The internal defence system in actinia equina (Linnaeus, 1758): The role of mucus and goblet cells

Mucus is a link between the organism and its environment, with unique and multifunctional properties in all metazoan classes. In metazoans it is secreted by highly specialized cells, the goblet cells, which are mainly scattered in the epithelia in contact with the external environment. Goblet cells also play a role in maintaining the health of the organism by acting as sentinels. Recent studies in vertebrates have suggested that some goblet cells may also be involved in antigen presentation to dendritic cells by activating Toll Like Receptor (TLR) signals, thereby contributing to the defense response by increasing mucus production. In aquatic organisms in particular, mucus is involved in several essential biological processes, including feeding, movement, reproduction, osmoregulation, defense against pathogens, xenobiotics and a variety of environmental stressors. The capability to create a functional layer of mucus is a significant evolutionary step that first emerged in cnidarians. Actinia equina (Linnaeus, 1758), is an anthozoan cnidarian of the family Actiniidae, which has a complex and unresolved taxonomic history. What was once thought to be a single polymorphic species with a wide geographical range is now defined as a species complex. The sea anemone lives in intertidal and subtidal rocky areas down to 20 meters depth. It is characterized by a highly variable coloration, with simple tentacles and a smooth or mottled column, ranging from red, brown and orange to green. The aim of this study was to investigate the role of goblet cells in the integument of A. equina and to provide further data on the internal defense system of this anthozoan from an evolutionary perspective. Our results on integument samples show scattered amoebocytes and goblet cells immunoreactive for TLR2, an evolutionarily conserved receptor that plays a key role in modulating the defense mechanisms of the body. Goblet cells also appear to be immunopositive for mucin2, a protein involved in the composition and formation of a layer of mucus that modulates external exchange and may prevent microbial invasion. Interestingly, the colocalization of both antibodies on goblet cells suggests that these cells function as sentinels and that their secretion, mucus, assists the body in protective mechanisms by mediating contact with the external environment. However, these data may also provide additional information to better understand and study the taxonomy of these cnidarians and how they interact and defend themselves against environmental insults.