Collar cells in planula and adult tentacle ectoderm of the solitary coral Balanophyllia regia (Anthozoa Eupsammiidae)

Reproductive characteristics and gametogenic cycle of the scleractinian coral Dendrophyllia ramea

The present study marks a pioneering investigation into the reproductive cycle of the scleractinian coral Dendrophyllia ramea. This is one of the first reproduction studies conducted in the Mediterranean Sea for a colonial azooxanthellate coral. Coral samples were collected in 2017 (May and October) and 2018 (February and July) in the Alborán Sea (SW Mediterranean). This location was selected due to its rarity as one of the few sites where this species thrives at depths shallower than 40 m. These samples were used to study the sexual patterns, fertilization mechanisms and gametogenic cycles by means of histological techniques. To broaden the scope, Sea Surface Temperature (SST) and Chlorophyll-a (Chl-a) data from open access databases have been considered to explore the potential influence of these environmental factors as triggers for gamete development and spawning time. The findings cast D. ramea as a gonochoric species, since no hermaphroditic specimens were observed among the analysed samples. Additionally, the lack of larvae and embryos in any of the analysed polyps, suggest that this species is fertilised externally. Two oocyte cohorts have been detected simultaneously, hinting at a yearly reproductive cycle, characterised by a prolonged oocyte maturation and seasonal spawning period taking place between August and October. Nevertheless, D. ramea display a low fecundity compared to other scleractinians inhabiting deep waters. Lastly, the early stages of gametogenesis seem to be coupled with the highest Chl-a values (i.e., March and December), whereas spawning takes place throughout the warmest period of the year (August to October).

Cnidarian hair cell development illuminates an ancient role for the class IV POU transcription factor in defining mechanoreceptor identity

Although specialized mechanosensory cells are found across animal phylogeny, early evolutionary histories of mechanoreceptor development remain enigmatic. Cnidaria (e.g. sea anemones and jellyfishes) is the sister group to well-studied Bilateria (e.g. flies and vertebrates), and has two mechanosensory cell types – a lineage-specific sensory effector known as the cnidocyte, and a classical mechanosensory neuron referred to as the hair cell. While developmental genetics of cnidocytes is increasingly understood, genes essential for cnidarian hair cell development are unknown. Here, we show that the class IV POU homeodomain transcription factor (POU-IV) – an indispensable regulator of mechanosensory cell differentiation in Bilateria and cnidocyte differentiation in Cnidaria – controls hair cell development in the sea anemone cnidarian Nematostella vectensis. N. vectensis POU-IV is postmitotically expressed in tentacular hair cells, and is necessary for development of the apical mechanosensory apparatus, but not of neurites, in hair cells. Moreover, it binds to deeply conserved DNA recognition elements, and turns on a unique set of effector genes – including the transmembrane receptor-encoding gene polycystin 1 – specifically in hair cells. Our results suggest that POU-IV directs differentiation of cnidarian hair cells and cnidocytes via distinct gene regulatory mechanisms, and support an evolutionarily ancient role for POU-IV in defining the mature state of mechanosensory neurons.

Diversity of cilia-based mechanosensory systems and their functions in marine animal behaviour

Sensory cells that detect mechanical forces usually have one or more specialized cilia. These mechanosensory cells underlie hearing, proprioception or gravity sensation. To date, it is unclear how cilia contribute to detecting mechanical forces and what is the relationship between mechanosensory ciliated cells in different animal groups and sensory systems. Here, we review examples of ciliated sensory cells with a focus on marine invertebrate animals. We discuss how various ciliated cells mediate mechanosensory responses during feeding, tactic responses or predator-prey interactions. We also highlight some of these systems as interesting and accessible models for future in-depth behavioural, functional and molecular studies. We envisage that embracing a broader diversity of organisms could lead to a more complete view of cilia-based mechanosensation. This article is part of the Theo Murphy meeting issue 'Unity and diversity of cilia in locomotion and transport'.

