Comparative analyses of morphological characters in Sphaerodoridae and allies (Annelida) revealed by an integrative microscopical approach

Abstract

Sphaerodoridae is a group of benthic marine worms (Annelida) characterized by the presence of spherical tubercles covering their whole surface. They are commonly considered as belonging to Phyllodocida although sistergroup relationships are still far from being understood. Primary homology assessments of their morphological features are lacking, hindering the appraisal of evolutionary relationships between taxa. Therefore, our detailed morphological investigation focuses on different Sphaerodoridae as well as on other members of Phyllodocida using an integrative approach combining scanning electron microscopy (SEM) as well as immunohistochemistry with standard neuronal (anti-5-HT) and muscular (phalloidin-rhodamine) markers and subsequent CLSM analysis of whole mounts and sections. Furthermore, we provide histological (HES) and light microscopical data to shed light on the structures and hypothetical function of sphaerodorid key morphological features. We provide fundamental details into the sphaerodorid morphology supporting a Phyllodocida ancestry of these enigmatic worms. However, the muscular arrangement and the presence of an axial muscular pharynx is similar to conditions observed in other members of the Errantia too. Furthermore, nervous system and muscle staining as well as SEM and histological observations of different types of tubercles indicate a homology of the so called microtubercles, present in the long-bodied sphaerodorids, to the dorsal cirri of other Errantia. The macrotubercles seem to represent a sphaerodorid autapomorphy based on our investigations. Therefore, our results allow comparisons concerning morphological patterns between Sphaerodoridae and other Phyllodocida and constitute a starting point for further comparative investigations to reveal the evolution of the remarkable Sphaerodoridae.

Myoanatomy and anterior muscle regeneration of the fireworm Eurythoe cf. complanata (Annelida: Amphinomidae)

“…cf. complanata is in concordance with investigations by Gustafson () and Storch (), and comparable to conditions known from biramous Errantia, such as Glyceridae, Aphroditidae, and Nereididae, but can be also observed in the uniramous Sphaerodoridae, Phyllodocidae, Syllidae, and Myzostomida (Storch, ; Tzetlin and Filippova, ; Filippova et al, ; Helm et al, ; Aguado et al, ; Helm and Capa, ). A prominent acicular flexor muscle is present in the notopodial branch of E .…”

On the role of the proventricle region in reproduction and regeneration in Typosyllis antoni (Annelida: Syllidae)

“…The presence of a proventricle has been proposed as a synapomorphy for Syllidae, and its ancestral function is seen in the role of a sucking pump [ 81 , 82 ]. Comparable structures have not been found in the digestive tract of any other group of annelids [ 83 ]. The proposed sucking function may be particularly useful in many syllids that feed on hydroids, bryozoans or other invertebrates [ 75 , 84 ].…”

Revision of the Australian Sphaerodoridae (Annelida) including the description of four new species

“…Primary homology statements are in most of these cases lacking. There are only evidences in favor of the homology of dorsal cirri of other Phyllodocida and the 'microtubercles' found dorsal to parapodia typical of Ephesiella, Ephesiopsis and Sphaerodorum (Helm & Capa 2015). The term macrotubercles is used for the large protuberances, generally located on the dorsal surface, regardless if they are sessile or stalked, or if they bear a terminal papillae or not.…”