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

Abstract

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.

Sensory cells in tunicates: insights into mechanoreceptor evolution

“…There is no evidence directly refuting that CENs and assorted Trunk Epidermal Neurons are mechanosensory cells, either. However, the larval neuron most widely accepted as a mechanosensitive cell type is the Papilla Neuron (PN) ( Figure 2A ) ( Manni et al, 2021 ). In Ciona, PNs (sometimes called Papilla Sensory Neurons or Primary Sensory Neurons of the Papillae) are found surrounding the three adhesive/sensory papillae at the very anterior end of the larva ( Zeng et al, 2019 ).…”

Vertebrate origins are informed by larval lampreys (ammocoetes): a response to Miyashitaet al., 2021

“…The clue is that the stiff notochord of modern amphioxus extends far rostral to the forebrain (Lacalli, 2008) -even rostral to a possible 'telencephalon' (Benito-Gutierrez et al, 2021) -where it helps to penetrate the sand during burrowing (Willey, 1894: 125). By comparison, in all other chordates, including tunicates, the notochord ends farther back, at the front of the midbrain (Manni et al, 2021;Ferran et al, 2022: 8-9). Although not absolutely conclusive, this amphioxus extension looks suspiciously like a secondary add-on because its geneexpression patterns reveal a unique embryonic origin as a 'variant sort of prechordal plate derivative that imitates the notochord in solidity and cell typology' (Albuixech-Crespo et al, 2017;Ferran et al, 2022: 5).…”

Larval Settlement on Marine Surfaces: The Role of Physico-Chemical Interactions

“…Manni et al ( 2022) [4] present a fine 3D-reconstruction of the tadpole-shaped larval anatomy of the solitary ascidian Halocynthia roretzi, a species reared for commercial purposes, with particular attention paid to the anterior adhesive papillae, which exhibit dynamic changes in their response to contact with a surface during larval settlement.…”