Vision in marine mollusks

In marine animals, the evolution of vision is – at it is on land - guided by the sort of light that is available, as well as the behavioral and ecological needs of each species [1]. Due to some of the physical constraints (optical and others) of life underwater, vision is mostly not as important for marine animals as it is for some land organisms. Instead, other sensory mechanisms, including hearing or olfaction (smell) are more important underwater, due to the greater density of this medium.

Nevertheless, developed some animals that we can find underwater some pretty amazing structures to ‘see’ and it is especially interesting that many of these are actually known for mollusks.

Scallops, for example, have a large number of small eyes arranged along the edge of their mantle (the shiny dots you see close to the shell). These eyes rely on a concave, parabolic mirror of crystals to focus and retro-reflect light. Additionally, their eyes possess a double-layered retina, the outer retina responding most strongly to light and the inner to abrupt darkness. While these eyes are unable to resolve shapes with high fidelity, the combined sensitivity of both retinas to light grants scallops exceptional contrast definition as well as the ability to detect changing patterns of light and motion [2]!

Scallops from the North Atlantic - their eyes can be seen along the edge of their mantle

And also giant clams (Tridacninae) are known to possess vision organs [3, 4]. Likewise to scallops, do their pinhole eyes allow them to distinguish between shadow and light so the clam can retract its mantle as soon as something is coming close or move fast above it. It has been even suggested that their eyes can help the clams as a behavioral reflex where the light stimulus results in an 'orientation response', during which the clam exposes its mantle towards the sun. This would be a very amazing mechanism as giant clams (due to their photosymbiotic relationship with unicellular algae) need light to thrive – this behavior would thus potentially maximize the absorption of light by the algal symbionts [4].

Outer mantle of a Red Sea giant clam

Also well known for their amazing and advanced vision are Cephalopods, including cuttlefishes. Scientists at UC Berkeley and Harvard investigated the cephalopod eye and found something very astonishing: Cephalopods are known to have eyes containing only one type of light receptor, which basically means they see only black and white. But still, they are able to change their color in perfect accordance with their surrounding!

The key is an unusual pupil – U-shaped, W-shaped, or dumbbell-shaped – that allows light to enter the eye through the lens from many directions, rather than just straight into the retina. Their wide pupils accentuate the chromatic aberration and might have the ability to judge color by bringing specific wavelengths to a focus on the retina. By focusing these wavelengths by changing the depth of their eyeball, altering the distance between the lens and the retina, and moving the pupil around to changes its off-axis location and thus the amount of chromatic blur [5]!

A cuttlefish in the Mediterranean sea

Although they might look like the marine version of a pill bug, Chitons of the Polyplacophora class are also marine mollusks. Their eyes are especially cool because contrary to the vast majority of animal lenses, that are made of proteins - chiton lenses are clearly different. They are made of calcium carbonate, or limestone, meaning that the chiton’s eyes – or the lenses, at least – are made from the same substance as their armored shell! Chitons may be the only living animals with rocky eyes of this sort! [6]

A chiton (right) and a sea urchin in Chilean Patagonia.

References: [1] Marshall, J. "Vision and lack of vision in the ocean." Current Biology 27.11 (2017): R494-R502 [2] Palmer, B. A., et al., “The image-forming mirror in the eye of the scallop” Science, 358(6367) (2017), 1172-1175. [3] Land, Michael F. "The spatial resolution of the pinhole eyes of giant clams (Tridacna maxima)." Proceedings of the Royal Society of London. Series B: Biological Sciences 270.1511 (2003): 185-188. [4] Wilkens, L.A. "The visual system of the giant clam Tridacna: behavioral adaptations." The Biological Bulletin 170.3 (1986): 393-408. [5] Temple, S. E., et al. "High-resolution polarisation vision in a cuttlefish." Current Biology 22.4 (2012): R121-R122. [6] Speiser, D., et al., "A chiton uses aragonite lenses to form images." Current Biology 21.8 (2011): 665-670.