Paedomorphic adaptations in a new species: a novel strategy for ascidians to live in soft-bottom habitats.

Inhabiting soft substrates presents complex challenges for some groups of sessile filter feeders. Among these, ascidians have independently evolved traits and strategies to live in such habitats. Paedomorphosis, the retention of juvenile features of the ancestor into the adult stage, has been associated with taxa living freely within the sediment, including species of the pyurid genus Heterostigma Ärnbäck-Christie-Linde, 1924 (Ascidiacea: Stolidobranchia: Pyuridae). These poorly known solitary ascidians display peculiar morphological adaptations deemed for an interstitial lifestyle, although complete knowledge of their biology is still lacking. We hereby describe Heterostigma monniotae sp. nov., a new pyurid species from the littoral soft bottoms of Napoli (central Tyrrhenian Sea, Mediterranean Sea). An updated taxonomic table, built on the literature and the screening of types and unpublished material, clarified differences within the known species of the genus. Multilocus phylogenetic analyses based on single genes and the complete mitochondrial genome of the new species provided the first molecular information on this group and resolved its position as sister to the other Stolidobranchia. Finally, morphological and behavioural acquired adaptations were observed in live specimens of H. monniotae , revealing the shift from a sessile to a paedomorphic motile phenotype in mature specimens. This adaptive strategy was never documented before in sessile ascidians, appearing as an extreme strategy to survive in unstable habitats, although its inducing factors are still unclear. The motility of adults was filmed here for the first time. These findings challenge previous assumptions of these species' lifestyle and behaviour, contributing to the understanding of the development and ecology of this group of sand-living ascidians. Finally, the comparison with closely related species highlighted how ontogenetic processes may have contributed to the radiation of sand-living tunicates. ZooBank: urn:lsid:zoobank.org:pub:FBC89E26-9213-4E40-81D1-36E686B118FC.