Moray Eels & Visual Adaptations

(Image from Wikipedia)

This study wanted to look at the eyes of different species of Moray Eels, and how their eyes have evolved to adapt to their light-changing environments that limit their visual capabilities.

As we know, marine organisms experience a variety of photic conditions in their environments, which in turn affect their sight capabilities. Therefore, the organisms have to adapt to these conditions to survive. This study describes two types of photoreceptors that are found in most vertebrate retinas: rods and cones. (You might remember that I touched on this topic in my Mantis Shrimp blog.) In a little more detail, rods have long segments that tend to dictate scotopic vision, while cones are shorter segments that dictate photic, high activity vision. These two kinds of photoreceptors contain pigments that are made up of an opsin protein and a “chromophoric group” that are based on vitamins A1 or A2.

There are a few things that help marine organisms adapt and cope to their surroundings. First- and probably the most obvious- is that their eye and/or retina have a specific structure; think of it this way: fish that are in little to no light tend to have larger eyes, or they have the ability to reflect light. Second, they have longer segments. Lastly, they have the ability to “switch chromosome class”, or manipulate what kinds of colors they see with their eyes.

In this study, it is mentioned that Moray Eels are thought to be nocturnal predators, with smaller eyes and “well-developed olfactory sense and sensory pores”. These characteristics aid them in their foraging abilities at night. However, some reports have said that Moray Eels forage during the day. (Well that’s contradictory…) If these reports are true, and Moray Eels forage during the day, then that means that they have different visual capabilities in terms of responding to light.

For this experiment, four species of Moray Eels were studied: the Ribbon Eel (Rhinomuraena quaesita), the Laced Moray (Gymnothorax favagineus), the Dusk-banded Moray (Gymnothorax reticularis), and the Slender Giant Moray (Strophidon sathete). These four species were then divided into two groups; the Ribbon Eel and the Laced Moray were the shallow-water group, and the Dusk-banded Moray and the Slender Giant Moray were the deep-water group. The reason that these eels were divided into two groups was because they both live in completely different kinds of habitats- some with more light than others. When the differences were compared, it provided insight as to how the Moray Eels have evolved and adapted to their visual constraints due to their environments.

A few methods were used for this experiment. Tests were done to measure the thickness of each retinal layer; they hypothesized that dim light conditions would produce an increase in photoreceptor and outer layer thickness. Next, they absorbed the spectra that the photoreceptor cells took in by means of microspectrophotometry (MSP). Lastly, the opsin genes of the eels were cloned and sequenced.

*I have added the URL to the pdf version of this paper if you would like to know the details of how the samples were collected, prepared, and followed through. I would go into detail, but there is too much to include in one blog.

The results of the testing showed that the moray eels had what they call a “duplex” retina- one with rods and at least one type of cone cell. Between the four species studied, there was a similar basic structure in their retinas, but there were also differences in the thicknesses in each layer. Below, you can see how the structure of the four species is relatively similar, but the thickness of each layer varies between species.

(Image from the journal article)

Overall, the authors concluded that there was evidence that not all Moray Eels are nocturnal as thought prior to this experiment. The species that they found to be nocturnal were G. favagineus and G. reticularis. The MSP testing concluded that Moray Eels’ photoreceptor sensitivity  were related to the photic characteristics of the specific habitat that they lived in. As a general conclusion, the results of the study proved that Moray Eels have developed an adaptation to different light levels in their environments.

Source:

Wang, Feng Yu, Meng Yun Tang, and Hong Young Yan. "A Comparative Study on the Visual Adaptations of Four Species of Moray Eel." Vision Research 51.9 (2011): 1099-108.

(View the pdf here)

The Mantis Shrimp

photo from chicagonow.com

Kingdom: Animalia

Phylum: Arthropoda

Subphylum: Crustacea

Class: Malacostraca

Subclass: Hoplocarida

Order: Stomatopoda

(dept.lamar.edu)

The Mantis Shrimp- very beautiful to look at, but a complete terror in the marine world.

