Cloacae, Cloacal Glands and Female Sperm Storage in Giant Salamanders

Most of the 620+ species of salamanders are relatively small animals, rarely exceeding 200 mm total length, but members of the families Cryptobranchidae (1800 mm), Sirenidae (978 mm), Amphiumidae (1162 mm), Proteidae (486 mm) and Dicamptodontidae (351 mm) can truly be considered "giant salamanders." These families have few species (Proteidae with six species is the most speciose) and are aquatic paedomorphs except for Dicamptodontidae, in which the four species can metamorphose. All families except Cryptobranchidae, which has single species in China and Japan, are limited in distribution to North America. Cryptobranchidae and Sirenidae reproduce with external fertilization, whereas species in the other families have internal fertilization and females store sperm in cloacal spermathecae. Males of the internal fertilizers possess cloacal glands that make spermatophores. The only sister group relationship that has been proposed is between Sirenidae and Proteidae. This paper brings together and summarizes information from a number of separate studies on histology of the cloaca and cloacal glands and ultrastructure of sperm storage, when it occurs, in the giant salamanders. Proteids have cloacae similar to those of other internal fertilizing salamanders, and if the sister group relationship with sirenids is valid, this means that internal fertilization and associated cloacal structures were lost in sirenids or they evolved independently in proteids. The cloacal evidence, however, supports the proposal in one recent paper to return the Dicamptodon, the only extant genus in the Dicamptodontidae, to its traditional position in the Ambystomatidae. Introduction “Giant salamanders” do not form a clade or a taxonomic group, but the large size attained by many species in these five disparate families makes it interesting to consider their comparative anatomy to