Hooked on zombie worms? Genetic blueprints of bristle formation in Osedax japonicus (Annelida).

BACKGROUND: This study sheds light on the genetic blueprints of chaetogenesis (bristle formation), a complex biomineralization process essential not only for the diverse group of bristle worms (annelids) but also for other spiralians. We explore the complex genetic mechanisms behind chaetae formation in Osedax japonicus, the bone-devouring deep-sea worm known for its unique ecological niche and morphological adaptations. RESULTS: We characterized the chaetal structure and musculature using electron microscopy and immunohistochemistry, and combined RNAseq of larval stages with in-situ hybridization chain reaction (HCR) to reveal gene expression patterns integral to chaetogenesis. Our findings pinpoint a distinct surge in gene expression during the larval stage of active chaetogenesis, identifying specific genes and cells involved. CONCLUSIONS: Our research underscores the value of studying on non-model, "aberrant" organisms like Osedax, whose unique, temporally restricted chaetogenesis provided insights into elevated gene expression across specific larval stages and led to the identification of genes critical for chaetae formation. The genes identified as directly involved in chaetogenesis lay the groundwork for future comparative studies across Annelida and Spiralia, potentially elucidating the homology of chaetae-like chitinous structures and their evolution.

Circumtropical distribution and cryptic species of the meiofaunal enteropneust Meioglossus (Harrimaniidae, Hemichordata).

Hemichordata has always played a central role in evolutionary studies of Chordata due to their close phylogenetic affinity and shared morphological characteristics. Hemichordates had no meiofaunal representatives until the surprising discovery of a microscopic, paedomorphic enteropneust Meioglossus psammophilus (Harrimaniidae, Hemichordata) from the Caribbean in 2012. No additional species have been described since, questioning the broader distribution and significance of this genus. However, being less than a millimeter long and superficially resembling an early juvenile acorn worm, Meioglossus may easily be overlooked in both macrofauna and meiofauna surveys. We here present the discovery of 11 additional populations of Meioglossus from shallow subtropical and tropical coralline sands of the Caribbean Sea, Red Sea, Indian Ocean, and East China Sea. These geographically separated populations show identical morphology but differ genetically. Our phylogenetic reconstructions include four gene markers and support the monophyly of Meioglossus. Species delineation analyses revealed eight new cryptic species, which we herein describe using DNA taxonomy. This study reveals a broad circumtropical distribution, supporting the validity and ecological importance of this enigmatic meiobenthic genus. The high cryptic diversity and apparent morphological stasis of Meioglossus may exemplify a potentially common evolutionary 'dead-end' scenario, where groups with highly miniaturized and simplified body plan lose their ability to diversify morphologically.

Postembryonic development and male paedomorphosis in Osedax (Siboglinidae, Annelida).

Most species of the bone-devouring marine annelid, Osedax, display distinct sexual dimorphism with macroscopic sedentary females rooted in bones and free-living microscopic dwarf males. The paedomorphic male resembles the non-feeding metatrochophore larva in size, presence of eight pairs of chaetae, and a head ciliation potentially representing a residual prototroch. The male development may thus uniquely reiterate and validate the theoretical heterochrony process "progenesis", which suggests that an accelerated sexual maturation and early arrest of somatic growth can lead to a miniaturized and paedomorphic adult. In this study, we describe the postembryonic larval and juvenile organogenesis of Osedax japonicus to test for a potential synchronous arrest of somatic growth during male development. Five postembryonic stages could be distinguished, resembling day one to five in the larval development at 10°C: (0D) first cleavage of fertilized eggs (embryos undergo unequal spiral cleavage), (1D) pre-trochophore, with apical organ, (2D) early trochophore, + prototroch, brain, circumesophageal connectives and subesophageal commissure, (3D) trochophore, + telotroch, four ventral nerves, (4D) early metatrochophore, + protonephridia, dorsal and terminal sensory organs, (5D) metatrochophore, + two ventral paratrochs, mid-ventral nerve, posterior trunk commissure, two dorsal nerves; competent for metamorphosis. The larval development largely mirrors that of other lecithotrophic annelid larvae but does not show continuous chaetogenesis or full gut development. Additionally, O. japonicus larvae exhibit an unpaired, mid-dorsal, sensory organ. Female individuals shed their larval traits during metamorphosis and continue organogenesis (including circulatory system) and extensive growth for 2-3 weeks before developing oocytes. In contrast, males develop sperm within a day of metamorphosis and display a synchronous metamorphic arrest in neural and muscular development, retaining a large portion of larval features post metamorphosis. Our findings hereby substantiate male miniaturization in Osedax to be the outcome of an early and synchronous offset of somatic development, fitting the theoretical process "progenesis". This may be the first compelling morpho-developmental exemplification of a progenetic origin of a microscopic body plan. The presented morphological staging system will further serve as a framework for future examination of molecular patterns and pathways determining Osedax development.

