The energetic cost of filtration by demosponges and their behavioural response to ambient currents

Molecular profiling of sponge deflation reveals an ancient relaxant-inflammatory response

A hallmark of animals is the coordination of whole-body movement. Neurons and muscles are central to this, yet coordinated movements also exist in sponges that lack these cell types. Sponges are sessile animals with a complex canal system for filter-feeding. They undergo whole-body movements resembling "contractions" that lead to canal closure and water expulsion. Here, we combine 3D optical coherence microscopy, pharmacology, and functional proteomics to elucidate anatomy, molecular physiology, and control of these movements. We find them driven by the relaxation of actomyosin stress fibers in epithelial canal cells, which leads to whole-body deflation via collapse of the incurrent and expansion of the excurrent system, controlled by an Akt/NO/PKG/A pathway. A concomitant increase in reactive oxygen species and secretion of proteinases and cytokines indicate an inflammation-like state reminiscent of vascular endothelial cells experiencing oscillatory shear stress. This suggests an ancient relaxant-inflammatory response of perturbed fluid-carrying systems in animals.

Microbiome diversity and host immune functions influence survivorship of sponge holobionts under future ocean conditions

The sponge-associated microbial community contributes to the overall health and adaptive capacity of the sponge holobiont. This community is regulated by the environment and the immune system of the host. However, little is known about the effect of environmental stress on the regulation of host immune functions and how this may, in turn, affect sponge-microbe interactions. In this study, we compared the bacterial diversity and immune repertoire of the demosponge, Neopetrosia compacta, and the calcareous sponge, Leucetta chagosensis, under varying levels of acidification and warming stress based on climate scenarios predicted for 2100. Neopetrosia compacta harbors a diverse microbial community and possesses a rich repertoire of scavenger receptors while L. chagosensis has a less diverse microbiome and an expanded range of pattern recognition receptors and immune response-related genes. Upon exposure to RCP 8.5 conditions, the microbiome composition and host transcriptome of N. compacta remained stable, which correlated with high survival (75%). In contrast, tissue necrosis and low survival (25%) of L. chagosensis was accompanied by microbial community shifts and downregulation of host immune-related pathways. Meta-analysis of microbiome diversity and immunological repertoire across poriferan classes further highlights the importance of host-microbe interactions in predicting the fate of sponges under future ocean conditions.

Suspension feeders: diversity, principles of particle separation and biomimetic potential

Suspension feeders (SFs) evolved a high diversity of mechanisms, sometimes with remarkably convergent morphologies, to retain plankton, detritus and man-made particles with particle sizes ranging from less than 1 µm to several centimetres. Based on an extensive literature review, also including the physical and technical principles of solid-liquid separation, we developed a set of 18 ecological and technical parameters to review 35 taxa of suspension-feeding Metazoa covering the diversity of morphological and functional principles. This includes passive SFs, such as gorgonians or crinoids that use the ambient flow to encounter particles, and sponges, bivalves or baleen whales, which actively create a feeding current. Separation media can be flat or funnel-shaped, built externally such as the filter houses in larvaceans, or internally, like the pleated gills in bivalves. Most SFs feed in the intermediate flow region of Reynolds number 1-50 and have cleaning mechanisms that allow for continuous feeding. Comparison of structure-function patterns in SFs to current filtration technologies highlights potential solutions to common technical design challenges, such as mucus nets which increase particle adhesion in ascidians, vanes which reduce pressure losses in whale sharks and changing mesh sizes in the flamingo beak which allow quick adaptation to particle sizes.

