At last, this alien-looking sea creature spotted near Okinawa, Japan, has finally been identified. The picture was taken by the professional underwater photographer Ryo Minemizu in 2018.
Credit: Ryo Minemizu.
Experts were baffled and debated whether it was a worm, a mollusk, a crustacean, or a larval stage of an animal. After posting the images of this animal to social networks, hundreds of professional biologists failed at identification to the level of a phylum.
Also intrigued was Igor Adameyko, a developmental neurobiologist at the Karolinska Institute in Sweden. After obtaining a specimen of the pea-sized creature, he and his team subjected the specimen to an exhaustive analysis and were able to identify that it consisted of two versions of the same parasite.
Using techniques to recover badly degraded ancient DNA the mystery creature (the size of a pea) was revealed to belong to a group of parasitic flatworms called digenean flukes, a diverse group of parasites known as trematodes. Some flukes have evolved a behaviour in which the larvae join into shapes that mimic small organisms. In doing so they entice a fish to eat the larvae, so they can continue their life cycle inside the host.
Polymorphic parasitic larvae cooperate to build swimming colonies luring hosts https://www.cell.com/current-biology/fulltext/S0960-9822(23)01170-3
What’s interesting about this parasite is that the two versions of the larvae cooperate using two different forms. The outer ones are the infectious agents ready to latch onto the gills of a fish that swallows them. The ones in the centre have the job of moving the creature through the water but take no part in the actual reproduction.
Abstract.
Parasites have evolved a variety of astonishing strategies to survive within their hosts, yet the most challenging event in their personal chronicles is the passage from one host to another. It becomes even more complex when a parasite needs to pass through the external environment. Therefore, the free-living stages of parasites present a wide range of adaptations for transmission. Parasitic flatworms from the group Digenea (flukes) have free-living larvae, cercariae, which are remarkably diverse in structure and behavior. One of the cercariae transmission strategies is to attain a prey-like appearance for the host. This can be done through the formation of a swimming aggregate of several cercariae adjoined together by their tails. Through the use of live observations and light, electron, and confocal microscopy, we described such a supposedly prey-mimetic colony comprising cercariae of two distinct morphotypes. They are functionally specialized: larger morphotype (sailors) enable motility, and smaller morphotype (passengers) presumably facilitate infection. The analysis of local read alignments between the two samples reveals that both cercaria types have identical 18S, 28S, and 5.8S rRNA genes. Further phylogenetic analysis of these ribosomal sequences indicates that our specimen belongs to the digenean family Acanthocolpidae, likely genus Pleorchis. This discovery provides a unique example and a novel insight into how morphologically and functionally heterogeneous individuals of the same species cooperate to build colonial organisms for the purpose of infection. This strategy bears resemblance to the cooperating castes of the same species found among insects.