Echinoderms – seastars, urchins, cucumbers and others – are often used as symbols of the sea; they are found neither on land nor in freshwater so this is appropriate.  So those of us who have a lifelong enchantment with marine critters would often list these as some of our favorite organisms – certainly I do.  But wait, they have eyes and are looking at us?  Julia describes her work on this question, and explains a bit about the challenging field of research that tries to determine what other animals can sense.  We can’t ask them, so how can we know?

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Dr. Megan Dethier, FHL Director
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Seeing Stars: Into the World of Echinoderm Vision

by Julia Notar

Julia Notar is a postdoctoral scholar at FHL studying the sensory biology and visual ecology of sea stars.  She completed her PhD in Biology, researching sea urchins and brittle stars in Sönke Johnsen’s lab at Duke University and received a MS in Biology, researching sea urchins with Malcolm Gordon at UCLA.  She swears she loves other phyla, too, but the echinoderms have (clearly) captured her heart.

Who’s really watching who, here?  I’m often left pondering this as I stare at a sea star – and it, me.  If you’ve never seen the eyes on a sea star before, take a look at the very tips of its arms next time you see one.  If the tip of the arm is curled up and you look closely, you should be able to see a tiny red or orange dot there, as small as the period at the end of a sentence or a speck of pepper.  Now you and a sea star are eye to eye.

We’ve known for a long time that sea stars have eyes.  Mentions of their eyes (sometimes referred to as “optic cushions”) can be found in the literature going back to at least the late 1800s.  Ok, these active predators of the sea floor have eyes – so what can they see and what do they do with that visual information?  It might surprise you to find that these questions have only recently begun to get some attention.  As a lover of echinoderms, I am of course biased, but I think ignoring sea star vision is like ignoring the fact that wolves have eyes when we’re interested in how they hunt and the ecological effects they have on their prey populations.

Although other aspects of sea star biology have been well-studied, it was only twelve years ago that Garm and Nilson (2014) showed the Indo-Pacific sea star Linckia laevigata used its eyes to navigate back to its home reef after being displaced.  They used an elegant and simple experimental manipulation to demonstrate this: sea stars that had their eyes removed were unable to navigate back; stars with their eyes still intact (but who received a sham surgery) were able to perform the task successfully.  The paper remains one of my favorites to share with students to this day.  Looking back, I eventually realized that it was really seminal in piquing my interest in sensory biology.  Here is a charismatic, well-known marine creature who sees and senses the world and makes decisions, all in ways that are so different to our approach as humans.

I had just started my master’s degree studying sea urchins, but the ideas in the paper hooked me and over the past 10+ years I have found myself working on all sorts of echinoderm vision-related questions.  After spending my master’s and PhD studying urchin vision and brittle star learning, I was eager to tackle yet another member of the echinoderms and get back to those mysterious sea stars.  What better place than Friday Harbor Labs?

My first experience at FHL was as a first-year grad student, when I took the Comparative Invertebrate Embryology class with Drs. Billie Swalla and Andreas Heyland in 2017.  I was inspired by the Labs and its incredible natural setting, learned a ton about embryology, and made some friends for life (one of whom, Dr. Rebecca Varney, was another student in that class and has now taught the Marine Invertebrate Zoology class for a second summer, which feels very full-circle).  When I saw the listing for a postdoc position at FHL, I jumped at the chance.  I knew I would be able to get my hands on so many sea stars.  SO many.

Despite the fact that we’ve known for over a decade that sea stars have true spatial vision and can perform visual navigation, only a fraction of species have been studied with regards to their vision (Garm et al., 2023).  Most of the work has been done on the Crown-of-Thorns sea star of the Indo-Pacific and deep-sea species from the North Atlantic.  The Salish Sea is home to dozens of sea star species whose vision has not been studied.  Given that sea stars play an important role in our local ecosystems in keeping urchin populations under control, and in the wake of last decade’s outbreak of Sea Star Wasting Disease, it felt urgent to me that we understand more about these critically important animals as their populations recover.

