A new polychaete species "Cryptochaetosyllis imitatio," which mimics nudibranchs, has been discovered by a research group at a graduate school of Nagoya University and their collaborators. The discovery was made in marine areas of Mie and Wakayama Prefectures and Vietnam. The new species was living on "alcyonarian" corals, but it had a color and shape that stood out rather than a pattern that would blend in. Its morphological characteristics might provide protection from foreign enemies by mimicking the appearance of venomous aeolid nudibranchs that live in the same waters. Moving forward, the group will continue to study the ecology of C. imitatio and the detailed reasons for its mimicry.
Provided by Nagoya University
A research group led by Lecturer Naoto Jimi, (invertebrate systematics), Sugashima Marine Biological Laboratory, Graduate School of Science, Nagoya University, has been studying polychaetes, which live on corals. Initially, a fisherman from Sugashima Island in Toba City, Mie Prefecture, contacted the research group to say that "a sea slug-like organism is attached" to alcyonarian corals, and he gave a specimen to the research group. A similar specimen was also found by a diver while diving in Wakayama. Moreover, the group received a report that a joint research team comprising Malaysian, Russian, and French scientists discovered the same organism while diving in Vietnamese waters. All of the individuals were found in temperate waters at depths of 20−40 meters.
Individuals from Mie and Wakayama were available as specimens and two were used in this study. It should be noted that divers have confirmed that a large number of individuals are present in the waters of Wakayama.
The collaborators thought it was a nudibranch when it was collected, but Jimi thought it was a "polychaete." The research groups continued their research and identified it as a polychaete species. Because its appearance resembled the aeolid nudibranchs living around the collection sites, Jimi decided to investigate this nudibranch-mimicking organism in further detail.
Aeolid nudibranchs can store venom in their large tentacles called "cerata," and the new species found in this study had similar large tentacular structures. The similarities include the alternate arrangement of large and small tentacles, white-colored tips of the tentacles, and darker subdistal parts. The new species also looked exactly like aeolid nudibranchs in that the tips of their parapodial chaetae were concealed within the body.
Nudibranchs are mollusks, which include squids, octopuses, and shellfish as typical examples. Meanwhile, polychaetes belong to the phylum Annelida, which includes earthworms and leeches. Polychaetes are named after chaetae on the parapodia. Jimi and his colleagues considered that the nudibranch-mimicking organism found in this study was a new species of polychaete because it had characteristics different from those of previously known polychaetes. The species was then named C. imitatio (Japanese name: keshou-syllis) after its appearance that is beautiful as if the organism wears makeup and the Japanese term "keshou," which means changing the appearance. "Syllis" is the generic name for the family Syllidae of polychaetes.
Provided by Naofumi Ueda of DIVE KOOZA; the yellow circle was inserted by editorial staff
Jimi and his colleagues decided to characterize the mimicry of C. imitatio. Venomous organisms have two forms of mimicry: Batesian mimicry and Müllerian mimicry. Batesian mimicry is a form of mimicry where a non-venomous organism imitates the appearance of a venomous organism. On the other hand, Müllerian mimicry is a form of mimicry where different venomous organisms evolve to have similar patterns. These types of mimicry are considered rare in annelids.
Examination of the "dorsal cirri," which closely resemble the sea slug's cerata with venom of aeolid nudibranchs, revealed that they contained no venom. However, they could prepare only two specimens of C. imitatio, and they consider that the possibility of being venomous cannot be ruled out completely.
Jimi said, "If we could find more individuals, we could accurately determine whether this species is venomous. We will study the evolutionary process of C. imitatio to clarify which form (Batesian or Müllerian) was used to successfully acquired mimicry and why it mimics the venomous aeolid nudibranchs."
The study was supported by the Showa Seitoku Memorial Foundation and a grant-in-aid for scientific research from the Japan Society for the Promotion of Science. The results were published in the British scientific journal Scientific Reports on July 29, and Nagoya University issued a press release related to this study on July 25.
Original article was provided by the Science Portal and has been translated by Science Japan.