Sepiolid Squid Symbiosis

As a student in microbiology I ask questions about how beneficial
bacteria associate with animals hosts: how they associate, what
benefits are exchanged through the association, and why sometimes
specific bacteria are included in an association while others are
excluded. The spin and the angle is usually to relate this back to
humans (think: why are certain species of bacteria in our mouths or
our intestines?), but there are many associations that are interesting
to study either for the novelty of the association (microbes that
provide nutrition from hydrothermal vents on the ocean floor to
tubeworm hosts), or for purposes that have economic value of their own
(a classic example is how nitrogen-fixing bacteria interact with their
plant hosts to fix gaseous nitrogen, which plants can't use, into
compounds that plants can use for growth).

In the field of beneficial microbial associations there are a number
of model animal-bacterial associations that are studied to help us
understand how partners interact in a more complex association.
Complexity can be measured as many things, but one complex factor is
the number of partners in the association. Humans carry greater than
5,000 different kinds of bacteria in our intestinal tract alone, so if
we wanted to ask how or if a specific species is maintained within the
intestine, and how, that's a difficult question to ask. In a model
where there is one animal host and one partner, you know exactly who
to look and and that the selective pressures are strong (perhaps - who
knows? - stronger than the forces our intestinal cells exert on
microbes they interact with).

One of the classic models for asking questions about how hosts and
symbionts recognize and interact with each other is the association
between a specific squid species and a specific bacterial species.
Adult sepiolid squids are about the size of a golf ball and associate
with bacteria that produce light in a special organ (called the light
organ) on the ventral side of the squid. The squid collects bacteria
from the environment through a series of selective processes, and once
inside the light organ the bacteria produce light. The light organ is
flanked by reflective tissue and lens tissue similar to our eyes that
moderates the amount of light that is reflected from the dorsal side
of the squid.

What's the benefit of the association to each partner? The light organ
is a beneficial place for the bacteria since they can reproduce more
inside the squid than outside the squid, and can be spread more widely
when released from the squid than if they were not associated. The
squid uses the light produced by the bacteria in a really cool way to
counterilluminate its shadow. The squid actively feed at night and
burrow in the sand during the day. While active at night, predators
looking up to the ocean surface from below the squid would see a
shadow cast by the squid against the moonlight. Light production by
the squid counterilluminates the shadow, and some very nice work has
showed that if you shine brighter light above the squid it will put
out more light from the organ by controlling how widely the lenses to
the organ are open.

Its a pretty neat biological phenomenon, and is just one of the ways
that bacteria interact with their animal hosts in beneficial ways. One
interesting thing is that the squid recognizes its beneficial bacteria
in many of the same ways we recognize pathogenic bacteria. This and
other work is starting to blur the distinction between beneficial and
pathogenic interactions.