Experimental Elucidation of the Life Cycle of Rhinebothrium urobatidium (Cestoda: Rhinebothriidea) from the Round Stingray (Urobatis halleri: Myliobatiformes) to First and Second Intermediate Hosts

Abstract

The life cycle of the cestode Rhinebothrium urobatidium, whose final host is the round stingray, Urobatis halleri, includes a copepod as the first intermediate host and small benthic fishes as second intermediate hosts. Hexacanth embryos within nonoperculate, untanned eggs collected from round stingray developed in the tidepool copepod Tigriopus californicus into caudate procercoids with an apical organ/sucker and cercomer, and rarely, bothridia. The procercoids, which developed in the body cavity within a membrane, adhered to the copepod intestine, were infective to arrow gobies, Clevelandia ios, within 15 d of exposure at 21°C. When infected copepods were fed to arrow gobies, procercoids developed into nonlacunate plerocercoids each bearing individually retractable bothridia and an apical sucker. Within 10 d of exposure to the second intermediate host, the larvae had migrated up the bile/cystic duct into the gall bladder, where they developed bothridia similar to those of adult worms. Between 30 to 51 d post-infection in the goby, plerocercoids approached the size of larvae found in natural infections, and the scolex became morphologically similar to that of adult worms from round stingrays. Only presumptive filiform microtriches (filitriches) were present on procercoids while both filitrichs and spiniform microtriches (spinitrichs) were present on plerocercoids. Identification of plerocercoids from experimental infections as those of R. urobatidium was confirmed through morphology of the scolex and using cytochrome c oxidase I sequences. The experimental transmission of R. urobatidium to first and second intermediate hosts provides improved understanding of the transmission and ontogeny of shark tapeworms. The biological characteristics of U. halleri, with its diverse parasite fauna, provide significant opportunities to examine the biology of an array of elasmobranch tapeworm taxa.