Biology subjects

Worsaae, K.

Publications and source records attributed to Worsaae, K..

5 recordsLinked to original sources

bioRxiv2025

Convergent evolution of sensory palps in Scalibregmatidae (Annelida)

Scalibregmatidae are deposit feeders inhabiting muddy or sandy sea floors at considerable depths, and usually lack head appendages common in many other marine annelids. Surprisingly, one lineage within this family bears palp-like structures comparable to anterior head appendages in other Annelida. Nevertheless, a homology of the scalibregmatid appendages with those of other annelids is highly questionable. Using an integrative morphological approach including immunohistochemistry, azan-histology, and 3D reconstruction, we examined the neural innervation of the anterior head region in one travisiid and two scalibregmatid species: Travisia sp. (without appendages), Scalibregma celticum (lateral, prostomial horns), and Axiokebuita cavernicola (palp-like appendages). Scalibregma and Axiokebuita show an innervation pattern of their appendages different from that of palps in other annelids, while no prominent prostomial innervation is present in Travisia. The short, lateral horn-like appendages of S. celticum possess a quite diffuse neuronal scaffold extending from distinct neurite bundles originating from multiple regions of the brain. The elongated palp-like head appendages of A. cavernicola exhibit a neuronal innervation pattern comparable with that of thin sensory antennae known from other annelid groups. Thus, innervating neurites solely originate in the dorsal part of the brain, a pattern not observable for "true" feeding- or sensory palps of other taxa. We also shed new light on the mysterious "neck organ" in scalibregmatids, which are shown to be ventral dublications of the olfactory nuchal organs considering the level of external features and nervous innervation. These results, combined to previous phylogenetic studies, show that Scalibregmatidae are the only so far known annelids to have re-evolved palp-like sensory head structures after a secondary loss. Thus, this is the first example of convergent evolution of palps, a character complex hitherto showing similar overall innervation patterns and considered homologous across all annelids.

evolutionary biology↗

bioRxiv2025

New insights into the evolution of Charlie Chaplin worms (Histriobdellidae, Eunicida, Annelida)

Histriobdellidae, the so-called Charlie Chaplin worms, is an enigmatic group of microscopic commensal annelids associated with crustaceans. They crawl by alternately attaching their adhesive anterior appendages and left and right huge lateral feet, and bear a complex jaw apparatus in the ventral muscular pharynx. Although histriobdellids were always thought to be a part of the jaw-bearing clade Eunicida, their exact placement within the annelid tree is still debated due to their highly derived external morphology and long branch attraction artefacts in molecular analyses. In this study we employ morphological and molecular comparative approaches in order to gain new insights into the evolution of Histriobdellidae and its aberrant traits. Our phylogenetic analyses of Eunicida including 52 species and four molecular markers yield further support for Histriobdellidae being the sister group to the eunicid family Dorvilleidae. The detailed morphology of Histriobdella homari Van Beneden, 1858, a commensal of the European lobster, was examined using standard immunohistochemical stainings and subsequent confocal laser scanning microscopy (CLSM) as well as scanning electron microscopy (SEM). Integrative analyses allow us to compare in detail with other eunicidans and unravel extensive anatomical transformations in Histriobdellidae. Neural innervation patterns help verify the presence of antennae and true annelid palps on the histriobdellid prostomium. The arrangement of ganglia and the neuronal scaffold innervating the anterior end supports the presence of a buccal segment (peristomium) in Histriobdella. Additionally, based on our comprehensive investigations we newly propose their adhesive anterior locomotory appendages to be homologous with parapodia, and their posterior-most adhesive locomotory appendages to be homologous with the pygidial lobes of other Annelida. Detailed studies of this highly deviating family of annelids not only exemplify how to reconstruct extreme transformation of canonical annelid characters such as parapodia, but again also highlight the exceptional evolutionary plasticity of the annelid body plan.

