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Demosponge

Demospongiae, commonly referred to as demosponges, represent the largest and most diverse within the Porifera, comprising over 90% of the approximately 9,800 known living (as of 2025). These sessile, are defined by their skeletons, which typically consist of siliceous spicules (with four axes or rays not at right angles) and/or organic spongin fibers, enabling a variety of growth forms from encrusting sheets to branching structures. Exhibiting the leucon grade of body organization, demosponges are efficient that pump water through specialized chambers to capture microscopic food particles, playing crucial roles in nutrient cycling and across marine and freshwater environments. The class Demospongiae is subdivided into three main subclasses: Keratosa, Verongimorpha, and Heteroscleromorpha, with the latter encompassing about 90% of demosponge diversity across 17 orders. Keratosa species feature skeletons primarily of spongin fibers, often lacking siliceous spicules, while Verongimorpha are distinguished by the absence of siliceous asters and the presence of unique brominated compounds in their spongin. Heteroscleromorpha, the most speciose subclass, display complex siliceous spicules including monaxons and tetraxons, along with diverse microscleres that aid in taxonomic identification. Recent phylomitogenomic studies have refined this , supporting a sister-group relationship between Heteroscleromorpha and the combined Keratosa-Verongimorpha clade, with divergences among major Heteroscleromorpha lineages tracing back to the Cambrian period. Biologically, demosponges are found in nearly all aquatic habitats, from intertidal zones and coral reefs to abyssal depths exceeding 8,000 meters, and they uniquely include all known freshwater species. Their reproduction is versatile, involving both asexual budding and sexual strategies that produce parenchymella larvae containing spongin, a feature distinguishing them from other classes. Ecologically, these organisms support by providing habitats for , filtering vast volumes of water daily, and contributing to carbon and nutrient dynamics in reefs and sediments. Evolutionarily, demosponges boast a rich fossil record from the onward, though soft-bodied forms are underrepresented due to preservation biases.

Description and Morphology

Physical Characteristics

Demospongiae represents the most diverse class within the Porifera, encompassing nearly 8,000 accepted and comprising over 90% of the approximately 9,760 valid extant worldwide. These s exhibit a wide array of external morphologies adapted to diverse substrates and flow regimes, including encrusting forms that thinly cover rocks or shells, tubular or finger-like projections, fan-shaped or branching structures, and massive or barrel-like bodies. Some achieve substantial sizes, with individuals exceeding 1 meter in and occasionally reaching over 2 meters in largest dimension. The external surface often features oscules for expulsion and may include surface projections or papillae that enhance filtration efficiency. Demosponges display vibrant coloration, ranging from reds, yellows, and oranges to blues, purples, and greens, primarily derived from pigments such as and melanins produced by the sponge itself or acquired through symbiotic relationships with like or dinoflagellates. These pigments, along with secondary metabolites like alkaloids and terpenoids, serve ecological roles including and UV protection. Certain exhibit environmental responsiveness in coloration; for instance, light availability influences the retention or expulsion of symbiotic , leading to shifts from to orange hues when symbionts are lost under low-light conditions. The predominant body plan in demosponges is leuconoid, characterized by a of canals and chambers that maximizes water flow through the body for feeding and . This architecture features incurrent canals leading to choanocyte chambers via prosopyles, with water exiting through excurrent canals and oscules, enabling efficient filtration in a matrix that supports cellular diversity. Some demosponge species exhibit exceptional ; for example, the (Xestospongia muta) is estimated to live up to 2,300 years based on models from size measurements. rates vary significantly by , remaining slow in cold, nutrient-limited deep waters—often less than 2 cm per year—compared to faster rates in warmer, shallow environments. A distinctive biochemical feature of demosponges is the of at the 26-position of the , producing compounds like 26-methylstigmastane, which is unique to this class among extant metazoans and serves as a for their evolutionary history. This modification arises from specialized sterol methyltransferase enzymes and is absent in other classes or most eukaryotes.

