Role of mucous cells in the defence system of the cnidarian <i>Actinia equina</i> (Linnaeus, 1758): The keystone of evolution

Abstract Cnidarians are the first phylum of metazoans to exhibit a functional mucus layer. In evolution, mucous cells perform the classical secretory function of mucus and act as sentinels for external insults, particularly in mammals. However, this topic has not been much analysed in the lower metazoans. This study aims to investigate the role of mucous cells in Actinia equina (Anthozoa, Hexacorallia) (Linnaeus, 1758) by using confocal microscopy and bioinformatics. The results show colocalization of Toll‐like receptor 2 (TLR2) and Mucin2 (MUC2) on these cells, suggesting a crosstalk between defence response and mucus secretion in cnidarians. Furthermore, the presence of TLR2 on the surface of mucous cells suggests their involvement in host defence responses, as occurs in vertebrates. Moreover, through in silico analysis, we confirmed the presence of orthologous genes for the tested antibodies, providing additional evidence to support our results from an evolutionary perspective. These data suggest that mucous cells play a defensive role throughout evolution, helping us understand how these cells have developed and adapted across phylogenesis. This information also provides new insights into the defensive strategies of invertebrates and clarifies their evolutionary history.