zoology↗

bioRxiv2020

Conservative route to extreme genome compaction in a miniature annelid

The causes and consequences of genome reduction in animals are unclear, because our understanding of this process mostly relies on lineages with often exceptionally high rates of evolution. Here, we decode the compact 73.8 Mb genome of Dimorphilus gyrociliatus, a meiobenthic segmented worm. The D. gyrociliatus genome retains traits classically associated with larger and slower-evolving genomes, such as an ordered, intact Hox cluster, a generally conserved developmental toolkit, and traces of ancestral bilaterian linkage. Unlike some other animals with small genomes, the analysis of the D. gyrociliatus epigenome revealed canonical features of genome regulation, excluding the presence of operons and trans-splicing. Instead, the gene dense D. gyrociliatus genome presents a divergent Myc pathway, a key physiological regulator of growth, proliferation, and genome stability in animals. Altogether, our results uncover a conservative route to genome compaction in annelids, reminiscent of that observed in the vertebrate Takifugu rubripes.

genomics↗

bioRxiv2018

Nerves innervating copulatory organs show common FMRFamide, FVRIamide, MIP and serotonin immunoreactivity patterns across Dinophilidae (Annelida) indicating their conserved role in copulatory behaviour

BackgroundMales of the microscopic annelid family Dinophilidae use their prominent glandomuscular copulatory organ (penis) to enzymatically dissolve the females epidermis and thereafter inject sperm. In order to test for putative conserved copulatory structures and neural orchestration across three dinophilid species, we reconstructed the reproductive myo- and neuroanatomy and mapped neurotransmitter immunoreactivity patterns of two specific markers with reported roles in invertebrate male mating behaviour (FVRIamide, MIP) and three general neural markers (acetylated -tubulin, serotonin, FMRFamide).\n\nResultsSeminal vesicles (one or two pairs), surrounded by a thin layer of longitudinal and circular muscles and innervated by nerve fibres, are found between testes and copulatory organ in the larger males of Dinophilus vorticoides and Trilobodrilus axi, but are missing in the only 0.05 mm long D. gyrociliatus dwarf males. The midventral copulatory organ is in all species composed of an outer muscular penis sheath and an inner penis cone. Nerves encircle the organ equatorially, either as a ring-shaped circumpenial fibre mass or as dorsal and ventral commissures, which are connected to the ventrolateral nerve cords. All three examined dinophilids show similar serotonin-, FMRFamide-, and FVRIamide-like immunoreactivity patterns in the nerves surrounding the penis, supporting the general involvement of these neurotransmitters in copulatory behaviour in meiofaunal annelids. MIP-like immunoreactivity is restricted to the circumpenial fibre mass in D. gyrociliatus and commissures around the penis in T. axi (but not found in D. vorticoides), indicating its role in controlling the copulatory organ.\n\nConclusionsThe overall myo- and neuroanatomy of the reproductive organs is rather similar in the three studied species, suggesting a common ancestry of the unpaired glandomuscular copulatory organ and its innervation in Dinophilidae. This is furthermore supported by the similar immunoreactivity patterns of the tested neurotransmitters around the penis. Smaller differences in the immunoreactivity patterns around the seminal vesicles and spermioducts might account for additional, individual traits. We thus show morphological support for the putatively conserved role of FMRFamide, FVRIamide, MIP and serotonin in dinophilid copulatory behaviour.

evolutionary biology↗

bioRxiv2018

Breaking the ladder: Evolution of the ventral nerve cord in Annelida

A median, segmented, annelid nerve cord has repeatedly been compared to the arthropod and vertebrate nerve cords and became the most used textbook representation of the annelid nervous system. Recent phylogenomic analyses, however, challenge the hypothesis that a subepidermal rope-ladder-like ventral nerve cord (VNC) composed of a paired serial chain of ganglia and somata-free connectives represents neither a plesiomorphic nor a typical condition in annelids.\n\nUsing a comparative approach by combining phylogenomic analyses with morphological methods (immunohistochemistry and CLSM, histology and TEM), we compiled a comprehensive dataset to reconstruct the evolution of the annelid VNC. Our phylogenomic analyses generally support previous topologies. However, the so far hard-to-place Apistobranchidae and Psammodrilidae are now incorporated among the basally branching annelids with high support. Based on this topology we reconstruct an intraepidermal VNC as ancestral state in Annelida. Thus, a subepidermal ladder-like nerve cord clearly represents a derived condition.\n\nBased on the presented data, a ladder-like appearance of the ventral nerve cord evolved repeatedly, and independently of the transition from an intraepidermal to a subepidermal cord during annelid evolution. Our investigations thereby question a common origin of the bilaterian median ganglionated VNC and propose an alternative set of neuroanatomical characteristics of the last common ancestor of Annelida or perhaps even Spiralia.

evolutionary biology↗