In bilaterian organisms, structures can be symmetrical or asymmetrical, the latter being subdivided into three forms: antisymmetry, directional asymmetry, and fluctuating asymmetry (Endler, 1986; Lahti et al., 2009). Fluctuating asymmetry (FA) consists of random deviations from perfect bilateral symmetry on a population of organism (Graham et al., 2010). This variation around the perfect symmetrical distribution represents a measure of development noise or developmental instability (Rott, 2003). As both sides of a bilateral trait develop under the control of the same genome, the developmental phenotypic target of a population should be perfect symmetry (De Coster et al., 2013), but developmental noise results in deviations from perfect bilateral symmetry or an increase in FA
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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