Using eDNA to find Micrognathozoa.

Over the past decades the sampling of environmental DNA (eDNA) - encompassing the DNA of all organisms present in an environmental sample1 - has emerged as a technique for biodiversity monitoring and discovery in a diversity of environments. Avoiding the physical collection and identification of biota, this approach is praised for its independence of taxonomic expertise and has changed the way biologists study biodiversity. However, a common result in eDNA studies is the finding of unexpected taxa which are often removed by conservative bioinformatic filters or disregarded, since the authors are uncertain about the result and rarely have the interest, time, skills, and/or resources to return to the field and confirm with actual specimens2. Here, we report a case in which an eDNA discovery led to the physical localization of a member of the Micrognathozoa (Figure 1B) - a rare group of limnic micrometazoans, and the animal phylum to be discovered last3, which is the sister group to rotifers4,5. To this day, Micrognathozoa still comprises only a single named species from Greenland and a few additional disparate places.

A transcriptome-based phylogeny for Polynoidae (Annelida: Aphroditiformia).

Polynoidae is the most diverse radiation of Aphroditiformia and one of the most successful groups of all Annelida in terms of diversity and habitats colonized. With such an unmatched diversity, phylogenetic investigations have struggled to understand their evolutionary relationships. Previous phylogenetic analyses have slowly increased taxon sampling and employed methodologies, but despite their diversity and biological importance, large genomic sampling is limited. To investigate the internal relationships within Polynoidae, we conducted the first phylogenomic analyses of the group based on 12 transcriptomes collected from species inhabiting a broad array of habitats, including shallow and deep waters, as well as hydrothermal vents, anchialine caves and the midwater. Our phylogenomic analyses of Polynoidae recovered congruent tree topologies representing the clades Polynoinae, Macellicephalinae and Lepidonotopodinae. Members of Polynoinae and Macellicephalinae clustered in well-supported and independent clades. In contrast, Lepidonotopodinae taxa were always recovered nested within Macellicephalinae. Though our sampling only covers a small proportion of the species known for Polynoidae, our results provide a robust phylogenomic framework to build from, emphasizing previously hypothesized relationships between Macellicephalinae and Lepidonotopodinae taxa, while providing new insights on the origin of enigmatic cave and pelagic lineages.

Phylogenomic analyses of mud dragons (Kinorhyncha).

Mud dragons (Kinorhyncha) are microscopic invertebrates, inhabiting marine sediments across the globe from intertidal to hadal depths. They are segmented, moulting animals like arthropods, but grouping with the unsegmented priapulans and loriciferans within Ecdysozoa. There are more than 300 species of kinorhynchs described within 31 genera and 11 families, however, their evolutionary relationships have so far only been investigated using morphology and a few molecular markers. Here we aim to resolve the relationships and classification of major clades within Kinorhyncha using transcriptomic data. In addition, we wish to revisit the position of three indistinctly segmented, aberrant genera in order to reconstruct the evolution of distinct segmentation within the group. We conducted a phylogenomic analysis of Kinorhyncha including 21 kinorhynch transcriptomes (of which 18 are new) representing 15 genera, and seven outgroups including priapulan, loriciferan, nematode and nematomorph transcriptomes. Results show a congruent and robust tree that supports the division of Kinorhyncha into two major clades: Cyclorhagida and Allomalorhagida. Cyclorhagida is composed of three subclades: Xenosomata, Kentrorhagata comb. nov. (including the aberrant Zelinkaderes) and Echinorhagata. Allomalorhagida is composed of two subclades: Pycnophyidae and Anomoirhaga nom. nov. Anomoirhaga nom. nov. accommodates the aberrant genera Cateria (previously nested within Cyclorhagida) and Franciscideres together with five additional genera. The distant and derived positions of the aberrant Zelinkaderes, Cateria and Franciscideres species suggest that their less distinct trunk segmentation evolved convergently, and that segmentation evolved among kinorhynch stem groups.

