Does the skeleton of a sponge provide a defense against predatory reef fish?

Sponge tissue often contains two structural components in high concentrations: spicules of silica, and refractory fibers of protein (spongin). Some terrestrial plants contain analogous structures, siliceous inclusions and refractory lignins, that have been demonstrated to deter herbivory. We performed feeding experiments with predatory reef fish to assess the deterrent properties of the structural components of three common Caribbean demosponges, Agelas clathrodes, Ectyoplasia ferox, and Xestospongia muta. The concentrations of spicules and spongin in the tissues varied widely between the three species, but when assayed at their natural volumetric concentrations, neither spicules (all three species assayed) nor the intact spiculated spongin skeleton (A. clathrodes and X. muta assayed) deterred feeding by reef fish in aquarium or field assays using prepared foods of a nutritional quality similar to, or higher than, that of sponge tissue. Spicules deterred feeding in aquarium assays when incorporated into prepared foods of a nutritional quality lower than that of sponge tissue (15–19 times less protein), but spiculated spongin skeleton was still palatable, even in prepared foods devoid of measurable protein, and even though spicules embedded in spongin were oriented in their natural conformation. Based on comparisons of the nutritional qualities of the tissues of the three sponge species and of the prepared foods, sponge tissue would have to be much lower in food value (5 times less protein or lower) for spicules to provide an effective defense, and spicules in combination with the spongin skeleton would be unlikely to provide an effective defense regardless of the nutritional quality of the tissue. Unlike terrestrial plants, marine sponges may use silica and refractory fibers solely for structural purposes.     

Skeleton construction upon local regression of the sponge body

We previously revealed that the mechanism of demosponge skeleton construction is self-organization by multiple rounds of sequential mechanical reactions of player cells. In these reactions, “transport cells” dynamically carry fine skeletal elements (spicules) on epithelia surrounding the inner body space of sponges (basal epithelium (basopinacoderm) and the endodermal epithelium (ENCM)). Once spicules pierce ENCM and apical pinacoderm, subsequently they are cemented to the substratum under the sponge body, or connected to other skeleton-constructing spicules. Thus, the “pierce” step is the key to holding up spicules in the temporary periphery of growing sponges’ bodies. Since sponges can regress as well as grow, here we asked how skeleton construction occurs during local regression of the body. We found that prior to local basopinacoderm retraction (and thus body regression), the body became thinner. Some spicules that were originally carried outward stagnated for a while, and were then carried inwards either on ENCM or basopinacoderm. Spicules that were carried inwards on ENCM pierced epithelia after a short transport, and thus became held up at relatively inward positions compared to spicules carried on outwardly extending basopinacoderm. The switch of epithelia on which transport cells migrate efficiently occurred in thinner body spaces where basopinacoderm and ENCM became close to each other. Thus, the mechanisms underlying this phenomenon are rather mechanical: the combination of sequential reactions of skeleton construction and the narrowed body space upon local retraction of basopinacoderm cause spicules to be held up at more-inward positions, which might strengthen the basopinacoderm's attachment to substratum.     

Intra-epithelial spicules in a homosclerophorid sponge

Attempts to understand the intricacies of biosilicification in sponges are hampered by difficulties in isolating and culturing their sclerocytes, which are specialized cells that wander at low density within the sponge body, and which are considered as being solely responsible for the secretion of siliceous skeletal structures (spicules). By investigating the homosclerophorid Corticium candelabrum, traditionally included in the class Demospongiae, we show that two abundant cell types of the epithelia (pinacocytes), in addition to sclerocytes, contain spicules intracellularly. The small size of these intracellular spicules, together with the ultrastructure of their silica layers, indicates that their silicification is unfinished and supports the idea that they are produced "in situ" by the epithelial cells rather than being incorporated from the intercellular mesohyl. The origin of small spicules that also occur (though rarely) within the nucleus of sclerocytes and the cytoplasm of choanocytes is more uncertain. Not only the location, but also the structure of spicules are unconventional in this sponge. Cross-sectioned spicules show a subcircular axial filament externally enveloped by a silica layer, followed by two concentric extra-axial organic layers, each being in turn surrounded by a silica ring. We interpret this structural pattern as the result of a distinctive three-step process, consisting of an initial (axial) silicification wave around the axial filament and two subsequent (extra-axial) silicification waves. These findings indicate that the cellular mechanisms of spicule production vary across sponges and reveal the need for a careful re-examination of the hitherto monophyletic state attributed to biosilicification within the phylum Porifera.     