Gastric pouches and the mucociliary sole: setting the stage for nervous system evolution

Prerequisite for tracing nervous system evolution is understanding of the body plan, feeding behaviour and locomotion of the first animals in which neurons evolved. Here, a comprehensive scenario is presented for the diversification of cell types in early metazoans, which enhanced feeding efficiency and led to the emergence of larger animals that were able to move. Starting from cup-shaped, gastraea-like animals with outer and inner choanoflagellate-like cells, two major innovations are discussed that set the stage for nervous system evolution. First, the invention of a mucociliary sole entailed a switch from intra- to extracellular digestion and increased the concentration of nutrients flowing into the gastric cavity. In these animals, an initial nerve net may have evolved via division of labour from mechanosensory-contractile cells in the lateral body wall, enabling coordinated movement of the growing body that involved both mucociliary creeping and changes of body shape. Second, the inner surface of the animals folded into metameric series of gastric pouches, which optimized nutrient resorption and allowed larger body sizes. The concomitant acquisition of bilateral symmetry may have allowed more directed locomotion and, with more demanding coordinative tasks, triggered the evolution of specialized nervous subsystems. Animals of this organizational state would have resembled Ediacarian fossils such as Dickinsonia and may have been close to the cnidarian–bilaterian ancestor. In the bilaterian lineage, the mucociliary sole was used mostly for creeping, or frequently lost. One possible remnant is the enigmatic Reissner's fibre in the ventral neural tube of cephalochordates and vertebrates.

Bacterial influences on animal origins

Animals evolved in seas teeming with bacteria, yet the influences of bacteria on animal origins are poorly understood. Comparisons among modern animals and their closest living relatives, the choanoflagellates, suggest that the first animals used flagellated collar cells to capture bacterial prey. The cell biology of prey capture, such as cell adhesion between predator and prey, involves mechanisms that may have been co-opted to mediate intercellular interactions during the evolution of animal multicellularity. Moreover, a history of bacterivory may have influenced the evolution of animal genomes by driving the evolution of genetic pathways for immunity and facilitating lateral gene transfer. Understanding the interactions between bacteria and the progenitors of animals may help to explain the myriad ways in which bacteria shape the biology of modern animals, including ourselves.

Development of ichthyosporeans sheds light on the origin of metazoan multicellularity

To understand the mechanisms involved in the transition from protists to multicellular animals (metazoans), studying unicellular relatives of metazoans is as important as studying metazoans themselves. However, investigations remain poor on the closest unicellular (or colonial) relatives of Metazoa, i.e., choanoflagellates, filastereans and ichthyosporeans. Molecular-level analyses on these protists have been severely limited by the lack of transgenesis tools. Their genomes, however, contain several key genes encoding proteins important for metazoan development and multicellularity, including those involved in cell–cell communication, cell proliferation, cell differentiation, and tissue growth control. Tools to analyze their functions in a molecular level are awaited. Here we report techniques of cell transformation and gene silencing developed for the first time in a close relative of metazoans, the ichthyosporean Creolimax fragrantissima. We propose C. fragrantissima as a model organism to investigate the origin of metazoan multicellularity. By transgenesis, we demonstrate that its colony develops from a fully-grown multinucleate syncytium, in which nuclear divisions are strictly synchronized. It has been hypothesized that metazoan multicellular development initially occurred in the course of evolution through successive rounds of cell division, which were not necessarily be synchronized, or alternatively through cell aggregation. Our findings point to another possible mechanism for the evolution of animal multicellularity, namely, cellularization of a syncytium in which nuclear divisions are synchronized. We believe that further studies on the development of ichthyosporeans by the use of our methodologies will provide novel insights into the origin of metazoan multicellularity.

Choanocyte ultrastructure in Halisarca dujardini (Demospongiae, Halisarcida)

Understanding poriferan choanocyte ultrastructure is crucial if we are to unravel the steps of a putative evolutionary transition between choanoflagellate protists and early metazoans. Surprisingly, some aspects of choanocyte cytology still remain little investigated. This study of choanocyte ultrastructure in the halisarcid demosponge Halisarca dujardini revealed a combination of minor and major distinctive traits, some of them unknown in Porifera so far. Most significant features were 1) an asymmetrical periflagellar sleeve, 2) a battery of specialized intercellular junctions at the lateral cell surface complemented with an array of lateral interdigitations between adjacent choanocytes that provides a particular sealing system of the choanoderm, and 3) a unique, unexpectedly complex, basal apparatus. The basal apparatus consists of a basal body provided with a small basal foot and an intricate transverse skeleton of microtubules. An accessory centriole, which is not perpendicular to the basal body, is about 45 degrees . In addition, a system of short striated rootlets (periodicity = 50-60 nm) arises from the proximal edge of the basal body and runs longitudinally to contact the nuclear apex. This is the first flagellar rootlet system ever found in a choanocyte. The accessory centriole, the rootlet system, and the nuclear apex are all encircled by a large Golgi apparatus, adding another distinctive feature to the choanocyte cytology. The set of distinct features discovered in the choanocyte of H. dujardini indicates that the ultrastructure of the poriferan choanocyte may vary substantially between sponge groups. It is necessary to improve understanding of such variation, as the cytological features of choanocytes are often coded as characters both for formulation of hypotheses on the origin of animals and inference of phylogenetic relationships at the base of the metazoan tree.

