Monopterus

Abstract: Genetic analysis of introduced populations, especially in morphologically conservative taxa, can clarify introduction histories, identify management units and source populations, provide a more realistic es- timate of the frequency of successful invasion, and suggest strategies for preventing further introductions. In the last 7 years, populations of swamp eels, referred to the Asian genus Monopterus (Family Synbranchidae) on the basis of external morphology, have been discovered in aquatic habitats near Atlanta, Georgia; Tampa, Florida; North Miami, Florida; and most recently in close proximity to Everglades National Park in Home- stead, Florida. Swamp eels are large predators capable of dispersal over land and have the potential to dis- rupt already threatened ecosystems. We analyzed mitochondrial DNA sequences from four known popula- tions in the continental United States and samples from Malaysia, Indonesia, Vietnam, and two locations in China to determine introduction histories, source populations, genetic diversity, and relationships among populations. Our results indicate that there have been at least three independent introductions of genetically distinct forms. Introduced populations in close proximity (separated by � 40 km) are genetically distinct. The level of sequence difference among introduced populations reaches levels seen among sister families of teleost fishes for the same region of the mitochondrial genome. These genetically distinct introduced populations in all likelihood represent at least two and possibly three species. Regardless of species status, these genetically distinct lineages may be expected to vary in ecological or life-history traits, representing different potential threats to the ecosystems where they have been introduced. Given the success of swamp eels in invading many habitats around the world, further study of these eels is warranted to elucidate the characteristics of successful invaders and invasions.

Abstract: BY the publication in 1944 of a description of the gonads of Monopterus javanensis Lac. (Symbranchii; Teleostei), Liu1 has furnished new and interesting evidence concerning the mechanism of sex determination in the lower vertebrates, and has opened a fresh field for research into this subject. It is well known that, while the separate nature of the sexes is a general rule among fishes and amphibians, there are many species in which this separation is neither stable nor complete, and a great deal of research, both descriptive and experimental, has been devoted to the problem. In these circumstances it is exceptionally interesting to find that Monopterus is peculiar—and so far apparently unique—among vertebrates in being a true hermaphrodite, which functions as a female during the first half of its life and as a male during the second. It appears that this discovery may be one of considerable theoretical significance, and in order to place it into its proper context, the present state of our knowledge concerning sex determination in the lower vertebrates may be briefly summarized.