Morphological study of the sclerites of the species Renilla muelleri Kölliker, 1872 (Anthozoa, Octocorallia, Pennatulacea)

Abstract

Some Octocorals (Cnidaria, Anthozoa) can produce a sub-millimeter to a millimeter-range skeleton ofcalcium carbonate, known as the sclerites. Sclerites have diverse morphological characters, such assize, and shape features that give them relevance as taxonomic characters at different levels. RenillaLamarck, 1816, is a genus of the order Pennatulacea (Octocorallia), most of whose colonies arecharacterized by two distinct zones: a peduncle, which anchors them to the substrate, and a rachisthat supports the polyps. In the case of Renilla muelleri Kölliker, 1872, prior research has establishedthe presence of similar sclerites in both: the peduncle and the rachis. Nevertheless, the potentialmorphological variations of these sclerites among colonies and within different colony regions haveyet to be assessed to determine the stability of these characteristics at the species level. This studyaimed to describe and compare the external microscopic morphology and biometry of the sclerites ofRenilla muelleri, enhancing their identification and assessing their consistency within the species.Sclerite composition was examined across the entire colony, and we analyzed length variation amongcolonies using generalized linear models (GLM) and within colony zones using generalized linear mixedmodels (GLMM). Additionally, the external microscopic morphology of all sclerites was examinedthrough scanning electron microscopy (SEM). Based on size, two types of sclerites, namely large andsmall, were identified within the colonies. Both types exhibited significant size differences amongcolonies. Furthermore, the large sclerites displayed notable variations between zones, with those inthe rachis being the largest and holding the highest rank within the colonies. In terms of externalmicroscopic morphology, the sclerites exhibited considerable variability, making it challenging toestablish clear groupings based on these characteristics. Based on the obtained results, it can beconcluded that both the biometry and external microscopic morphology of sclerites do not exhibitconsistency as characteristics in Renilla muelleri highlight ambiguity in defining Renilla species.