Muscular adaptations in swimming scale worms (Polynoidae, Annelida).

Annelids are predominantly found along with the seafloor, but over time have colonized a vast diversity of habitats, such as the water column, where different modes of locomotion are necessary. Yet, little is known about their potential muscular adaptation to the continuous swimming behaviour required in the water column. The musculature and motility were examined for five scale worm species of Polynoidae (Aphroditiformia, Annelida) found in shallow waters, deep sea or caves and which exhibit crawling, occasional swimming or continuous swimming, respectively. Their parapodial musculature was reconstructed using microCT and computational three-dimensional analyses, and the muscular functions were interpreted from video recordings of their locomotion. Since most benthic scale worms are able to swim for short distances using body and parapodial muscle movements, suitable musculature for swimming is already present. Our results indicate that rather than rearrangements or addition of muscles, a shift to a pelagic lifestyle is mainly accompanied by structural loss of muscle bundles and density, as well as elongation of extrinsic dorsal and ventral parapodial muscles. Our study documents clear differences in locomotion and musculature among closely related annelids with different lifestyles as well as points to myoanatomical adaptations for accessing the water column.

Revisiting kinorhynch segmentation: variation of segmental patterns in the nervous system of three aberrant species.

BACKGROUND: Kinorhynch segmentation differs from the patterns found in Chordata, Arthropoda and Annelida which have coeloms and circulatory systems. Due to these differences and their obsolete status as 'Aschelminthes', the microscopic kinorhynchs are often not acknowledged as segmented bilaterians. Yet, morphological studies have shown a conserved segmental arrangement of ectodermal and mesodermal organ systems with spatial correspondence along the anterior-posterior axis. However, a few aberrant kinorhynch lineages present a worm-like body plan with thin cuticle and less distinct segmentation, and thus their study may aid to shed new light on the evolution of segmental patterns within Kinorhyncha. RESULTS: Here we found the nervous system in the aberrant Cateria styx and Franciscideres kalenesos to be clearly segmental, and similar to those of non-aberrant kinorhynchs; hereby not mirroring their otherwise aberrant and posteriorly shifted myoanatomy. In Zelinkaderes yong, however, the segmental arrangement of the nervous system is also shifted posteriorly and misaligned with respect to the cuticular segmentation. CONCLUSIONS: The morphological disparity together with the distant phylogenetic positions of F. kalenesos, C. styx and Z. yong support a convergent origin of aberrant appearances and segmental mismatches within Kinorhyncha.

Geographical sampling bias on the assessment of endemism areas for marine meiobenthic fauna.

Species distribution patterns are constrained by historical and ecological processes in space and time, but very often the species range sizes are geographical sampling biases resulting from unequal sampling effort. One of the most common definitions of endemism is based on the "congruence of distributional areas" criterion, when two or more species have the same distributional limits. By acknowledging that available data of marine meiobenthic species are prone to geographical sampling bias and that can affect the accuracy of the biogeographical signals, the present study combines analyses of inventory incompleteness and recognition of spatial congruence of Gastrotricha, Kinorhyncha, meiobenthic Annelida and Tardigrada in order to better understand the large-scale distribution of these organisms in coastal and shelf areas of the world. We used the marine bioregionalization framework for geographical operative units to quantify the inventory incompleteness effect (by modelling spatial predictions of species richness) and to recognize areas of endemism. Our models showed that the difference between observed and expected species richness in the Southern Hemisphere is much higher than in the Northern Hemisphere. Parsimony Analysis of Endemicity delimited 20 areas of endemism, most found in the Northern Hemisphere. Distribution patterns of meiobenthic species are shown to respond to events of geographical barriers and abiotic features, and their distribution is far from homogeneous throughout the world. Also, our data show that ecoregions with distinct biotas have at least some cohesion over evolutionary time. However, we found that inventory incompleteness may significantly affect the explanatory power of areas of endemism delimitation in both hemispheres. Yet, whereas future increases in sampling efforts are likely to change the spatial congruence ranges in the Southern Hemisphere, patterns for the Northern Hemisphere may prove to be relatively more resilient.

