Aquaculture is a growing agricultural sector that due to the actual market saturation by sea bass and gilthead sea bream, needs for new fish species diversification. Sparid fishes are important candidates for species diversification in the Mediterranean aquaculture. Under culture conditions, parasites may cause serious problems. This thesis aims to improve our knowledge of myxozoan and aporocotylid parasites in sparids which are new candidates or existing Mediterranean aquaculture species. The present study seeks to identify, describe and clarify the taxonomic position of the parasite taxa by morphological and molecular approaches, and by defining their phylogenetical position and relationship with previously described species. Development, habitat selection in the host and pathology are analysed in order to evaluate the pathogenic potential of the parasites described. Also this thesis focuses on the study of parasite life cycles, seasonality, transmission and routes of infection in the fish host, with the final aim to provide information allowing for the development of management strategies for aquaculture systems. The study led to the following findings and conclusions: -A new genus and species of blood fluke, Skoulekia meningialis n. gen., n. sp. was described from the common two-banded seabream Diplodus vulgaris in a non common location, the ectomeningeal veins surrounding the optic lobes of the brain. The adults of S. meningialis were found to cause mild, chronic, localised meningitis. Its special location surrounding a vital organ should be kept in mind when establishing D. vulgaris in aquaculture systems. -Large cysts of myxosporean spores of the species Unicapsula pflugfelderi were detected in the fillets of sand steenbras Lithognathus mormyrus. Myoliquefaction process was not found to occur in U. pflugfelderi. However, the massive presence of macroscopic plasmodia in the fillets of L. mormyrus could produce marketing problems, affecting to the potential of the sand steenbras as an aquaculture candidate. -A new myxozoan parasite Ceratomyxa puntazzi n. sp. was found to cause histopathological changes in the liver and in the gall bladder of the sharpsnout seabream Diplodus puntazzo and has to be considered as an opportunistic parasite in immunocompromised hosts. The genus Ceratomyxa seems to be very host-specific in sparids and high species diversity can be expected in Mediterranean sparids. The phylogenetical analyses revealed that all ceratomyxid species of sparids from the Mediterranean rise from a common ancestor. -This thesis provides unique insights into the development and cellular organisation of the myxozoan C. puntazzi. This study is the first use of confocal laser scanning microscopy with specific stains on developmental stages of a myxozoan. C. puntazzi seems to follow a sequence of development with two pathways in the bile: pre-sporogonic proliferative development where stages show high degree of motility, cellular divisions and budding by plasmotomy, and sporogonic development, characterized by the formation and maturation of spores. For the first time in myxozoans, motility and the distribution of potential effectors of motility were analysed. F-actin rich cytoskeletal elements were found to concentrate at one end of the parasite, which produces localised filopodia, thus facilitating parasite motility. The mechanism of budding was identified, for the first time, as an active F-actin dependent separation process. -Natural exposures of sharpsnout seabream to a C. puntazzi enzootic habitat are the first attempt of conducting a seasonality study in marine myxozoans and provide important insights into the seasonal patterns of myxozoan abundance in a temperate marine environment. C. puntazzi infection in the bile shows a marked temperature-related seasonality, with visual detection of parasites exclusively between April - November. However, PCR detection in the bile is possible all year round. Blood stages were detected all year round by PCR. This indicates that actinospores are present in the sea water and the blood of exposed fish is invaded throughout the whole year. Implications for D. puntazzo culture are discussed.La acuicultura mediterránea necesita diversificación de especies de peces para el cultivo debido a la saturación del mercado con dorada y lubina. En condiciones de cultivo, los peces pueden verse afectados por diversos patógenos, entre ellos los parásitos. Entre los parásitos mas problemáticos en la acuicultura mediterránea, figuran los mixozoos y los aporocotílidos. En esta tesis doctoral se presentan nuevas y se aportan nuevos datos de especies ya descritas de estos dos grupos parásitos, se estudia la relación parasito-hospedador y se incluyen nuevos datos del desarrollo, rutas de infección, ciclo de vida y estacionalidad en especies de peces candidatas para el cultivo. Un nuevo género y especie de aporocotílido Skoulekia meningialis n. gen. n. sp. se halló en un hábitat poco común, las venas ectomeningeales del cerebro de la mojarra Diplodus vulgaris, donde los adultos provocan una meningitis moderada pero crónica. Plasmodios alargados con esporas del mixozoo Unicapsula pflugfelderi se hallaron en la musculatura de la herrera Lithognathus mormyrus y aunque no se observó mioliquefaccion, la presencia masiva de estas estructuras en los filetes puede afectar al potencial de la herrera como especie para la acuicultura. Ceratomyxa puntazzi n. sp. es una nueva especie de mixozoo descrita de la vesícula biliar del sargo picudo Diplodus puntazzo. C. puntazzi provoca cambios histopatológicos en el hígado y la vesícula biliar del sargo picudo y debe considerarse como un patógeno oportunista. C. puntazzi forma parte del complejo de especies del género Ceratomyxa hallado en espáridos del Mediterráneo los cuales provienen de un ancestro común. Dentro del estudio de C. puntazzi, se describen los componentes celulares, y los mecanismos que intervienen en la motilidad de los mixozoos y como estos elementos están implicados en los procesos de gemación y citocinesis en los mixozoos. Experimentos de transmisión con jaulas en el mar a un medio natural enzoótico para C. puntazzi reveló una estacionalidad marcada, con presencia de esporas en la bilis de Abril a Noviembre, pero con detección por PCR durante todo el año tanto en la bilis como en la sangre. Estos resultados tienen implicaciones para el manejo del cultivo del sargo picudo.