Wednesday, 30 September 2009

Very Fishy

From the Abstract of Widespread occurrence of intersex in black basses (Micropterus spp.) from U.S. rivers, 1995–2004 by Hinck et al, Aquatic Toxology 2009.08.001
Intersex occurrence in freshwater fishes was evaluated for nine river basins in the United States. Testicular oocytes (predominantly male testes containing female germ cells) were the most pervasive form of intersex observed, even though similar numbers of male (n = 1477) and female (n = 1633) fish were examined. Intersex was found in 3% of the fish collected. The intersex condition was observed in four of the 16 species examined (25%) and in fish from 34 of 111 sites (31%). Intersex was not found in multiple species from the same site but was most prevalent in largemouth bass (Micropterus salmoides; 18% of males) and smallmouth bass (M. dolomieu; 33% of males). The percentage of intersex fish per site was 8–91% for largemouth bass and 14–73% for smallmouth bass.
Intersex was not found in largemouth bass older than five years and was most common in 1–3-year-old male largemouth bass. The cause(s) of intersex in these species is also unknown, and it remains to be determined whether the intersex we observed in largemouth and smallmouth bass developed during sex differentiation in early life stages, during exposure to environmental factors during adult life stages, or both.
The fact that no IS largemouth bass were found suggests two alternative possibilities: the first, that the IS condition reduces lifespan in this species; the second, that something new has happened in the last 5 years.

Signs of jumbled hormones that trouble creatures from turtles to alligators have now turned up in a superstar of sportfishing - the largemouth bass.

Almost one in five male largemouths from rivers around the country had microscopic egg cells inside their testes, said Jo Ellen Hinck, a biologist at the U.S. Geological Survey who worked on a nine-year project to test fish from more than 100 locations.

Bass were among just a few kinds of fish where the study found those characteristics, which researchers labeled as intersex features.

And though she doesn't know why, Hinck said bass in the Southeast were especially affected.
The Apalachicola River was the only Florida waterway checked. Sixty percent of the bass tested at Blountstown were intersex.
While other scientists have tested obviously polluted rivers for damaged wildlife, this was the most comprehensive fish research yet, said Hinck.

His last research was published last month in the journal Aquatic Toxicology.

And it raised new questions, because bass in remote areas that seemed to have little pollution had the same issues as fish caught in cities.
The rates differ markedly geographically, suggesting a variable common to these IS "hotspots". But pollution per se doesn't appear to be responsible. A particular chemical may be. Or some other common environmental factor. And the causal mechanism is unknown, but appears species-specific.

Science is like that sometimes, more questions than answers.


.:dyssonance:. said...

from my perspective, it is this sort of evidence and the questions it raises that is wondrous and incredible.

Because it reminds us that the world is both ever changing and ever complex, that the patterns are great and small, and that beauty exists in all of it.

Sara said...

Many fish carry genes to switch from large gamete to small (FtM) or vice versa (MtF) when it is advantageous to do so (typically only once). Just about all known examples are marine, such as Goby and Clownfish. But consider that Striped Bass (often found in the ocean) often coexist with Largemouth Bass in freshwater lakes. Is it so inconceivable that Largemouth Bass and perhaps all or most fish may contain genes to enable such a switch? Most Biologists that study the phenomenon are baffled as to why it isn't more prevalent as the benefits to the species far outweigh the cost to the individual. My first guess upon reading the original was that some environmental shift had occurred that is awakening the long dormant gene(s). Salinity, water temperature, or yes, even endocrine disruptors could be the trigger, but not the actual mechanism. Can't wait to see the follow ups!

Sara said...

Possible update, including possible mechanism for flip:

Environmental Effects on Fish Sex Determination and Differentiation

Environmental Effects on Fish Sex Determination and Differentiation
J.F. Baroillera, H. D'Cottaa, E. Saillantb
Sex Dev 2009;3:118-135 (DOI: 10.1159/000223077)
Vol. 3, No. 2-3, 2009

Environmental factors affect the sex ratio of many gonochoristic fish species. They can either determine sex or influence sex differentiation. Temperature is the most common environmental cue affecting sex but density, pH and hypoxia have also been shown to influence the sex ratio of fish species from very divergent orders. Differential growth or developmental rate is suggested to influence sex differentiation in sea bass. Studies in most fish species used domestic strains reared under controlled conditions. In tilapia and sea bass, domestic stocks and field-collected populations showed similar patterns of thermosensitivity under controlled conditions. Genetic variability of thermosensitivity is seen between populations but also between families within the same population. Furthermore, in the Nile tilapia progeny testing of wild male breeders has strongly suggested the existence of XX males in 2 different natural populations. Tilapia and Atlantic silverside studies have shown that temperature sensitivity is a heritable trait which can respond to directional (tilapia) or frequency dependent selection. In tilapia, transitional forms within a genetic sex determination (GSD) and environmental sex determination (ESD) continuum seem to exist. Temperature regulates the expression of the ovarian-aromatase cyp19a1 which is consistently inhibited in temperature masculinized gonads. Foxl2 issuppressed before cyp19a1. Recent in vitro studies have shown that foxl2 activates cyp19a1, suggesting that temperature acts directly on foxl2 or further upstream. Dmrt1 up-regulation is correlated with temperature-induced male phenotypes. Temperature through apoptosis or germ cell proliferation could be a critical threshold for male or female sex differentiation.

...And a little Googling reveals that each of the rivers mentioned in the original post have seen reduced flows during the time mentioned, with a corresponding increase in water temperature (many sources & studies; search for: rivername average water temperature.)