Morphology and life cycle of an epiphytic pherusellid ctenostome bryozoan from the Mediterranean Sea

A Dark Horse: Colonial System of Integration in Ctenostome Bryozoans (Gymnolaemata: Ctenostomata)

The colonial system of integration (CSI) provides intracolonial nutrient supply in many gymnolaemate bryozoans. In Ctenostomata, its presence is known for species with stolonal colonies, for example, vesicularioideans, but its structure is almost unexplored. The CSI is thought to be absent in alcyonidioideans and other ctenostomes. Here, we present the first detailed description of the CSI ultrastructure in both autozooids and kenozooids of two vesicularioideans, Buskia nitens and Amathia gracilis, and two alcyonidioideans, Alcyonidium hirsutum and Flustrellidra hispida. We revealed differences in the endocyst structure: in studied alcyonioidioideans, it comprises the epidermis, extracellular matrix and coelomic lining, while in the studied vesicularioideans, it includes only the epidermis. In vesicularioidean autozooids, the main CSI cord and the most distal part of the muscular funiculus originate together as a single structure near the caecum apex. However, at a short distance basally, they separate and run to different sites: the main CSI cord reaches the communication pore, and the muscular funiculus attaches to the cystid wall in the proximal part of the autozooids. The CSI in alcyonidioidean autozooids includes a central part, comprising several strands running from the caecum and pylorus to the cystid walls, and a peripheral part, which is located between the epidermis and peritoneum of the cystid walls and reaches the communication pores. The autozooidal CSI in the studied alcyonidioids never reaches kenozooidal communication pores. Nevertheless, the CSI is present in kenozooids of F. hispida; its structure corresponds to that of the peripheral part of the CSI in autozooids. These findings suggest that the CSI likely originated rather early in bryozoan evolution, and its putative initial function is nutrient transport to budding sites and zooids undergoing degeneration-regeneration cycle.

Rediscovering the unusual, solitary bryozoan Monobryozoon ambulans Remane, 1936: first molecular and new morphological data clarify its phylogenetic position

BACKGROUND

One of the most peculiar groups of the mostly colonial phylum Bryozoa is the taxon Monobryozoon, whose name already implies non-colonial members of the phylum. Its peculiarity and highly unusual lifestyle as a meiobenthic clade living on sand grains has fascinated many biologists. In particular its systematic relationship to other bryozoans remains a mystery. Despite numerous searches for M. ambulans in its type locality Helgoland, a locality with a long-lasting marine station and tradition of numerous courses and workshops, it has never been reencountered until today. Here we report the first observations of this almost mythical species, Monobryozoon ambulans.

RESULTS

For the first time since 1938, we present new modern, morphological analyses of this species as well as the first ever molecular data. Our detailed morphological analysis confirms most previous descriptions, but also ascertains the presence of special ambulatory polymorphic zooids. We consider these as bud anlagen that ultimately consecutively separate from the animal rendering it pseudo-colonial. The remaining morphological data show strong ties to alcyonidioidean ctenostome bryozoans. Our morphological data is in accordance with the phylogenomic analysis, which clusters it with species of Alcyonidium as a sister group to multiporate ctenostomes. Divergence time estimation and ancestral state reconstruction recover the solitary state of M. ambulans as a derived character that probably evolved in the Late Cretaceous. In this study, we also provide the entire mitogenome of M. ambulans, which-despite the momentary lack of comparable data-provides important data of a unique and rare species for comparative aspects in the future.

CONCLUSIONS

We were able to provide first sequence data and modern morphological data for the unique bryozoan, M. ambulans, which are both supporting an alcyonidioidean relationship within ctenostome bryozoans.

