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Open Access Highly Accessed Correspondence

The giant eyes of giant squid are indeed unexpectedly large, but not if used for spotting sperm whales

Dan-E Nilsson1*, Eric J Warrant1, Sönke Johnsen2, Roger T Hanlon3 and Nadav Shashar4

Author Affiliations

1 Department of Biology, Lund University, Lund 22362, Sweden

2 Biology Department, Duke University, Durham, NC 27708, USA

3 Marine Biological Laboratory, Woods Hole, MA 02543, USA

4 Department of Life Sciences, Eilat Campus, Ben Gurion University of the Negev, Eilat 88000, Israel

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BMC Evolutionary Biology 2013, 13:187  doi:10.1186/1471-2148-13-187

Published: 8 September 2013

Abstract

Background

We recently reported (Curr Biol 22:683–688, 2012) that the eyes of giant and colossal squid can grow to three times the diameter of the eyes of any other animal, including large fishes and whales. As an explanation to this extreme absolute eye size, we developed a theory for visual performance in aquatic habitats, leading to the conclusion that the huge eyes of giant and colossal squid are uniquely suited for detection of sperm whales, which are important squid-predators in the depths where these squid live. A paper in this journal by Schmitz et al. (BMC Evol Biol 13:45, 2013) refutes our conclusions on the basis of two claims: (1) using allometric data they argue that the eyes of giant and colossal squid are not unexpectedly large for the size of the squid, and (2) a revision of the values used for modelling indicates that large eyes are not better for detection of approaching sperm whales than they are for any other task.

Results and conclusions

We agree with Schmitz et al. that their revised values for intensity and abundance of planktonic bioluminescence may be more realistic, or at least more appropriately conservative, but argue that their conclusions are incorrect because they have not considered some of the main arguments put forward in our paper. We also present new modelling to demonstrate that our conclusions remain robust, even with the revised input values suggested by Schmitz et al.

Keywords:
Vision; Eyes; Giant squid; Sperm whale; Bioluminescence; Mesopelagic