Spicules and skeletons: mantle musculature of two species of dorid nudibranchs (Gastropoda: Nudibranchia: Doridina)

Abstract

Molluscs often possess complex calcified elements in addition to the shell, but how these elements function and relate to other tissues is often poorly understood. Dorid nudibranchs typically possess innumerable calcareous spicules arranged in complex networks. To describe how these spicules interact with muscles and connective tissue, we reconstructed tomographic digital models using serial sections and synchrotron micro-CT. In two species with dramatically different spicule network morphologies, musculature was divided into a dorsal layer of crossed fibres, a ventral layer of branching radial fibres, and scattered dorsoventral fibres in between. These two species differed in the size of their dorsal tubercles, which was reflected in the organization of dorsal musculature, and in the amount and organization of connective tissue. In Platydoris sanguinea Bergh, 1905, dense mats of spicules sandwiched a layer of connective tissue with fewer spicules and muscle insertions only onto the ventral spicules. In Cadlina luteomarginata MacFarland, 1966, thick tracts of spicules are surrounded by a sheath of connective tissue. Muscles surround and insert into the dorsal tubercle spicule layer. Thus, both species appear to use the spicule network for muscle antagonism and transfer of motion, but the different arrangement of elements suggests that they use this skeleton in quite different ways. 

Doris adrianae sp. nov. (Heterobranchia; Nudibranchia; Doridina) from the Galician coasts (NW Iberian Peninsula).

“…Dorids are known to have tegumentary spicules, although most papers describe their presence as spicules radiating from the base of the tubercles and other vertical spicules supporting the tubercles (kress, 1981;cAttAneo-vietti, et al 1993;Brodie, 2004;ehrlich, 2019;orteA & esPinosA, 2017;among others). Some authors make more detailed descriptions of the skeletal structure of dorids (gArcíA, et al 1986;Penney, 2006), in some use micro-computed tomography (AlBAtercedor & sAnchez-tocino, 2011;Penney, et al 2018;Penney, et al 2020) and include species of the genus Doris (Penney, 2008) such as Doris montereyensis Cooper, 1863, but none of these studies describe a skeletal structure as complex as that of Doris adrianae sp. nov.…”