An integrative approach to the anatomy of Syllis gracilis Grube, 1840 (Annelida) using micro-computed X-ray tomography

Abstract

Background The overall anatomy of the genus Syllis (Annelida: Syllidae) has been largely studied; however, an integrative approach considering different anatomical techniques has never been considered. Here, we use micro-computed X-ray tomography (micro-CT) to examine the internal anatomy of Syllis gracilis Grube, 1840, along with other widely available techniques. Methods We studied the anatomy of the marine annelid S. gracilis through an integrative approach, including micro-CT along with stereo and light compound microscopy (STM, LCM), scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM) and histological sectioning (HIS). In this manner, we evaluated the applicability of micro-CT for the examination of annelid anatomy by testing whether the images obtained make it possible to visualize the main body structures, in comparison with other current techniques, of the various elements of its internal anatomy. Results Overall external and internal body elements are clearly shown by the integrative use of all techniques, thus overcoming the limitations of each when studied separately.Any given method shows disparate results, depending on the body part considered. For instance, micro-CT provided good images of the external anatomy, including relevant characters such as the shape, length and number of articles of dorsal parapodial cirri. However, it is especially useful for the examination of internal anatomy, thus allowing for 3D visualization of the natural spatial arrangement of the different organs. The features best visualized are those of higher tissue density (i.e., body musculature, anterior parts of the digestive tract), particularly in 3D images of unstained specimens, whereas less electrodense tissues (i.e., the peritoneal lining of septa and nervous system) are less clearly visualized. The use of iodine stain with micro-CT has shown advantages against non-staining for the adequate observation of delicate elements of low density, such as the segmental organs, the connective between the ganglia, the ventral nerve cord and segmental nerves. Discussion Main external anatomical elements of S. gracilis are well shown with micro-CT, but images show lesser optical resolution and contrast when compared to micrographs provided by SEM and CLSM, especially for fine structural features of chaetae. Comparison of micro-CT and HIS images revealed the utility and reliability of the former to show the presence, shape and spatial disposition of most internal body organs; the resolution of micro-CT images at a cellular level is, however, much lower than that of HIS, which makes both techniques complementary.

Comparative transcriptomics in Syllidae (Annelida) indicates that posterior regeneration and regular growth are comparable, while anterior regeneration is a distinct process

“…Eusyllis blomstrandi Malmgren, 1867 [49–51]; while others additionally regenerate all missing segments and also a characteristic differentiation of the digestive tube called proventricle (e.g. Syllis gracilis Grube, 1840 [37, 52–55]). Interestingly, the molecular background of regeneration in syllids has not been explored.…”

Estudio microtomográfico de la anatomía de Laubieriopsis cabiochi (Amoureux, 1982) (Annelida, Fauveliopsidae)

“…El examen de poliquetos por medio de microtomografía computarizada (µCT) (p. ej. Parapar et al, 2015aParapar et al, , 2015bParapar et al, , 2016Parapar et al, , 2018aParapar et al, , 2018bParapar et al, , 2019 ha revelado que las imágenes tanto de su anatomía externa como interna son un buen complemento de aquellas obtenidas mediante el estudio histológico tradicional. Este hecho resulta particularmente relevante para estudios de reconstrucción anatómica, la cual, tanto en poliquetos como en Clitelados (p. ej.…”

“…Asimismo, es destacable la baja definición de las imágenes obtenidas de las sedas, de calidad muy inferior a las logradas con el MEB o la microscopía óptica, lo cual corrobora lo visto en otros casos con otras especies de poliquetos (p.ej. Parapar et al, 2017Parapar et al, , 2018aParapar et al, , 2018bParapar et al, , 2019.…”

“…La presencia potencial de un plexo circulatorio asociado al intestino observado en el individuo sin teñir podría ser un aspecto novedoso en el conocimiento anatómico de los fauveliópsidos. Sin embargo, la falta de réplicas no permite confirmar su existencia de forma definitiva, pues la posibilidad de artefactos en las imágenes es un hecho constatado en estudios recientes con otras especies de poliquetos mediante esta técnica (Parapar et al, 2018a(Parapar et al, , 2018b(Parapar et al, , 2019. En este sentido, hemos detectado un nuevo ejemplo de artefacto en relación con los estrechamientos del esófago.…”

“…Así, Dinley et al (2010), Faulwetter et al (2013aFaulwetter et al ( , 2013bFaulwetter et al ( , 2014 y Paterson et al (2014) emplearon este método demostrando que las imágenes obtenidas pueden aportar detalles anatómicos muy relevantes. Recientemente, Parapar & Hutchings (2014) y Parapar et al (2015aParapar et al ( , 2015bParapar et al ( , 2016Parapar et al ( , 2018aParapar et al ( , 2019 han empleado el µCT en el estudio morfo-anatómico de varias especies de Anélidos (correspondientes a la antigua clase Polychaeta, ahora sin validez sistemática dada su naturaleza parafilética), continuando la exploración de sus posibilidades frente a otras técnicas anatómicas clásicas como la microscopía óptica y electrónica de barrido y las secciones histológicas.…”