Skeletal Structure

The skeletons of demosponges are primarily composed of siliceous spicules, which are divided into megascleres (larger structural elements) and microscleres (smaller supportive elements), spongin (a collagenous protein ), or a combination of these materials. Siliceous spicules consist mainly of hydrated silica (), formed through the enzymatic action of silicatein within specialized sclerocytes, providing a rigid framework embedded in a matrix. Spongin, in contrast, forms flexible, fibrous networks that contribute to the overall elasticity and resilience of the sponge body. Demosponge spicules exhibit diverse shapes, including monaxons (single-axis forms such as styles, oxeas, and strongyles) and tetraxons (four-rayed forms like triaenes and calthrops), with megascleres typically forming the primary architectural supports and microscleres such as sigmas, chelae, and asters dispersed for reinforcement. In certain species, such as Haliclona penicillata and Phorbas areolatus, spicules incorporate recycled silica from ingested frustules; this process involves enzymatic disassembly by silicase in amoeboid cells followed by reassembly via silicatein, enabling efficient nutrient utilization in silica-rich polar waters. However, spicules are absent in orders like Keratosa (including Dendroceratida and Dictyoceratida), where the relies entirely on laminated spongin fibers—either homogeneous or with a pithy core—for flexibility and structural integrity, as seen in species like bath sponges. Spicules in demosponges are often arranged in tracts or bundles, with megascleres forming ascending or radial primary tracts (e.g., 2–8 spicules per bundle) that branch into secondary networks, enhancing load distribution and supporting varied body forms from encrusting to erect. This organization, combined with spongin cements, allows for architectural diversity while maintaining stability. Evolutionarily, silica-based spicules confer rigidity and toughness superior to the calcareous spicules of other sponge classes like Calcarea, adapting demosponges to the mechanical stresses of environments through exceptional and .

Habitat and Distribution

Marine Environments

Demosponges are predominantly organisms, inhabiting a wide array of benthic environments from intertidal zones to abyssal depths exceeding 6,000 meters. They exhibit the highest in tropical coral reefs and temperate rocky shores, where environmental conditions support prolific growth and structural complexity. In these habitats, demosponges contribute to architecture by encrusting or forming erect structures on various substrates. In deep-sea environments, demosponges demonstrate remarkable adaptations to low oxygen levels and high hydrostatic pressures, including reduced metabolic rates that conserve energy in nutrient-scarce conditions. These adaptations enable survival at depths up to 8,840 meters, the recorded depth limit for sponges, primarily through efficient and symbiotic microbial associations that enhance resilience. Such physiological adjustments allow demosponges to thrive in the cold, dark abyssal plains where water temperatures drop below and pressures exceed 600 atmospheres. Demosponges typically associate with hard substrates such as rocks, , and coral rubble, though some tolerate or even flourish in soft sediments by extending root-like holdfasts. In regions like the , certain demosponge form dense aggregations that contribute to reef framework stability by binding sediments and providing complexity. These associations underscore their role in stabilizing marine substrates across shallow to mesophotic zones. Demosponges exhibit broad tolerance to fluctuating salinities and temperatures, enabling of diverse settings from hypersaline lagoons to hyposaline coastal areas. Polar , such as those in waters, endure subzero temperatures and persist under seasonal cover, relying on proteins and low metabolic demands for survival. This thermal resilience contrasts with the rarer freshwater , which require specialized not typical of forms. Globally, demosponges display a , with hotspots concentrated in the and regions due to latitudinal gradients favoring in warm, stable waters. These areas reflect historical evolutionary pressures and current oceanographic connectivity.