Myogenesis of Siboglinum fiordicum sheds light on body regionalisation in beard worms (Siboglinidae, Annelida).

BACKGROUND: Many annelids, including well-studied species such as Platynereis, show similar structured segments along their body axis (homonomous segmentation). However, numerous annelid species diverge from this pattern and exhibit specialised segments or body regions (heteronomous segmentation). Recent phylogenomic studies and paleontological findings suggest that a heteronomous body architecture may represent an ancestral condition in Annelida. To better understand the segmentation within heteronomous species we describe the myogenesis and mesodermal delineation of segments in Siboglinum fiordicum during development. RESULTS: Employing confocal and transmission electron microscopy we show that the somatic longitudinal musculature consists of four separate strands, among which ventrolateral one is the most prominent and is proposed to drive the search movements of the head of the late metatrochophore. The somatic circular musculature lies inside the longitudinal musculature and is predominantly developed at the anterior end of the competent larva to support the burrowing behaviour. Our application of transmission electron microscopy allows us to describe the developmental order of the non-muscular septa. The first septum to form is supported by thick bundles of longitudinal muscles and separates the body into an anterior and a posterior region. The second group of septa to develop further divides the posterior body region (opisthosoma) and is supported by developing circular muscles. At the late larval stage, a septum reinforced by circular muscles divides the anterior body region into a forepart and a trunk segment. The remaining septa and their circular muscles form one by one at the very posterior end of the opisthosoma. CONCLUSIONS: The heteronomous Siboglinum lacks the strict anterior to posterior sequence of segment formation as it is found in the most studied annelid species. Instead, the first septum divides the body into two body regions before segments are laid down in first the posterior opisthosoma and then in the anterior body, respectively. Similar patterns of segment formation are described for the heteronomous chaetopterid Chaetopterus variopedatus and serpulid Hydroides elegans and may represent an adaptation of these annelids to the settlement and transition to the sedentarian-tubiculous mode of life.

Have the eyes of bioluminescent scale worms adapted to see their own light? A comparative study of eyes and vision in Harmothoe imbricata and Lepidonotus squamatus.

Annelids constitute a diverse phylum with more than 19,000 species, which exhibit greatly varying morphologies and lifestyles ranging from sessile detritivores to fast swimming active predators. The lifestyle of an animal is closely linked to its sensory systems, not least the visual equipment. Interestingly, many errantian annelid species from different families, such as the scale worms (Polynoidae), have two pairs of eyes on their prostomium. These eyes are typically 100-200 µm in diameter and structurally similar judged from their gross morphology. The polynoids Harmothoe imbricata and Lepidonotus squamatus from the North Atlantic are both benthic predators preying on small invertebrates but only H. imbricata can produce bioluminescence in its scales. Here, we examined the eye morphology, photoreceptor physiology and light-guided behaviour in these two scale worms to assess their visual capacity and visual ecology. The structure and physiology of the two pairs of eyes are remarkably similar within each species, with the only difference being the gaze direction. The photoreceptor physiology, however, differs between species. Both species express a single opsin in their eyes, but in H. imbricata the peak sensitivity is green shifted and the temporal resolution is lower, suggesting that the eyes of H. imbricata are adapted to detect their own bioluminescence. The behavioural experiments showed that both species are strictly night active but yielded no support for the hypothesis that H. imbricata is repelled by its own bioluminescence.

Morphological convergence and adaptation in cave and pelagic scale worms (Polynoidae, Annelida).

