tails and a brownish color within the numerous passengers.
When fragments of the creature’s genome were sequenced, the closest matches were found in the Pleorchis genus of trematode worms. However, the specific species remains unknown.
The aggregation of over 1,000 worms forming the shape of a jellyfish reflects an evolutionary strategy where multiple individual creatures mimic prey to increase their chances of being eaten. This phenomenon is seen in other species of trematode larvae, some of which have developed remarkably long tails. Another disturbing strategy called “Rattenkönig” occurs when worms join together by their tails, forming a squirming pinwheel similar to a rat-king.
The researchers believe that the jellyfish-like aggregation not only aids the worms in swimming but also provides additional benefits such as increasing the number of parasites swallowed by a host in a single gulp and ensuring the worms reach their correct host. Polymorphism, or multiple forms within one species, is rare in parasitic flatworms, making the presence of distinct sailors and passengers in this aggregation a new case of divergent forms within a single trematode.
While the researchers discovered well-developed penetration glands in the passengers, which allow them to invade their host’s tissues, the larger sailors lacked such organs. This leaves their ultimate purpose a mystery. It is unclear if the sailors sacrifice themselves to transport the passengers to their final destination, showcasing a division of labor among the baby worms, or if they engage in a massive mating event within the host’s digestive system.
The study highlights the mysteries that still exist in the sea and serves as a reminder that appearances can often deceive. Some seemingly innocuous creatures, like the bobbing jellyfish, turn out to be complex clusters of parasitic worms united by their desire to be consumed and subsequently expelled. The underwater world continues to hold surprises, and further research is needed to unlock its secrets.