Reafference and the origin of the self in early nervous system evolution

Discussions of the function of early nervous systems usually focus on a causal flow from sensors to effectors, by which an animal coordinates its actions with exogenous changes in its environment. We propose, instead, that much early sensing was reafferent; it was responsive to the consequences of the animal's own actions. We distinguish two general categories of reafference-translocational and deformational-and use these to survey the distribution of several often-neglected forms of sensing, including gravity sensing, flow sensing and proprioception. We discuss sensing of these kinds in sponges, ctenophores, placozoans, cnidarians and bilaterians. Reafference is ubiquitous, as ongoing action, especially whole-body motility, will almost inevitably influence the senses. Corollary discharge-a pathway or circuit by which an animal tracks its own actions and their reafferent consequences-is not a necessary feature of reafferent sensing but a later-evolving mechanism. We also argue for the importance of reafferent sensing to the evolution of the body-self, a form of organization that enables an animal to sense and act as a single unit. This article is part of the theme issue 'Basal cognition: multicellularity, neurons and the cognitive lens'.

Hydrodynamics of sponge pumps and evolution of the sponge body plan

Sponges are suspension feeders that filter vast amounts of water. Pumping is carried out by flagellated chambers that are connected to an inhalant and exhalant canal system. In 'leucon' sponges with relatively high-pressure resistance due to a complex and narrow canal system, pumping and filtering are only possible owing to the presence of a gasket-like structure (forming a canopy above the collar filters). Here, we combine numerical and experimental work and demonstrate how sponges that lack such sealing elements are able to efficiently pump and force the flagella-driven flow through their collar filter, thanks to the formation of a 'hydrodynamic gasket' above the collar. Our findings link the architecture of flagellated chambers to that of the canal system, and lend support to the current view that the sponge aquiferous system evolved from an open-type filtration system, and that the first metazoans were filter feeders.

Interocean patterns in shallow water sponge assemblage structure and function

Sponges are a major component of benthic ecosystems across the world and fulfil a number of important functional roles. However, despite their importance, there have been few attempts to compare sponge assemblage structure and ecological functions across large spatial scales. In this review, we examine commonalities and differences between shallow water (<100 m) sponges at bioregional (15 bioregions) and macroregional (tropical, Mediterranean, temperate, and polar) scales, to provide a more comprehensive understanding of sponge ecology. Patterns of sponge abundance (based on density and area occupied) were highly variable, with an average benthic cover between ~1 and 30%. Sponges were generally found to occupy more space (percentage cover) in the Mediterranean and polar macroregions, compared to temperate and tropical macroregions, although sponge densities (sponges m^-2 ) were highest in temperate bioregions. Mean species richness standardised by sampling area was similar across all bioregions, except for a few locations that supported very high small-scale biodiversity concentrations. Encrusting growth forms were generally the dominant sponge morphology, with the exception of the Tropical West Atlantic, where upright forms dominated. Annelids and Arthropods were the most commonly reported macrofauna associated with sponges across bioregions. With respect to reproduction, there were no patterns in gametic development (hermaphroditism versus gonochorism), although temperate, tropical, and polar macroregions had an increasingly higher percentage of viviparous species, respectively, with viviparity being the sole gamete development mechanism reported for polar sponges to date. Seasonal reproductive timing was the most common in all bioregions, but continuous timing was more common in the Mediterranean and tropical bioregions compared to polar and temperate bioregions. We found little variation across bioregions in larval size, and the dominant larval type across the globe was parenchymella. No pattens among bioregions were found in the limited information available for standardised respiration and pumping rates. Many organisms were found to predate sponges, with the abundance of sponge predators being higher in tropical systems. While there is some evidence to support a higher overall proportion of phototrophic species in the Tropical Austalian bioregion compared to the Western Atlantic, both also have large numbers of heterotrophic species. Sponges are important spatial competitors across all bioregions, most commonly being reported to interact with anthozoans and algae. Even though the available information was limited for many bioregions, our analyses demonstrate some differences in sponge traits and functions among bioregions, and among macroregions. However, we also identified similarities in sponge assemblage structure and function at global scales, likely reflecting a combination of regional- and local-scale biological and physical processes affecting sponge assemblages, along with common ancestry. Finally, we used our analyses to highlight geographic bias in past sponge research, and identify gaps in our understanding of sponge ecology globally. By so doing, we identified key areas for future research on sponge ecology. We hope that our study will help sponge researchers to consider bioregion-specific features of sponge assemblages and key sponge-mediated ecological processes from a global perspective.

