Nudibranchs are colorful marine slugs that are found in almost all shallow water areas around the world. Sadly, they cannot enjoy the beautiful colors and patterns that they have, because their eyes do not allow them to see in the same way humans (and many other animals) do. This leads to a few questions. Since they can't see the color patterns, what exactly can their eyes do? What do these slugs use their eyes for? And finally, are there other ways they can sense their environment? They have to get around somehow, right?
Nudibranch eyes are located on the head, just at the base of structures called rhinophores (we'll look at these in more detail in a bit). They are extremely small, and as such are difficult to see in photographs (you can see a couple on this Sea Slug Forum page: www.seaslugforum.net/find/2377). From research that has been done in the past on eyes in nudibranchs, we know that the eyes are primarily used in a very simple way, to detect dark and light. Once they sense a change in the light intensity, nudibranchs exhibit a behavior called phototaxis, referring to the movement of an organism toward or away from a light stimulus. Whether they move toward or away depends on the species, since some are active during the day (move toward the light) and others at night (move away from the light). While it is possible that these slugs use their eyes to detect the shadow of predators, it is more likely that their eyes are primarily their way of keeping their internal clock tuned to day and night.
Since they can't see beyond being able to distinguish light and dark, how do nudibranchs find food or avoid predators? This is done primarily by use of rhinophores (See below) and oral and/or propodial tentacles (See below), additional structures that these animals possess that allow them to sense chemical changes in the water. The rhinophores (aka the nose of the nudibranch) come out of the top of the head of the slug and look a lot like antennae in insects. These structures have special receptors that allow the nudibranch to sense chemical cues in the water, enabling them to find food and avoid predators. Oral and propodial tentacles, however, are more like hands. These structures act as feelers, allowing the slugs to touch the environment around them and determine if the direction they are moving is safe, or if they need to change their path.
Though nudibranchs can't see very well, this is obviously not a detriment to their ability to survive. These slugs have evolved multiple adaptations (rhinophores, oral tentacles and propodial tentacles) that allow them to sense their environment, and they have done very well for themselves. So, while we enjoy the beautiful color patterns that these slugs have, we should also admire the amazing adaptations that they have in order to perceive their environment.
For more information on (and pictures of) nudibranch rhinophores, see the Sea Slug Forum’s rhinophore page.