Freshwater Species

Freshwater demosponges represent a small minority of the , with approximately 268 belonging exclusively to the monophyletic order Spongillida, which has colonized lentic and lotic freshwater systems worldwide. These inhabit rivers, lakes, and wetlands, with distributions primarily limited to temperate and tropical freshwater bodies across continents, including pancontinental ranges in genera like Ephydatia. For instance, exhibits a broad distribution from to , where it is among the most common freshwater sponges. Physiological adaptations enable these sponges to thrive in freshwater environments, including a strong capacity for to maintain internal ion balances against hypotonic conditions. A key adaptation is the formation of —dormant, resistant structures produced asexually that encapsulate undifferentiated cells to survive , freezing, and other harsh conditions such as winter extremes (detailed further in the section on ). These allow persistence through seasonal drying or low temperatures, facilitating recolonization in spring. In morphology, freshwater demosponges are generally smaller than their counterparts, which can reach several meters in size, with examples like forming clusters typically 30–55 cm in diameter and 30–45 cm in height. They often adopt encrusting growth forms on submerged plants, rocks, or other hard , adapting to the dynamic flow and substrate availability in freshwater habitats. These sponges face significant threats from pressures, particularly and habitat alteration, as their filter-feeding lifestyle exposes them to contaminants, leading to of metals and toxins. Such vulnerabilities have raised conservation concerns in regions with deteriorating , prompting calls for monitoring their populations as indicators of .

Classification and Systematics

Orders and Families

The modern taxonomic classification of Demospongiae is based on the revised system proposed by Morrow and Cárdenas in 2015, which integrates molecular phylogenetic data from markers such as 18S rRNA, 28S rRNA, and CO1 to recognize 22 orders grouped into three main subclasses: Verongimorpha (characterized by spongin with and bromotyrosine-derived compounds), Keratosa (featuring keratin-like spongin fibers without siliceous spicules), and the most diverse, Heteroscleromorpha (with varied siliceous spicules and spongin). This framework has been adopted by the World Porifera Database, which incorporates ongoing molecular analyses to refine family and genus assignments. Demospongiae includes over 7,000 described species organized into more than 80 families, accounting for approximately 85-90% of all extant sponge diversity. Among the key orders, Haplosclerida (Heteroscleromorpha) comprises common marine species with siliceous diactinal spicules (e.g., oxeas) forming an isodictyal choanosomal skeleton, often including accessory microscleres like sigmas. Dictyoceratida (Keratosa) is distinguished by an exclusively spongin fiber skeleton without siliceous spicules, exemplified by commercial bath sponges in families like Spongiidae. Poecilosclerida (Heteroscleromorpha) exhibits high diversity in microscleres such as chelae and sigmas, with regionally differentiated ectosomal and choanosomal skeletons. Significant historical changes include the 2012 elevation of Homoscleromorpha from a subclass within Demospongiae to a distinct , driven by genomic evidence revealing unique features like true epithelial-like cells and differences in developmental genes. Phylogenomic analyses using mitochondrial genomes and nuclear markers have since resolved earlier debates on polyphyletic origins, robustly confirming the of Demospongiae as a cohesive within Porifera. Groups like sclerosponges represent ongoing taxonomic challenges within this framework due to their hypercalcified skeletons.

Sclerosponges and Chaetetids

Sclerosponges, also known as coralline sponges, were originally classified in the as a distinct class Sclerospongiae due to their massive skeletons, which contrasted with the siliceous spicules typical of most demosponges. These skeletons often consist of deposited in organized layers, providing a hard basal structure covered by . Molecular and morphological analyses have since confirmed that sclerosponges are polyphyletic lineages nested within Demospongiae, with representatives such as Vaceletia crypta reclassified into the order Dictyoceratida in the subclass Keratosa, based on mitochondrial gene sequences and skeletal microstructure. Similarly, groups like stromatoporoids, previously considered separate, are now recognized as hypercalcified demosponges in orders such as Stromatoporida, featuring -based skeletons formed through similar processes. Chaetetids represent an extinct Paleozoic group, primarily from the Silurian to Carboniferous periods, characterized by dense assemblages of long, contiguous tubules that formed rigid, branching or massive structures. These tubules, often 0.6–1.2 mm in diameter, created a microstructure resembling flowing hair, with rare siliceous spicules embedded in some specimens, indicating a demosponge affinity. Reclassification as hypercalcified demosponges has been supported by detailed examinations of skeletal microstructure, including revealing pseudomorphs of monoaxon and polyaxon spicules, aligning them with modern Merliida taxa that retain chaetetid-like basal skeletons. Post-2015 molecular phylogenies, incorporating and mitochondrial genomes, have reinforced the placement of both sclerosponges and chaetetids within Demospongiae, demonstrating their and in hypercalcification with the class Calcarea through independent evolution of skeletons. Cladistic analyses of spicule morphology and genetic sequences resolve these groups into subclasses like Keratosa and Heteroscleromorpha, overturning earlier separations based solely on hard skeletal traits. This historical misclassification stemmed from the emphasis on versus siliceous skeletons, but integrated evidence now confirms their integration into demosponge . Ecologically, extant sclerosponges such as Ceratoporella nicholsoni in reefs contribute to carbonate production by cementing and stabilizing structures, forming encrusting layers that support bioherm development and nutrient cycling in mesophotic zones.

