Morphological Study and 3D Reconstruction of the Larva of the Ascidian Halocynthia roretzi

The swimming larva represents the dispersal phase of ascidians, marine invertebrates belonging to tunicates. Due to its adhesive papillae, the larva searches the substrate, adheres to it, and undergoes metamorphosis, thereby becoming a sessile filter feeding animal. The larva anatomy has been described in detail in a few species, revealing a different degree of adult structure differentiation, called adultation. In the solitary ascidian Halocynthia roretzi, a species reared for commercial purposes, embryogenesis has been described in detail, but information on the larval anatomy is still lacking. Here, we describe it using a comparative approach, utilizing 3D reconstruction, as well as histological/TEM observations, with attention to its papillae. The larva is comparable to those of other solitary ascidians, such as Ciona intestinalis. However, it displays a higher level of adultation for the presence of the atrium, opened outside by means of the atrial siphon, and the peribranchial chambers. It does not reach the level of complexity of the larva of Botryllus schlosseri, a phylogenetically close colonial ascidian. Our study reveals that the papillae of H. roretzi, previously described as simple and conform, exhibit dynamic changes during settlement. This opens up new considerations on papillae morphology and evolution and deserves to be further investigated.

Questions on the taxonomic status of species of Protoholozoa Kott 1969 (Ascidiacea, Aplousobranchia, Holozoidae) with a description of a new genus

Among numerous ascidians, two colonies of the deep species Protoholozoa pedunculata Kott, 1969 were collected in 2017 by the French Poker 4 survey, Indian Ocean. The larvae of this species are described here for the first time. The present material shows the disposition of adhesive papillae in a line and incubation in the atrial cavity. That is different from what was first reported and from what is known in other species of the genus, which show triradiate adhesive papillae and incubation in a brood pouch. This new data leads us to discuss the taxonomic status of different species, leaving Protoholozoa Kott, 1969 for the previous ones and creating Scotiazoa gen. nov. for species with triradiate larval papillae and brood pouch.

Microstomum (Platyhelminthes, Macrostomorpha, Microstomidae) from the Swedish west coast: two new species and a population description

Two new species of marine Platyhelminthes, Microstomum laurae sp. nov. and Microstomum edmondi sp. nov. (Macrostomida: Microstomidae) are described from the west coast of Sweden. Microstomum laurae sp. nov. is distinguished by the following combination of characters: rounded anterior and posterior ends; presence of approximately 20 adhesive papillae on the posterior rim; paired lateral red eyespots located level with the brain; preoral gut extending anterior to brain and very small sensory pits. Microstomum edmondi sp. nov. is a protandrous hermaphrodite with a single ovary, single testis and male copulatory organ with stylet. It is characterized by a conical pointed anterior end, a blunt posterior end with numerous adhesive papillae along the rim, and large ciliary pits. The stylet is shaped as a narrow funnel with a short, arched tip. In addition, the first records of fully mature specimens of Microstomum rubromaculatum von Graff, 1882 from Fiskebäckskil and a phylogenetic analysis of Microstomum Schmidt, 1848 based on the mitochondrial cytochrome oxidase I (COI) gene are presented.

On the phylogenetic relationships of Axiokebuita, Travisia and Scalibregmatidae (Polychaeta)

We provide a description of newly collected specimens of Axiokebuita Pocklington & Fournier from Norway, previously known only from east Canada and the Antarctic. Due to delineation problems between the only two described species, A. minuta (Hartman) and A. millsi Pocklington & Fournier, these new specimens cannot unambiguously be referred to either species. Previously unnoticed adhesive papillae on the pygidium are present in both species and may constitute an apomorphy for Axiokebuita. The taxon lacks many morphological features otherwise characteristic for scalibregmatids, and to assess its affinities we present 18S rDNA and 28S rDNA-based analyses together with six other scalibregmatids and twenty other polychaetes. A nemertean is used as outgroup. All analyses support that Axiokebuita is a scalibregmatid. Furthermore, Travisia Johnston, traditionally referred to the Opheliidae, is nested within the scalibregmatids, as sister to Neolipobranchius Hartman & Fauchald. Arenicolidae and Maldanidae may constitute the sister group of scalibregmatids.

Anteroposterior patterning of the epidermis by inductive influences from the vegetal hemisphere cells in the ascidian embryo

Patterning along the anteroposterior axis is a critical step during animal embryogenesis. Although mechanisms of anteroposterior patterning in the neural tube have been studied in various chordates, little is known about those of the epidermis. To approach this issue, we investigated patterning mechanisms of the epidermis in the ascidian embryo. First we examined expression of homeobox genes (Hrdll-1, Hroth, HrHox-1 and Hrcad) in the epidermis. Hrdll-1 is expressed in the anterior tip of the epidermis that later forms the adhesive papillae, while Hroth is expressed in the anterior part of the trunk epidermis. HrHox-1 and Hrcad are expressed in middle and posterior parts of the epidermis, respectively. These data suggested that the epidermis of the ascidian embryo is patterned anteroposteriorly. In ascidian embryogenesis, the epidermis is exclusively derived from animal hemisphere cells. To investigate regulation of expression of the four homeobox genes in the epidermis by vegetal hemisphere cells, we next performed hemisphere isolation and cell ablation experiments. We showed that removal of the vegetal cells before the late 16-cell stage results in loss of expression of these homeobox genes in the animal hemisphere cells. Expression of Hrdll-1 and Hroth depends on contact with the anterior-vegetal (the A-line) cells, while expression of HrHox-1 and Hrcad requires contact with the posterior-vegetal (the B-line) cells. We also demonstrated that contact with the vegetal cells until the late 32-cell stage is sufficient for animal cells to express Hrdll-1, Hroth and Hrcad, while longer contact is necessary for HrHox-1 expression. Contact with the A-line cells until the late 32-cell stage is also sufficient for formation of the adhesive papillae. Our data indicate that the epidermis of the ascidian embryo is patterned along the anteroposterior axis by multiple inductive influences from the vegetal hemisphere cells and provide the first insight into mechanisms of epidermis patterning in the chordate embryos.