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Crawfish Frog, Lithobates areolatus
Photo Credit: Stanley Trauth
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Amphibian populations around the world
have been decimated by the chytrid fungus, Batrachochytrium
dendrobatidis (Bd), but not all species or individuals in all regions are
equally susceptible. Vanessa Kinney and colleagues (2011) report the first case
of Bd in Crawfish Frogs (Lithobates
areolatus). But, more importantly, describe the nature and the course of
this disease in Crawfish Frogs which has an unusual natural history. They
investigated whether there is a life history pattern or a seasonal pattern to
infection by this fungus, and if it is possible to determine when and where the
infection is being acquired and shed. Given the concern for the conservation of
Crawfish Frogs, they examined the potential of Bd to cause fatalities in this
species and to determine if they are carriers of the fungus, as are other large
North American ranids. Crawfish Frogs are a typical North American
pond-breeding species that have explosive spring breeding aggregations in
seasonal and semi-permanent wetlands. However, when they are not breeding
Crawfish Frogs are solitary, isolated from other individuals in burrows dug by
crayfish. The burrows penetrate the water table, providing a permanent aquatic
habitat when the frogs are not breeding. Over the course of two years Kinney et
al. sampled for the presence of Bd in Crawfish Frog adults. Sampling was
conducted seasonally, as animals moved from post-winter emergence through
breeding migrations, then back into upland burrow habitats. During the study,
53% of Crawfish Frog breeding adults tested positive for Bd in at least one
sample; 27% entered breeding wetlands Bd positive; 46% exited wetlands Bd
positive. Five emigrating Crawfish Frogs (12%) developed chytridiomycosis and
died. In contrast, all 25 adult frogs sampled while occupying upland crayfish
burrows during the summer tested Bd negative. One percent of postmetamorphic
juveniles sampled were Bd positive. Zoospore equivalents/swab ranged from 0.8
to 24,436; five out of eight frogs with zoospore equivalents near or more than
10,000 died. The data suggest Crawfish Frogs acquire Bd during breeding
activities. A Bd-positive female entered a breeding pond on 8 April, 2010 with
a low infection intensity (20 zoospore equivalents) and exited 15 days later
with a high infection intensity (8,607 zoospore equivalents). A similar
situation occurred on 19 April 2010, when a Bd-positive subadult Crawfish Frog
entered the breeding pond with 119 zoospore equivalents and exited 5 days later
with 23,006 zoospore equivalents. Overall, 46% (42/91) of samples from Crawfish
Frogs exiting breeding wetlands on our study site were Bd positive. Thus,
infection rates in Crawfish Frog populations increased from near zero during
the summer to over 25% following overwintering; rates nearly double again during
and after breeding—when mortality occurs—before the infection wanes during the
summer. Bd-negative postmetamorphic juveniles may not be exposed again to this
pathogen until they take up residence in crayfish burrows, or until their first
breeding, some years later.
Citation
Kinney VC, Heemeyer JL, Pessier AP,
Lannoo MJ (2011) Seasonal Pattern of Batrachochytrium dendrobatidis Infection
and Mortality in Lithobates areolatus:
Affirmation of Vredenburg's “10,000 Zoospore Rule”. PLoS ONE 6(3): e16708. doi:10.1371/journal.pone.0016708
Labels: amphibian conservation, Batrachochytrium dendrobatidis, Lithobates areolatus