Cryptic clams: Biologists find species hiding in plain view

Mar 12, 2013
Map of Australia showing the three biogeographic provinces and sampling sites included in the U-M clam study. Map by Jingchun Li.

Cryptic comments seem to have an ambiguous, obscure or hidden meaning. In biology, cryptic species are outwardly indistinguishable groups whose differences are hidden inside their genes.

Two University of Michigan have identified three of tiny , long believed to be members of the same species, which have been hiding in plain view along the rocky shores of for millions of years.

The unusual convergence of a climate-cooling event and the peculiarities of local geography caused the three cryptic species to split from a common ancestor more than 10 million years ago, the U-M researchers propose in a paper to be published next month in the journal .

The U-M scientists conducted a after collecting thousands of the crevice-dwelling, rice grain-sized clams from hundreds of miles of southern Australia coastline over the past decade. Their findings provide insights about the forces that shape evolution and solve a puzzle that has stumped marine biologists for decades.

"This study provides important clues about how marine regional biotas can evolve, including our observation that these processes can involve major modulated by local geography," Jingchun Li, lead author of the report and a doctoral student in the U-M Department of Ecology and .

Li conducted the research as part of her dissertation with co-author Diarmaid O'Foighil, Li's adviser and director of the U-M Museum of Zoology.

Map of Australia showing the three biogeographic provinces and sampling sites included in the U-M clam study. Map by Jingchun Li.

"You cannot tell them apart physically, but their genes indicate that their evolutionary divergence predates that of humans from ," O'Foighil said of the three clam groups, which are currently classified as members of the same species, Lasaea australis.

Australia's southern coastline is home to three evolutionarily distinct assemblages of known as biogeographic provinces. Each province contains hundreds of species of invertebrates, fish, algae and other organisms, and there are substantial differences between the species living in each province.

Here's the riddle that has perplexed biologists for decades: How did these three distinct biogeographic provinces evolve along a continuous coastline? The emergence of new species often begins when gene flow between populations is reduced or eliminated. This type of genetic isolation happened routinely throughout evolutionary history when populations became physically separated—when a new physical barrier such as a mountain or a river split the geographic range of a species, for example.

But what force could drive speciation along an unbroken coastline with no obvious barriers to gene flow?

The genetic analysis by Li and O'Foighil, which is backed by evidence from the fossil record, shows that the three cryptic clam species began splitting away from a 13 or 14 million years ago.

That's about the same time that a major climate-cooling event called the middle Miocene climate transition permanently lowered sea-surface temperatures in the southwest Pacific Ocean—including the southern coast of Australia—by 10.8 to 12.6 degrees Fahrenheit.

Li and O'Foighil propose that the cooling event partitioned Australia's southern coastline into three zones with a cool region, including the present-day southeastern state of Victoria and the island of Tasmania, flanked on either side by two relatively warm ones.

The emergence of three temperature zones created opportunities for local adaptation that isolated the organisms living within each zone. That isolation led, in turn, to the evolution of the three biogeographic provinces seen today, according to Li and O'Foighil.

In their study, Li and O'Foighil showed that each of the three cryptic clam species is found in only one of the three biogeographic provinces.

"I know of no other case where you start out with one marine biota, then a climate-change event results in the generation of three biotas from that one," O'Foighil said. "A key finding of the study is that relatively ancient climate-change events can shape marine biotas."

Explore further: Arctic expedition pioneers technique for polar bear research

Related Stories

Species detectives track unseen evolution

Jul 19, 2007

New species are evading detection using a foolproof disguise – their own unchanged appearance. Research published in the online open access journal, BMC Evolutionary Biology, suggests that the phenomenon of different animal ...

Reproductive isolation driving evolution of species

May 16, 2012

Evolution of species remains a hot topic since Darwin’s theory of natural selection. A European initiative addressed the issue of speciation from the viewpoint of reproductive isolation.

Ocean heatwave decimates vital seaweed habitat

Jan 22, 2013

The decimation of a seaweed that provides vital habitat for an interdependent web of marine species off the WA coast, as a consequence of a record ocean heatwave, has been revealed in a paper published in ...

Recommended for you

An uphill climb for mountain species?

12 hours ago

A recently published paper provides a history of scientific research on mountain ecosystems, looks at the issues threatening wildlife in these systems, and sets an agenda for biodiversity conservation throughout ...

Extinctions during human era worse than thought

13 hours ago

It's hard to comprehend how bad the current rate of species extinction around the world has become without knowing what it was before people came along. The newest estimate is that the pre-human rate was ...

Robotics to combat slimy pest

17 hours ago

One hundred years after they arrived in a sack of grain, white Italian snails are the target of beleaguered South Australian farmers who have joined forces with University of Sydney robotics experts to eradicate ...

Migratory fish scale to new heights

18 hours ago

WA scientists are the first to observe and document juvenile trout minnow (Galaxias truttaceus Valenciennes 1846) successfully negotiating a vertical weir wall by modifying their swimming technique to 'climb' ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

baudrunner
5 / 5 (1) Mar 12, 2013
Someone needs to have a word with Jingchun Li. Is he getting paid for two maps?