Reproduction and Development

Sexual Reproduction

Demosponges exhibit diverse sexual strategies, being either hermaphroditic—often simultaneous, with both male and female s produced concurrently—or gonochoristic, with separate sexes and no pronounced . In gonochoristic species, sex ratios frequently favor females, as observed in populations of subtidal tetractinomorph demosponges. Spermatocytes typically derive from choanocytes, the flagellated cells lining the aquiferous system, through a process where choanocytes transform into spermatogonia; in contrast, oocytes originate from archaeocytes, totipotent amoeboid cells within the . This ensures efficient gamete production integrated with the sponge's filter-feeding apparatus. Fertilization in demosponges is predominantly internal, occurring within the parental in viviparous , where sperm are drawn in through the pores and transported via choanocyte chambers to oocytes. This leads to the of parenchymella larvae, compact, ciliated structures characterized by an outer layer of flagellated cells for and an inner mass of cells including prospective choanocytes and archaeocytes. These larvae, measuring 100–500 μm in diameter in like Spongia officinalis, feature posterior tufts of longer cilia that enable swimming and dispersal over distances of several meters to kilometers. is prevalent, with embryos brooded internally until larval competence, as seen in ceractinomorph demosponges where larvae are released through the osculum after 2–4 weeks of . Ovipary, involving external egg release and fertilization, occurs less commonly, such as in some tetractinomorphs like Cinachyra tarentina. Post-release, parenchymella larvae exhibit phototactic behavior, often negative phototaxis guiding them toward shaded substrates, and respond to bacterial biofilms as inductive cues for settlement. In Ircinia felix, for instance, a photoreceptor organ with ciliated cells directs sinking to the seafloor, while microbial films on hard surfaces trigger metamorphosis, with settlement rates enhanced by specific bacteria like Pseudoalteromonas species. Upon attachment, larvae undergo rapid metamorphosis into juveniles, reorganizing cells to form a functional aquiferous system within hours to days, establishing the sessile adult form. Sexual reproduction in marine demosponges is typically seasonal, peaking in summer to autumn and synchronized with rising water temperatures (e.g., 18–25°C) and phytoplankton blooms that support gametogenesis, as documented in Mediterranean species like Tethya meloni. This timing optimizes larval survival and dispersal in nutrient-rich conditions.

Asexual Reproduction

Demosponges utilize as a key strategy for population maintenance and , particularly in environments subject to physical or seasonal fluctuations. This process generates clonal offspring through mechanisms such as , formation, and fragmentation, bypassing to retain identical genotypes and facilitate swift recovery from disturbances. Budding in demosponges involves the development of propagules—small aggregates of —that emerge externally or internally on the parent's surface and eventually detach to form independent individuals. External , common in marine species like Tethya wilhelma, proceeds through distinct stages: initial formation of a homogenous mass on a stalk, followed by into a globular structure with early aquiferous canals, then maturation of and choanosome layers, culminating in a functional juvenile after approximately 48 hours. Internal occurs less frequently but similarly produces propagules that migrate outward before detachment. These processes predominate in stable or moderately stressed habitats, allowing localized population expansion without reliance on external dispersal agents. In freshwater demosponges, particularly the family Spongillidae such as Ephydatia fluviatilis, serve as dormant, resistant structures for surviving harsh conditions like or freezing. form within the as clusters of totipotent archeocytes (thesocytes), which are binucleated s rich in nutrient reserves like vitelline platelets, encased in a protective spongin layer reinforced with silica spicules embedded in a chitin-spongin matrix. Upon return of favorable conditions, thesocytes undergo to yield uninucleated archeocytes that proliferate and differentiate into all necessary types, hatching a new without meiotic division. This adaptation ensures persistence in ephemeral aquatic environments, enabling rapid re-colonization. Fragmentation is prevalent among demosponges in turbulent habitats, where body parts break off due to wave action, predation, or storms and regenerate into complete individuals. In species like those in coral reefs, fragments as small as 1–2 mm can survive dispersal by currents and reorganize via archeocyte to restore full , including aquiferous systems and skeletal elements. This method supports rapid population recovery in dynamic coastal zones, with fragments often exhibiting high viability and initial growth rates that bolster clonal persistence.

