The effect of cohabitation of Tubifex tubifex (Oligochaeta: Tubificidae) populations on infections to Myxobolus cerebralis (Myxozoa: Myxobolidae).

The competitive interactions between susceptible and resistant Tubifex tubifex (Oligochaeta: Tubificidae) exposed to Myxobolus cerebralis (Myxozoa: Myxobolidae) infections were investigated in two laboratory trials. Competition was assessed by the total parasite production over the course of the trials in mixed and pure cultures of M. cerebralis exposed worms, and by the genetic analyses of worms from the control and experimental groups at the beginning and end of the experiments. Mixed cultures of resistant and susceptible worms showed a 70% reduction in production of parasites released when compared with pure cultures of susceptible worms. In studies with laboratory and field-collected oligochaetes the mixed cultures at the end of the cohabitation experiments were dominated by resistant Tubifex from lineage V (HB strain) this strain of Tubifex has a competitive advantage over worms from other lineages. The results of this study suggest that certain species of Tubifex may be dead-end hosts to M. cerebralis by absorbing or inactivating the parasite and may also show greater survival compared to susceptible oligochaetes in certain whirling disease enzootic habitats.

The severity of whirling disease among wild trout corresponds to the differences in genetic composition of Tubifex tubifex populations in central Colorado.

We analyzed the geographic distribution of Tubifex tubifex from various river drainages in central Colorado by genetic screening with specific mitochondrial 16S ribosomal DNA (mt 16S rDNA) markers. Four distinct mt 16S rDNA lineages are evident. The sites varied with respect to land- and water-use practices. All sites represented habitats presumed capable of supporting oligochaetes. At the locations where whirling disease has had the greatest impact on resident rainbow trout, T. tubifex, representing lineages I and III (genotypes known to be susceptible to Mxyobolus cerebralis), were most commonly found. In contrast, at sites less affected by whirling disease, T. tubifex of lineages V and VI that are more resistant to M. cerebralis infections were more abundant. The predominance of resistant T. tubifex worms (lineages V and VI) at low-impact sites supports the conclusion that when these genotypes are in greater abundance, the potential for more severe effects of whirling disease on wild rainbow trout populations may be diminished.

Phylogenetic comparison of the myxosporea based on an actin cDNA isolated from Myxobolus cerebralis.

The full-length actin gene from Myxobolus cerebralis (McerAct-1), the first characterized from representatives in the phylum Myxozoa, encodes a 378-amino acid polypeptide with an estimated molecular weight of 41,580-Da. A phylogenetic comparison found M. cerebralis to branch outside the metazoans. This finding contrasts with previous reports that suggest an evolutionary affinity of the Myxozoa with either the Bilateria or Cnidaria.

Prevalence and susceptibility of infection to Myxobolus cerebralis, and genetic differences among populations of Tubifex tubifex.

The prevalence of infection and susceptibility of the aquatic oligochaete Tubifex tubifex to Myxobolus cerebralis, was examined in 2 studies on the upper Colorado River, Colorado, USA, where whirling disease occurs in wild trout populations. In the first study, the prevalence of infection ranged from 0.4 to 1.5%, as determined by counting the number of T. tubifex releasing triactinomyxons of M. cerebralis directly following their collection from the field. The susceptibility of those T. tubifex not releasing triactinomyxons was assessed by the number of these oligochaetes releasing triactinomyxons 3 mo following experimental exposures to spores of M. cerebralis. The prevalence of infection following experimental exposures of these T. tubifex ranged from 4.2 to 14.1%. In a second study, all T. tubifex collected at 2 different times directly from the 2 field sites in Colorado were exposed to spores of M. cerebralis. Individual oligochaetes representing those groups of T. tubifex releasing and those groups not releasing triactinomyxons at 3 mo were screened with molecular genetic markers. T. tubifex populations found at the 2 study sites consisted of 4 genetically distinct lineages that varied with respect to their susceptibility to experimental exposure to M. cerebralis. Lineages I and III contained the most oligochaetes susceptible to M. cerebralis and were the most prominent lineages at Windy Gap Reservoir, a site of high infectivity for wild rainbow trout on the upper Colorado River. In contrast, at the Breeze Bridge site which is below Windy Gap Reservoir and where M. cerebralis infections are less severe in wild trout, oligochaetes in lineages V and VI that are resistant to M. cerebralis were more prominent. These results suggest that certain habitats, such as Windy Gap Reservoir, are conducive to large and more homogenous populations of susceptible T. tubifex lineages that may serve as point sources of infection for M. cerebralis. Although not a direct objective of this study, there was no evidence of M. cerebralis infections among any oligochaetes other than those that would be classified as T. tubifex by standard morphological characteristics.

DEVELOPMENT, METAMORPHOSIS, AND SEASONAL ABUNDANCE OF EMBRYOS AND LARVAE OF THE ANTARCTIC SEA URCHIN STERECHINUS NEUMAYERI.

The development to metamorphosis of the shallow-water antarctic sea urchin, Sterechinus neumayeri, is described for the first time. Developmental stages are similar to those of closely related temperate species with feeding larvae, but the rate of development is extremely slow. Hatching of ciliated blastulae occurs approximately 140, 128, and 110 hours after fertilization at -1.8, -1.0, and -0.5°C, respectively, more than twice the time required for closely related temperate species near their normal ambient temperature. Larvae reared at -1.8 to -0.9°C are capable of feeding 20 days after fertilization and are competent to metamorphose after 115 days. Early cleavage embryos, blastulae, gastrulae, and prism larvae of this species were collected from the plankton adjacent to McMurdo Station, Antarctica, in early November and December, 1984 and 1985. Echinoplutei were not found during this study, but they have been collected from the plankton in other years; there is no evidence that the larvae are demersal. The timing of spawning ensures that feeding larvae are in the plankton during the abbreviated summer peak of phytoplankton abundance in McMurdo Sound. Recruitment of juveniles into the benthos most likely occurs in synchrony with the subsequent period of high levels of benthic chl a concentrations.