Feeding behavior, epidermal structure and mucus cytochemistry of the scleractinian Mycetophyllia reesi, a coral without tentacles

The scleractinian Mycetophyllia reesi lacks even the vestiges of tentacles, but quickly captures particulate food by mucus entanglement. Mesenterial filaments emerge through the oral opening, collect the mucus-embedded particulates, and withdraw to the gastrovascular system within 15 min. Mucocytes dominate the outer epidermis with about 3,000 cells/mm(2) and are capable of apocrine discharge en masse. Mucocytes are spumous, typically with web-like inclusions, which for the most part lack electron opacity with ordinary staining, and are only weakly PAS positive. In contrast, the mucus reacts strongly to diamine and other reagents that suggest an appreciable acidic mucopolysaccharide component. The strongest staining reaction occurs in the presence of high iron diamine, suggesting with other tests that the mucus contains significant quantities of sulfated polysaccharides. Cells with cilia anchored by spiriliform microvilli flank the mucocytes and possess small, spumous inclusions that contain acidic, sulfated, and neutral polysaccharides that do not appear to discharge during feeding. These support cells are closely intertwined with narrow, sinuous, secretory cells containing an electron-opaque cytoplasm of unknown composition that is discharged along with mucus during feeding. The outer epidermis also contains scattered cnidae, rather than the clusters or batteries typical of tentacles. The overwhelming abundance of mucocytes is consistent with their importance in feeding. Likewise, the small number of epidermal cnidae suggest they play a minor role in acquiring food. An inner epidermal layer associated with the mesoglea contains epitheliomuscular cells, nerve cells and pigment cells. The two epidermal layers form an essentially pseudostratified, architecturally simple epithelium.

Gastrodermal structure and feeding responses in the scleractinian Mycetophyllia reesi, a coral with novel digestive filaments

Mycetophyllia reesi Wells is a colonial scleractinian coral whose outer surface consists of a series of oral-pharyngeal openings that lack tentacles. The polyps also lack a column and cannot protrude from the colonial surface. Correspondingly, there is no central digestive cavity. Instead, the pharynx is directly connected to a series of radially arranged mesenterial ducts lying parallel to the skeleton. The ducts, composed primarily of ciliated cells with small mucus inclusions and large, compartmentalized mucocytes, house filaments that protrude through the oral apertures during feeding. The filaments may or may not be directly connected with or originate from the mesenterial ducts and are histologically distinct from them. They are therefore referred to as digestive, rather than mesenterial filaments. In contrast with other scleractinians, the digestive filaments are thin, unequally bilobed stalks with spatulate ends. The cnidoglandular (CG) lobe, the larger of the two, exhibits a distinct cellular zonation. Large mastigophore cnidae and elongated zymogen-like cells are clustered at its distal end. Neither of these cells appear to respond to particulate food material, suggesting that they may be employed in alternative modes of nutrition and/or competition. Behind the distal region, the CG lobe exhibits typical zymogen, mucus, and collar cells as well as numerous atrichous nematocysts. The atrichs and zymogen cells discharge during particulate feeding. Tracts of collar cells with particularly well-defined cilia, elongated rootlets, and mucus inclusions are found at the outer edge of the CG lobe. These cells disgorge their contents during feeding and appear to function in food transport. The smaller lobe of the filament is a muscular sheet containing well-defined fields of circular and longitudinal myofibrils along with associated neurons. Collar cells with lysosome-like inclusions and large, compartmentalized mucocytes are also characteristic of this region. There are no zooxanthellae in the filaments, but these endosymbionts are present as a thin layer in the oral-most portion of the gastrodermis. The cellular zonation and multi-functionality of these digestive filaments suggest another example of a cnidarian structure at the organ level of complexity.

The microvilli and hyaline layer of embryonic asteroid epithelial collar cells: a sensory structure to determine the position of locomotory cilia?