To be a transit link: Similarity in the structure of colonial system of integration and communication pores in autozooids and avicularia of Terminoflustra membranaceotruncata (Bryozoa: Cheilostomata)

Bryozoan colonies consist of zooids, which can differ in structure and function. Most heteromorphic zooids are unable to feed and autozooids supply them with nutrients. The structure of the tissues providing nutrient transfer is poorly investigated. Here, I present a detailed description of the colonial system of integration (CSI) and communication pores in autozooids and avicularia of the cheilosome bryozoan Terminoflustra membranaceotruncata. The CSI is the nutrient transport and distribution system in the colony. In both autozooids and avicularia it consists of a single cell type, that is, elongated cells, and has a variable branching pattern, except for the presence of a peripheral cord. The general similarity in the CSI structure in avicularia and autozooids is probably due to the interzooidal type of the avicularium. Interzooidal avicularia are likely to consume only a part of the nutrients delivered to them by the CSI, and they transit the rest of the nutrients further. The variability and irregularity of branching pattern of the CSI may be explained by the presence of single communication pores and their varying number. The structure of communication pores is similar regardless of their location (in the transverse or lateral wall) and the type of zooid in contact. Rosette complexes include a cincture cell, a few special cells, and a few limiting cells. Along each zooidal wall, there are communication pores with both unidirectional and bidirectional polarity of special cells. However, the total number of nucleus-containing lobes of special cells is approximately the same on each side of any zooidal wall. Supposing the polarity of special cells reflects the direction of nutrient transport, the pattern of special cells polarity is probably related to the need for bidirectional transport through each zooidal wall. The possibility for such transport is important in large perennial colonies with wide zones of autozooids undergoing polypide degeneration.

Colonial system of integration and communication pores in a polymorphic bryozoan Dendrobeania fruticosa (Bryozoa: Cheilostomata)

Bryozoan colonies are composed of zooids, which can differ in structure and function. Autozooids supply heteromorphic zooids with nutrients, which are usually unable to feed. To date, the ultrastructure of the tissues providing nutrient transfer is almost unexplored. Here, we present a detailed description of the colonial system of integration (CSI) and the different types of pore plates in Dendrobeania fruticosa. All cells of the CSI are joined by tight junctions that isolate its lumen. The lumen of the CSI is not a single structure, but a dense network of small interstices filled with a heterogeneous matrix. In autozooids, the CSI is composed of two types of cells: elongated and stellate. Elongated cells form the central part of the CSI, including two main longitudinal cords and several main branches to the gut and pore plates. Stellate cells compose the peripheral part of the CSI, which is a delicate mesh starting from the central part and reaching various structures of autozooids. Autozooids have two tiny muscular funiculi, which start from the caecum apex and run to the basal wall. Each funiculus includes a central cord of extracellular matrix and two longitudinal muscle cells; together they are enveloped with a layer of cells. The rosette complexes of all types of pore plates in D. fruticosa display a similar cellular composition: a cincture cell and a few special cells; limiting cells are absent. Special cells have bidirectional polarity in interautozooidal and avicularian pore plates. This is probably due to the need for bidirectional transport of nutrients during degeneration-regeneration cycles. Cincture cells and epidermal cells of pore plates contain microtubules and inclusions resembling dense-cored vesicles, which are typical of neurons. Probably, cincture cells are involved in the signal transduction from one zooid to another and can be a part of the colony-wide nervous system.

Boring bryozoans: an investigation into the endolithic bryozoan family Penetrantiidae

An endolithic lifestyle in mineralized substrates has evolved multiple times in various phyla including Bryozoa. The family Penetrantiidae includes one genus with ten extant and two fossil species. They predominantly colonize the shells of molluscs and establish colonies by chemical dissolution of calcium carbonate. Based on several morphological characters, they were described to be either cheilostome or ctenostome bryozoans. For more than 40 years, neither the characters of species identity and systematics nor the problem of their phylogeny was approached. Consequently, the aim of this study is to reevaluate species identities and the systematic position of the genus Penetrantia by analyzing at least six different species from eight regions with the aid of modern methods such as confocal laser scanning microscopy and 3D-reconstruction techniques. This study demonstrates that the musculature associated with the operculum and brood chamber shows significant differences from the cheilostome counterparts and seems to have evolved independently. Together with the presence of other ctenostome-like features such as true polymorphic stolons and uncalcified body wall, this finding supports a ctenostome affinity. Operculum morphology reveals many new species-specific characters, which, together with information about gonozooid morphology, tentacle number, and zooid size ranges, will enhance species identification. It also revealed a probable new species in Japan as well as potential cryptic species in France and New Zealand. In addition, this study increases the known distribution range of the family and its substrate diversity. Altogether, the new information collated here provides the basis for future work on a neglected taxon.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13127-023-00612-z.