Short-term responses of tropical lagoon sponges to elevated temperature and nitrate

Sponges are often important components of coastal lagoons, however their responses to anthropogenic stressors remain poorly understood. Here, we tested the responses of three lagoon sponges, Neopetrosia exigua, Amphimedon navalis and Spheciospongia vagabunda from Mauritius (Western Indian Ocean), to nine temperature and nitrate combinations for 14 days. We found that elevated seawater temperature resulted in significant physiological responses in all species, but there was generally little negative effect of elevated nitrate. At the end of the experiment, the buoyant weight of all three species were significantly reduced, while for the two chlorophyll a-containing species, N. exigua and S. vagabunda, effective quantum yield (ΔF/Fm') of photosystem (PS) II, photosynthetic pigment concentrations, gross photosynthetic rate and gross photosynthesis to respiration (P:R) ratio were also reduced. Dark respiration rates were higher in all three species at elevated temperature. While these lagoon sponges appeared to be impacted by elevated temperature, here, we demonstrate that these species are physiologically tolerant to excess nitrate concentrations.

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'.

Sponge Behavior and the Chemical Basis of Responses: A Post-Genomic View

Sponges perceive and respond to a range of stimuli. How they do this is still difficult to pin down despite now having transcriptomes and genomes of an array of species. Here we evaluate the current understanding of sponge behavior and present new observations on sponge activity in situ. We also explore biosynthesis pathways available to sponges from data in genomes/transcriptomes of sponges and other non-bilaterians with a focus on exploring the role of chemical signaling pathways mediating sponge behavior and how such chemical signal pathways may have evolved. Sponge larvae respond to light but opsins are not used, nor is there a common photoreceptor molecule or mechanism used across sponge groups. Other cues are gravity and chemicals. In situ recordings of behavior show that both shallow and deep-water sponges move a lot over minutes and hours, and correlation of behavior with temperature, pressure, oxygen, and water movement suggests that at least one sponge responds to changes in atmospheric pressure. The sensors for these cues as far as we know are individual cells and, except in the case of electrical signaling in Hexactinellida, these most likely act as independent effectors, generating a whole-body reaction by the global reach of the stimulus to all parts of the animal. We found no evidence for use of conventional neurotransmitters such as serotonin and dopamine. Intriguingly, some chemicals synthesized by symbiont microbes could mean other more complex signaling occurs, but how that interplay might happen is not understood. Our review suggests chemical signaling pathways found in sponges do not reflect loss of a more complex set.

In situ determination of Si, N, and P utilization by the demosponge Tethya citrina: A benthic-chamber approach

Sponges consume dissolved silicon (DSi) to build their skeletons. Few studies have attempted to quantify DSi utilization by these organisms and all available determinations come from laboratory measurements. Here we measured DSi consumption rates of the sponge Tethya citrina in its natural habitat, conducting 24h incubations in benthic chambers. Sponges consumed DSi at an average rate of 0.046 ± 0.018 μmol h^-1 mL^-1 when DSi availability in its habitat was 8.3 ± 1.8 μM. Such DSi consumption rates significantly matched the values predicted by a kinetic model elsewhere developed previously for this species through laboratory incubations. These results support the use of laboratory incubations as a suitable approach to learn about DSi consumption. During the field incubations, utilization of other dissolved inorganic nutrients by this low-microbial-abundance (LMA) sponge was also measured. The sponges were net sources of ammonium (-0.043 ± 0.031 μmol h^-1 mL^-1), nitrate (-0.063 ± 0.031 μmol h^-1 mL^-1), nitrite (-0.007 ± 0.003 μmol h^-1 mL^-1), and phosphate (-0.004 ± 0.005 μmol h^-1 mL^-1), in agreement with the general pattern in other LMA species. The detected effluxes were among the lowest reported for sponges, which agreed with the low respiration rates characterizing this species (0.35 ± 0.11 μmol-O₂ h^-1 mL^-1). Despite relatively low flux, the dense population of T. citrina modifies the availability of dissolved inorganic nutrients in the demersal water of its habitat, contributing up to 14% of nitrate and nitrite stocks. Through these effects, the bottom layer contacting the benthic communities where siliceous LMA sponges abound can be partially depleted in DSi, but can benefit from inputs of N and P dissolved inorganic nutrients that are critical to primary producers.

