By Ethan Bensinger (Pitzer College) and Josh Weiss (Pitzer College) [Edited by Lars Schmitz, as part of BIOL 167 “Sensory Evolution”, an upper division class at the W.M. Keck Science Department. Written for educational purposes only.]
Most starfish species possess compound eyes with no lens at the tip of their arms, which, except for a lack of true optics, resembles an arthropod compound eye.
The presence of compound eyes on starfish have been known for almost two centuries but have had no visually guided behavior associated with them until now. A recent study by Garm and Nilsson (2014) investigates the importance of these eyes for navigation. They studied Linckia laevigata, a species of starfish that lives predominantly on coral reefs. Garm and Nilsson determined that these starfish have eyes of low resolution, meaning that they are unable to distinguish objects that are “too small” for them to see. This information led the investigators hypothesize that starfish use vision to find their way back to the reef and navigate towards it.
Garm and Nilsson indicate that the compound eyes of starfish are involved in negative phototaxis, which is defined as the orientation and movement of an organism in response to light stimulus. The results of Garm and Nilsson’s experiment provide an example of starfish eyes supporting only low-resolution vision, which is believed to be an essential stage in eye evolution before high-resolution vision was necessary for prey/predator detection. Yoshida & Ohtsuki (1968) researched phototaxis in starfish in a prior study and found only some indications that the eyes are possibly involved in this sort of behavior. There had been no behavior directly associated with the eyes, and it was still uncertain why they possess such prominent eyes. As such, Garm and Nilsson’s paper may be a breakthrough in this area.
Let’s provide some background to starfish vision. Starfish eyes are unique, as they use a combination of microvilli and ciliary type photoreceptors. Most derived deuterostomes use ciliary type photoreceptors only, whereas microvilli photoreceptors are typically associated with arthropod eyes. Microvilli photoreceptors are likely favored by most invertebrates because they are very versatile and adaptable to many light conditions, whereas vertebrates utilize two different kinds of ciliary receptors largely because this duplex retina may be more efficient (Fain, Hardie, & Laughlin 2010). Given the occurrence of both microvilli- and ciliary-type photoreceptors in many metazoans, including basal chordates, it is likely that the ancestral echinoderm had both photoreceptor types as well. Hence sea urchins, characterized by microvillar photoreceptors only (Ullrich-Lüter & Dupont et. al 2010) would have lost the ciliary photoreceptors secondarily.
So, starfish have two types of photoreceptors but what are they good for? Finding a function for compound eyes in starfish is rather significant. Otherwise, it would make little sense for an organism to have such delicate structures. Garm and Nilsson hypothesized that starfish eyes would not be acute enough to detect organisms around them; rather, they use their eyes to navigate towards and through coral reefs. Since echinoderms are one of the most basal groups of deuterostomes the navigation of coral reefs as the primary behavioral function of the starfish eyes reveals an important initial drive for eye evolution. Starfish are also generally not mobile enough to use sensory systems to avoid predators, so the only visual guided behavior they would need is to find their habitat.
Garm and Nilsson investigated how starfish used their vision by blinding several starfish and comparing their navigational abilities to that of non-blinded ones (Figure 1). Five adult-sized subjects had their eyes removed, while five other subjects had their eyes left intact. The animals were then left to recover and tested one day later. The subjects were tested by simultaneously placing each subject one meter away from the reef. All starfish were unable to navigate back to the reef if the reef took up less than 30 degrees vertically, which closely matches their estimated resolution determined by extracellular electroretinogram and a goniometer. Their movements were monitored for 25 minutes, and photos were taken at 5-minute intervals to calculate their speed of movement at each time. The researchers found that the animals that were blinded were completely unable to find the reef, while those that were not blinded were able to without trouble in under a half an hour. Apparently, the dark appearance of the reef as opposed to the bright signal received from the open ocean (as inferred by spectral sensitivities, Figure 2) is providing a sufficient visual cue. The blinded starfish were determined to be just as active as the non-blinded starfish but instead moved in random directions. Although this behaviorally shows the importance of eyes in starfish, it remains unknown if the behavioral importance of sea urchin photoreceptors is more than simply avoiding light.
What else could be done in this area? Garm and Nilsson would have created a far greater understanding of the specific function of the two different photoreceptors, had the authors blinded only one type of photoreceptor rather than both. However, this would have substantially increased the difficulty of the experiment. Additionally, they performed a very superficial analysis of how this study illuminates eye evolution, with no comparison made between starfish and other echinoderms. From an evolutionary perspective, the morphology of eyes in L. laevigata, in addition to their optical quality, is close to the theoretical “primitive eye” that developed when image formation first appeared evolutionarily. Further investigation of this topic can clarify what the first visual tasks were that motivated such an important step in optical evolution, specifically regarding navigation. Future research on other species of starfish would also be necessary to broaden the scope of this paper, especially whether starfish that live in tide pools utilize their vision when there are no large structures that define their habitat. Nevertheless, Garm and Nilsson’s paper is a big step towards a better understanding of vision in starfish.
Garm, A. & Nilsson, D. 2014 Visual navigation in starfish: first evidence for the use of vision and eyes in starfish. Proc. R. Soc. B 281. (DOI 10.1098/rspb.2013.3011)
Fain, G. & Hardie, R. & Laughlin, S. 2010 Phototransduction and the evolution of Photoreceptors. Curr Biol. 116. (DOI: 10.1016/j.cub.2009.12.006.)
Ullrich-Lüter, E. & Dupont, S. et. al. Unique system of photoreceptors in sea urchin tube feet. Proc. Nat. Acad. Sci. 8367. (DOI:10.1073/pnas.1018495108)
Yoshida M., Ohtsuki H. 1968 The phototactic behavior of the starfish Asterias amurensis Lutken. Biol. Bull. 134. (DOI:10.2307/1539869)