Morphology of ctenostome bryozoans: 6. Amphibiobeania epiphylla

Ctenostome bryozoans are unmineralized and mostly marine. Their lack of calcified skeletal features requires other characters to be considered for systematic and phylogenetic considerations. As a continuation of an ongoing series of studies, we herein investigate the morphology of Amphibiobeania epiphylla, a unique bryozoan inhabiting mangrove leaves that are highly exposed to tidal cycles and regular dry events according to the tidal cycle. Besides this interesting mode of life, the species was originally interpreted to be a weakly mineralized cheilostome bryozoan, whereas molecular data place it among ctenostome bryozoans. To elucidate the systematic and phylogenetic position of the genus and also find morphological adaptations to an extreme habitat, we investigated the morphology of A. epiphylla in detail. Zooids show a lophophore with eight tentacles and a simple gut with a prominent caecum, lophophoral anus and most notably a distinct gizzard in the cardiac region. Gizzard teeth are multiple, simple homogeneous cuticular structures. The cuticle of the zooid is rather uniform and shows no respective thickenings into opercular flaps or folds. Likewise, apertural muscles are represented by a single pair of muscles. There are no specific closing muscles in the apertural area like the operculum occlusors of cheilostomes. Most prominent within zooids is a spongiose tissue filling most of the body cavity. Although not properly understood, this tissue may aid in keeping animals moist and hydrated during prolonged dry times. In summary, all morphological characters support a ctenostome rather than a cheilostome affinity, possibly with Vesicularioidea or Victorelloidea. In addition, we provide new molecular data that clearly supports such a closer relationship.

Morphology of ctenostome bryozoans: 5. Sundanella, with description of a new species from the Western Atlantic and the Multiporata concept

Ctenostome bryozoans are a small group of gymnolaemates that comprise only a few hundred described species. Soft-tissue morphology remains the most important source for analysing morphological characters and inferring relationships within this clade. The current study focuses on the genus Sundanella, for which morphological data is scarce to almost absent. We studied two species of the genus, including one new to science, using histology and three-dimensional reconstruction techniques and confocal laser scanning microscopy. Sundanella generally has a thick, sometimes arborescent cuticle and multiporous interzooidal pore plates. The lophophore is bilateral with an oral rejection tract and generally has 30 or 31 tentacles in both species. The digestive tract shows a large cardia in S. floridensis sp. nov. and an extremely elongated intestine in Sundanella sibogae. Both terminate via a vestibular anus. Only parietodiaphragmatic muscles are present and four to six duplicature bands. Both species show a large broad frontal duplicature band further splitting into four individual bands. The collar is vestibular. Sundanella sibogae shows highly vacuolated cells at the diaphragm, whereas S. floridensis sp. nov. has unique glandular pouches at the diaphragmal area of the tentacle sheath. Such apertural glands have never been encountered in other ctenostomes. Both species of Sundanella are brooders that brood embryos either in the vestibular or cystid wall. Taken together, the current analysis shows numerous characteristics that refute an assignment of Sundanella to victorellid ctenostomes, which only show superficial resemblance, but differ substantially in most of their soft-body morphological traits. Instead, a close relationship with other multiporate ctenostomes is evident and the families Pherusellidae, Flustrellidrae and Sundanellidae should be summarized as clade 'Multiporata' in the future.

A comparative analysis of the nervous system of cheilostome bryozoans

Background

Bryozoans are sessile aquatic suspension feeders in mainly marine, but also freshwater habitats. Most species belong to the marine and calcified Cheilostomata. Since this taxon remains mostly unstudied regarding its neuroanatomy, the focus of this study is on the characterization and ground pattern reconstruction of the autozooidal nervous system based on six representatives.