Histological investigation of organisms with hard skeletons: a case study of siliceous sponges

Siliceous and calcareous sponges commonly are treated with acid to remove the spicules prior to embedding and cutting for histological investigations. Histology of spiculated sponge tissue represents a challenging problem in sponge histotechnology. Furthermore, fluorescence in situ hybridization (FISH), a key method for studying sponge-associated microbes, is not possible after acid treatment. For a broad range of siliceous sponge species, we developed and evaluated methods for embedding in paraffin, methylmethacrylate resins, LR White resin and cryomatrix. Different methods for cutting tissue blocks as well as mounting and staining sections also were tested. Our aim was to enable histological investigations and FISH without prior removal of the spicules. To obtain an overview of tissue and skeleton arrangement, we recommend embedding tissue blocks with LR White resin combined with en bloc staining techniques for large specimens with thick and numerous spicules, but paraffin embedding and subsequent staining for whole small specimens. For FISH on siliceous sponges, we recommend Histocryl embedding if the spicule content is high, but paraffin embedding if it is low. Classical histological techniques are used for detailed tissue examinations.     

Histological investigation of organisms with hard skeletons: a case study of siliceous sponges.

Siliceous and calcareous sponges commonly are treated with acid to remove the spicules prior to embedding and cutting for histological investigations. Histology of spiculated sponge tissue represents a challenging problem in sponge histotechnology. Furthermore, fluorescence in situ hybridization (FISH), a key method for studying sponge-associated microbes, is not possible after acid treatment. For a broad range of siliceous sponge species, we developed and evaluated methods for embedding in paraffin, methylmethacrylate resins, LR White resin and cryomatrix. Different methods for cutting tissue blocks as well as mounting and staining sections also were tested. Our aim was to enable histological investigations and FISH without prior removal of the spicules. To obtain an overview of tissue and skeleton arrangement, we recommend embedding tissue blocks with LR White resin combined with en bloc staining techniques for large specimens with thick and numerous spicules, but paraffin embedding and subsequent staining for whole small specimens. For FISH on siliceous sponges, we recommend Histocryl embedding if the spicule content is high, but paraffin embedding if it is low. Classical histological techniques are used for detailed tissue examinations.     

深海六放海绵大骨针的结构与特性

在海绵动物(多孔动物)中,六放海绵和寻常海绵为硅质骨骼.生活在深海(1 000 m)中的六放海绵是最古老的海绵动物,其中间单根海绵和春氏单根海绵有长达3 m的骨针,是地球上最长的生物硅结构.利用电子显微技术观测, 这些直径达8 mm的巨大根须骨针具有同心层状结构,其横截面显示明显的构造分界:中间为含有轴丝的轴管,外围是一50-150 μm厚的轴筒,最外面为区状区(300-500层,每层厚度3-5 μm).生物化学研究显示其主要的蛋白质为35 kD大分子,另外,还检测到23-24 kD 多肽,可能是硅蛋白相关蛋白.依据现有的红血球凝聚活性,从骨针提取物中也检测到了凝集素.由电子探针获得其化学成分主要为Si,K和Na.此外,骨针的光传输实验表明,该巨大根须骨针用作光纤可传输600 nm至1 400 nm范围的光,而滤掉小于600 nm的光(类似高通滤波器)和大于1 400 nm 的红外光(类似低通滤波器).另外,从六放海绵的空囊泡沫海绵中分离出一个基因并确证了其推导的编码蛋白序列,该蛋白编码一个光裂合酶相关蛋白,蛋白相似性比较结果显示属于光裂合酶相关蛋白中多细胞动物隐色素一类.基于以上数据给出了六放海绵硅质骨针形成的示意图.另外,由单根海绵骨针可作为波导传输光/电和/或化学信号,推断在海绵动物中有类似神经系统的网络系统[动物学报 53(3):557-569,2007].     

