A scientific team, including researchers from CEAB-CSIC, has documented for the first time active movement in adult individuals of a type of ascidian. This newly discovered species employs this previously unknown strategy to survive in sandy seabeds. Until now, it was believed that these filter-feeding animals were completely immobile after their larval stage.
Researchers from the Università degli Studi di Napoli, the Stazione Zoologica Anton Dohrn (also in Naples, Italy), and the Centre for Advanced Studies of Blanes (CEAB-CSIC) have described a new species of ascidian discovered on the seabed of the Gulf of Naples (Tyrrhenian Sea, Mediterranean). It inhabits sandy substrates — an unusual habitat for ascidians — and displays a surprising characteristic: it is capable of movement, albeit very slowly. This behaviour, previously unrecorded in adult individuals of this animal group, challenges the traditional notion that ascidians remain fixed to a substrate once they complete metamorphosis (ascidian larvae do swim for a time until they find a suitable place to settle).
The new species has been named Heterostigma monniotae, as a homage to the French ascidian expert Françoise Monniot (1936–2024), a pioneer in the study of this group of animals. It lives freely in shallow sandy seabeds, between 2 and 20 metres deep, and as an adult displays a remarkable trait: it is able to move independently. The scientific team has captured this movement on video. Under laboratory conditions, numerous individuals freed themselves from the sand that typically covers them and adopted a “naked” form, with opposing siphons and rhythmic contractions that enabled them to slowly crawl along the sandy substrate. Within a few days, they had moved up to 17 millimetres.
One member of the scientific team, CEAB-CSIC ascidian expert Xavier Turon, explains:“It might not seem like much, but the mere fact that they move actively completely changes what we knew about these animals. We thought that once developed, they were entirely sessile, unable to move at all, fixed for life where the larvae had settled. Voluntary movement in adulthood, even if very slow, could be key to survival in certain contexts: it may help them regroup or find more favourable conditions. This discovery compels us to rethink their biology, ecology, and evolution.”
Riccardo Virgili, PhD student at the University of Naples Federico II and at the Stazione Zoologica Anton Dohrn and first autor of the article, emphasises:“Ascidians show a variety of body plans and lifestyles. However, what we observed in this new species was formerly unreported and quite surprising. The mechanisms observed in this little ascidian revealed the unexpected capacity of certain ascidians to actively react to changes of external conditions. These findings open new questions on on how ascidians can adapt to unstable habitats.”
The discovery of this new species and its unique adaptation has been documented in a scientific article recently published in the journal Invertebrate Systematics, titled “Paedomorphic adaptations in a new Heterostigma species: a novel strategy for ascidians to live in soft-bottom habitats” (Virgili et al., 2025, doi:10.1071/IS24103).
The researchers highlight that this evolutionary strategy is completely novel in ascidians. It is based on paedomorphosis — the retention of juvenile traits into adulthood. In H. monniotae, characteristics of the juvenile form — such as a flexible body, opposing siphons, and a thin tunic with little sand (facilitating movement) — persist into the adult stage. This is an adaptive strategy for life among sand grains, an uncommon substrate for these marine animals. According to the scientists, this active movement may be an extreme response to living in unstable habitats, helping individuals escape localised threats such as currents and burial. They stress that this is the first documented case of adult ascidians developing a mobile phenotype, opening new perspectives for the study of these organisms.
The team also warns of the fragility of this population: H. monniotae has low genetic diversity and a very limited distribution, in heavily human-impacted coastal areas (so far located around Naples and specific sites in southern France). They therefore call for monitoring and conservation measures — both direct and indirect — to protect this particularly unique species.
Filter-feeding marine animals
Ascidians are marine invertebrates, classified among the tunicates. They feed by filtering water, extracting nutrients. They are commonly found on the seafloor, particularly on rocky substrates, where they attach and remain for their entire adult lives. Despite their simple appearance, ascidians are the group of animals most closely related to vertebrates in evolutionary terms. Most undergo a mobile larval stage, during which they swim for a few hours before finding a suitable place to attach. Once fixed, they develop into a fully sedentary adult form. This recent discovery challenges the long-standing belief that all ascidians are sessile once developed, revealing a new species capable of movement in adulthood — an evolutionary strategy for adapting to the sandy Mediterranean seabeds where it lives.