Meiosis and Genetic Recombination

Demosponges, like other sponges in the phylum Porifera, possess a suite of conserved meiotic genes that facilitate the reduction division essential for sexual reproduction. Key genes such as SPO11, which initiates meiotic recombination by generating double-strand breaks, and DMC1, involved in homologous chromosome pairing and strand invasion, are present and expressed in demosponge genomes, including those of the haplosclerid Amphimedon queenslandica and the deep-sea species in the genus Geodia. Additional meiotic machinery components, such as MSH4, MSH5, MLH1, MLH3, RAD51, HOP1, HOP2, and SYCP1-3, are also identified in Geodia demosponges, indicating functional homologous recombination pathways similar to those in more complex eukaryotes. These genes' presence underscores the early eukaryotic origins of meiosis, with orthologs traceable to the last common ancestor of animals, predating the emergence of multicellularity in Porifera. During in demosponges, meiotic recombination occurs in the I stage, where homologous pair and exchange genetic material, promoting diversity in the resulting gametes. This process is regulated by genes like CCNA1, MNS1, and SMC proteins, which control meiotic progression and arrest, ensuring proper chromosome before fertilization. In ovoviviparous demosponge species that produce parenchymella larvae, recombination during contributes to in these free-swimming offspring, enhancing adaptability despite the larvae's brief dispersive phase. Genomic studies of demosponges reveal highly conserved despite their relatively simple body plan lacking true tissues. Transcriptomic analyses of Geodia species demonstrate upregulation of signaling genes (e.g., RDH, ALDH, RXR) that trigger entry into , alongside and recombination factors like RAD50, PMS2, and BRIP1, mirroring mechanisms. These findings, from five gonochoristic and oviparous Geodia demosponges, highlight the molecular complexity of in basal metazoans. The conservation of meiotic machinery in demosponges implies that , including recombination, evolved before multicellularity in the Porifera lineage, providing a foundational genetic system for animal diversification. This ancient toolkit supports the hypothesis that originated in unicellular ancestors and was co-opted for formation in early metazoans.

