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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