This was supposed to be a short, simple post about how brittle stars use their skeletons to help them see. It turns out, however, that the brittle stars have misled us, and what we thought we knew twenty years ago has been overturned by new observations. The question of what and how various creatures see merges physics and biology in fascinating ways, getting a spot on my list of “What is Biophysics?” topics. (This is #11 in the series; here’s #1 and #10.).
Brittle stars are sea creatures, similar to sea stars, with which they’re often confused. (Yes, my post title contributes to the confusion.) Brittle stars have no eyes. They can, however, detect light using photoreceptor cells distributed throughout their bodies. A mineral skeleton that includes bulbous bits of calcite supports the animal.
In a beautiful paper published in 2001, Joanna Aizenberg et al.  (links at the end) showed that these calcite blobs behave like lenses, focusing light to a point underneath. Through both optical calculations and direct experimentation — shining light through cleaned skeletons onto a light-sensitive film — they demonstrated not only that focusing occurs, but also that the shape of the calcite structures minimizes spherical aberration, a form of distortion. Each lenses focuses light to the depth at which the photoreceptor cells reside. It’s a beautiful story about an elegant, functional form nature has evolved. The paper, published in Nature, caused a splash. It has been cited nearly 700 times to date.
The shocking twist came in 2018, with a paper crisply titled, “Whole-body photoreceptor networks are independent of ‘lenses’ in brittle stars,”  The authors (Lauren Sumner-Rooney et al.) looked carefully at brittle star anatomy and realized that the photoreceptors don’t lie at the calcite lens focal points. In fact, the photoreceptors have no discernible spatial relationship with the putative lenses at all. The authors write, “The structural optics of these crystal ‘lenses’ are an exaptation and do not fulfil any apparent visual role.” I’m not sure what exaptation refers to here — the term means a trait that plays a role different than what it originally evolved to do, and perhaps the authors mean that there may originally have been an optical function that isn’t currently the case. Or perhaps the current function is to get materials scientists excited… The paper is thorough, readable, and surprising. I didn’t see it in 2018, by the way — I don’t get notifications about breaking echinoderm news — but rather just a week ago when I thought about writing about the 2001 paper. There’s also a nice Nature news piece about Sumner-Rooney et al.’s paper .
I don’t know if there’s a deep lesson to learn here. It’s challenging to try to understand the natural world, and it’s tempting to conclude that because something could have a particular function, it does have that function. The temptation is especially strong when the function seems clever or elegant. We can’t, in the case of the brittle star, design different calcite shapes and see if the organism responds to light differently; we’re stuck simply looking at whether the proposed function is reasonable. Of course, we should look critically and try to poke holes in our explanations. This is, in fact, what happened, though it’s remarkable that it took 17 years, and a completely different research group, to dive into the anatomy of the brittle star.
More relevant to this “What is Biophysics?” series: the question of how organisms see is a wonderful one, and though many of the scientists exploring it (probably including Sumner-Rooney et al.) wouldn’t characterize themselves as biophysicists, the topic clearly lies at the intersection of physics and biology. (It already surfaced in our series, in #6.) There’s an amazing diversity of eyes and vision in nature — nicely summarized in a recent article, “The Diversity of Eyes and Vision”  — and undoubtedly lots to explore.
Even the brittle stars have more optical tricks up their (quintuple?) sleeves: They sense light, but do they “see?” Can they discern that there’s a dark region “over there” that they’d like to get to, for example? Apparently, for some brittle star species, the answer is “yes,” and the animals’ vision is controlled by the motion of pigmented cells (Ref. , Sumner-Rooney & co, again). I haven’t read much about this, but I think my summary is correct.
Finally, to illustrate that everything is stranger at the bottom of the ocean, there are bioluminescent deep-sea brittle stars with glowing arms . (The links are below; see esp. Fig. 5a).
Papers [2, 5,] and  all have Esther Ullrich-Lüter, of the Museum für Naturkunde, Berlin, as an author (the corresponding author in [2, 5]); I’m sure hers is a fascinating lab to visit!
There’s nothing about brittle stars in my pop-science biophysics book, though sea urchins come up in the context of animal development. And perhaps, combining insights into embryogenesis (Chapter 7) and tools for rewriting genomes (Chapter 15), we can in the future make brittle stars with lenses at the right spots. Who knows what they’ll see! Here are the usual links: My description, Publisher, Amazon.
Today’s illustration: A brittle star, which I based on this photograph and an illustration from page 40 of the 1910 textbook, “First Course in Biology” by L. H. Bailey and W. M. Coleman (MacMillan, 1910), magically freely available here.
 J. Aizenberg, A. Tkachenko, S. Weiner, L. Addadi, G. Hendler, Calcitic microlenses as part of the photoreceptor system in brittlestars. Nature.412, 819-822 (2001). Link
 L. Sumner-Rooney, I. A. Rahman, J. D. Sigwart, E. Ullrich-Lüter, Whole-body photoreceptor networks are independent of ‘lenses’ in brittle stars. Proceedings of the Royal Society B: Biological Sciences.285, 20172590 (2018). Link
 G. Guglielmi, How brittlestars “see” without eyes. Nature (2018). Link
 D.-E. Nilsson, The Diversity of Eyes and Vision. Annual Review of Vision Science.7, 19-41 (2021). Link
 L. Sumner-Rooney, J. D. Kirwan, E. Lowe, E. Ullrich-Lüter, Extraocular Vision in a Brittle Star Is Mediated by Chromatophore Movement in Response to Ambient Light. Current Biology. 30, 319-327.e4 (2020). Link
 J. Delroisse, E. Ullrich-Lüter, S. Blaue, O. Ortega-Martinez, I. Eeckhaut, P. Flammang, J. Mallefet, A puzzling homology: a brittle star using a putative cnidarian-type luciferase for bioluminescence. Open Biology. 7, 160300 Link
— Raghuveer Parthasarathy; October 21, 2022