Results

A common neuronal innervation pattern is present in the investigated species: a cerebral ganglion is located at the base of the lophophore, from where neurite bundles embrace the mouth opening to form a circumoral nerve ring. Four neurite bundles project from the cerebral ganglion to innervate peripheral areas, such as the body wall and parietal muscles via the tentacle sheath. Five neurite bundles comprise the main innervation of the visceral tract. Four neurite bundles innervate each tentacle via the circumoral nerve ring. Mediofrontal tentacle neurite bundles emerge directly from the nerve ring. Two laterofrontal- and one abfrontal tentacle neurite bundles emanate from radial neurite bundles, which originate from the cerebral ganglion and circumoral nerve ring in between two adjacent tentacles. The radial neurite bundles terminate in intertentacular pits and give rise to one abfrontal neurite bundle at the oral side and two abfrontal neurite bundles at the anal side. Similar patterns are described in ctenostome bryozoans.

Conclusions

The present results thus represent the gymnolaemate situation. Innervation of the tentacle sheath and visceral tract by fewer neurite bundles and tentacular innervation by four to six tentacle neurite bundles support cyclostomes as sister taxon to gymnolaemates. Phylactolaemates feature fewer distinct neurite bundles in visceral- and tentacle sheath innervation, which always split in nervous plexus, and their tentacles have six neurite bundles. Thus, this study supports phylactolaemates as sistergroup to myolaemates.

Ultrastructure of rhizoids in the marine bryozoan Dendrobeania fruticosa (Gymnolaemata: Cheilostomata)

Bryozoans form colonies of iterated modules, termed zooids, and display varying degrees of polymorphism. Polymorphic colonies comprise autozooids (or feeding zooids) and heteromorphic zooids, among which the most common types are avicularia and kenozooids. Kenozooids differ in shape, size, and presumed function. Among this diversity, there are rhizoids, which serve to attach colonies to the substrate or to lift them above it. To date, only general data on anatomy of kenozooids at light microscopy level are available. Here, we present the first description of the ultrastructure of the holdfast-like rhizoids of the cheilostome bryozoan Dendrobeania fruticosa. The rhizoid wall is composed of a single-layered epidermis, which produces the ectocyst. The voluminous cavity is acoelomate: it has no special cellular lining, nor any signs of an extracellular matrix toward the epidermis. It is traversed by delicate branching funicular strands that originate from the pore plate. The only cells in contact with the epidermis are the cells of the funicular system and the storage cells. The pore plate between the rhizoid and autozooid includes a variable number of communication pores. Each pore is plugged with a rosette complex, which includes a cincture cell and four special cells extending through the pore. The limiting cells are absent, and the special cells are in direct contact with the funicular strands. Cell contacts between special cells are absent; moreover, there are spaces between their proximal lobes filled with a heterogeneous matrix similar to that in the lumen of the funicular strands. Such matrix is also found outside of the extracellular matrix surrounding the special cells. These findings allow us to suggest that nutrient transport most likely occurs between, rather than through, the special cells. However, further studies are needed to understand how the rosette complex functions.