Hierarchical assembly of the siliceous skeletal lattice of the hexactinellid sponge Euplectella aspergillum

Despite its inherent mechanical fragility, silica is widely used as a skeletal material in a great diversity of organisms ranging from diatoms and radiolaria to sponges and higher plants. In addition to their micro- and nanoscale structural regularity, many of these hard tissues form complex hierarchically ordered composites. One such example is found in the siliceous skeletal system of the Western Pacific hexactinellid sponge, Euplectella aspergillum. In this species, the skeleton comprises an elaborate cylindrical lattice-like structure with at least six hierarchical levels spanning the length scale from nanometers to centimeters. The basic building blocks are laminated skeletal elements (spicules) that consist of a central proteinaceous axial filament surrounded by alternating concentric domains of consolidated silica nanoparticles and organic interlayers. Two intersecting grids of non-planar cruciform spicules define a locally quadrate, globally cylindrical skeletal lattice that provides the framework onto which other skeletal constituents are deposited. The grids are supported by bundles of spicules that form vertical, horizontal and diagonally ordered struts. The overall cylindrical lattice is capped at its upper end by a terminal sieve plate and rooted into the sea floor at its base by a flexible cluster of barbed fibrillar anchor spicules. External diagonally oriented spiral ridges that extend perpendicular to the surface further strengthen the lattice. A secondarily deposited laminated silica matrix that cements the structure together additionally reinforces the resulting skeletal mass. The mechanical consequences of each of these various levels of structural complexity are discussed.     

The dorid nudibranchs Peltodoris lentiginosa and Archidoris odhneri as predators of glass sponges

The dorid nudibranchs Peltodoris lentiginosa and Archidoris odhneri were found on glass sponges (Porifera, Hexactinellida) during remotely operated vehicle surveys of three reefs in the Strait of Georgia, British Columbia, Canada. Eight nudibranchs were sampled from 2009 to 2011. Identification of sponge spicules found in their gut and fecal contents confirmed the nudibranchs to be predators of the reef‐forming hexactinellids Aphrocallistes vastus and Heterochone calyx, as well as of the demosponge Desmacella austini, which encrusts skeletons of the glass sponges. Four of five nudibranchs dissected for gut content analysis had stomachs containing sponge spicules. Counts from high‐definition video footage taken during systematic surveys done in 2009 showed that nudibranchs were found in only two of the three glass sponge reefs. These data provide the first quantitative evidence of a molluscan predator on glass sponges found outside of Antarctica, and establish the first trophic link between glass sponges and their associated community of animals in a sponge reef ecosystem on the western Canadian continental shelf.     

Indigenous demosponge spicules in a Late Devonian stromatoporoid basal skeleton from the Frasnian of Belgium

This paper records the first example of a demosponge spicule framework in a single specimen of a Devonian stromatoporoid from the Frasnian of southern Belgium. The small sample (2.5 × 2 cm) is a component in a brecciated carbonate from a carbonate mound in La Boverie Quarry 30 km east of Dinant. Because of the small size of the sample, generic identification is not confirmed, but the stromatoporoid basal skeleton is similar to the genus Stromatopora. The spicules are arranged in the calcified skeleton, but not in the gallery space, and are recrystallized as multi‐crystalline calcite. The spicules fall into two size ranges: 10–20 μm diameter and 500–2000 μm long for the large ones and between 5–15 μm diameter and 50–100 μm length for the small ones. In tangential section, the spicules are circular, they have a simple structure, and no axial canal has been preserved. The large spicules are always monaxons, straight or slightly curved styles or strongyles. The spicules most closely resemble halichondrid/axinellid demosponge spicules and are important rare evidence of the existence of spicules in Palaeozoic stromatoporoids, reinforcing the interpretation that stromatoporoids were sponges. The basal skeleton may have had an aragonitic spherulitic mineralogy. Furthermore, the spicules indicate that this stromatoporoid sample is a demosponge.     

Calcification processes of siliceous sponges in Viséan Limestones (Counties Sligo and Leitrim,Northwestern Ireland)

