Post‐metamorphic ontogeny of Zoroaster fulgens Thomson, 1873 (Asteroidea, Forcipulatacea)

Abstract

The complete ontogenetic development of an asteroid skeleton has never been described formally for any species. Here, we describe in detail the post‐metamorphic ontogeny of Zoroaster fulgens Thomson, 1873. The major novelty of our work is the description of patterns of plate addition, the ontogeny of the internal ossicles, as well as the variability of ossicles according to their position along series. Seven specimens collected in the Rockall Basin (North Atlantic) were dissected with bleach and their anatomy was documented using a scanning electron microscope. The external anatomy was additionally observed on more than 30 specimens. We found that the overall structure of the skeleton does not change much between juveniles and adults, but the shape of individual ossicle changes during growth. Allometric scaling was particularly visible on the orals, ambulacrals and adambulacrals. The shape of an ossicle is more dependent of its position along the arm series than of its individual size. Many morphological features differentiate progressively during ontogeny, while others are expressed consistently among specimens. The study of this ontogenetic series allows discussing the homology between the structures present on the ossicles of Z. fulgens in particular and other forcipulatacean sea stars in general (i.e. muscles insertions and articulation areas). The new data obtained in this study provide a comprehensive framework of the anatomy and ontogeny of Z. fulgens that will help resolve taxonomic and phylogenetic controversies in the future.

Comparative anatomy and phylogeny of the Forcipulatacea (Echinodermata: Asteroidea): insights from ossicle morphology

“…The dissections followed protocols as described by Fau & Villier (2018). Specimens were immersed in a dilute solution of sodium hypochlorite (bleach), rinsed, dried and the isolated ossicles were then mounted on scanning electron microscope (SEM) stubs.…”

“…To describe ossicle orientation in the asteroid skeleton, we used anatomical conventions as described in Fau & Villier (2018): (1) the actinal surface (adoral) vs. the abactinal surface (aboral); (2) the abradial direction that points toward the radial plane of symmetry vs. the abradial direction that points away from the radial plane of symmetry; and (3) the proximal direction toward the centre of the disc vs. the distal direction toward the tip of the arm.…”

“…The anatomy of asteroid ossicles was described previously only by a few authors (Turner & Dearborn, 1972;Blake, 1973;Gale, 2011). As Blake (1973) and Turner & Dearborn (1972) based their nomenclature on only three families, all of them belonging to the order Paxilosida, we followed the nomenclature proposed in Gale (2011), including the modifications proposed by Fau & Villier (2018), as well as new modifications and additions proposed herein (Table 2). Anatomical terms and abbreviations present in Table 2 are provided in italics in the text.…”

“…Table 2. List of the terms and abbreviations used in this paper, modified from Fau & Villier (2018). Terms and abbreviations preceded by an asterisk * are specific to this paper.…”

“…Phylogenetic characters have been developed and implemented for echinoids and ophiuroids (Kroh & Smith, 2010;Thuy & Stöhr, 2016). Despite these promising results, asteroid ossicles have only been studied in detail by a few authors (Cuénot, 1887;Blake, 1972Blake, , 1973Turner & Dearborn, 1972;Gale, 2011;Fau & Villier, 2018) and were seldom used in phylogenetic analysis, mainly in palaeontological studies (Blake, 1987;Gale, 1987;Mah, 2000;Blake & Elliott, 2003;Villier et al, 2004;Mah et al, 2010;Gale, 2011). The most recent phylogenetic hypotheses for Forcipulatacea were based on molecular data (Mah & Foltz, 2011a;Mah et al, 2015) and have provided new insights into the history of this group.…”