Parasite diversity of Nyctiphanes simplex and Nematoscelis difficilis (Crustacea:Euphausiacea) along the Northwestern coast of Mexico

Abstract

The diversity of parasites found on Nyctiphanes simplex and Nematoscelis difficilis (Order Euphausiacea) was compared during ten oceanographic cruises made off both coasts of Baja California Peninsula, Mexico. We tested the hypothesis that N. simplex has a more diverse parasitic assemblage than N. difficilis because it is a neritic species, has larger population abundance, and tends to form denser and more compact swarms than N. difficilis. These biological and behavioral features may enhance parasite transmission within swarms. We detected five types of ectoparasites: (1) epibiotic diatoms Licmophora sp., (2) Ephelotidae suctorian ciliates, (3) Foettingeriidae exuviotrophic apostome ciliates, (4) an unidentified Epicaridean cryptoniscus larvae (isopoda), and two castrators (5) the ectoparasitic Dajidae isopod Notophryxus lateralis; and (1) the ellobiopsid mesoparasite Thalassomyces fagei. We also detected seven types of endoparasites: (1) an undescribed endoparasitoid Collinia ciliate (Apostomatida), three types of Cestoda (2) a Tetrarhynchobothruium sp. (Trypanorhyncha), (3) Echinobothrium sp. (Diphyllidea: Echinobothyriidae), and (4) unidentified metacestode, (5) a Trematoda Paronatrema-like metacercaria (Syncoeliidae), (6) the nematode Anisakis simplex (L3), and (7) Polymorphidae Acantocephalan larvae (acanthor, acanthella, and cystacanth larval stages). We demonstrated that N. simplex is affected by all types of parasites, except the isopod N. lateralis, having a considerably larger parasitic diversity and prevalence rates than N. difficilis only infested with three types of ectoparasites and the ellobiopsid mesoparasite. Euphausiid swarming is an adaptive behavior for reproduction, protection against predators, and increased efficiency in food searching, but has a negative effect due to parasitism. Although the advantages of aggregation must overcome the reduction of population individual fitness induced by parasites, we demonstrated that all types of parasites can affect about 14% of N. simplex individuals. Collinia endoparasitoids must occasionally have a significant influence on population mortality with potential epizootic events.