Morphology of ctenostome bryozoans: 3. Elzerina, Flustrellidra, Bockiella

Ctenostome bryozoans are a small group of bryozoans whose soft‐tissue morphology has received only little attention. The present study represents the third in a series of articles dealing with the morphology of this clade of bryozoans. The morphology of three genera of Alcyonidioidea, that is, Bockiella (Alcyonidiidae), Elzerina and Flustrellidra (both Flustrellidridae), are analyzed using histology and 3D‐reconstruction techniques. The general zooidal morphology is similar and externally differs by the shape of the aperture. Zooids of Elzerina binderi are elongated in the fronto‐basal axis, whereas the other two are more flattened in this axis. All species show multiple pore‐complexes in their zooidal walls ranging from ~66 in E. binderi, to ~30 in F. hispida and to less than 10 in Bockiella. The aperture is bilabiate in flustrellidrids and roundish in Bockiella. Apertural muscles are present as parieto‐diaphragmatic muscles. The flustrellidrids have a large frontal duplicature band that further splits into four separate bands. The collar is diaphragmatic in Bockiella, but vestibular in the flustrellidrids. Lophophores are similar among the investigated species with a rejection tract in the flustrellidrids. The digestive tract shows differences in the extent and proportions of the caecum, which is large in the flustrellidrids and small in Bockiella; the anus is vestibular in all species. A funicular muscle of variable location is present in each species. Elzerina binderi has additional thin strands emanating from the digestive tract to the body wall. The parietal muscles show a unique situation in E. binderi with five bundles being present, two laterals and one distal. Several features aid in defining characters for the entire superfamily and the families Flustrellidridae and Alcyonidiidae. Besides the shape of the aperture, the frontal duplicature band, the vestibular collar and the large caecum are important. The set of characters also confirms recent notions that Bockiella belongs to the Alcyonidiidae.

Morphology of ctenostome bryozoans: 2. Haywardozoon pacificum, with implications of the phylogenetic position of the genus

The genus Haywardozoon represent a little known genus of ctenostome bryozoans that has only been found in the deep-sea. It forms small, mostly uniserial colonies lacking polymorphs. Zooids have a conspicuous apertural closure mechanism consisting of a cuticular lower lip that closes the aperture. The systematic placement of the genus remains uncertain, detailed morphological studies that include soft-body morphological traits are missing. Consequently, this is the first study analyzing H. pacificum by means of histological serial sections and 3d-reconstruction. Zooids are ovoid and in some cases solitary, that is, showing no interconnected zooids. Most prominent is the large vestibular wall that can be more than half of the total length of the zooid. Its vestibular wall is particularly lined by a complex, multilayered and branched cuticle. A single pair of lateral parieto-diaphragmatic muscles is present. The polypide is small and comprises about 17 tentacles. The digestive tract is short, has an elongated cardia, a vestigial caecum and a vestibular anus. An ovipositor/intertentacular organ and several oligolecithal oocytes were detected. Several aspects of zooidal morphology, including the structure of the bilateral aperture, parieto-diaphragmatic muscles, general structure of the gut and the thick cuticle, clearly indicate an association to the ctenostome superfamily Alcyonidioidea. Therefore, we reject the previous placement into Hislopioidea and suggest a possible association to pherusellid ctenostomes. New reproductive characters show that H. pacificum is a broadcaster contrary to some other deep-sea forms that are brooding. RESEARCH HIGHLIGHT: Morphology of ctenostome bryozoans remain little investigated. This contribution is the second of a series of detailed morphological analyses of this understudied clade of bryozoans. The morphological investigation of Haywardozoon pacificum revealed numerous characters that show a closer relationship to Flustrellididrae rather than Hislopiidae as previously assumed.

Morphology of ctenostome bryozoans: 1. Arachnidium fibrosum

The morphology of ctenostome bryozoans remains little investigated with only few species having been subject to more detailed studies. From all the seven main different superfamilies, only few representatives have been studied. The superfamily Arachnidioidea has particularly been neglected concerning detailed morphological and histological details. So far, not a single analysis specifically studied a representative of the family Arachnidiidae. Arachnidium‐like forms have, however, often been regarded as potential cheilostome ancestors, the most successful group of bryozoans to date. The lack of any morphological data on this family called for a detailed investigation of one of its representatives. Hence, we analysed the general morphology and histology of Arachnidium fibrosum. Most striking morphological features previously unrecognized are a cardiac constrictor, previously almost unknown in the family, a single pair of apertural muscles consisting of proximal parieto‐diaphragmatic and distal parieto‐vestibular muscles, six pairs of duplicature bands, a lophophoral anus and retractor muscles attaching to the foregut. Although comparative data are limited, there seem to be two distinct different clades of arachnidiid ctenostomes that are characterized by their aperture and details of gut morphology. Further analysis of additional arachnidioidean species are required to confirm this.