Spatial Distribution of Medusa Cunina octonaria andFrequency of Parasitic Association with Liriope tetraphylla (Cnidaria:Hydrozoa: Trachylina) in Temperate Southwestern Atlantic Waters

摘要

This study examined the spatial distribution of the medusae phase of Cuninaoctonaria (Narcomedusae) in temperate Southwestern Atlantic waters using atotal of 3,288 zooplankton lots collected along the Uruguayan and Argentine waters(34–56°S), which were placed in the Medusae collection of the Universidad Nacional de Mardel Plata, Argentina. In addition, we reported the peculiar parasitic association betweentwo hydrozoan species: the polypoid phase (stolon and medusoid buds) of C.octonaria (parasite) and the free-swimming medusa of Liriopetetraphylla (Limnomedusae) (host) over a one-year sampling period (February2014 to March 2015) in the coasts of Mar del Plata, Argentina. We examined theseasonality, prevalence, and intensity of parasitic infection. Metadata associated withthe medusa collection was also used to map areas of seasonality where such association wasobserved. Cunina octonaria was found from southern Uruguay to the coastof Mar del Plata (34.8–38.2°S, 57.2–54.0°W), with the highest abundances and frequency ofoccurrence in the Río de la Plata estuary. The parasitic association was identified fromthe austral warm period (spring-summer season) until mid-autumn. Out of the 21,734L. tetraphylla specimens that were examined, 316 were parasitized(prevalence = 1.5%) exclusively in the manubrium and gastric peduncle, with an infectionintensity of 1 to 2 stolons per host. Furthermore, the medusoid buds per stolon rangedfrom 11 and 29 at different stages of development. No significant differences wereobserved between the umbrella diameter of parasitized and non-parasitized L.tetraphylla specimens, nor was any significant correlation identified betweenumbrella diameter and prevalence, and intensity of infection. According to the aggregationcoefficient, C. octonaria had an overdispersed distribution in the hostpopulation. All parasitized hosts showed stomach vacuity due to the location of thestolon, which blocked the mouth of the host. We identified the parasitic association inthe coasts of Mar del Plata, as well as in both coasts of the Río de la Plata Estuary(Uruguayan-Argentinean coasts). In the Southwestern Atlantic, several biologicalinteractions between medusae and other groups have been identified; however, the specifichost selectivity of C. octonaria for L. tetraphylla wasnot previously identified. Here we discuss the ecological importance of this associationduring the holoplanktonic life history of the narcomedusae. Additionally, we report thesouthern limit of the spatial distribution of this particular parasitic association in theSouthwestern Atlantic, thus increasing the knowledge of biological associations ofgelatinous zooplankton (Cnidaria and Ctenophora) on Uruguayan and Argentinean coasts.