Larouche-Bilodeau C and Cameron CB , Acorn worm ossicle ultrastructure and compositi...

ECB-ART-50876

R Soc Open Sci null;99:220773. doi: 10.1098/rsos.220773.

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Acorn worm ossicle ultrastructure and composition and the origin of the echinoderm skeleton.

Larouche-Bilodeau C , Cameron CB .

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Here, we describe the shape and mineral composition of ossicles from eight acorn worm species, bringing the total known biomineralizing enteropneusts to 10 and confirming that ossicles are widespread in Enteropneusta. Three general forms were identified including a globular form that occurs in all three major enteropneust families. The biomineral compositions included all three polymorphs of calcium carbonate; calcite, aragonite and vaterite, and low to high magnesium concentrations. Calcite was the most common and characteristic of echinoderm ossicles. Based on these findings we hypothesize that an enteropneust-like ancestor to the Ambulacraria had ectodermal ossicles, formed in an extracellular occluded space bordered by a sheath of sclerocyte cells. The ossicles were microscopic, monotypic globular shaped, calcite ossicles with low to high Mg content and MSP130 proteins. The ossicles lacked intercalation with other ossicles. The function of acorn worm ossicles is unknown, but the position of ossicles in the trunk epithelia and near to the surface suggests predator deterrence, to provide grip on the walls of a burrow or tube, as storage of metabolic waste, or to regulate blood pH, rather than as an endoskeleton function seen in fossil and crown group Echinodermata.

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	Figure 1. . Photographs of acorn worms analysed in this study. (a) Saccoglossus kowalevskii. (b) Saccoglossus pusillus. (c) Harrimania planktophilus. (d) Protoglossus graveolens. (e) Schizocardium californicum. (f) Glossobalanus berkeleyi. (g) Balanoglossus occidentalis. (c, collar; dt, dark trunk; gs, gill slits; h, hepatic sacs; pt, pale trunk; pr, proboscis) (a,b,c,d,e,g) were photographed live, and (f) was fixed and stored in ethanol.
	Figure 2. . Scanning electron micrographs of ossicles from Saccoglossus kowalevskii. (a) Side view of a typical double-broccoli ossicle formed of a prismatic shaft (s) and two terminal lobes (t). (b) Close-up on a terminal lobe showing the arrangement and spacing of trabeculae (tb). The trabeculae end in a point (p). (c) Broken terminal lobes showing the brick-like arrangement of the cortex (c) and the porous medulla (m). (d) Side view of a typical prism ossicle formed of a prismatic shaft (s) and pyramidal tips (t). Most of the ossicle is the shaft. The surface of both the shaft and tip is porous. (e,f) Broccoli ossicle with small and large terminal lobes showing the variation in size.
	Figure 3. . Scanning electron micrographs of ossicles from Saccoglossus pusillus. (a) Lone terminal lobe from a broccoli ossicle. (b) Close up on the same terminal lobe showing the arrangement and spacing of trabeculae (tb). The trabeculae end in a low point (p) and are rounded in cross-section. (c) Angled view of a typical prism ossicle. The triangular faces (tf) are rough, and the rectangular faces (rf) are wrinkly. This ossicle has no crack. (d) Prism ossicle with pores (p). Typical ossicle with a squared prism and pyramidal ends. (e) Prism ossicle with a bowtie-shaped crack. The crack shows a dense cortex (c) and a porous medulla (m).
	Figure 4. . Scanning electron micrographs of ossicles from Harrimania planktophilus. (a) Sideview of a typical ossicle (m, medial lobe; s, shaft; t, terminal lobe). (b) Close-up on the shaft region of (a) showing its laminar organization. (c) The largest ossicle found, showing that the shaft is completely outgrown by medial lobes. (d) The structure of medial and terminal lobes are identical. Individual trabeculae are indistinguishable. (e) Broken terminal lobe of the ossicle shown in (d). There are no pores inside the ossicle.
	Figure 5. . Scanning electron micrographs of ossicles from Protoglossus graveolens. (a) Two typical ossicles formed of two terminal lobes (t) separated by an equatorial groove (eg). (b) A larger ossicle with many pores (p). (c) The terminal lobe trabeculae are indistinguishable.
	Figure 6. . Scanning electron micrographs of ossicles from Schizocardium californicum. (a) An ossicle with a single lobe and terminal polyhedral outgrowths (o). (b) An ossicle with three lobes (l), separated by a groove (g). Polyhedral outgrowths are present. (c) Close-up on the surface of a lobe. The lobes may be an aggregate of smaller crystals and have pores (p). (d) Close-up of a broken polyhedral outgrowth showing the porous inner organization.
	Figure 7. . Scanning electron micrographs of ossicles from Balanoglossus occidentalis. (a) Sideview of a typical ossicle formed of two conical terminal lobes (t) joined at a point. (b) A smaller ossicle showing clear trabeculae (tb). (c) Close-up on the terminal lobe of (a) showing the aggregation of polyhedral crystals.
	Figure 8. . Scanning electron micrographs of ossicles from Balanoglossus aurantiacus. (a) A typical ossicle with an axis (dashed line) around which the lamellae (l) are organized. (b) An ossicle without such axis. (c) Close-up on the lamellae showing how they intersect in a network fashion. (d) Close-up on a broken ossicle showing the inner network of pores.
	Figure 9. . Scanning electron micrographs of ossicles from Glossobalanus berkeleyi. (a) A typical ossicle formed of two spherical terminal lobes (t) separated by an equatorial groove (eg). (b) A broken ossicle with a solid interior. (c) Close-up of the edge of the broken ossicle. Minute pores (p) can be seen inside the ossicle. Scale bar: (a) 5 µm, (b) 5 µm and (c) 1 µm.
	Figure 10. . Raman spectra of enteropneusts ossicles. The peak pattern is characteristic of each polymorph. (a) The spectra of G. berkeleyi, B. occidentalis, S. kowalevskii, P. graveolens and S. pusillus all show the same peak pattern as the calcite ossicle from the seastar Pisaster ochraceus. (b) The ossicles of B. aurantiacus and H. planktophilus show the same peaks as the aragonite shell of the chitin Katharina tunicata. (c) The ossicles of S. californicum has peaks that correspond to vaterite (fig. 3 from [8]). The background noise was removed by subtracting a simple polynomial function to the spectra which does not alter the positioning or relative amplitude of the peaks. An arbitrary value was also added to each spectrum to prevent them from overlapping when plotted.
	Figure 11. . Phylogenetic tree of biomineralization in Ambulacraria. The phylogenetic tree is based on [20]. The CaCO3 mineral polymorph of each species is represented by a coloured tree branch. Parsimony shows calcite as the ancestral state (L = 5).

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Aizenberg, Calcitic microlenses as part of the photoreceptor system in brittlestars. 2001, Pubmed, Echinobase