Summary In the Lower Carboniferous limestones and shales of the Benbulben Range, Counties Sligo and Leitrim, northwestern Ireland, a suite of carbonate nodules, about 1 to 4 cm in diameter, has been sampled and investigated by thin sectioning. The nodules consist of micritic, peloidal and fenestral fabrics. Many of them contain relics of desma bearing demosponges and hexactinellid sponge skeletons. The nodules are interpreted as calcified siliceous sponges. Micrite and peloids have been formed via microbial activity during the decay of the soft sponge tissue. The actual processes are deduced from Recent examples investigated at Lizard Island, Autralia, byReitner (1993). The skeletal opal was dissolved very early. In places where the skeleton was already embedded in micrite the spicules are preserved as molds cemented by granular ferroan calcite. The nodules were extensively inhabited by agglutinating polychaetes and bored by sponges. Micrite clasts have been exported to the surrounding seafloor before the sponges were completely covered by sediments. Fenestral fabrics represent primary sponge cavities, that may be enlarged due to volume reduction of the soft tissue during calcification. Some originated from non-calcification of decaying tissue. The granular calcite cement, filling the fenestral fabrics, contains relics of spicules and faintly visible peloids floating unsupportedly in the cement. These peloids were probably produced in situ by calcification of organic mucilages that filled the cavities almost entirely. It is evident that most diagenetic processes occurring within the sponges happened on the seafloor, most likely within the still living individuals. Possibly the nodules represent a precursor stage of mud mound development.     

Middle and Late Cambrian sponge spicules from Hunan, China

Abundant and well-preserved assemblages of disarticulated sponge spicules occur in Middle and Late Cambrian platform carbonates of western Hunan, China. Assemblages recovered from 11 stratigraphic horizons include calcisponges, demosponges, and hexactinellids. Hexactinellida, in particular, are both abundant and diverse in Upper Cambrian carbonates. Comparison with spicule assemblages from Australia indicates that many of these taxa have long stratigraphic ranges, limiting their use in correlation. The morphological diversity of these spicules exceeds that known for living siliceous sponges, supporting the observation that during the Cambrian radiation, sponges, like other metazoans, evolved a variety of architectural forms not observed in later periods. Like conodonts, individual sponges can produce more than one spicule form; thus, an "apparatus genus" concept based on multiple co-occurring elements may eventually prove useful in the biostratigraphic and paleobiological interpretation of disarticulated sponge spicules. Four distinctive forms are recognized as new taxa: Australispongia sinensis new genus and species, Flosculus gracilis new genus and species, Pinnatispongia bengtsoni new genus and species, and Nabaviella paibiensis new species.     

Light inside sponges

Sponges are the most basal metazoan organisms. As sessile filter feeders in marine or freshwater habitats, they often live in close association with phototrophic microorganisms. Active photosynthesis by the associated microorganisms has been believed to be restricted to the outer tissue portion of the sponge hosts. However, phototrophic microorganisms have also been detected in deeper tissue regions. In many cases they are found around spicules, siliceous skelettal elements of demosponges and hexactinellids. The finding of phototrophic organisms seemingly assembled around spicules led to the hypothesis of a siliceous light transmission system in sponges. The principle ability to conduct light was already shown for sponge derived, explanted spicules. However it was not shown until now, that in deed sponges have a light transmission system, and can harbour photosynthetically active microorganisms in deeper tissue regions.Here we show for the first time, that, as hypothesized 13 year ago, sponge spicules in living specimens transmit light into deeper tissue regions. Our results demonstrate that in opposite to the actual opinion, photosynthetically active microorganisms can also live in deeper tissue regions, and not only directly beneath the surface, when a light transmission system (spicules) is present.Our results show the possibility of massive or globular sponges being supplied with photosynthetic products or pathways throughout their whole body, implying not only a more important role of these endobioses. Our findings also elucidate the in-situ function of a recently more and more interesting biomaterial, which is unique not only for its mechanical, electrical and optical properties. Biosilica is of special interest for the possibility to produce it enzymatically under environmental conditions.     

Morphological,optical, and structural characteristics of the spicules of glass sponges and the photoreceptor hypothesis of their survival

The morphology, structure, and optical characteristics of the spicules of some marine glass sponges have been studied. The results obtained are interpreted from the point of view of their possible role in the vital provision of the sponge organism.     

