First Report of Calceostoma glandulosum (Monogenea) in Argyrosomus regius: Morphological and Molecular Characterization and Temperature Effects on Life Cycle

The increase in fish production in aquaculture reflects the growth of the world population. However, this expansion is accompanied by challenges, such as the intensification of production systems through increased stocking density, which induces stress in fish, weakening their immune system and making them more susceptible to disease outbreaks, particularly those caused by parasites. Monogeneans are ectoparasites that attach to the external surfaces of fish and are characterised by their host-specificity. One of the emerging species in aquaculture, much appreciated in the Mediterranean region, is the meagre (Argyrosomus regius), whose production has been increasing due to its high-quality meat and excellent growth rates. However, the information available on the parasites that affect this species is limited, which makes it difficult to prevent and control disease outbreaks in aquaculture systems effectively. This study presents the first morphological and molecular characterisation of Calceostoma glandulosum, an ectoparasite of the Monopisthocotylea subclass, characterised by its lapel (L) in the cephalic area and an anchor-shaped haptor (Hp). The parasite’s life cycle, including eggs, larvae (oncomiracidium), and adults, has also been described. It was demonstrated that water temperature could influence the development of the eggs; at more favourable temperatures for the parasite, there was a higher percentage of developed and hatched eggs. The occurrence of the parasite in the farming tanks was higher when the temperature reached 20°C, and in vitro experiments have shown a hatching rate of 100% at 23°C. C. glandulosum reaches sexual maturity between 11 and 14 days and has a fertility period of 10 days. To assess the host-specificity of C. glandulosum for A. regius, eggs were investigated in other species (Sparus aurata, Diplodus sargus, and Seriola rivoliana). This work provides valuable insights into the behavior of monogeneans concerning environmental conditions and host interactions, offering critical information for implementing preventive measures in aquaculture.