The pelagic Cnidaria of the Gulf of Mexico: zoogeography, ecology, and systematics.

Zoogeographic, ecologic and systematic studies conducted on the pelagic Cnidaria of the Gulf of Mexico revealed 18 species of scyphozoans (including an undescribed species of Nausithoe), approximately 56 species of hydromedusae (including six new species and one new genus) and 45 species of siphonophores. Eight species of siphonophores (Amphicaryon ernesti, A. peltifera, Clausophyes ovata, Maresearsia praeclara, Marrus orthocannoides, Nectopyramis diomedeae, N. natans and Prava dubia) previously known only from the Indo-Pacific are reported for the first time for the western Atlantic. Other records of forms previously known only from the Indo-Pacific include the scyphozoans Deepstaria enigmatica and Periphyllopsis braueri as well as the deep water continental shelf hydromedusae Bythotiara depressa and Calycopsis simulans. The hydromedusan genus Halistaura previously known only from three Pacific species now includes a new species from the Gulf of Mexico. East Atlantic medusae reported for the western Atlantic for the first time include the hydromedusae Dipurena ophiogaster, Sarsia gemmifera, Steenstrupia nutans and Zygocanna vagans and the scyphomedusa Atolla vanhoeffeni. In addition to the well defined bathypelagic cnidarian assemblage the Gulf of Mexico cnidarian fauna may be divided zoogeographically into a northern Gulf neritic and estuarine assemblage, which appears to be a Pleistocene relict fauna, and a tropical neritic and epipelagic fauna of the southern Gulf which is primarily of West Indian origin, but includes also some Indo-Pacific and East Atlantic components. The Carolinian fauna of the northern Gulf was isolated in the Pleistocene from the Carolinian region by the penisula of Florida. The occurrence of the east Atlantic forms and Pacific forms in the Gulf indicates a previous continuity of the world's tropical waters across the Isthmus of Central America. Isolation of the Gulf of Mexico from the Indo-Pacific probably occurred last in the Pliocene, dividing the fauna into Atlantic and Pacific populations. Hydromedusae which will serve as indicators of neritic water in the epipelagic have been defined. Northern and southern Gulf species are easily distinguished. Analysis of northern Gulf estuarine and neritic medusae and siphonophores indicate that they are excellent indicators of estuarine flushing and mixing of neritic and oceanic waters. Seasonality in northern Gulf neritic and estuarine leptolinid medusae has also been elucidated and has been found to be primarily temperature dependent. Quantitative analysis of medusan growth patterns in selected species indicates a wide variety of allmetric and isometric growth patterns in closely related and widely divergent forms. Infraspecific analysis of Bougainvillia carolinensis in the northern Gulf reveals that neritic and estuarine populations are morphologically distinct. Examination of cnidarian-symbiont systems revealed metazoan parasites and symbionts including an unusual cestode-like larva parasitic in the cabbagehead jellyfish, Stomolophus meleagris.