Evidence for spicule homology in calcareous and siliceous sponges: biminerallic spicules in Lenica sp. from the Early Cambrian of South China

Botting, J.P., Muir, L.A., Xiao, S., Li, X. & Lin, J.‐P. 2012: Evidence for spicule homology in calcareous and siliceous sponges: biminerallic spicules in Lenica sp. from the Early Cambrian of South China. Lethaia, Vol. 45, pp. 463–475. The relationships of the extant sponge classes, and the nature of the last common ancestor of all sponges, are currently unclear. Early sponges preserved in the fossil record differ greatly from extant taxa, and therefore information from the fossil record is critical for testing hypotheses of sponge phylogenetic relationships that are based on modern taxa. New specimens of the enigmatic sponge Lenica sp., from the Early Cambrian Hetang Biota of South China, exhibit an unusual spicule structure. Each spicule consists of a siliceous core with an axial canal, an organic outer layer and a middle layer interpreted to have been originally calcium carbonate. This finding confirms previous work suggesting the existence of biminerallic spicules in early sponges. Combined with data from other early sponges, the new findings imply that the two fundamental spicule structures of modern sponges were derived from a compound, biminerallic precursor. Spicules are therefore homologous structures in Calcarea and Silicea, and if sponges are paraphyletic with respect to Eumetazoa, then spicules may also have been a primitive feature of Metazoa. □Calcarea, Early Cambrian, Hetang Biota, phylogeny, Silicea, taphonomy.     

Bathyal sponges from the late Early Miocene of the Vienna Basin (central Paratethys,Slovakia)

Here we report, for the first time, a very rich and diversified sponge assemblage from late Early Miocene deposits of a central part of the Vienna Basin (Paratethys) in Slovakia. Bodily preserved sponges are described as a new genus and species Paracinachyrella fossilis (Tetiliidae, Demospongiae). Dissociated spicules reveal the presence of the “soft” demosponges that belong to families Tetillidae, Theneidae, Geodiidae, Samidae, Thrombidae, Thoosidae, Agelasidae, Myxillidae, Bubaridae, and Tedaniidae, the lithistid family Pleromidae, and an undetermined rhizoclone-bearing lithistid. Fragments of dictyonal skeleton indicate the presence of hexactinellid sponges that belong to the families Farreidae and Euretidae, and lychniscosan sponges. We estimate that at least 16–19 different species of siliceous sponges inhabited this region of the Central Paratethys during the latest Burdigalian. Most of these sponges are reported for the first time from the Miocene of the Paratethys. This sponge fauna has clear Tethyan affinities and indicates the existence of connection between Paratethys and Tethys during the latest Burdigalian, as well as the presence of open marine, deep-water, bathyal conditions in this part of the Vienna Basin.     

Carbonaceous preservation of Cambrian hexactinellid sponge spicules

Early fossil sponges offer a direct window onto the evolutionary emergence of animals, but insights are limited by the paucity of characters preserved in the conventional fossil record. Here, a new preservational mode for sponge spicules is reported from the lower Cambrian Forteau Formation (Newfoundland, Canada), prompting a re-examination of proposed homologies and sponge inter-relationships. The spicules occur as wholly carbonaceous films, and are interpreted as the remains of robust organic spicule sheaths. Comparable sheaths are restricted among living taxa to calcarean sponges, although the symmetries of the fossil spicules are characteristic of hexactinellid sponges. A similar extinct character combination has been documented in the Burgess Shale fossil Eiffelia. Interpreting the shared characters as homologous implies complex patterns of spicule evolution, but an alternative interpretation as convergent autapomorphies is more parsimonious. In light of the mutually exclusive distributions of these same characters among the crown groups, this result suggests that sponges exhibited an early episode of disparity expansion followed by comparatively constrained evolution, a pattern shared with many other metazoans but obscured by the conventional fossil record of sponges.     

