Email to a friend
Thin skin, slow-growing
gills protect larval stage of Antarctic fish
Life Sciences Editor
photo to enlarge
by Bryan Palmintier
A. Cziko, left) and Kevin Hoefling, a scientific research
prepare to place an oceanic current meter (yellow
ball) and its
home-made stand into the water. The 250-pound equipment
off the edge of the ice floe to rest on the sea floor.
meter allows scientists to obtain information on the
seawater at a single location.
— Very thin but hardy, unblemished skin and slow developing gills
appear to be keys to survival for newly hatched Antarctic notothenioids,
a group of fish whose adults thrive in icy waters because of antifreeze
proteins (AFPs) in their blood.
Such adaptations are important, researchers at the University of Illinois
at Urbana-Champaign say, because the larval fish of at least two species
of notothenioids that inhabit the Ross Sea at McMurdo Sound and Terra
Nova Bay surprisingly lack sufficient antifreeze to protect them through
their first three months of life.
The unexpected discovery, reported online by the Journal of Experimental
Biology ahead of regular publication, counters the assumption that these
vital proteins must be present from the time of hatching – a view
held by scientists since fish AFPs were found in the 1960s.
Internal fluids such as blood in many notothenioids are about half as
salty as seawater. While seawater reaches its freezing point at –1.91
degrees Celsius, fish fluids will freeze at about –1 degree Celsius.
The water where these species dwell rarely rises above the freezing
point and is regularly filled with ice crystals.
photo to enlarge
by Chrissie Callison
A. Cziko carries his compressed air tanks after a
dive near McMurdo
station (barely visible in the background) in Antarctica.
"The way that
we’ve understood how adult polar fishes survive has been based
on their use of these antifreeze proteins to lower the freezing point
of their internal fluids," said lead author Paul A. Cziko, a research
specialist in the department of animal
biology. "We finally got a chance to look at the larval fish,
and it seems that they don’t always have to have antifreeze proteins
Cziko, who earned bachelor’s degrees in honors biology and biochemistry
in 2004 from Illinois, studied in Antarctica as an undergraduate with
animal biology professors Chi-Hing (Christina) Cheng and Arthur L. DeVries,
who discovered AFPs in notothenioids.
The research team, which also included Clive W. Evans of the University
of Auckland in New Zealand, studied three notothenioid species: Gymnodraco
acuticeps (naked dragonfish); Pagothenia borchgrevinki (bald notothen);
and Pleuragramma antarcticum (Antarctic silverfish). All species develop
as eggs for between five and 10 months before hatching in icy waters
in the Austral spring. Five years of data, collected from 2000 to 2004,
While each species spawned at different depths, all larvae swam upward
into platelet ice, located just below several meters of surface ice,
when they hatched, seeking perhaps a safe area to hide from predators,
The average freezing point of the larval fish fluids was about –
1.3 degrees Celsius, according to testing with a nanoliter osmometer.
Yet the fish hatch into water at almost –2 degrees Celsius. "With
all this ice around, there is no way they can prevent freezing,"
Cheng said. "At –2 degrees Celsius, internal fluids would
freeze instantly and the baby fish would die."
"This 0.7 of a degree Celsius is small but very significant,"
Cziko said. "In adults, we find ice in their bodies but these small
crystals don’t grow because of antifreeze proteins. Finding that
larval fish don’t have enough antifreeze really threw off how
we understand survival in fish."
While the larvae of one species, the bald notothen, survives using high
levels of AFPs like the adults, the researchers were astonished to find
that the dragonfish and silverfish hatchlings have too little to allow
survival during direct contact with ice. Looking more closely, the researchers
discovered that the gills of all three species were undeveloped at hatching,
minimizing the risk of ice passing through them to get inside.
The delicately thin skin of the larval fish may offer additional protection,
because their skin hasn’t yet been exposed to environmental damages,
Cheng said. The skin and undeveloped gills, Cziko said, may combine
to allow time for antifreeze levels to rise.
The production of AFPs did not show much increase in the larval fish
until 84 days after hatching, the researchers found. Adult values weren’t
reached for 147 days.
"Amazingly," DeVries said, "for about three months the
larval fish must rely only on their skin and gills to prevent ice from
entering, and to keep them from freezing solid."
The National Science Foundation supported the research through grants
to Cheng and DeVries. Cziko was supported by a grant (Special Undergraduate
Research on the Environment) from the U. of I. Environmental