Across Annelida, accessing the water column drives morphological and lifestyle modifications-yet in the primarily "benthic" scale worms, the ecological significance of swimming has largely been ignored. We investigated genetic, morphological and behavioural adaptations associated with swimming across Polynoidae, using mitogenomics and comparative methods. Mitochondrial genomes from cave and pelagic polynoids were highly similar, with non-significant rearrangements only present in cave Gesiella. Gene orders of the new mitogenomes were highly similar to shallow water species, suggestive of an underlying polynoid ground pattern. Being the first phylogenetic analyses to include the holopelagic Drieschia, we recovered this species nested among shallow water terminals, suggesting a shallow water ancestry. Based on these results, our phylogenetic reconstructions showed that swimming evolved independently three times in Polynoidae, involving convergent adaptations in morphology and motility patterns across the deep sea (Branchipolynoe), midwater (Drieschia) and anchialine caves (Pelagomacellicephala and Gesiella). Phylogenetic generalized least-squares (PGLS) analyses showed that holopelagic and anchialine cave species exhibit hypertrophy of the dorsal cirri, yet, these morphological modifications are achieved along different evolutionary pathways, i.e., elongation of the cirrophore versus style. Together, these findings suggest that a water column lifestyle elicits similar morphological adaptations, favouring bodies designed for drifting and sensing.

Conservative route to 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-megabase 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.

Transitions in functional morphology from "large branchiopods" to Cladocera: Video and confocal microscopic studies of Cyclestheria hislopi (Cyclestherida) and Sida crystallina (Cladocera: Ctenopoda).

Great diversity is found in morphology and functionality of arthropod appendages, both along the body axis of individual animals and between different life-cycle stages. Despite many branchiopod crustaceans being well known for displaying a relatively simple arrangement of many serially post-maxillary appendages (trunk limbs), this taxon also shows an often unappreciated large variation in appendage morphology. Diplostracan branchiopods exhibit generally a division of labor into locomotory antennae and feeding/filtratory post-maxillary appendages (trunk limbs). We here study the functionality and morphology of the swimming antennae and feeding appendages in clam shrimps and cladocerans and analyze the findings in an evolutionary context (e.g., possible progenetic origin of Cladocera). We focus on Cyclestheria hislopi (Cyclestherida), sister species to Cladocera and exhibiting many "large" branchiopod characters (e.g., many serially similar appendages), and Sida crystallina (Cladocera, Ctenopoda), which likely exhibits plesiomorphic cladoceran traits (e.g., six pairs of serially similar appendages). We combine (semi-)high-speed recordings of behavior with confocal laser scanning microscopy analyses of musculature to infer functionality and homologies of locomotory and filtratory appendages in the two groups. Our morphological study shows that the musculature in all trunk limbs (irrespective of limb size) of both C. hislopi and S. crystallina comprises overall similar muscle groups in largely corresponding arrangements. Some differences between C. hislopi and S. crystallina, such as fewer trunk limbs and antennal segments in the latter, may reflect a progenetic origin of Cladocera. Other differences seem related to the appearance of a specialized type of swimming and feeding in Cladocera, where the anterior locomotory system (antennae) and the posterior feeding system (trunk limbs) have become fully separated functionally from each other. This separation is likely one explanation for the omnipresence of cladocerans, which have conquered both freshwater and marine free water masses and a number of other habitats.

Broad North Atlantic distribution of a meiobenthic annelid - against all odds.

DNA barcoding and population genetic studies have revealed an unforeseen hidden diversity of cryptic species among microscopic marine benthos, otherwise exhibiting highly similar and simple morphologies. This has led to a paradigm shift, rejecting cosmopolitism of marine meiofauna until genetically proven and challenging the "Everything is Everywhere, but the environment selects" hypothesis that claims ubiquitous distribution of microscopic organisms. With phylogenetic and species delimitation analyses of worldwide genetic samples of the meiofaunal family Dinophilidae (Annelida) we here resolve three genera within the family and showcase an exceptionally broad, boreal, North Atlantic distribution of a single microscopic marine species with no obvious means of dispersal besides vicariance. With its endobenthic lifestyle, small size, limited migratory powers and lack of pelagic larvae, the broad distribution of Dinophilus vorticoides seems to constitute a "meiofaunal paradox". This species feasts in the biofilm among sand grains, but also on macroalgae and ice within which it can likely survive long-distance rafting dispersal due to its varying lifecycle stages; eggs encapsulated in cocoons and dormant encystment stages. Though often neglected and possibly underestimated among marine microscopic species, dormancy may be a highly significant factor for explaining wide distribution patterns and a key to solving this meiofaunal paradox.

