From Coelomocytes to Colored Aggregates: Cellular Components and Processes Involved in the Immune Response of the Holothuroid Cucumaria frondosa

Characterization of microbiota and histology of cultured sea cucumber Isostichopus badionotus juveniles during an outbreak of skin ulceration syndrome

Due to the dramatic reduction of sea cucumber Isostichopus badionotus populations in the Yucatan Peninsula by overfishing and poaching, aquaculture has been encouraged as an alternative to commercial catching and restoring wild populations. However, the scarcity of broodstock, the emergence of a new disease in the auricularia larvae stage, and the development of skin ulceration syndrome (SUS) in the culture have limited aquaculture development. This study presents the changes in the intestine and skin microbiota observed in early and advanced stages of SUS disease in cultured juvenile I. badionotus obtained during an outbreak in experimental culture through 16S rRNA gene sequencing and histological evidence. Our results showed inflammation in the intestines of juveniles at both stages of SUS. However, more severe tissue damage and the presence of bacterial clusters were detected only in the advanced stages of SUS. Differences in the composition and structure of the intestinal and skin bacterial community from early and advanced stages of SUS were detected, with more evident changes in the intestinal microbial communities. These findings suggest that SUS was not induced by a single pathogenic bacterium. Nevertheless, a decrease in the abundance of Vibrio and an increase in Halarcobacter (syn. Arcobacter) was observed, suggesting that these two bacterial groups could be keystone genera involved in SUS disease.

Rainbow bodies: Revisiting the diversity of coelomocyte aggregates and their synthesis in echinoderms

The innate immunity of echinoderms has been a research focus since the early twentieth century, consistently providing ever deeper knowledge of its complexity and evolutionary aspects. At its core are coelomocytes, which are diverse cells collectively known to respond in a variety of ways, including via movement, phagocytosis, and aggregation. However, features of cellular immunity have never been compared in echinoderms from phylogenetic and distributional perspectives, to provide insight into ecological and evolutionary patterns. The present study catalyzed and characterized the formation of coelomocyte aggregates in members of all five extant classes of echinoderms. The morphological characteristics of these aggregates (including their colour, shape, texture, size) were assessed, as well as the major cells composing them. Coelomocyte diversity (both as free and aggregated forms) was determined to be maximum in class Holothuroidea, followed by Echinoidea, with the other classes showing similar levels of diversity. The colours of coelomocyte aggregates appeared to be more closely linked to phylogeny (classes, orders) rather than geographic range, or external colour of the species themselves. Asteroids and ophiuroids displayed primarily light-coloured aggregates, from transparent to green; while holothuroids, echinoids and crinoids demonstrated more vivid variants, from red to deep purple. The kinetics of aggregate formation and expulsion were monitored in selected species, showing immediate cellular response to foreign particulate matter in the form of encapsulation and various methods of expulsion, including through the dermal papillae of asteroids and the anus (cloaca) of holothuroids. The findings support that coelomocyte aggregate formation is a conserved immune response across all five extant classes of echinoderms with variations in their cell catalysts, complexity, shape, colour, and size.

Evidence of anticipatory immune and hormonal responses to predation risk in an echinoderm

Recent efforts have been devoted to the link between responses to non-physical stressors and immune states in animals, mostly using human and other vertebrate models. Despite evolutionary relevance, comparatively limited work on the appraisal of predation risk and aspects of cognitive ecology and ecoimmunology has been carried out in non-chordate animals. The present study explored the capacity of holothuroid echinoderms to display an immune response to both reactive and anticipatory predatory stressors. Experimental trials and a mix of behavioural, cellular and hormonal markers were used, with a focus on coelomocytes (analogues of mammalian leukocytes), which are the main components of the echinoderm innate immunity. Findings suggest that holothuroids can not only appraise threatening cues (i.e. scent of a predator or alarm signals from injured conspecifics) but prepare themselves immunologically, presumably to cope more efficiently with potential future injuries. The responses share features with recently defined central emotional states and wane after prolonged stress in a manner akin to habituation, which are traits that have rarely been shown in non-vertebrates, and never in echinoderms. Because echinoderms sit alongside chordates in the deuterostome clade, such findings offer unique insights into the adaptive value and evolution of stress responses in animals.