THE GIST
In this photo, we can see the velvet belly lantern shark's glowing
underside. Click
on the image to see a slide show about super shark
senses.
Jérôme Mallefet
In open water, there is often no place to hide. Some sharks have
overcome this problem by making themselves invisible to both prey
and predators, according to a new study.
Light trickery permits the optical illusion, described in the
current issue of the Journal
of Experimental Marine Biology and Ecology. The findings
represent the first experimental tests of shark luminescence.
Lead author Julien Claes explained to Discovery News that about 50
different shark species, or more than 10 percent of all known
sharks, are luminous. This means they can produce and emit light
from their bodies.
Claes and his colleagues chose to focus on one particular luminous
shark, nicknamed "the phantom hunter of the fjords": the velvet
belly lantern shark.
This shark's shimmer originates from light emitting organs called
photophores from underneath its body, "effectively creating a glow
from that region," said Claes, a researcher in the Laboratory of
Marine Biology, Earth and Life Institute at the Catholic University
of Louvain.
"Since many predators have upward-looking eyes, it is a common
method of camouflage in the mesopelagic zone (from 656 to 3,281
feet below the surface), although it is the first time it is
demonstrated in sharks," he added.
For the study, Claes and his team collected male and female velvet
belly lantern sharks in Bergen, Norway. The sharks were then
brought to Espeland Marine Station, where they were maintained in
cold, dark water tanks, replicating conditions of their natural
habitat.
Immediately after being caught, most of the sharks produced
a spontaneous and long-lasting luminescence, occasionally lasting
over an hour. The spectrum of this light closely matched that of
the shark's usual deepwater fjord home.
The scientists next measured the luminescence intensity
of each shark. Measurements were again taken after an overhead
light simulation, some days later, in order to test the sharks'
response to light.
The sharks were able to adjust slightly their emitted light in
response to external light changes. This ability suggests that they
use both their eyes and a small gland in the brain to monitor
information on light shining down from above. Like most sharks, the
mouth of this species is on its underside, so the camouflage system
allows the shark to grab prey, such as krill and pearlfish, with
invisible ease.
Similar to how lipstick makes a woman's lips stand out more, the
shark's light may also turn on members of the opposite sex.
"Communication is also a function of the luminescence, since some
parts of the animal appear brighter at close range, such as the
pelvic part containing the sexual organs," Claes suggests.
Bernard Seret, a shark expert at the National Museum of Natural
History in France, told Discovery News that he agrees with the new
study. Seret hopes the research team will explore the many other
possible functions of shark bioluminescence.
Rui Coelho, a shark research scientist at the Florida Museum of
Natural History, also supports the new paper's conclusions.
"I believe that what most surprised and excited me about this paper
was the finding that the emission of light on the ventral surface
of the sharks closely resembles the environmental light," Coelho
said, "allowing the sharks to efficiently camouflage themselves by
counter-illumination, remaining invisible to both possible
predators and potential prey."
Luckily humans are not on the prey list for the velvet belly
lantern shark. Even if we were, this glowing predator would
probably pose little threat.