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 of calcium carbonate, known as sclerites. Sclerites have diverse morphological characters, such as size, and shape features that give them relevance as taxonomic characters at different levels. Renilla Lamarck, 1816, is a genus of the order Pennatulacea (Octocorallia), in which colonies are characterized by two distinct zones: a peduncle, which anchors them to the substrate, and a rachis that supports the polyps. In the case of Renilla muelleri Kölliker, 1872, prior research established the presence of similar sclerites in both the peduncle and the rachis. Nevertheless, the potential morphological variations of these sclerites among colonies and within different colony regions have yet to be assessed to determine the stability of these characteristics at the species level. This study aimed to describe and compare the external microscopic morphology and biometry of the sclerites of Renilla muelleri, enhancing their identification and assessing their consistency within the species. Sclerite composition was examined across the entire colony, and we analyzed length variation among colonies using generalized linear models (GLM) and within colony zones using generalized linear mixed models (GLMM). Additionally, the external microscopic morphology of all sclerites was examined through scanning electron microscopy (SEM). Based on size, two types of sclerites, namely large and small, were identified within the colonies. Both types exhibited significant size differences among colonies. Furthermore, the large sclerites displayed notable variations between zones, with those in the rachis being the largest and holding the highest rank within the colonies. In terms of external microscopic morphology, the sclerites exhibited considerable variability, making it challenging to establish clear groupings based on these characteristics. Based on the obtained results, it can be concluded that both the biometry and external microscopic morphology of sclerites do not exhibit consistency as characteristics in Renilla muelleri highlighting the ambiguity in defining Renilla species.