The fine structure of myocytes in the sponges microciona prolifera (Ellis and Solander) and tedania ignis (Duchassaing and Michelotti)

Cell contractility in early animal evolution

Tissue deformation mediated by collective cell contractility is a signature characteristic of animals. In most animals, fast and reversible contractions of muscle cells mediate behavior, while slow and irreversible contractions of epithelial or mesenchymal cells play a key role in morphogenesis. Animal tissue contractility relies on the activity of the actin/myosin II complex (together referred to as 'actomyosin'), an ancient and versatile molecular machinery that performs a broad range of functions in development and physiology. This review synthesizes emerging insights from morphological and molecular studies into the evolutionary history of animal contractile tissue. The most ancient functions of actomyosin are cell crawling and cytokinesis, which are found in a wide variety of unicellular eukaryotes and in individual metazoan cells. Another contractile functional module, apical constriction, is universal in metazoans and shared with choanoflagellates, their closest known living relatives. The evolution of animal contractile tissue involved two key innovations: firstly, the ability to coordinate and integrate actomyosin assembly across multiple cells, notably to generate supracellular cables, which ensure tissue integrity but also allow coordinated morphogenesis and movements at the organism scale; and secondly, the evolution of dedicated contractile cell types for adult movement, belonging to two broad categories respectively defined by the expression of the fast (striated-type) and slow (smooth/non-muscle-type) myosin II paralogs. Both contractile cell types ancestrally resembled generic contractile epithelial or mesenchymal cells and might have played a versatile role in both behavior and morphogenesis. Modern animal contractile cells span a continuum between unspecialized contractile epithelia (which underlie behavior in modern placozoans), epithelia with supracellular actomyosin cables (found in modern sponges), epitheliomuscular tissues (with a concentration of actomyosin cables in basal processes, for example in sea anemones), and specialized muscle tissue that has lost most or all epithelial properties (as in ctenophores, jellyfish and bilaterians). Recent studies in a broad range of metazoans have begun to reveal the molecular basis of these transitions, powered by the elaboration of the contractile apparatus and the evolution of 'core regulatory complexes' of transcription factors specifying contractile cell identity.

Actual and Model-Predicted Growth of Sponges—With a Bioenergetic Comparison to Other Filter-Feeders

Sponges are one of the earliest-evolved and simplest groups of animals, but they share basic characteristics with more advanced and later-evolved filter-feeding invertebrates, such as mussels. Sponges are abundant in many coastal regions where they filter large amounts of water for food particles and thus play an important ecological role. Therefore, a better understanding of the bioenergetics and growth of sponges compared to other filter-feeders is important. While the filtration (pumping) rates of many sponge species have been measured as a function of their size, little is known about their rate of growth. Here, we use a bioenergetic growth model for demosponges, based on the energy budget and observations of filtration (F) and respiration rates (R). Because F versus dry weight (W) can be expressed as F = a1Wb1 and the maintenance respiratory rate can be expressed as Rm = a2Wb2, we show that if b1~ b2the growth rate can be expressed as: G = aWb1, and, consequently, the weight-specific growth rate is µ = G/W = aWb1−1 = aWb where the constant a depends on ambient sponge-available food particles (free-living bacteria and phytoplankton with diameter < ostia diameter). Because the exponent b1 is close to 1, then b ~ 0, which implies µ = a and thus exponential growth as confirmed in field growth studies. Exponential growth in sponges and in at least some bryozoans is probably unique among filter-feeding invertebrates. Finally, we show that the F/R-ratio and the derived oxygen extraction efficiency in these sponges are similar to other filter-feeding invertebrates, thus reflecting a comparable adaptation to feeding on a thin suspension of bacteria and phytoplankton.

