Formation and degeneration of scaled capillary notochaetae in Owenia fusiformis Delle Chiaje, 1844 (Oweniidae, Annelida)

Abstract

Phylotranscriptomic studies of the past decade have repeatedly placed Oweniidae together with Magelonidae, as the sister group to remaining annelids. This newly established placement clearly makes them a key-lineage for understanding annelid evolution and morphology. One of the most prominent morphological features of all annelids are their chaetae. The arrangement and formation process (chaetogenesis) of these chitinous bristles have been studied extensively in hooked chaetae that are arranged in rows. However, the information on other types of chaetae is still scarce. In this study, we investigated the scaled capillary notochaetae of Owenia fusiformis, looking both into the formation process that causes the scaly surface ornamentation and into their arrangement within tight bundles. Our results demonstrate the incredible plasticity of chaetogenesis that allows forming a vast array of three-dimensional structures. The capillary chaetae of Owenia fusiformis are unique in lacking an enamel coating and the scales covering the apical surface of each chaeta are formed by a single microvillus of the chaetoblast. Furthermore, the bundle of chaetae has a peripherally located formative site and a central degenerative site and it appears to result from a secondary curling of the chaetal sac.

The development of the adult nervous system in the annelidOwenia fusiformis

“…The dorsal and ventral root of the brain creates an FVamide-lir, RYamide-lir, RGWamide-lir, MIP-lir and tubulin + neuropil (np; Figure 6c, f, i, l; Additional File 7: Supplementary Figure 7a–b), which connects to the thorax of the evaginating trunk via circumesophageal connectives (or lateral medullary cords (22); see discussion) (Figure 6; Additional File 6: Supplementary Figure 6; Additional File 7: Supplementary Figure 7a–b). In the juvenile and adult, the thorax is composed of three fused trunk segments, which we name ciliated thoracic segments (cts), and differentiate from the other trunk segments by having capillary chaetae (35, 36) and abundant cilia in the epidermis (Additional File 7: Supplementary Figure 7a–b). Paired RGWamide-lir parapodial glandular organs (pgos) up to the seventh segment (27, 42) facilitate the distinction between the three thoracic and the seven abdominal segments (27, 28) (Figure 6g–h; Additional File 6: Supplementary Figure 6e–f).…”

“…Adult specimens were relaxed in 8% MgCl 2 and fixed overnight at 4°C. Adults were then placed in 60 mm dishes in 1x phosphate buffer saline (PBS), and their heads were dissected with a razor blade between the thoracic and the abdominal segments (between segments three and four (35,36)). Adult heads were treated post-fixation with 1% collagenase D (Merk-Sigma, # COLLD-RO) overnight at 4 °C and permeabilised through several washes with 1x PBS + 0.5% Triton X-100 (PTx).…”

Chaetal arrangement and type diversity in two Magelona species (Magelonidae, Annelida) with ultrastructural details of the internal support chaetae

“…The oweniid neuropodial patches arise by the multiplication of a single transverse row with a dorsal formative site, which is visible in young, just metamorphosized specimens (Meyer & Bartolomaeus, 1996; Müller, Bartolomaeus, et al, 2021). The notopodial bundles, however, constitute a spirally curled‐up row with one formative site as well, which primarily lies ventrally (Müller et al, 2021). Altogether, both Magelonidae and Oweniidae show various chaetal arrangement patterns, all of which are derived from a single transverse row with a ventral formative site in the notopodium and a dorsal one in the neuropodium.…”

“…Oweniidae, as the sister group of Magelonidae, has notopodial bundles of capillary chaetae and large patches of neuropodial hooks (Meyer & Bartolomaeus, 1996). The oweniid neuropodial patches arise by the multiplication of a single transverse row with a dorsal formative site, which is visible in young, just metamorphosized specimens (Meyer & Bartolomaeus, 1996; Müller, Bartolomaeus, et al, 2021). The notopodial bundles, however, constitute a spirally curled‐up row with one formative site as well, which primarily lies ventrally (Müller et al, 2021).…”