Ctenophores and siphonophores are alternative hosts in the life cycle of didymozoids in the Gulf of California

Background. Few studies exist of parasites of zooplanktonic organisms. This situation increases the odds to discover new species of parasites, new types of parasitic interactions, and reports that increase the geographic distribution ranges worldwide. Goals. We report a new parasitic interaction between the metacercariae of didymozoids (Digenea) with a ctenophore and a siphonophore species (gelatinous zooplankton) based on observations of live zooplankton in the Gulf of California. Methods. Zooplankton was collected during day and night using a standard Bongo net in an oceanographic cruise carried out during June-July 2016 in the Gulf of California. Results. We observed four ctenophore specimens of Pleurobrachia bachei (intensity of parasitism = 2-3 parasites per host) and a siphonophore specimen of Muggiaea atlantica (intensity = 1) parasitized with a total of nine metacercariae Didymozoidae morphologically identified as larval type Monilicaecum. Molecular analysis of didymozoid metacercariae was not attempted here, but a molecular identification of morphologically similar didymozoids was previously reported infecting chaetognaths of the Gulf of California. Didymozoids were found in three different microhabitats in the P. bachei and in the subumbrellar cavity of M. atlantica. Conclusions. We do the first record of didymozoid parasitizing ctenophores and siphonophores that also represent new geographic records for these parasites in the Gulf of California extending its distribution range from the Atlantic Ocean to the Pacific Ocean.