Early stage embryos of the starfish Pisaster ochraceus exhibit one cilium per cell which is primarily involved in locomotion. SEM observations have demonstrated two types of microvilli "stage horn"-like and "finger-like" microvilli (CMs), both of which probably serve to anchor and support the hyaline layer (HL). The CMs arise from the cellular membrane a short distance from the base of the ciliary shaft and form a circle around the base of each cilium. This arrangement is found in embryos and larvae as well as in adult tissues of many other marine organisms. TEM observations of material prepared by freeze substitution has demonstrated that the HL unites the circle of CMs and forms two collars. The outer ECM collar is single and attached directly to the CMs, while the inner collar consists of multiple rings of ECM located between the cilium and the CMs. The inner collar elements are not attached to the cilium but are attached to the inner aspects of the CMs by a complex arrangement consisting of a loop of ECM and two short ECM fibers. The arrangement of the ECM of the collars could provide an excellent way to transmit the movements of the cilium to the surrounding microvilli. Although the bases of the CMs always encircle the ciliary shaft, the shafts of the CMs are seen in different positions. This suggests that the CM/ECM collar may be able to change position relative to the cilium. Confocal laser scanning microscopy demonstrates that the CMs contain phalloidin positive material which extends into a phalloidin positive region located in the apex of the cells. The CMs and apical web contain microfilaments which are probably actin and could be involved in movement of the CMs. A movable circle of CMs with their associated ECM could represent a mechanism to sense the position of the cilium and/or to define the direction and extent of the stroke.

Fine structure of a proprioceptor in the body wall of the marine nematode Deontostoma californicum Steiner and Albin, 1933 (Enoplida: Leptosomatidae)

The structure of a proprioceptor in the lateral hypodermal chords of Deontostoma californicum has been studied by light and electron microscopy. It is comprised of a sensory cell provided with a cilium situated in a terminal invagination. An accompanying dendrite forms a synaptic junction at the distal end of the sensory cell. This is the first fine structural description of this proprioceptor in the Enoplida.

Fine structural studies of the nervous system and the apical organ in the planula larva of the sea anemone Anthopleura elegantissima

The nervous system of the planula larva of Anthopleura elegantissima consists of an apical organ, one type of endodermal receptor cell, two types of ectodermal receptor cells, central neurons and nerve plexus. Both interneural and neuromuscular synapses are found in the nerve plexus. The apical organ is a collection of about 100 long, columnar cells each bearing a long cilium and a collar of about 10 microvilli. The cilia of the apical organ are twisted together to form an apical tuft. The ciliary rootlets of the apical organ cells are extremely long, reaching to the basal processes of the cells adjacent to the mesoglea. All three types of sensory cells are tall and slender in profile and are identified by the presence of one or more of the following features: microtubules, small vesicles, membrane-bound granules and synapses. The interneurons are bipolar cells with somas restricted to the aboral end, adjacent to the apical organ. All synapses observed are polarized or asymmetrical. A diagram including all the elements of the nervous system is presented and the possible functions of the nervous system are discussed in relation to larval behavior.

Fine structural observations on the ciliary receptors in the epidermis of three otoplanid species (Turbellaria proseriata)

In Notocaryoturbella bigermaria, Otoplana truncaspina and Paroto-planella heterorhabditica three types of epidermal receptors are recognized. Type I: with a single cilium running in a duct, piercing the distal dendrite process of the receptor. The internal wall of the dendrite process has eight ridges with longitudinal filaments lying inside them. The ciliary basal body lacks a longitudinal rootlet but is encircled by a thin annular formation. Type II: with a single (A) or several (B) cilia which protrude from the outer epithelial surface and are provided with a large and striped rootlet. Both types are considered as mechanoreceptors. Type III: with two or more short and stumpy cilia devoid of rootlets and displaying the usual 9 + 2 pattern in the proximal part only. They are considered as chemoreceptors.

The ciliary-cone sensory cell of anemones and cerianthids

An ultrastructural study of the tentacles of Stomphia and of Ceriantheopsis has revealed that the so-called 'ciliary-cone sensory cell' consists of a cluster of five to seven apparent receptors rather than just one cell as reported previously. At the center of a cluster is a single cell, whose dendrite bears one cilium surrounded by about ten large stereocilia. Surrounding this cell are a number of peripheral cells whose dendrites bear large numbers of small stereocilia and, in Ceriantheopsis, one cilium. The sensory apparatuses of all cells in a cluster unite to form a single unit projecting above the tissue surface: the ciliary cone. Their possible physiological role is discussed in relation to new behavioural observations.