Muscle systems and motility of early animals highlighted by cnidarians from the basal Cambrian

Although fossil evidence suggests that various animal groups were able to move actively through their environment in the early stages of their evolution, virtually no direct information is available on the nature of their muscle systems. The origin of jellyfish swimming, for example, is of great interest to biologists. Exceptionally preserved muscles are described here in benthic peridermal olivooid medusozoans from the basal Cambrian of China (Kuanchuanpu Formation, ca. 535 Ma) that have direct equivalent in modern medusozoans. They consist of circular fibers distributed over the bell surface (subumbrella) and most probably have a myoepithelial origin. This is the oldest record of a muscle system in cnidarians and more generally in animals. This basic system was probably co-opted by early Cambrian jellyfish to develop capacities for jet-propelled swimming within the water column. Additional lines of fossil evidence obtained from ecdysozoans (worms and panarthropods) show that the muscle systems of early animals underwent a rapid diversification through the early Cambrian and increased their capacity to colonize a wide range of habitats both within the water column and sediment at a critical time of their evolutionary radiation.

pH Regulation and Tissue Coordination Pathways Promote Calcium Carbonate Bioerosion by Excavating Sponges

Coral reefs are threatened by a multitude of environmental and biotic influences. Among these, excavating sponges raise particular concern since they bore into coral skeleton forming extensive cavities which lead to weakening and loss of reef structures. Sponge bioerosion is achieved by a combination of chemical dissolution and mechanical chip removal and ocean acidification has been shown to accelerate bioerosion rates. However, despite the ecological relevance of sponge bioerosion, the exact chemical conditions in which dissolution takes place and how chips are removed remain elusive. Using fluorescence microscopy, we show that intracellular pH is lower at etching sites compared to ambient seawater and the sponge’s tissue. This is realised through the extension of filopodia filled with low intracellular pH vesicles suggesting that protons are actively transported into this microenvironment to promote CaCO₃ dissolution. Furthermore, fusiform myocyte-like cells forming reticulated pathways were localised at the interface between calcite and sponge. Such cells may be used by sponges to contract a conductive pathway to remove chips possibly instigated by excess Ca²⁺ at the boring site. The mechanism underlying CaCO₃ dissolution by sponges provides new insight into how environmental conditions can enhance dissolution and improves predictions of future rates of coral dissolution due to sponge activity.

Interacting-heads motif has been conserved as a mechanism of myosin II inhibition since before the origin of animals

Electron microscope studies have shown that the switched-off state of myosin II in muscle involves intramolecular interaction between the two heads of myosin and between one head and the tail. The interaction, seen in both myosin filaments and isolated molecules, inhibits activity by blocking actin-binding and ATPase sites on myosin. This interacting-heads motif is highly conserved, occurring in invertebrates and vertebrates, in striated, smooth, and nonmuscle myosin IIs, and in myosins regulated by both Ca²⁺ binding and regulatory light-chain phosphorylation. Our goal was to determine how early this motif arose by studying the structure of inhibited myosin II molecules from primitive animals and from earlier, unicellular species that predate animals. Myosin II from Cnidaria (sea anemones, jellyfish), the most primitive animals with muscles, and Porifera (sponges), the most primitive of all animals (lacking muscle tissue) showed the same interacting-heads structure as myosins from higher animals, confirming the early origin of the motif. The social amoeba Dictyostelium discoideum showed a similar, but modified, version of the motif, while the amoeba Acanthamoeba castellanii and fission yeast (Schizosaccharomyces pombe) showed no head-head interaction, consistent with the different sequences and regulatory mechanisms of these myosins compared with animal myosin IIs. Our results suggest that head-head/head-tail interactions have been conserved, with slight modifications, as a mechanism for regulating myosin II activity from the emergence of the first animals and before. The early origins of these interactions highlight their importance in generating the inhibited (relaxed) state of myosin in muscle and nonmuscle cells.

Elements of a 'nervous system' in sponges

Genomic and transcriptomic analyses show that sponges possess a large repertoire of genes associated with neuronal processes in other animals, but what is the evidence these are used in a coordination or sensory context in sponges? The very different phylogenetic hypotheses under discussion today suggest very different scenarios for the evolution of tissues and coordination systems in early animals. The sponge genomic 'toolkit' either reflects a simple, pre-neural system used to protect the sponge filter or represents the remnants of a more complex signalling system and sponges have lost cell types, tissues and regionalization to suit their current suspension-feeding habit. Comparative transcriptome data can be informative but need to be assessed in the context of knowledge of sponge tissue structure and physiology. Here, I examine the elements of the sponge neural toolkit including sensory cells, conduction pathways, signalling molecules and the ionic basis of signalling. The elements described do not fit the scheme of a loss of sophistication, but seem rather to reflect an early specialization for suspension feeding, which fits with the presumed ecological framework in which the first animals evolved.

