Science News - How hogfish see with their skin

There are a number of animals in the ocean that can flash colors right before your very eyes. When threatened, a common cuttlefish may get a black stripe, like a masked bandit, over its eyes as its tentacles flair angrily. An octopus will change its color and texture to match its surroundings, making it practically invisible in the landscape. 

How do these animals know when to change color? Are they expertly taking in their surroundings with their eyes, or is there something else at work? In an effort to get to the bottom of this phenomenon, researchers Schweikert, Fitak, and Johnsen from Duke University in Durham, North Carolina have decided to take a look at another one of nature's magicians, the hogfish.

Taken by Albert Kok - Wikimedia Commons

Hogfish are brilliantly colored reef fish famous for their mating behaviors. All hogfish are born female, but after a female matures and reaches a certain level of social dominance, the female changes her sex and becomes a male. While hogfish live in groups, there is only one male, guarding his own personal harem. If something should happen to him, a dominant female will rise up, change sexes, and replace him. 

Hogfish are just as adept at changing their color as they are at changing their sex, shown below in a video uploaded by Mark Karl in 2014.

The key to these changes lie in pigmented skin cells called chromatophores. Chromatophores rapidly rearranged pigments in the skin to change the color, morphing the appearance of either a small area on the animal or even the entire animal itself. However, scientists are still trying to figure out what tells chromatophores to change color. Rather than the hogfish seeing with their eyes, researchers believe hogfish are, in a sense, seeing with their skin, a process called dermal photoreception.

To test out this theory, the researchers at Duke University looked at the genes hogfish have in both their skin and in their eyes. Examining genes is one of the best ways to look at what is happening throughout the body because they serve as the blueprint for proteins in the cells. Proteins are essentially the workers of each cell, so by looking at genes, the researchers were looking at the job descriptions of proteins in the eyes and skin of hogfish. After these proteins do their job, however, the rest of the workers in the cell need to know what to do. Messages can be spread through the cell by other proteins in a signal pathway, a phenomenon similar to a game of telephone. Scientists can also see the code for this kind of communication in the genes. Essentially, the scientist looked at the job description of various workers in each cell as well as how these workers communicate to describe what was going on. 

Photo by Bernard Dupont - Wikimedia Commons

What the researchers found is that hogfish can see with their skin, although not in the same way they can see with their eyes. Hogfish have a protein in their skin called SWS1 that can sense ultraviolet light; in contrast, hogfish eyes have five separate proteins that can be used to see. In addition, the signal pathway that SWS1 uses to communicate, called cAMP, is different from the signal pathway used by the proteins in the eyes, or cGMP. This means that while hogfish use both their eyes and their skin to see, the eyes and the skin don't have the same proteins, and these proteins don't communicate in the same way. While both the eyes and the skin of hogfish can be used to sense the light around them, both parts of the body are doing it in a completely different way.

This discovery is groundbreaking; it is the first time that scientists have been able to show that color-changing fish can sense visual signal, light, with something other than their eyes. There is a lot more work to be done to figure out how much the color of the fish is impacted by the eyes versus the skin and how this factors into how other animals change color. Still, this is an exciting first step in exploring one of nature's coolest phenomenon.

Article Citation:

Lorian E. Schweikert, Robert R. Fitak, Sönke Johnsen. De novo transcriptomics reveal distinct phototransduction signaling components in the retina and skin of a color-changing vertebrate, the hogfish (Lachnolaimus maximus). Journal of Comparative Physiology A, 2018; DOI: 10.1007/s00359-018-1254-4

Blue-bloods in America - How horseshoe crabs can save your life

The horseshoe crab looks menacing. Its dark body crawls along the sand with the creeping speed of a movie monster running through molasses. If picked up, its spikelike tail swings wildly, trying to right itself as a tangle of legs and small pinchers thrash around slippery gills. When it bleeds, it bleeds blue. It looks like a small, ancient monster, and in many ways it is. The horseshoe crab has remained virtually unchanged through human history and is considered a living fossil by scientists. It's not surprising that many squeal and run when they see the horseshoe crab on the beach, but everything about the horseshoe crab is harmless. The tail so many people think will sting them is only used to flip the animal back over when the waves knock it on its back. The claws are tiny and used to grab food and one another while mating. There is nothing in this animal that can hurt you. In fact, chances are this animal has actually helped you.

Mating horseshoe crabs.
Females are typically larger. Males use boxing glove shaped claws to grab onto the female and hitch a ride.
Photo credit Wikipedia

Horseshoe crab blood is more unique than even it's blue color would let on. Limulus polyphemus (the scientific name for horseshoe crabs) has special cells called amoebocytes. If you are thinking "wow, that sounds like something I looked at in high school biology," you are probably pretty close. Amebocytes get their name from the way they move like amoebas, a one-celled organism that moves by changing its shape and extending foot-like bulges in its body. Basically you can think of amoebocytes as little blood cells creeping through horseshoe crabs looking for particular types of bacteria and toxins, like E. coli. When an amoebocyte finds what it is looking for, it starts to ooze proteins that cause the blood to congeal. This effectively traps toxins in their tracks and leaves a conspicuous marker that the bacteria are there in the first place. Long story short, if you are trying to stop E. coli, this is a dream.

An amoeba, for which amoebocytes are named
 Photo credit Wikimedia Commons

The medical industry relies on this kind of a test. Referred to as the LAL test (named for Limulus, the horseshoe crab's scientific name, and amoebocyte, the cell) people have used the blood of horseshoe crabs to test medicines and medical products for decades. With a bit of horseshoe crab blood, the industry can help ensure that bacteria aren't being placed directly into patients' bloodstreams. According to a PBS documentary on horseshoe crabs, every drug certified by the FDA must be tested using LAL. Great wonders of nature don't necessarily come cheap. Horseshoe crab blood sells for as much as $14,000 a quart.

When there is money to be made and lives to be saved, people will follow. The horseshoe crabs are taken out of the water, placed on a table, and bled from a weak part of their shells. The animals are then rereleased into the ocean and assumed to go on their merry way. However, nobody actually knows how many horseshoe crabs this practice kills, and there is some reason for concern. Horseshoe crabs bled by labs can be out of the ocean for up to 72 hours, and returning the crabs back to the water after such a long time and the loss of blood may leave them disoriented. While most groups that harvest horseshoe crab blood estimate that only 3% of the horseshoe crabs they bleed die, some scientists say it may be as high as 10-15%. There have been few efforts to monitor what actually happens to horseshoe crabs once they are returned to the water, so in a lot of ways the practice is left in the dark. In the meantime, horseshoe crabs have been steadily disappearing from beaches. One study noted horseshoe crab populations decreased by more than 80% in Cape Cod between 1984 and 1999 (Widener and Barlow 1999). Horseshoe crabs have continued to disappear from New England waters, and last year the status of the animal was changed to "vulnerable," one level below "endangered."

Photo credit Wikimedia Commons

It may be that as we continue to increase, and thus our need for medicine increases, horseshoe crabs may not be able to keep up with this demand. When volunteering at the New England Aquarium, I talked to a number of people who have visited the Cape every summer throughout their lives. Some of them would get a little misty eyed when they would see a horseshoe crab; they used to see beaches covered in them, and now, showing them to their kids was a rare treat. The exact cause of the decline of these animals isn't pinpointed, but as long as we are dependent on them, this decline is bad news for both of us. If we could develop an alternative LAL test that would work without capturing these unwilling blood donors, maybe we could leave these gentle crawlies alone. I can't think of a better way to thank the species that has unwittingly saved so many lives.