Ultrastructure of the larval compound setae of the polychaeteNereis vexillosa grube

The cellular 3D printer of a marine bristle worm-chaetogenesis in Platynereis dumerilii (Audouin & Milne Edwards, 1834) (Annelida)

Annelid chaetae are extracellular chitinous structures that are formed in an extracellular epidermal invagination, the chaetal follicle. The basalmost cell of this follicle, the chaetoblast, serves like a 3D-printer as it dynamically shapes the chaeta. During chaetogenesis apical microvilli of the chaetoblast form the template for the chaeta, any structural details result from modulating the microvilli pattern. This study describes this process in detail in the model organism Platynereis dumerilii and clarifies some aspects of chaetogenesis in its close relative Nereis vexillosa, the first annelid in which the ultrastructure of chaetogenesis had been described. Nereid species possess compound chaetae characteristic for numerous subgroups of errant annelids. The distal most section of these chaetae is movable; a hinge connects this part of the chaeta to the shaft. Modulation of the microvilli and differences in their structure, diameter and number of microvilli, and their withdrawal and reappearance determine the shape of these compound chaetae. Chaetal structure and pattern also change during life history. While larvae possess a single type of chaeta (in addition to internal aciculae), juveniles and adults possess two types of chaetae that are replaced by large paddle-shaped chaetae in swimming epitokous stages. Chaetogenesis is a continuous process that lasts during the entire lifespan. The detailed developmental sequence of chaetae and their site of formation are very similar within species and species groups. We expect that similarity results from a conserved gene regulatory network making this an optimal system to test the phylogenetic affinity of taxa and the homology of their chaetae.

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

The systematics of Annelida has repeatedly been changed based on morphological data, but more recently established transcriptomic approaches yielded a stable and widely accepted phylogenetic tree, placing Magelonidae and Oweniidae as sister group to all remaining annelids. This led to an increased interest in these groups in terms of morphological traits and their phylogenetic significance. As one of the most characteristic morphological characters of annelids, the chaetae of Magelonidae, are well investigated regarding their shape, but phylogenetically relevant aspects like their general arrangement are still poorly studied. Furthermore, some species possess abdominal internal support chaetae that no study has addressed in detail thus far. The chaetal arrangement and position of formative sites were studied in the differently expressed parapodia of the thorax, the ninth chaetiger and the abdomen of Magelona mirabilis and Magelona johnstoni. Our results show that all chaetigers primarily bore one row of chaetae per parapodial ramus, each with a single formative site. We also present the first histological as well as ultrastructural data on the magelonid support chaetae, showing that they represent internal hooded hooks, with which they share a common chaetal sac. Their distribution within Magelonidae, however, still requires a broader examination in order to evaluate their presence as a convergent or homologous trait. This article is protected by copyright. All rights reserved.

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

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.

Biomimetics and the case of the remarkable ragworms

Biomimetics is a rapidly growing field both as an academic and as an applied discipline. This paper gives a short introduction to the current status of the discipline before it describes three approaches to biomimetics: the mechanism-driven, which is based on the study of a specific mechanism; the focused organism-driven, which is based on the study of one function in a model organism; and the integrative organism-driven approach, where multiple functions of a model organism provide inspiration. The first two are established approaches and include many modern studies and the famous biomimetic discoveries of Velcro and the Lotus-Effect, whereas the last approach is not yet well recognized. The advantages of the integrative organism-driven approach are discussed using the ragworms as a case study. A morphological and locomotory study of these marine polychaetes reveals their biomimetic potential, which includes using their ability to move in slippery substrates as inspiration for novel endoscopes, using their compound setae as models for passive friction structures and using their three gaits, slow crawling, fast crawling, and swimming as well as their rapid burrowing technique to provide inspiration for the design of displacement pumps and multifunctional robots.