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Off the coast of Hawaii live majestic humpback whales, mysterious argonauts, and myriad mahi-mahis. A uniquely isolated archipelago with many habitat types and 137 islands, Hawaii is a biodiversity hotspot with over 25,000 species that call it home, at least 10,000 of which cannot be found anywhere else in the world. But today, we’ll focus on an animal over 500 times smaller than the humpback whale: the Hawaiian bobtail squid (Euprymna scolopes). This squid, though it grows to a mere 35 millimeters—under half the diameter of a baseball!—has been purposefully studied in the laboratory since at least 1972 and is now the subject of hundreds of academic papers. Clearly (or not), there must be something special about it. How can a species so physically small attract scientific attention so disproportionally large? What has it, for lack of a better word, inked ?

What makes the Hawaiian bobtail squid special?

Did a scientist decide on a whim that the Hawaiian bobtail squid was worth studying? Chance may have played a role, but there are other reasons too. The squid has several traits that make it an ideal model organism—the kind of creature scientists like to work with, and work with often: It’s small. It grows rapidly. And it’s available year-round. This means that scientists can reliably access, manipulate, and study it in a lab. Importantly, the squid only associates with a single bacterial species – Vibrio fischeri – making their relationship easier to understand than, say, a relationship involving one of the over 10,000 microbes living in and on humans. The close, and often beneficial, interaction between two species is termed symbiosis. In return for nutrients supplied by the squid, the bacteria produce light that helps camouflage its host. (Downwelling light casts shadows that predators can see; the bacteria’s light minimizes the squid’s shadow.)

Because the squid is born without any bacteria within it, and because it can survive independently, isolating newborn squid within an hour of hatching allows researchers to study the squid without the influence of its symbiont. Researchers can similarly study the bacteria when it is outside of the squid. And, predictably, they can study the squid and bacteria’s deep partnership and interconnection.

How do squid and bacteria find their respective partner?

From the outset, the lives of squid and bacteria are intertwined. Even though Vibrio fischeri is relatively rare, accounting for under 0.1% of bacteria in Hawaiian seawater, juvenile squid capture enough bacteria to become bioluminescent within a single day. And despite that relative rarity, the Hawaiian bobtail squid does not host any other bacterial species, including those that are much more abundant. These two organisms seem to be made for each other, and only each other. How can this be?

The Hawaiian bobtail squid uses both physical and chemical deterrents to create a very unpleasant environment for most bacterial species. Any potential inhabitant encounters numerous hazards. Immune cells gobble up foreign invaders, constant water flow washes away trespassers, mucus traps and sheds microorganisms, and enzymes help produce toxic, bacteria-killing molecules. With these layers of defense, it may be a bit of a surprise that V. fischeri can even colonize the Hawaiian bobtail squid.

Like humans, squids need oxygen to sustain their tissues and survive. Squids obtain this oxygen by drawing in seawater—which teems with a multitude of bacteria—through their mantle cavity, passing it through their gills, and expelling it through their siphon. The Hawaiian bobtail squid is no exception. However, its anatomy is slightly different; intaken seawater is routed past a duct that leads to the light organ where V. fischeri eventually thrives.

Most bacteria will never interact with the squid and will leave on the same stream of water that carried them in. Nevertheless, some bacteria will be captured by mucus secreted by surface-lining cells – epithelial cells in biological parlance – at the entrance of the duct. V. fischeri senses and migrates towards certain complex carbohydrates, amino acids, and sugars present in the mucus. This precise attraction is specific to V. fischeri and even other Vibrio species do not exhibit the same behavior. Epithelial cells with cilia, which are small, hair-like protrusions, create currents that further help V. fischeri aggregate in the mucus.

Aggregating allows V. fischeri to attempt a push across one of 6 ducts (in the juvenile squid; adults have 2), through an antechamber, past a microns-wide bottleneck, and into one of 6 crypts of the light organ, a task made more arduous by duct-lining cilia that continuously sweep particles outward. Unlike most bacteria, V. fischeri can make it through the duct because it has a tuft of flagella, a tail-like structure that allows the bacteria to move freely.

Physical challenges aren’t all; the squid secretes harsh chemicals that V. fischeri uniquely has the ability to degrade. Other molecules produced by V. fischeri also help it embed itself within the light organ, such as by forming biofilms critical to bacterial aggregation. In the end, for each of the 6 light organ crypts, only one or two V. fischeri prevails in establishing itself.

So the squid subject the bacteria to a rigorous selection process. Do the bacteria affect the squid too?

Just as the squid plays an outsized role in the bacteria’s life, the bacteria affect the juvenile squid’s development, too. Case in point: Ever heard of “structure determines function?” In the juvenile squid, the light organ functions to capture V. fischeri. In the adult squid, its primary role is to retain and support the bacteria. It makes sense, then, that the light organ is structurally different at different life stages.

The bacteria produce lipopolysaccharides (LPS) and peptidoglycans (PGN), molecules which are classically pathogenic and disease-causing in other organisms, that instead play a critical role in maturation of the light organ. While other bacteria produce these molecules as well, they are primarily secreted by the rotation of V. fischeri’s flagella, an uncommon structure that most bacteria lack. When the light organ is populated, the ciliated epithelial cells that were essential for V. fischeri to gain entry in the first place are killed through apoptosis, or programmed cell death, over a 4-day period. This irreversible change in the light organ’s structure, akin to shutting the door on other bacteria, only happens with signaling from V. fischeri.

In an experiment where mutated V. fischeri could not secrete the transformation-inducing peptidoglycans, the mutated V. fischeri faced increased competition from other bacteria, which is undesirable as other bacteria do not produce light that helps the squid. After introducing a second strain of V. fischeri to squids that had already been exposed to either normal or mutated V. fischeri, the researchers found the new bacteria was 10 times more prevalent in the light organ when squids were previously colonized by the mutated V. fischeri.

Once colonization happens, the squid also stops secreting mucus, which is necessary to trap bacteria at the entrance of the light organ. Notably, the squid is extremely sensitive to the bacteria’s presence. When just 3 to 5 bacterial cells interact with cilia in the squid’s light organ, immune cells migrate to the light organ. As a final example of how the squid and bacteria are dynamically linked, the bacteria changes the expression of squid genes in not only the light organ itself, to favor the migration of more V. fischeri, but in the eyes and gills as well, two areas relatively remote from the light organ. This is light-dependent as mutating V. fischeri to eliminate its light production reduced the changes in gene expression by two-thirds.

Since it was first described in 1913, the Hawaiian bobtail squid has captured intense scientific attention. We now better understand communication across species, which can be incredibly and unexpectedly versatile, with the bacteria physically altering the squid, transforming its gene expression, and driving its internal circadian rhythm. In addition, we now know that lipopolysaccharides, traditionally thought to be pro-inflammatory and solely “bad,” can be anti-inflammatory too. We can also study tissues that interact with light, which has already had technological implications; quorum sensing, which may have relevance to diseases such as pneumonia; and even squid development, which is possible because the juvenile squid is transparent. And it is all thanks to this unassuming squid and its unrivaled “special relationship” with an equally unassuming bacterium.