Oxygen and the Energetic Requirements of the First Multicellular Animals

The appearance of multicellular animals during the Neoproterozoic Era is thought to have coincided with oxygenation of the oceans; however, we know little about the physiological needs of early animals or about the environment they lived in. Approaches using biomarkers, fossils, and phylogenomics have provided some hints of the types of animals that may have been present during the Neoproterozoic, but extant animals are our best modern links to the theoretical ancestors of animals. Neoproterozoic oceans were low energy habitats, with low oxygen concentrations and sparse food availability for the first animals. We examined tolerance of extant ctenophores and sponges-as representatives of extant lineages of the earliest known metazoan groups-to feeding and oxygen use. A review of respiration rates in species across several phyla suggests that suspension feeders in general have a wide range of metabolic rates, but sponges have some of the highest of invertebrates and ctenophores some of the lowest. Our own studies on the metabolism of two groups of deep water sponges show that sponges have different approaches to deal with the cost of filtration and low food availability. We also confirmed that deep water sponges tolerate periods of hypoxia, but at the cost of filtration, indicating that normal feeding is energetically expensive. Predictions of oxygen levels in the Neoproterozoic suggest the last common ancestor of multicellular animals was unlikely to have filtered like modern sponges. Getting enough food at low oxygen would have been a more important driver of the evolution of early body plans.

Hydrodynamics of the leucon sponge pump

Leuconoid sponges are filter-feeders with a complex system of branching inhalant and exhalant canals leading to and from the close-packed choanocyte chambers. Each of these choanocyte chambers holds many choanocytes that act as pumping units delivering the relatively high pressure rise needed to overcome the system pressure losses in canals and constrictions. Here, we test the hypothesis that, in order to deliver the high pressures observed, each choanocyte operates as a leaky, positive displacement-type pump owing to the interaction between its beating flagellar vane and the collar, open at the base for inflow but sealed above. The leaking backflow is caused by small gaps between the vaned flagellum and the collar. The choanocyte pumps act in parallel, each delivering the same high pressure, because low-pressure and high-pressure zones in the choanocyte chamber are separated by a seal (secondary reticulum). A simple analytical model is derived for the pump characteristic, and by imposing an estimated system characteristic we obtain the back-pressure characteristic that shows good agreement with available experimental data. Computational fluid dynamics is used to verify a simple model for the dependence of leak flow through gaps in a conceptual collar-vane-flagellum system and then applied to models of a choanocyte tailored to the parameters of the freshwater demosponge Spongilla lacustris to study its flows in detail. It is found that both the impermeable glycocalyx mesh covering the upper part of the collar and the secondary reticulum are indispensable features for the choanocyte pump to deliver the observed high pressures. Finally, the mechanical pump power expended by the beating flagellum is compared with the useful (reversible) pumping power received by the water flow to arrive at a typical mechanical pump efficiency of about 70%.

Hydrodynamic functionality of the lorica in choanoflagellates

Choanoflagellates are unicellular eukaryotes that are ubiquitous in aquatic habitats. They have a single flagellum that creates a flow toward a collar filter composed of filter strands that extend from the cell. In one common group, the loricate choanoflagellates, the cell is suspended in an elaborate basket-like structure, the lorica, the function of which remains unknown. Here, we use Computational Fluid Dynamics to explore the possible hydrodynamic function of the lorica. We use the choanoflagellate Diaphaoneca grandis as a model organism. It has been hypothesized that the function of the lorica is to prevent refiltration (flow recirculation) and to increase the drag and, hence, increase the feeding rate and reduce the swimming speed. We find no support for these hypotheses. On the contrary, motile prey are encountered at a much lower rate by the loricate organism. The presence of the lorica does not affect the average swimming speed, but it suppresses the lateral motion and rotation of the cell. Without the lorica, the cell jiggles from side to side while swimming. The unsteady flow generated by the beating flagellum causes reversed flow through the collar filter that may wash away captured prey while it is being transported to the cell body for engulfment. The lorica substantially decreases such flow, hence it potentially increases the capture efficiency. This may be the main adaptive value of the lorica.

