A New Species of Myxidium (Myxosporea: Myxidiidae), from the Western Chorus Frog, Pseudacris triseriata triseriata, and Blanchard's Cricket Frog, Acris crepitans blanchardi (Hylidae), from Eastern Nebraska: Morphology, Phylogeny, and Critical Comments on Amphibian Myxidium Taxonomy

During March 2001-April 2004, 164 adult anurans of 6 species (47 Rana blairi, 35 Rana catesbeiana, 31 Hyla chrysoscelis, 31 Pseudacris triseriata triseriata, 11 Bufo woodhousii, and 9 Acris crepitans blanchardi) from Pawnee Lake, Lancaster County, Nebraska, were surveyed for myxozoan parasites. Of these, 20 of 31 (65%) P. triseriata triseriata and 1 of 9 (11%) A. crepitans blanchardi were infected with a new species of Myxidium. Myxidium melleni n. sp. (Myxosporea) is described from the gallbladder of the western chorus frog, P. triseriata triseriata (Hylidae). This is the second species of Myxidium described from North American amphibians. Mature plasmodia are disc-shaped or elliptical 691 (400-1,375) × 499 (230-1,200) × 23 (16-35) μm, polysporic, producing many disporic pansporoblasts. The mature spores, 12.3 (12.0-13.5) × 7.6 (7.0-9.0) × 6.6 (6.0-8.0) μm, containing a single binucleated sporoplasm, are broadly elliptical, with 2-5 transverse grooves on each valve, and contain two equal polar capsules 5.2 (4.8-5.5) × 4.2 (3.8-4.5) μm positioned at opposite ends of the spore. Myxidium melleni n. sp. is morphologically consistent with other members of Myxidium. However, M. melleni n. sp. was phylogenetically distinct from other Myxidium species for which DNA sequences are available. Only with improved morphological analyses, accompanied by molecular data, and the deposit of type specimens, can the ambiguous nature of Myxidium be resolved. Guidelines for descriptions of new species of Myxidium are provided.

Population Growth of Antarctic Fur Seals: Limitation by a Top Predator, The Leopard Seal?

Antarctic fur seals (Arctocephalus gazella) in the South Shetland Islands are recovering from 19th-century exploitation more slowly than the main population at South Georgia. To document demographic changes associated with the recovery in the South Shetlands, we monitored fur seal abundance and reproduction in the vicinity of Elephant Island during austral summers from 1986/1987 through 1994/1995. Total births, mean and variance of birth dates, and average daily mortality rates were estimated from daily live pup counts at North Cove (NC) and North Annex (NA) colonies on Seal Island. Sightings of leopard seals (Hydrurga leptonyx) and incidents of leopard seal predation on fur seal pups were recorded opportunistically during daily fur seal research at both sites. High mortality of fur seal pups, attributed to predation by leopard seals frequently observed at NC, caused pup numbers to decline rapidly between January and March (i.e., prior to weaning) each year and probably caused a long-term decline in the size of that colony. The NA colony, where leopard seals were never observed, increased in size during the study. Pup mortality from causes other than leopard seal predation appeared to be similar at the two sites. The number of pups counted at four locations in the Elephant Island vicinity increased slowly, at an annual rate of 3.8%, compared to rates as high as 11% at other locations in the South Shetland Islands. Several lines of circumstantial evidence are consistent with the hypothesis that leopard seal predators limit the growth of the fur seal population in the Elephant Island area and perhaps in the broader population in the South Shetland Islands. The sustained growth of this fur seal population over many decades rules out certain predator–prey models, allowing inference about the interaction between leopard seals and fur seals even though it is less thoroughly studied than predator–prey systems of terrestrial vertebrates of the northern hemisphere. Top-down forces should be included in hypotheses for future research on the factors shaping the recovery of the fur seal population in the South Shetland Islands.