One question I’m beginning to answer is how the eyes of the sea star grow as the animal develops from a tiny juvenile into an adult.  Sea stars have compound eyes, similar to those of insects and crustaceans, formed by multiple facets that point slightly different directions in space and help collectively gather a picture of their world.  The existing work on juvenile sea star eyes shows that Crown-of-Thorns juveniles have smaller eyes than adults, and that their eyes are too small to resolve images.  Essentially, because of the limitations of physics, eyes in general need to be a certain size in order to gather enough light to form an image.  These results were backed up by behavioral experiments showing this to be true: juveniles are sensitive to light but do not respond to spatial images.

So far, this appears to be true in a couple of local species, too.  Recently-settled juveniles of Leptasterias hexactis and Solaster stimpsoni are sensitive to light but do not orient to spatial images.  A great team of FHL undergraduates helped collect these behavioral data: many thanks to Haji Higuchi, Kiera Zuniga and Anouk Janess for their work on this!  We are able give the stars a behavioral “eye exam” by putting them in a grey cylindrical tank with a black-on-white stripe on one side (Figure 2).  If the black-on-white stripe is very skinny (too small to see), it will blend in with the grey background.  If it is large enough to see, the animal will be able to see the edges of the black stripe against its white borders.  The idea is that if an animal can see the stripe (and the stripe ‘matters’ to the sea star), it will move towards this visual target; if the stripe is not visible, the star moves randomly.  We think that the sea stars interpret the stripe as a large navigational landmark, like a reef, kelp forest, pier piling, etc., but of course, only the stars know for sure.

When we photograph the eyes, we can see how the tiny compound eyes grow by adding facets one at a time.  As these tiny juveniles grow, there is a dramatic difference in number of facets by size: 2mm L. hexactis usually only have one facet in their compound eye, and as they approach 3-4mm in size they grow a second, third, and then fourth facet (Figure 3).  This is consistent with what we know about sea star eyes, but is fairly unusual in the animal kingdom.  Many animals are born with eyes that are nearly full-size (especially in mammals), and others start with fully functioning, smaller eyes that increase all their components in size as they grow.  Sea stars seem to be adding one adult-sized facet at a time, like adding tiles to a mosaic.

In the next phase of this project, I am measuring the optics of the adults’ eyes in these two species and giving them their own behavioral ‘eye exam’ to see how they compare to the juveniles.  Though her experiments are still underway, an REU student working with me this summer, Charlotte Calero, is building a great dataset on the L. hexactis adults showing that they too have true spatial vision.

So, the next time you see a sea star crawling around, lean in and take a peek at its eyes.  I bet you’ll find that you’re already being watched.

This work was supported by the Calvin Postdoc Term Fellowship at FHL and the Brooks and Suzanne Ragen FHL Endowed Program Support Fund, awarded to Julia Notar, and the FHL REU Program, awarded to Charlotte Calero.

References:

Garm A., and N. Dan-Eric.  2014.  Visual navigation in starfish: first evidence for the use of vision and eyes in starfish.  Proceedings of the Royal Society B: Biological Sciences, 281(1777).  https://doi.org/10.1098/rspb.2013.3011

Garm A., Sundberg D., and C.E. Korsvig-Nielsen.  (2023.)  Dispersed vision in starfish: a collection of semi-independent arms.  In Distributed Vision: From Simple Sensors to Sophisticated Combination Eyes (pp. 87-115).  Cham: Springer International Publishing.  https://doi.org/10.1007/978-3-031-23216-9

Korsvig-Nielsen C., Hall M., Mott C., and A. Garm.  2019.   Eyes and negative phototaxis in juvenile crown-of-thorns starfish, Acanthaster species complex.  Biology Open, 8(6).  https://doi.org/10.1242/bio.041814

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