The physiology and molecular biology of sponge tissues

Sponges have become the focus of studies on molecular evolution and the evolution of animal body plans due to their ancient branching point in the metazoan lineage. Whereas our former understanding of sponge function was largely based on a morphological perspective, the recent availability of the first full genome of a sponge (Amphimedon queenslandica), and of the transcriptomes of other sponges, provides a new way of understanding sponges by their molecular components. This wealth of genetic information not only confirms some long-held ideas about sponge form and function but also poses new puzzles. For example, the Amphimedon sponge genome tells us that sponges possess a repertoire of genes involved in control of cell proliferation and in regulation of development. In vitro expression studies with genes involved in stem cell maintenance confirm that archaeocytes are the main stem cell population and are able to differentiate into many cell types in the sponge including pinacocytes and choanocytes. Therefore, the diverse roles of archaeocytes imply differential gene expression within a single cell ontogenetically, and gene expression is likely also different in different species; but what triggers cells to enter one pathway and not another and how each archaeocyte cell type can be identified based on this gene knowledge are new challenges. Whereas molecular data provide a powerful new tool for interpreting sponge form and function, because sponges are suspension feeders, their body plan and physiology are very much dependent on their physical environment, and in particular on flow. Therefore, in order to integrate new knowledge of molecular data into a better understanding the sponge body plan, it is important to use an organismal approach. In this chapter, we give an account of sponge body organization as it relates to the physiology of the sponge in light of new molecular data. We focus, in particular, on the structure of sponge tissues and review descriptive as well as experimental work on choanocyte morphology and function. Special attention is given to pinacocyte epithelia, cell junctions, and the molecules present in sponge epithelia. Studies describing the role of the pinacoderm in sensing, coordination, and secretion are reviewed. A wealth of recent work describes gene presence and expression patterns in sponge tissues during development, and we review this in the context of the previous descriptions of sponge morphology and physiology. A final section addresses recent findings of genes involved in the immune response. This review is far from exhaustive but intends rather to revisit for non-specialists key aspects of sponge morphology and physiology in light of new molecular data as a means to better understand and interpret sponge form and function today.

Origin of the neuro-sensory system: new and expected insights from sponges

The capacity of all cells to respond to stimuli implies the conduction of information at least over short distances. In multicellular organisms, more complex systems of integration and coordination of activities are necessary. In most animals, the processing of information is performed by a nervous system. Among the most basal taxa, sponges are nerveless so that it is traditionally assumed that the integrated neuro-sensory system originated only once in Eumetazoa, a hypothesis not in agreement with some recent phylogenomic studies. The aim of this review is to show that recent data on sponges might provide clues for understanding the origin of this complex system. First, sponges are able to react to external stimuli, and some of them display spontaneous movement activities. These coordinated behaviors involve nervous system-like mechanisms, such as action potentials and/or neurotransmitters. Second, genomic analyses show that sponges possess genes orthologous to those involved in the patterning or functioning of the neuro-sensory system in Eumetazoa. Finally, some of these genes are expressed in specific cells (flask cells, choanocytes). Together with ultrastructural data, this gives rise to challenging hypotheses concerning cell types that might play neuro-sensory-like roles in sponges.

The contractile sponge epithelium sensu lato – body contraction of the demosponge Tethya wilhelma is mediated by the pinacoderm

Sponges constitute one of the two metazoan phyla that are able to contract their bodies despite a complete lack of muscle cells. Two competing hypotheses on the mechanisms behind this have been postulated to date: (1) mesohyl-mediated contraction originating from fusiform smooth muscle-like actinocytes (‘myocytes’) and (2) epidermal contraction originating in pinacocytes. No direct support exists for either hypothesis. The question of agonist–antagonist interaction in sponge contraction seems to have been completely neglected so far. In the present study we addressed this by studying sponge contraction kinetics. We also tested both hypotheses by carrying out volumetric studies of 3D synchrotron radiation-based x-ray microtomography data obtained from contracted and expanded specimens of Tethya wilhelma. Our results support the pinacoderm contraction hypothesis. Should mesohyl contraction be present, it is likely to be part of the antagonist system. We conclude that epithelial contraction plays a major role in sponges. Contractile epithelia sensu lato may be regarded as part of the ground pattern of the Metazoa.