The Mantis Shrimp can be found in shallow waters off the shores of Palos Verdes and Catalina Island in southern California (Cabrillo Marine Aquarium). They are carnivores, and a deadly predator to other marine animals; they hunt for prey by means of stalking and sneak attacks as well as burrowing and waiting motionless for their prey before they snatch them. The Mantis Shrimp's diet includes small crustaceans, snails, clams, and fish.

Mantis Shrimp are not shrimp at all- they're crustaceans. They get their name only because they look like both a praying mantis and a shrimp. Their average size is about 12 inches in length. Reproduction varies among Mantis Shrimp. Some are monogamous, but most are polygamous. All Mantis Shrimp reproduce by sexual reproduction. This is initiated when a male Mantis Shrimp does a courtship fane to let a female know his intentions. (dept.lamar.edu) When they come together, the male will transfer sperm to the female. The female can then choose to retain the fertilized eggs, immediately lay the eggs in her burrow, or keep them on her forelimbs.

There are two characteristics of the Mantis Shrimp that I really want to get down to, though- their eyes and eyesight, and their incredible strength behind their punch and attacks.

photo from National Geographic - Italy

First their eyes and sight. As you may know, humans have three types of color receptive "cones". These cones are green, blue, and red; the red cone allows us to see red and colors derived from red- like orange when yellow is added to red, or purple when blue is added to red. (The Oatmeal) Knowing this, imagine having TWELVE to TWENTY-ONE photoreceptors! ALL OF THE COLORS! According to an article by Sebastiaan Mathôt, he says that the upper and lower parts of the eye are like the typical compound eye, but it's the Mantis Shrimp's midband that gives the crustacean their incredible ability to see so many colors. However, even though Mantis Shrimp have the ability to see many different colors, they are TERRIBLE at distinguishing color differences. In the article written by Mathôt, he goes on to explain that scientists have done color discrimination tests on Mantis Shrimp to test their vision. This included using two optical cables in an aquarium that produced color; the crustaceans were trained so that they picked the cable with the specific color (and then they were rewarded with food!). The experiment showed that when the colors were very different, their task proved easy; when they were very close in color, the task proved hard. So it was concluded that Mantis Shrimp can only differentiate between colors that are about 12nm apart... humans can distinguish color differences as small as 1nm. (Awkward...) What a wasted talent! ...Maybe they're just overwhelmed with how many colors they can see. In all seriousness, there are many possible reasons for this, so I would definitely check out Mathôt's article, here, to read more on why the Mantis Shrimp have such trouble using their crazy color sight ability.

photo from UCMP Berkeley

Now onto their terrifying boxing gloves of appendages they have. You should ALWAYS take these little dudes seriously. Mantis Shrimp have what's called raptorial appendages on the front of their bodies; you can think of these like arms, if that helps. They can move these appendages extremely fast- two milliseconds, to be exact. (UCMP Berkeley) To put that into perspective, a blink is 100 milliseconds... but I wouldn't suggest blinking if you know one of these guys is stalking you. The other part of this appendage is what they call a "smasher", and they're freakishly powerful. (HULK SMASH!) Mantis Shrimp use this club-like part of their appendage to completely annihilate any animal with a hard shell. In fact, they can smash so hard, hard shells snap like butter being split by a knife. (Scared yet?) Get this: if their smasher was the size of a human fist, the force of its punch would be equivalent to a twenty-two caliber rifle (UCMP Berkeley); so, in other words... you're dead. If that's not a good comparison for you: a Mantis Shrimp can strike prey with 1,500 Newtons of force; if humans could throw at only ONE-TENTH that speed, we would be able to throw a baseball into orbit! (The Oatmeal)

There's more about their color vision and power punch in a great comic strip on The Oatmeal website. Check it out here: http://theoatmeal.com/comics/mantis_shrimp

Sure, the Mantis Shrimp may be beautiful, but these guys are insane. Have fun trying to sleep tonight.

I'm kidding. I will, though, leave you with this video from "True Facts". I would definitely watch this if you want to see their power punch in action (plus, the video is pretty funny).