Squid Use Polarized Light to Attract Mates, New Study Reveals

While many animals rely on bright colors, sounds, or movements to attract a mate, new scientific research reveals that some species of squid utilize a more subtle method of gaining the attention of females: the polarization of light.

An Unusual Courtship Display

A study published January 26th in the journal Proceedings of the National Academy of Sciences found that a squid species known as Doryteuthis andersoni possesses a remarkable ability to send light signals during courtship—signals invisible to the human eye, yet strikingly clear to other squid.

Researchers observed that male Doryteuthis andersoni have a pair of notably long arms, much longer than the arms of females. Normally, these arms are coiled in front of the head and inconspicuous. However, during courtship, the male extends these arms toward the female in a display, revealing lines that appear to shimmer.

According to lead study author Arata Nakayama, a postdoctoral research fellow in the Department of Living Marine Resources at the University of Tokyo, these animals are able to exchange secret visual messages that humans and even some predators cannot perceive. He notes that the squid uses this property as a form of hidden language during mating, rather than relying solely on traditional colors.

The Mystery of Vertical Polarization

Initially, the shimmering may appear as a simple glow. However, when researchers used specialized cameras capable of measuring light polarization, they discovered something more significant: the males display not just luminous lines, but a dual pattern of polarized light. The team found that the male’s arms produce adjacent areas emitting two different types of polarization—horizontally polarized light and vertically polarized light.

For squid, which have vision sensitive to polarization, the presence of these opposing patterns next to each other makes the signal exceptionally strong, akin to the distinct difference between two completely contrasting colors. This means the male’s display becomes “visually striking” to the female even in dim lighting or murky waters.

Researchers discovered specialized reflective cells called “iridophores” that function like tiny mirrors, initially reflecting light horizontally. However, the light doesn’t remain horizontal; it shifts to vertical as it passes through a transparent muscular layer within the arm. This muscular layer possesses a physical property known as birefringence, which changes the direction of polarized light as it passes through.

The muscles act as a “light transformer,” rotating the direction of polarization from horizontal to vertical, turning the squid’s arm into a natural “light screen” generating complex signals during courtship. The shape of the arm itself also plays a role; the male arm is thicker and more cylindrical than the female’s, providing the ideal pathway for light within the tissues and allowing the polarization angle to shift by 90 degrees.

Frequently Asked Questions

What is light polarization?

Light is composed of waves vibrating in different directions. When these vibrations become organized in a single direction, it’s known as light polarization.

Which animals can see polarized light?

Humans cannot distinguish this type of light, but many marine creatures like squid and octopus, as well as some insects, possess the ability to see it.

How do male Doryteuthis andersoni squid use polarized light?

Male squid use polarized light to create a complex visual signal during courtship, displaying both horizontally and vertically polarized light to attract females.

As research continues, scientists may uncover further intricacies of this unique communication method and potentially discover similar strategies employed by other marine species. It is possible that a deeper understanding of these hidden signals could reveal new insights into the mating behaviors and social dynamics of these fascinating creatures.