The central nervous system of Oweniidae (Annelida) and its implications for the structure of the ancestral annelid brain.

BACKGROUND: Recent phylogenomic analyses congruently reveal a basal clade which consists of Oweniidae and Mageloniidae as sister group to the remaining Annelida. These results indicate that the last common ancestor of Annelida was a tube-dwelling organism. They also challenge traditional evolutionary hypotheses of different organ systems, among them the nervous system. In textbooks the central nervous system is described as consisting of a ganglionic ventral nervous system and a dorsally located brain with different tracts that connect certain parts of the brain to each other. Only limited information on the fine structure, however, is available for Oweniidae, which constitute the sister group (possibly together with Magelonidae) to all remaining annelids. RESULTS: The brain of Oweniidae is ring- shaped and basiepidermal. Ganglia, higher brain centers or complex sensory organs do not exist; instead the central nervous system is medullary. Posterior to the brain the ventral medullary cord arises directly from the ventral region of the brain in Myriowenia sp. while in Owenia fusiformis two medullary cords arise perpendicular to the brain ring, extend caudally and fuse posterior. The central nervous system is composed of a central neuropil and surrounding somata of the neurons. According to ultrastructural and histological data only one type of neuron is present in the central nervous system. CONCLUSION: The central nervous system of Oweniidae is the simplest in terms of enlargement of the dorsal part of the brain and neuron distribution found among Annelida. Our investigation suggests that neither ganglia nor commissures inside the brain neuropil or clusters of polymorphic neurons were present in the annelid stem species. These structures evolved later within Annelida, most likely in the stem lineage of Amphinomidae, Sipuncula and Pleistoannelida. Palps were supposedly present in the last common ancestor of annelids and innervated by two nerves originating in the dorsal part of the brain. A broader comparison with species of each major spiralian clade shows the medullary nervous system to be a common feature and thus possibly representing the ancestral state of the spiralian nervous system. Moreover, ganglia and clusters of polymorphic neurons seemingly evolved independently in the compared taxa of Spiralia and Annelida.

Convergent evolution of the ladder-like ventral nerve cord in Annelida.

BACKGROUND: 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 either a plesiomorphic or a typical condition in annelids. RESULTS: Using 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 the ancestral state in Annelida. Thus, a subepidermal ladder-like nerve cord clearly represents a derived condition. CONCLUSIONS: Based 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 propose an alternative set of neuroanatomical characteristics for the last common ancestor of Annelida or perhaps even Spiralia.

Regeneration of the Rhopalium and the Rhopalial Nervous System in the Box Jellyfish Tripedalia cystophora.

Cubozoans have the most intricate visual apparatus within Cnidaria. It comprises four identical sensory structures, the rhopalia, each of which holds six eyes of four morphological types. Two of these eyes are camera-type eyes that are, in many ways, similar to the vertebrate eye. The visual input is used to control complex behaviors, such as navigation and obstacle avoidance, and is processed by an elaborate rhopalial nervous system. Several studies have examined the rhopalial nervous system, which, despite a radial symmetric body plan, is bilaterally symmetrical, connecting the two sides of the rhopalium through commissures in an extensive neuropil. The four rhopalia are interconnected by a nerve ring situated in the oral margin of the bell, and together these structures constitute the cubozoan central nervous system. Cnidarians have excellent regenerative capabilities, enabling most species to regenerate large body areas or body parts, and some species can regenerate completely from just a few hundred cells. Here we test whether cubozoans are capable of regenerating the rhopalia, despite the complexity of the visual system and the rhopalial nervous system. The results show that the rhopalia are readily regrown after amputation and have developed most, if not all, neural elements within two weeks. Using electrophysiology, we investigated the functionality of the regrown rhopalia and found that they generated pacemaker signals and that the lens eyes showed a normal response to light. Our findings substantiate the amazing regenerative ability in Cnidaria by showing here the complex sensory system of Cubozoa, a model system proving to be highly applicable in studies of neurogenesis.