The midwater region of the ocean houses the largest ecosystem on Earth. This region, which forms the link between the ocean surface and the bottom, is enormous — with depths ranging from 200 to 11,000 meters. Many of the inhabitants of this vast area have evolved peculiar abilities and features that allow them to survive. This includes camouflaging their bodies in darkness and in light.
Lateral aspect of hatchetfish showing general anatomy including ventral photophores, and appearance of the fish in diffuse lighting. Courtesy of the University of Pennsylvania.
One of these creatures is the hatchetfish. Named because of a body shape that resembles the blade of a hatchet, the hatchetfish has a special feature on its underside to aid in its survival — photophores. These disordered arrays of crystals are bioluminescent and can actually manipulate light.
“They spend their whole lives sort of suspended in the middle,” said Alison Sweeney, an assistant professor in the department of physics and astronomy in the School of Arts and Sciences at the University of Pennsylvania. “Because they live in this three-dimensional void, they have to deal with being potentially visible from every angle. There’s literally nothing to hide behind, and so they end up hiding within the light itself.”
Sweeney and fellow researchers Eric Rosenthal and Amanda Holt are trying to understand how and why the skin of the hatchetfish interacts with light. To do so, they boarded the National Science Foundation’s coastal research vessel, the R/V Hugh R. Sharp, and set out to catch some fish.
Darkfield reflected-light micrograph of hatchetfish skin. Individual skin cells appear as reflective wire-like structures with long axes running vertically. Courtesy of the University of Pennsylvania.
“I think there’s a fundamental curiosity of basically just how sophisticated nature is in terms of photonics,” said Sweeney.
The research team analyzed raw images of the fresh-caught hatchetfish using a Nikon D4S fitted with a 60-mm macro lens and compared the results with reflectance spectra measured using an Ocean Optics fiber optic spectrometer. They used a collimated blue LED from Thorlabs Inc. as the light source. They then used MEEP software to implement the finite-difference-time-domain method, a numerical method for solving Maxwell’s equations that govern the flow of light to understand how the guanine crystals within the skin scattered light.
“The individual scales in the hatchetfish, while wavelength sized in one plane, are micron sized in the other,” said Holt. “MEEP allows for input of irregular structures and is flexible in regards to data output.”
“We were expecting some sort of reflective crystals like fish scales,” Rosenthal told Photonics Media. “Specifically, we found two distinct layers of crystals and determined that the different layers had different properties. We argue that this difference is useful in the organism’s camouflage strategy.”
Hatchetfish, inhabitants of the vast midwater region of the ocean, have evolved the ability to manipulate light to camouflage in the deep sea. Courtesy of Josh More.
The researchers found that the high-refractive index scattering elements of the hatchetfish belly reflects light. They also discovered that, when they shined light directly onto the side of the fish, the structures there absorbed the photons and piped the light through the fish’s body to the photophores in its belly like a beam dump.
Because hatchetfish live in both shallow and deeper parts of the ocean, the versatility of their bioluminescence mechanism allows them to adapt and camouflage their bodies in either situation.
“It turns out that nature is generally quite a bit more sophisticated in how it has evolved to manipulate light,” said Sweeney. “In part this is because cells are fundamentally masterful at organizing things at length scales of tens to hundreds of nanometers, which is exactly the length scale that you need to operate at to manipulate light.”
Through light manipulation, the biology of the hatchetfish could inspire new technological applications such as synthetic materials with hatchetfish-skin-like optical properties, bringing the midwater inhabitants of the ocean up to the surface.