Early sponge evolution: A review and phylogenetic framework

Sponges are one of the critical groups in understanding the early evolution of animals. Traditional views of these relationships are currently being challenged by molecular data, but the debate has so far made little use of recent palaeontological advances that provide an independent perspective on deep sponge evolution. This review summarises the available information, particularly where the fossil record reveals extinct character combinations that directly impinge on our understanding of high-level relationships and evolutionary origins. An evolutionary outline is proposed that includes the major early fossil groups, combining the fossil record with molecular phylogenetics. The key points are as follows. (1) Crown-group sponge classes are difficult to recognise in the fossil record, with the exception of demosponges, the origins of which are now becoming clear. (2) Hexactine spicules were present in the stem lineages of Hexactinellida, Demospongiae, Silicea and probably also Calcarea and Porifera; this spicule type is not diagnostic of hexactinellids in the fossil record. (3) Reticulosans form the stem lineage of Silicea, and probably also Porifera. (4) At least some early-branching groups possessed biminerallic spicules of silica (with axial filament) combined with an outer layer of calcite secreted within an organic sheath. (5) Spicules are homologous within Silicea, but also between Silicea and Calcarea, and perhaps with Homoscleromorpha. (6) The last common ancestor of extant sponges was probably a thin-walled, hexactine-bearing sponge with biminerallic spicules. (7) The stem group of sponges included tetraradially-symmetric taxa that grade morphologically into Cambrian fossils described as ctenophores. (8) The protomonaxonid sponges are an early-branching group, probably derived from the poriferan stem lineage, and include the problematic chancelloriids as derived members of the piraniid lineage. (9) There are no definite records of Precambrian sponges: isolated hexactine-like spicules may instead be derived from radiolarians. Early sponges had mineralised skeletons and thus should have a good preservation potential: the lack of sponge fossils in Precambrian strata may be due to genuine absence of sponges. (10) In contrast to molecular clock and biomarker evidence, the fossil record indicates a basal Cambrian diversification of the main sponge lineages, and a clear relationship to ctenophore-like ancestors. Overall, the early sponge fossil record reveals a diverse suite of extinct and surprising character combinations that illustrate the origins of the major lineages; however, there are still unanswered questions that require further detailed studies of the morphology, mineralogy and structure of early sponges.     

First evidence of the presence of chitin in skeletons of marine sponges. Part II. Glass sponges (Hexactinellida: Porifera)

Sponges (Porifera) are presently gaining increased scientific attention because of their secondary metabolites and specific skeleton structures. In contrast to demosponges, whose skeletons are formed from biopolymer spongin, glass sponges (hexactinellids) possess silica-organic composites as the main natural material for their skeletal fibres. Chitin has a crystalline structure and it constitutes a network of organized fibres. This structure confers rigidity and resistance to organisms that contain it, including monocellular (yeast, amoeba, diatoms) and multicellular (higher fungi, arthropods, nematodes, molluscs) organisms. In contrast to different marine invertebrates whose exoskeletons are built of chitin, this polysaccharide has not been found previously as an endogenous biopolymer within glass sponges (Hexactinellida). We hypothesized that glass sponges, which are considered to be the most basal lineage of multicellular animals, must possess chitin. Here, we present a detailed study of the structural and physico-chemical properties of skeletal fragments of the glass sponge Farrea occa. We show that these fibres have a layered design with specific compositional variations in the chitin/silica composite. We applied an effective approach for the demineralization of glass sponge skeletal formations based on an etching procedure using alkali solutions. The results show unambiguously that alpha-chitin is an essential component of the skeletal structures of Hexactinellida. This is the first report of a silica-chitin's composite biomaterial found in nature. From this perspective, the view that silica-chitin scaffolds may be key templates for skeleton formation also in ancestral unicellular organisms, rather than silica-protein composites, emerges as a viable alternative hypothesis.     

Multi-scale mineralogical characterization of the hypercalcified sponge Petrobiona massiliana (Calcarea, Calcaronea)

The massive basal skeleton of a few remnant living hypercalcified sponges rediscovered since the 1960s are valuable representatives of ancient calcium carbonate biomineralization mechanisms in basal Metazoa. A multi-scale mineralogical characterization of the easily accessible Mediterranean living hypercalcified sponge belonging to Calcarea, Petrobiona massiliana (Vacelet and Lévi, 1958), was conducted. Oriented observations in light and electron microscopy of mature and growing areas of the Mg-calcite basal skeleton were combined in order to describe all structural levels from the submicronic to the macroscopic scale. The smallest units produced are ca. 50–100 nm grains that are in a mushy amorphous state before their crystallization. Selected area electron diffraction (SAED) further demonstrated that submicronic grains are assembled into crystallographically coherent clusters or fibers, the latter are even laterally associated into single-crystal bundles. A model of crystallization propagation through amorphous submicronic granular units is proposed to explain the formation of coherent micron-scale structural units. Finally, XRD and EELS analyses highlighted, respectively, inter-individual variation of skeletal Mg contents and heterogeneous spatial distribution of Ca ions in skeletal fibers. All mineralogical features presented here cannot be explained by classical inorganic crystallization principles in super-saturated solutions, but rather underlined a highly biologically regulated formation of the basal skeleton. This study extending recent observations on corals, mollusk and echinoderms confirms that occurrence of submicronic granular units and a possible transient amorphous precursor phase in calcium carbonate skeletons is a common biomineralization strategy already selected by basal metazoans.