Anatomy and ultrastructure of the tropical sponge Cladocroce caelum (Haplosclerida, Demospongiae)

The main characteristic of sponges (Porifera) is the presence of the aquiferous system-a system formed by canals and choanocyte chambers, in which the sponges carry out most of their physiological functions. Despite of the importance for the biology of the group, the knowledge about this structure is still incipient, even when morphological investigations are taken in account. Here, we investigated the anatomy and ultrastructure of the tropical demosponge Cladocroce caelum (Haplosclerida, Demospongiae) using light and electron microscopy. In the studied region, specimens of this species were repent or repent-branched, possessing one to several oscula. A uniform and reduced atrium was found just below each osculum. There was a thin ectosome and the choanosome presented meager mesohyl, but a high number of choanocyte chambers. The choanocyte chambers were rounded, and, as in other haplosclerids, they are found separated from the mesohyl by endopinacocytes, "hanging" in the inhalant canals. Even though the utility of the general organization of the aquiferous system has been advocated as a possible tool to understand the phylogeny of the group, we found that these characters might not be as useful as expected. The size of the particles ingested by the sponge and the amount of bacteria to sustain their bodies are discussed. In addition, we found that the density of choanocyte chambers was reduced when the specimens were carrying out the spermatogenesis, indicating that the reproduction may impair the filtering activity of the sponge. Our findings consist in a first step to better comprehend the physiology, development, and adaptation to the environmental conditions where the species is found.

First large-scale ecological impact study of desalination outfall reveals trade-offs in effects of hypersalinity and hydrodynamics

Desalination is an increasingly common method of meeting potable water demands, but the associated ecological risks are not well understood. Seawater desalination plants discharge large volumes of hypersaline brine directly into the ocean, raising concerns about potential impacts to marine life. In order to reduce impacts of brine, newer desalination outfalls are often fitted with high-pressure diffusers that discharge brine at high velocity into the water column, increasing the mixing and dilution of brine with ocean water. However, there are few published studies of marine impacts of desalination brine, and no well replicated before-after designs. Here we report a six-year study testing for impacts and subsequent recovery of sessile marine invertebrate recruitment near a desalination outfall with high-pressure diffusers. We used a Multiple Before-After-Control-Impact (MBACI) design to test for impacts and recovery at two distances (30 m and 100 m) from a 250 ML/day plant outfall, as well as a gradient design to test the strength of impacts relative to distance from the outfall. The diffusers achieved the target of less than 1 psμ salinity difference to surrounding ambient waters within 100 m of the discharge outfall, but sessile invertebrates were nonetheless impacted. Polychaetes, bryozoans and sponges reduced in cover as far as 100 m from the outfall, while barnacles showed the opposite pattern and were more abundant near the discharging outfall. Ecological impacts were disproportionate to the relatively minor change in salinity (∼1 psμ), suggesting a mechanism other than salinity. We propose that impacts were primarily driven by changes in hydrodynamics caused by the diffusers, such as higher near-bed flow away from the outfall. This is consistent with flow preferences of various taxonomic groups, which differ due to differences in settlement and feeding abilities. High-pressure diffusers designed to reduce impacts of hypersalinity may inadvertently cause impacts through hydrodynamics, leading to a trade-off in minimizing combined salinity and hydrodynamic stress. This study provides the first before-after test of ecological impacts of desalination brine on sessile marine communities, and rare insight into mechanisms behind impacts of a growing form of human disturbance.