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

Abstract

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 to evaluate their presence as a convergent or homologous trait.

First Report on Three Lesser-Known Magelona Species from Korean Waters: Details of All Thoracic Chaetigers and Methyl Green Staining Patterns

“…There is huge variation in the mucronate chaetae in this group, and it also depends on where the chaeta is in the rami. Often, those towards the outer rami are more slender, and those in the central part of the rami are more bulbous [20]. There needs to be a review of mucronate and pennoned chaetae to establish this, but these chaetae are more similar to the mirabilis group than those seen in species like Magelona hartmanae Jones, 1978 [21].…”