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)