The Trojan seahorse: citizen science pictures of a seahorse harbour insights into the distribution and behaviour of a long-overlooked polychaete worm.

Symbiotic marine invertebrates can be small, hidden or difficult to find, hampering the understanding of their distribution and ecological roles. Haplosyllis anthogorgicola is a polychaete inhabiting the gorgonian Anthogorgia bocki, where it lives in high densities within the host's coenenchyme and occupies burrows formed by host tissue near coral polyps. This study provides the first records of H. anthogorgicola since its description in 1956, from colonies of Anthogorgiidae in southern Japan. We observed that host gorgonians were also inhabited by the pygmy seahorse Hippocampus bargibanti, a popular species to observe and photograph among SCUBA divers. Therefore, we examined photographic records of H. bargibanti available on the citizen science website iNaturalist and screened for structures associated with infestation by H. anthogorgicola to gather information on this elusive species. Our analyses confirmed that this polychaete and/or similar species are widespread in the central Indo-Pacific region. In addition, we observed some polychaete behaviours, raising questions about the nature of the relationships between H. anthogorgicola, its gorgonian hosts and the pygmy seahorse. Our study demonstrates that citizen science can contribute to our knowledge not only on the distribution and behaviour of well known and charismatic species but also inadvertently on overlooked and neglected taxa.