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Nyctiphanes simplex and Nematoscelis difficilis are the most abundant euphausiids in the Gulf of California and the previously known parasite fauna includes 6 types of ectoparasites and 7 types of endoparasites, although in most cases their taxonomic identification is still imprecise. We report the discovery of four new records of parasites found in these two species from systematically collected aboard the R/V El Puma with bongo (500 ?m) and non-quantitative (for analysis of in vivo samples) zooplankton nets are reported for four oceanographic cruises (January and July 2007, August 2012 and June 2013). Four parasites not previously recorded in these species, three of which have been observed infecting only Nyctiphanes simplex is discovered: 1) cystacanths of Acanthocephala Bolbosoma spp. that infected females, one of them ovigerous with viable embryos although with small brood size, suggesting that acanthocephalans not necessarily cause castration but may decrease fertility (July 2007, intensity 1 and low prevalence 0.06-1.45 %), 2) Paronatrema spp. in N. simplex (July 2007; intensity = 1 and typically low prevalence 0.16 and 0.03 %). A record previously published Paronatrema in N. simplex was wrongly identified which was actually a cestode; thus this is the first confirmed record of this species of the euphausiid N. simplex. 3) metacercariae of trematode (family Syncoeliidae) discovered so far infecting only N. dificilis (January 2007) and the parasite that infected both euphausiid species; 4) likely three species of gregarinida (Phylum Apicomplexa Levine, 1970, Order Eugrerarinoida) not yet identified located in the hepatopancreas and in the intestine of the euphausiid. These are the first records of trematode and gregarinid infections for N. simplex and N. difficilis and in general from euphausiids of the Gulf of California. From a review of the known trematodes, gregarine, and acanthocephalans infecting euphausiids worldwide highlights that sporozoans infecting euphausiids are known only for the Antarctic, North Atlantic and Mediterranean Sea. Therefore the euphausiids infected with sporozoans in the Gulf of California is the first record in Pacific Ocean and subtropical latitudes. It is likely that these parasites are still unknown to science and its impact on the host is still poorly understood. However, histological evidence suggest that sporozoans and cercariae of cestoda that infect with high intensities and trematode metacercariae and the acanthocephalan cystacanth due its great size in comparison with the host affect vital rates and generally the health condition of the infected euphausiids.

Palabras clave: Euphausiids; Paralarval assemblages; parasites; trematoda; acantocephala; gregarinida

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