Monday, 19 September 2016

Stonefly lays eggs or has live births depending on the season

Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

By Emily Benson
Species: Capnia lacustra

Habitat: Lake Tahoe in California and Nevada, 30 to 90 metres below the lake’s surface
An autumn birthday means one of these rare aquatic insects was born live; but a spring birthday bash means it probably hatched from an egg.

The insect, a species of stonefly called Capnia lacustra, is one of 10 invertebrate species found only on the bed  of Lake Tahoe – which lies on the border between California and Nevada.

And it is one of a kind among stoneflies. Most species in this group live in streams or rivers as juveniles, before emerging into the air as winged adults to mate and lay eggs. C. lacustra has a different strategy: it’s the only known species of stonefly that spends its entire life under water, never developing wings.

Two other types of stonefly that live in Russia’s Lake Baikal also remain wingless – but even they crawl to shore and leave the water to lay their eggs, says Annie Caires at the University of Nevada, Reno. “There are so many amazing organisms at the bottom of these deep lakes,” she says.

But the creatures’ rarity and difficult-to-reach habitat make them hard to study. A lake-wide survey in 2008 and 2009revealed that C. lacustra – and the particular plant it lives on – appears to be much scarcer in Lake Tahoe now than in the 1960s.

Two birth seasons
To learn more about this species of stonefly before its population declines even further, Caires and her colleagues collected several females visibly stuffed with eggs to study their reproductive habits in the lab. This is when they received their first surprise, finding that females were heavy with eggs at two separate times of year – spring and autumn – rather than the usual one.

The researchers kept the female stoneflies in water that matched the temperature of the lake at each time of year: about 10 °C during the autumn, and a few degrees cooler in the spring.

In the spring, the stoneflies deposited eggs from which babies emerged, as the researchers had expected. But in the autumn, the insects hatched while the eggs were still inside their mothers’ bodies and then struggled out of their abdomens as juveniles.

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