Goldfish (Carassius auratus) gills function as both respiratory and immune-regulatory organs, integrating neuroendocrine and immune responses to environmental stimuli. This study explores the spatial organization and interaction of neuroendocrine cells (NECs) and immune cells within goldfish gills using confocal immunohistochemistry and transmission electron microscopy. NECs, identified near blood capillaries and nerve fibers, highlight their role in environmental sensing and physiological regulation. These cells express serotonin (5-HT), a neurotransmitter critical for neuroimmune communication. Two distinct macrophage subsets were observed: iNOS-positive macrophages, concentrated in the basal epithelium, suggest a pro-inflammatory role, whereas 5-HT-positive macrophages, dispersed in the subepithelium, likely contribute to immune modulation. The co-localization of MHC-II and CD68 in macrophages further supports an active antigen-processing system in the gills. Ultrastructural analysis revealed diverse immune cells, including rodlet cells, telocytes, and lymphocytes, within the gill epithelium. Telocytes formed intricate networks with immune cells, highlighting their role in immune coordination and tissue homeostasis. These findings provide new insights into the neuroimmune interactions in fish gills, contributing to a broader understanding of aquatic immune systems and environmental adaptability.

One of the most crucial biological indicators in tracking long-term variations in the reproductive cycle is sexual development. Scorpaena porcus (Linnaeus, 1758), commonly known as the black scorpionfish, is a small teleost from the family Scorpaenidae. Much is known about its ecology, but data on its reproductive and defense systems are still lacking. Antimicrobial peptides (AMPs), such as piscidins, are integral components of the innate immune system in fish. These peptides exhibit a wide range of activity against bacteria, fungi, viruses, and protozoa and act as the first line of host defense. This study aims to investigate the primary sexual development stages in male and female gonads of black scorpionfish, providing additional knowledge on the reproductive biology of this teleost while evaluating concomitant changes in the expression of a Piscidin-1 antimicrobial peptide. The results show a histological, morpho-structural change from the immature stage to the developing virgin stage. Immunohistochemical analyses show that germinal and somatic cells are strongly reactive to Piscidin-1 in both gonads at an early ontogeny stage. These data suggest that Piscidin-1 may play a key role in the local defense system of scorpionfish gonads at this delicate stage, which is critical for the continuation and maintenance of the species. The present findings are potentially useful for a better understanding of the reproductive cycle of this fish, improving our knowledge of the interaction between the immune system and reproduction.

The skin of vertebrates serves as a crucial interface with the external environment. In fish, it performs various functions, mainly offering protection against pathogens through the action of specialized cells. Cyclostomes, such as hagfish, lack scales and rely heavily on mucus for defense. These jawless vertebrates possess specialized glands that produce a unique mucous exudate when threatened, forming a thick slime that can clog the gills of predators. This substance, composed of mucus and filamentous proteins, offers hagfish a distinct evolutionary advantage and may explain their survival among extinct agnates. These proteins are produced in the cytoplasm of epidermal thread cells, which are unique to hagfish and contain coiled, intermediate filaments. Despite extensive research on thread cell morphology, their roles remain poorly understood. This study investigates the putative defense function of epidermal thread cells in three hagfish species, Eptatretus cirrhatus (J. R. Forster, 1801), Eptatretus stoutii (Lockington, 1878), and Myxine glutinosa (Linnaeus, 1758), using immunohistochemistry, confocal microscopy, and bioinformatics techniques to better understand their contribution to hagfish immunity and ecological resilience.

When it comes to predation, alarm signals enable individuals to assess risks and modulate their behavior accordingly. These signals, often chemical in aquatic environments, can be recognized across species boundaries and are typically released through injury-induced mechanisms in response to predation. While extensively documented in teleosts, particularly those possessing epidermal club cells, such mechanisms remain poorly understood in more basal vertebrates, such as lampreys, which possess unique epidermal structures called skein cells. The present study investigated the potential role of skein cells in the immune and alarm signaling systems of the sea lamprey (Petromyzon marinus), given their strategic location in the skin and distinctive ultrastructural characteristics, such as polarized nuclei and tonofilament-rich basal cytoplasm. Although originally misidentified as club cells, skein cells may be involved in mucus secretion and the release of compounds such as sialic acid and glycoconjugates, which provide defense against aquatic pathogens. This study employed histomorphological analysis, immunoperoxidase labeling, confocal microscopy, bioinformatics, and quantitative and statistical analysis to investigate the hypothesis that skein cells contribute to anti-predator defense via the release of alarm substances. These findings provide new insights into the evolutionary origins and functional diversity of chemical signaling in the early vertebrate.