Insights into the structure and morphogenesis of the giant basal spicule of the glass sponge Monorhaphis chuni

Abstract

Background A basal spicule of the hexactinellid sponge Monorhaphis chuni may reach up to 3 m in length and 10 mm in diameter, an extreme case of large spicule size. Generally, sponge spicules are of scales from micrometers to centimeters. Due to its large size many researchers have described its structure and properties and have proposed it as a model of hexactinellid spicule development. Thorough examination of new material of this basal spicule has revealed numerous inconsistencies between our observations and earlier descriptions. In this work, we present the results of detailed examinations with transmitted light and epifluorescence microscopy, SEM, solid state NMR analysis, FTIR and X-ray analysis and staining of Monorhaphis chuni basal spicules of different sizes, collected from a number of deep sea locations, to better understand its structure and function. Results Three morphologically/structurally different silica layers i.e. plain glassy layer (PG), tuberculate layer (TL) and annular layer (AL), and an axial cylinder (AC) characterize adult spicules. Young, immature spicules display only plain glassy silica layers which dominate the spicule volume. All three layers i.e. PG, TL and AL can substitute for each other along the surface of the spicule, but equally they are superimposed in older parts of the spicules, with AL being the most external and occurring only in the lower part of the spicules and TL being intermediate between AL and PG. The TL, which is composed of several thinner layers, is formed by a progressive folding of its surface but its microstructure is the same as in the PG layer (glassy silica). The AL differs significantly from the PG and TL in being granular and porous in structure. The TL was found to display positive structures (tubercles), not depressions, as earlier suggested. The apparent perforated and non-perforated bands of the AL are an optical artefact. The new layer type that we called the Ripple Mark Layer (RML) was noted, as well as narrow spikes on the AL ridges, both structures not reported earlier. The interface of the TL and AL, where tubercles fit into depressions of the lower surface of the AL, represent tenon and mortise or dovetail joints, making the spicules more stiff/strong and thus less prone to breaking in the lower part. Early stages of the spicule growth are bidirectional, later growth is unidirectional toward the spicule apex. Growth in thickness proceeds by adding new layers. The spicules are composed of well condensed silica, but the outermost AL is characterized by slightly more condensed silica with less water than the rest. Organics permeating the silica are homogeneous and proteinaceous. The external organic net (most probably collagen) enveloping the basal spicule is a structural element that bounds the sponge body together with the spicule, rather than controlling tubercle formation. Growth of various layers may proceed simultaneously in different locations along the spicule and it is sclerosyncytium that controls formation of silica layers. The growth in spicule length is controlled by extension of the top of the axial filament that is not enclosed by silica and is not involved in further silica deposition. No structures that can be related to sclerocytes (as known in Demospongiae) in Monorhaphis were discovered during this study. Conclusions Our studies resulted in a new insight into the structure and growth of the basal Monorhaphis spicules that contradicts earlier results, and permitted us to propose a new model of this spicule’s formation. Due to its unique structure, associated with its function, the basal spicule of Monorhaphis chuni cannot serve as a general model of growth for all hexactinellid spicules.

Combined complementary imaging techniques in morphological analysis of Spongilla lacustris (Linnaeus 1759)

“…Visualization 3D shows that collagen-like surface envelops skeletal elements and gemmules. The organic net enveloping the spicule is an important, structural element that bounds the sponge body together with the spicule (Pisera et al 2021). As we showed, the grids were supported by bundles of spicules that form vertical, horizontal and diagonal struts.…”

“…Spicules diameter diminishes toward the ending. According to Pisera et al (2021) growth in thickness of the spicules of Monorhaphis chuni is bidirectional and proceeds from the spicule center to the spicule tips proceed by adding new layers.…”

The terminology of sponge spicules

“…Collagen or chitin matrix may be also present inside the silica spicules (Ehrlich et al, 2007(Ehrlich et al, , 2017(Ehrlich et al, , 2018Fromont et al, 2019;Pisera et al, 2021). So far, the axial filament has never been observed in the spicules of Calcarea (Uriz, 2006).…”

“…Siliceous spicules are biocomposites that incorporate organic material in their structure—a special protein complex forming an axial filament situated in the interior of most siliceous spicules (Ehrlich et al, 2022; Görlich et al, 2020; Uriz, 2006; Uriz et al, 2003). Collagen or chitin matrix may be also present inside the silica spicules (Ehrlich et al, 2007, 2017, 2018; Fromont et al, 2019; Pisera et al, 2021). So far, the axial filament has never been observed in the spicules of Calcarea (Uriz, 2006).…”

Arrested in Glass: Actin within Sophisticated Architectures of Biosilica in Sponges

“…[ 5,6 ] The inorganic chemistry of poriferan skeletons with respect to silica in Hexactinellida, Demospongiae, and Homoscleromorpha, or calcium carbonates in Calcarea, is well understood, but the role of the bio‐organic phases in skeletogenesis of sponges is still debated. [ 7–10 ] Silica‐based structures in sponges have several roles, including protection, support of the body form, maintenance of posture in flow, as well as anchoring to sandy and muddy bottoms. The main players in poriferan biosilicification and spiculogenesis are low molecular weight proteins (i.e., silicateins, cathepsins) [ 10–12 ] in demosponges, accompanied by glassin, [ 13 ] collagen, [ 14 ] and chitin [ 15,16 ] in glass sponges (Hexactinellida).…”