A redescription of Syncarpacomposita (Ascidiacea, Stolidobranchia) with an inference of its phylogenetic position within Styelidae

Biodiversity and spatial distribution of ascidian using environmental DNA metabarcoding

Monitoring studies are necessary to understand the biodiversity of marine ecosystems and are useful for identifying and managing rare or invasive species. Because monitoring has traditionally relied only on visual surveys (e.g., trapping, netting, electrofishing, and SCUBA diving) with limited time and physical resources, environmental DNA (eDNA) analysis is being applied as an efficient monitoring method. This study compared whether the eDNA metabarcoding technique can replace the traditional visual survey in an ascidian fauna study. We designed ascidian-specific primers and identified a clear gap (3.75%) by barcoding gap analysis. Then, we collected seawater samples for eDNA analysis during the summer (August-September) of 2021 at three sites (Mokpo, Yeosu, and Uljin) in South Korea. In the survey sites of this study, 25 species were observed through literature and visual survey, among which 9 species were detected by metabarcoding and 16 species were not detected. On the other hand, 10 species were detected only by metabarcoding, and one of them was identified as Pyura mirabilis, an unrecorded species in South Korea. This study succeeded in detecting cryptic or rare species with one seawater collection, which can be used to determine their unexplored habitat. Therefore, we conclude that monitoring using eDNA is more efficient than visual surveys for detecting rare or cryptic ascidian species. We also suggest that, when combined with traditional monitoring methods, it could be a tool to complement ascidian fauna studies.

Phylogenetic comparison of egg transparency in ascidians by hyperspectral imaging

The transparency of animals is an important biological feature. Ascidian eggs have various degrees of transparency, but this characteristic has not yet been measured quantitatively and comprehensively. In this study, we established a method for evaluating the transparency of eggs to first characterize the transparency of ascidian eggs across different species and to infer a phylogenetic relationship among multiple taxa in the class Ascidiacea. We measured the transmittance of 199 eggs from 21 individuals using a hyperspectral camera. The spectrum of the visual range of wavelengths (400–760 nm) varied among individuals and we calculated each average transmittance of the visual range as bio-transparency. When combined with phylogenetic analysis based on the nuclear 18S rRNA and the mitochondrial cytochrome c oxidase subunit I gene sequences, the bio-transparencies of 13 species were derived from four different families: Ascidiidae, Cionidae, Pyuridae, and Styelidae. The bio-transparency varied 10–90% and likely evolved independently in each family. Ascidiella aspersa showed extremely high (88.0 ± 1.6%) bio-transparency in eggs that was maintained in the “invisible” larva. In addition, it was indicated that species of the Ascidiidae family may have a phylogenetic constraint of egg transparency.

The eventful history of nonembryonic development in tunicates

Tunicates encompass a large group of marine filter-feeding animals and more than half of them are able to reproduce asexually by a particular form of nonembryonic development (NED) generally called budding. The phylogeny of tunicates suggests that asexual reproduction is an evolutionarily plastic trait, a view that is further reinforced by the fact that budding mechanisms differ from one species to another, involving nonhomologous tissues and cells. In this review, we explore more than 150 years of literature to provide an overview of NED diversity and we present a comparative picture of budding tissues across tunicates. Based on the phylogenetic relationships between budding and nonbudding species, we hypothesize that NED diversity is the result of seven independent acquisitions and subsequent diversifications in the course of tunicate evolution. While this scenario represents the state-of-the-art of our current knowledge, we point out gray areas that need to be further explored to refine our understanding of tunicate phylogeny and NED. Tunicates, with their plastic evolution and diversity of budding, represent an ideal playground for evolutionary developmental biologists to unravel the genetic and molecular mechanisms regulating nonembryonic development, as well as to better understand how such a profound innovation in life-history has evolved in numerous metazoans.