Ecology and Evolutionary History

Ecological Roles

Demosponges serve as vital in ecosystems, processing vast quantities of to remove , , and , thereby enhancing clarity and quality. A single kilogram of demosponge can between 15,000 and 24,000 liters of per day, capturing particles and microorganisms that would otherwise accumulate. This activity not only supports the sponges' but also reduces bacterial loads significantly; for instance, species like Aplysina aerophoba and Geodia cydonium can clear Vibrio parahaemolyticus from with retention efficiencies approaching 99.99% over 72 hours. Clearance rates for these demosponges reach up to 84.84 mL per hour per gram dry weight, demonstrating their efficiency in . As habitat providers, demosponges create complex three-dimensional structures that shelter a diverse array of marine organisms, including , crustaceans, polychaetes, and ophiuroids. Massive demosponges along Mediterranean coasts host up to 61 macrofaunal taxa, with mesophotic species supporting higher richness (48 taxa) due to greater structural complexity, while shallow-water sponges provide denser refugia for non-indigenous . Examples include the Synalpheus gambarelloides and Ophiactis savignyi inhabiting demosponge interiors, which act as protective "hotels" fostering in otherwise exposed benthic environments. Many demosponges also form symbiotic relationships with photosynthetic , such as or , which reside in their tissues and contribute to nutrient exchange via , particularly in sunlit tropical habitats. Demosponges play a key role in nutrient cycling by processing and excreting essential elements, influencing surrounding benthic communities. They recycle and through microbial symbionts, with high-microbial-abundance (HMA) species like Geodia spp. converting their intake into bioavailable forms that subsidize deep-sea food webs. In nutrient-limited environments, such as sponge grounds at 600–700 m depth, demosponges facilitate via microbial symbionts and silicon uptake, sequestering dissolved silicon into durable biogenic silica skeletons at rates of 0.90 mmol Si m⁻² day⁻¹ in systems, thereby linking , carbon, and cycles. This excretion of processed nutrients supports associated , including sea urchins and , by providing a steady supply of . Due to their constant filtration of ambient water, demosponges function as effective bioindicators of , accumulating contaminants like , , and toxins that reflect . Species such as Halichondria panicea retain pathogens and microparticles such as , making them reliable sentinels for monitoring levels in coastal and systems. Additionally, many demosponges produce allelochemicals that deter predators like and ; for example, 68% of Antarctic demosponge concentrate these defenses in their outer layers to protect against predation. In tropical reef ecosystems, demosponges contribute to structural integrity and deposition by stabilizing substrates and fostering microbial communities that promote binding. Hypercalcified demosponges deposit organized skeletons, aiding in framework consolidation alongside corals, while their filtration and detritus production enhance overall reef productivity.

Fossil Record and Evolution

The fossil record of Demospongiae provides evidence of their ancient origins, with the oldest direct body fossils dating to Stage 3, approximately 515 million years ago (Ma), including well-preserved specimens like Vauxia gracilenta from the that retain organic components such as . These early fossils indicate that demosponges were already diverse during the , contributing to the initial diversification of marine ecosystems. evidence extends their history further back into the , with sterane lipids suggestive of demosponge-like organisms appearing as early as ~760 Ma, though more robust chemical signatures, such as 24-isopropylcholestanes, are documented from ~635 Ma in . Recent analyses of C31 sterols, including 26-methylstigmastane, confirm demosponge presence in late sediments, predating the biota and supporting an animal origin before the glaciations. These s, unique to demosponges due to their unusual side-chain methylation, have been crucial for identifying sponges in the absence of body fossils, though some abiotic alterations can mimic them, necessitating careful validation. Demosponges underwent major evolutionary radiations during the and periods, with diversification accelerating in the early to mid- as evidenced by increasing genus richness in reefal deposits. Their skeletal structures, often composed of siliceous spicules or hypercalcified layers, played a key role in ancient reef building; stromatoporoid demosponges, for instance, dominated reefs, forming expansive microbial-sponge frameworks that peaked in abundance during the middle to late (~382–359 Ma). These fossils reveal paleoenvironmental insights, as variations in skeletal mineralogy—such as layers in hypercalcified forms like Astrosclera willeyana—correlate with fluctuations in . Such records highlight demosponges' adaptability to changing chemistry during the . Phylogenetically, Demospongiae occupy a basal position within Metazoa, consistently supported as the to all other in molecular analyses, with their affirmed by multi-gene datasets and mitochondrial phylogenomics. estimates, calibrated against and data, place the crown-group origin of Demospongiae in the (~800–650 Ma), aligning with geochemical evidence for early metazoan evolution and predating the diversification by hundreds of millions of years. This timeline underscores demosponges' role as living s, retaining primitive traits while radiating into the most species-rich class.

