Molecular identification of metacestodes infecting bait shrimp in the Gulf of Mexico

Tapeworms (cestodes) have complex life cycles in which they may pass through several intermediate hosts before reaching sexual maturity in a definitive host. In marine systems, larval cestodes parasitize invertebrates and vertebrates with the adult forms usually using elasmobranchs (sharks and rays) as definitive hosts. Larvae are difficult, and often impossible, to identify through morphology. This is because many lack characteristics of identifiable adult cestodes. Gene sequences can be used for identification when morphology is not applicable. Many organisms, including cestodes, have been taxonomically placed by using nuclear ribosomal DNA (rDNA) gene sequences. Mitochondrial DNA (mtDNA) can also be used for the same purpose.;Bait shrimp (family Penaeidae) in the Gulf of Mexico are parasitized by cestode larvae that occur in the intestine and ventral nerve cord. These larvae are too morphologically simple to be identified through their physical features. It is also not clear which species of penaeids carry these larvae because of taxonomic instability and nomenclatural revisions of penaeids, and the difficulty of reliably identifying juvenile and subadult penaeids. Three questions were addressed by this project: (a) Are the larvae found in the intestine and in the nerve cord different stages in the life cycle of the same parasite? (b) Are the larvae found in different penaeid species the same or different cestode species? (c) What are the adult stages of these larvae -- to what taxonomic group(s) do they belong and in which hosts do they occur?;To answer these questions, rDNA sequences of larvae collected from shrimp were compared with each other and with rDNA sequences of adult cestodes collected in the Gulf of Mexico and adult cestode sequences deposited in GenBank. Shrimp mtDNA sequences were obtained to identify infected and uninfected penaeids. Three cestode species were found in bait shrimp. Farfantepenaeus duorarum from Panacea, FL was infected with a lecanicephalid. Three species of bait shrimp, F. duorarum, F. aztecus, and Litopenaeus setiferus, occurring in the northern most Gulf of Mexico were infected with a rhinebothriid species. F. duorarum occurring in the Florida Keys was infected with a different rhinebothriid species. The adult form of the lecanicephalid was discovered as a species of Polypocephalus in two species of ray, Dasyatis sabina and Dasyatis americana.