Physiology of coordination in sponges

All multicellular organisms need a means of communicating between cells and between regions of the body. The evolution of a nervous system, by the Cnidaria, provided a fast means of communication and enabled the colonization of rapidly changing environments. Sponges, the descendants of the first multicellular animals, lack nerves but nevertheless have a number of different systems that allow coordinated behaviour, albeit rather slow coordinated behaviour. It is from elements within these systems that the origins of the nervous and endocrine systems, the grand organizing principles of higher animals, seem likely to have appeared. Electrical activity has not been found in cellular sponges, yet local contractions are elicited in response to a variety of stimuli and, in some cases, contractions propagate across the body to control the hydrodynamics of the feeding current. The mechanism of propagation is thought to involve hormones or a combination of other signaling molecules and direct mechanical action of one cell on the next, leading to increased intracellular calcium. In other instances cellular sponges respond to stress, such as heat shock, by elevating intracellular calcium by way of second messengers such as cyclic ADP-ribose. Electrical communication, well known in plants and protists, was first demonstrated in a sponge in 1997. Hexactinellids (glass sponges), which arrest their feeding current within 20 s of mechanical or electrical stimulation, do so via an electrical impulse that propagates through syncytial tissues. These unusual syncytial tissues are cytoplasmically coupled from outside to inside and top to bottom so that there are no membrane boundaries to impede the electrical currents. Pharmacological tests suggest that Ca2+, rather than Na+, drives the action potential. The conduction velocity is slow (0.27 cm·s–1) and is highly temperature sensitive (Q10~3). At present, glass sponges are the only poriferans known to have propagated electrical signals. In addition, reports of directional swimming in sponge larvae, of the rapid and coordinated changes in the tensile strength of the extracellular matrix in Chondrosia Nardo, 1847, and of the rapid closure of ostia of some cellular sponges in response to mechanical stimuli further illustrate the variety of coordinating mechanisms that evolved in the Porifera in the absence of a nervous system.

Kinetics and rhythm of body contractions in the sponge Tethya wilhelma (Porifera: Demospongiae)

Sponges of the species Tethya wilhelma display rhythmic body contractions, which were analyzed by digital timelapse imaging and semi-automated image analysis. For the first time, differential, quantitative data on sponge behaviour could be obtained. The sponges are able to reduce their body volume by up to 73.3% during regular contractions. Each contraction cycle follows a characteristic pattern of four phases, permitting analysis of the kinetics of contraction and expansion. Long-term observations (for &gt;7 days) reveal that the sponge contractions display a day-night periodicity in which contraction cycles are significantly longer during the dark hours. The contractions seem to be mediated by the pinacoderm; they are triggered locally and spread over the sponge surface at 12.5 μm s-1. If two individuals of a clone are fused, the individual contraction rhythm of both sponges persists for several days, until a single new individual sponge is formed with a synchronized rhythm. The reported results and techniques establish T. wilhelma as a model organism for research on the development of aneural signal transduction and integration during early Metazoan evolution.

Muscular alteration of gill geometry in vitro: implications for bivalve pumping processes

In bivalves, water-pumping potential is determined both by ciliary activity and by the geometry of the system of passageways that acts as a conduit for water flow. Smooth muscles intrinsic to the gills of eulamellibranch bivalves possess the anatomical organization needed to regulate the dimensions of these water passageways. The tone of these muscles can be controlled experimentally using excitatory neurotransmitters to elicit muscle contraction and by removing Ca++ from the Ringer's solution to induce muscular relaxation. These experimental methods were used to investigate the effects of smooth muscle tone on the gill dimensions of two freshwater bivalves, Dreissena polymorpha and Corbicula fluminea, and one marine bivalve, Mercenaria mercenaria. In addition, endoscopic observations were made from the suprabranchial chamber of a freshwater unionid, Lampsilis anodontoides. Contraction of gill muscles led to a significant reduction in interfilament width, internal ostial area, and the cross-sectional area of the water tubes. Endoscopic observation from minimally disturbed L. anodontoides revealed rapid constriction of the water tubes upon contraction of the muscles of the gill and gill axis. Taken together, these data support the idea that alteration of smooth muscle tone in the gill provides a mechanism for controlling water-pumping activities.