Economic and Scientific Importance

Commercial Uses

Demosponges of the family , particularly species rich in spongin such as and , are harvested commercially for their fibrous skeletons, which are processed into bath sponges prized for their durability, softness, and high water absorption capacity—up to 20–35 times their dry weight. These properties stem from the keratin-like protein that forms the skeletal framework, providing resilience against compression and repeated use. Harvesting has occurred in the Mediterranean and regions for over 5,000 years, with ancient and Romans employing sponges for personal hygiene, cleaning, and even as drinking utensils or in helmets. Traditional methods included free-diving with lead weights to depths of 1–40 meters, later supplemented by hook-and-line or glass-bottomed viewing devices from boats, though modern practices have shifted toward more regulated diving to minimize habitat damage. Aquaculture techniques for bath sponges have been developed since the late , with early experiments in the Mediterranean involving cultivation by attaching sponge fragments (explants) to substrates like ropes or trays at depths of 5–20 meters, allowing growth over 12–24 months before harvest. In the , similar low-cost methods using mesh panels or ropes have been trialed since the mid-20th century, particularly in areas like the and , to propagate species like Spongia barbara. The annual global trade in natural bath sponges, primarily from wild harvest and , is valued at several million U.S. dollars, with major markets in (approximately $6 million), the ($1–1.8 million), and ($0.75–2.2 million) based on late-20th-century data from the ; production totals around 200–300 metric tons annually, though re-exports inflate import figures. Recent market reports indicate the natural sponge sector remains small-scale with limited updated global volume data as of 2023. Sustainable has helped reduce pressure on wild populations by providing an alternative supply, with farms in the Mediterranean and yielding viable crops while preserving natural stocks depleted by historical overharvesting. Beyond marine species, freshwater demosponges such as those in the genus Metania are utilized in artisanal crafts, notably as a source of siliceous spicules for tempering in communities of the Bolivian , where the spicules enhance clay durability and reduce cracking during firing. Commercial sponge populations face significant challenges, including outbreaks—such as epizootics linked to bacterial —that have decimated yields in the Mediterranean since the , and climate-driven impacts like rising sea temperatures and extreme events that exacerbate stress and mortality in species like Spongia officinalis. compounds these issues, prompting calls for enhanced to ensure long-term viability.

Biomedical Applications

Demosponges represent a prolific source of bioactive metabolites, with over 7,000 compounds isolated from this class, many exhibiting pharmacological potential. These secondary metabolites, often produced by the sponges or their associated microbiomes, include diverse classes such as terpenoids, alkaloids, and peptides that demonstrate antiviral, anticancer, and activities. A notable example is avarol, a sesquiterpenoid derived from the Mediterranean demosponge Dysidea avara, which has shown potent antiviral effects against and in preclinical studies. Such compounds arise from the chemical diversity facilitated by the varied marine habitats of demosponges, contributing to their adaptation across coral reefs, deep-sea vents, and temperate coasts. Brominated compounds from marine demosponges, particularly in orders like Verongiida, exhibit strong antimicrobial properties that address challenges posed by antibiotic-resistant bacteria. These halogenated metabolites, such as aerothionin and homoaerothionin from Aplysina aerophoba, inhibit the growth of pathogens including methicillin-resistant Staphylococcus aureus and multidrug-resistant Pseudomonas aeruginosa. Their bioactivity stems from disrupting bacterial cell membranes and enzyme functions, offering leads for novel antibiotics in an era of rising resistance. In the 2020s, research has increasingly targeted and neuroprotective agents from demosponge microbiomes, highlighting their therapeutic promise for diseases. For instance, contignasterines, 2-aminoimidazole steroids isolated from Neopetrosia cf. rava in 2024, suppress ROS production and NO release in macrophages, reducing in models of neuroinflammatory disorders. Similarly, terpenoids like dysiherbols D–E from the demosponge Dysidea avara exhibit effects by inhibiting TNF-α-induced activation. More than 75 neuroactive compounds have been isolated from marine sponges and their associated bacteria, targeting pathways like inhibition for potential applications in Alzheimer's and Parkinson's. Despite their potential, low natural yields of these bioactive compounds—often less than 0.1% of —pose significant challenges for scalable , prompting the of systems. Demosponges like Halichondria panicea have been successfully cultivated in ex situ bioreactors and setups, achieving increases of up to 300% while maintaining metabolite profiles through controlled microbial symbioses. These methods address supply limitations without depleting wild populations. The evolutionary basis for demosponge chemical defenses lies in their sessile lifestyle, which exposes them to constant threats from predators, competitors, and organisms in benthic environments. This has driven the production of toxic or repellent metabolites as a primary , with many demosponge chemically defended to deter herbivory and microbial overgrowth.