Between 200 and 1000 meters below the ocean's
surface lies the mesopelagic zone.
In this depth range, only a minute fraction
of sunlight is present.
Inhabitants of this dimly-lit realm face many
challenges; chief among these, they must avoid
being seen by predators.
To stay concealed from predators, many creatures
here use camouflage such as transparency,
reflective surfaces, or dark coloration.
Others use more deceptive means of disguise.
One such deception is visual mimicry.
This method of protection is commonly observed
in the insect world.
In mimicry systems, there are three participants:
the model, the mimic, and the dupe.
By copying the model's distinctive appearance,
the mimic evades detection by duping potential
predators.
For mimicry to be successful, the habitat
must contain an abundance of unpalatable models.
In the mesopelagic zone, the abundant siphonophore
Nanomia bijuga is one such potential model.
Siphonophores not only lack sufficient nutrient
density to make them worthwhile prey for most
active predators, but they also have numerous
stinging cells.
Recently, scientists at MBARI have used video
collected by remotely operated vehicles, or
ROVs, to describe the first case of siphonophore
mimicry by a cephalopod.
Chiroteuthis calyx, a common, solitary squid
inhabiting the temperate waters of the North
Pacific, is an ambush predator.
They hover, motionless in the water column,
and use bioluminescent lures on their tentacle
tips to attract crustaceans or small fish,
and then grab them with their tentacles and
arms.
We discovered that, like other cephalopods
inhabiting the mesopelagic zone, juvenile
and subadult Chiroteuthis use a diversity
of body patterns consisting of postures, colors,
bioluminescence, and locomotion that help
the squids communicate, capture prey, and
evade predation.
Chiroteuthis calyx is commonly known as the
swordtail squid, owing to the large, ornamented
tail that only the juvenile life stage possesses.
An intact tail more than doubles the length
of these squid, and allows them to save energy
by providing buoyancy and stability.
But the tail may also serve another function.
We discovered that Chiroteuthis juveniles,
when in the presence of Nanomia, orient and
color their tail and body to closely resemble
these siphonophores, and thus remain concealed
from predators (not to mention scientists).
However, this mimicry is not consistent across
life stages; as juveniles progress into the
subadult life stage, they lose their tail,
and also the ability to resemble Nanomia.
When observed by ROV cameras, subadults did
not demonstrate behaviors mimicking Nanomia.
It is likely that the smaller and more vulnerable
juvenile Chiroteuthis avoid predation as a
result of mimicking Nanomia's appearance and
behavior.
In this case, the model was the siphonophore
Nanomia, the mimic was the squid Chiroteuthis,
and the dupe was would-be predators - and
us!
