This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law. Access to this work was provided by the University of Maryland, Baltimore County (UMBC) ScholarWorks@UMBC digital repository on the Maryland Shared Open Access (MD-SOAR) platform. Please provide feedback Please support the ScholarWorks@UMBC repository by emailing [email protected] and telling us what having access to this work means to you and why it’s important to you. Thank you. Strange eyes, stranger brains: exceptional royalsocietypublishing.org/journal/rspb diversity of optic lobe organization in midwater crustaceans Chan Lin1, Henk-Jan T. Hoving2, Thomas W. Cronin3 and Karen J. Osborn1,4 Research 1Department of Invertebrate Zoology, Smithsonian National Museum of Natural History, Washington, Cite this article: Lin C, Hoving H-JT, Cronin DC 20013, USA 2GEOMAR, Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany TW, Osborn KJ. 2021 Strange eyes, stranger 3Department of Biological Sciences, University of Maryland Baltimore County, Baltimore, MD 21250, USA brains: exceptional diversity of optic lobe 4Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039, USA organization in midwater crustaceans. CL, 0000-0003-2527-8810; H-JTH, 0000-0002-4330-6507; TWC, 0000-0001-7375-9382; Proc. R. Soc. B 288: 20210216. KJO, 0000-0002-4226-9257 https://doi.org/10.1098/rspb.2021.0216 Nervous systems across Animalia not only share a common blueprint at the biophysical and molecular level, but even between diverse groups of ani- mals the structure and neuronal organization of several brain regions are Received: 26 January 2021 strikingly conserved. Despite variation in the morphology and complexity Accepted: 12 March 2021 of eyes across malacostracan crustaceans, many studies have shown that the organization of malacostracan optic lobes is highly conserved. Here, we report results of divergent evolution to this ‘neural ground pattern’ dis- covered in hyperiid amphipods, a relatively small group of holopelagic malacostracan crustaceans that possess an unusually wide diversity of com- Subject Category: pound eyes. We show that the structure and organization of hyperiid optic Neuroscience and cognition lobes has not only diverged from the malacostracan ground pattern, but is also highly variable between closely related genera. Our findings demon- Subject Areas: strate a variety of trade-offs between sensory systems of hyperiids and even within the visual system alone, thus providing evidence that selection neuroscience, evolution has modified individual components of the central nervous system to gener- ate distinct combinations of visual centres in the hyperiid optic lobes. Our Keywords: results provide new insights into the patterns of brain evolution among hyperiid amphipods, neuroanatomy, compound animals that live under extreme conditions. eyes, optic lobes, brain evolution 1. Introduction Author for correspondence: The vast column of water below the ocean’s surface and above the deep-sea Chan Lin floor, the midwater, harbours a diverse community of poorly documented ani- e-mail: [email protected] mals that display numerous adaptations to survival in this habitat like no other [1]. In the upper reaches of the midwater (100–1000 m), limited solar light penetration, an abundance of bioluminescence and the need to see without being seen have pushed the evolution of visual systems to the extreme [2]. Downloaded from https://royalsocietypublishing.org/ on 28 April 2021 Members of the amphipod suborder Hyperiidea (Arthropoda: Crustacea: Mala- costraca) live exclusively in the midwater and exhibit a particularly impressive diversity of eye designs. These include reduced or absent eyes (figure 1b) [3], reflective eye cups [4], dorsally directed eyes covering the entire head (figure 1a) [5], eyes with dorsally and laterally directed zones (figure 1c)[5–7], replicate eye pairs (figure 1d)[5–7], eyes with 360° fields of view [8] and eyes with numerous retinas [9]. Despite the broad variation seen in hyperiid external visual struc- tures, visual circuits and neural organization behind these eyes are largely under-investigated. In arthropods, visual information is relayed from photoreceptor cells in the eye to the central brain through a series of visual processing neuropils in the Electronic supplementary material is available optic lobes. In Malacostraca (the largest class of crustaceans with approx. online at https://doi.org/10.6084/m9.figshare. 40 000 extant species including shrimps, crabs, lobsters, krill, isopods and amphi- c.5359411. pods), the organization of the optic lobes is typified by a distinct ground pattern of three nested optic neuropils connected with two successive optic chiasmata © 2021 The Author(s) Published by the Royal Society. All rights reserved. (a) (b) 2 royalsocietypublishing.org/journal/rspb (c) (d) Proc. R. Soc. B Figure 1. Four hyperiid amphipod eye morphologies. (a) Cystisoma magna, huge dorsally directed compound eyes with a diffuse retinal sheet. Ventral view of the brain and the retinal sheet (left) and whole animal (right). (b) Lanceola sayana, tiny compound eyes (white arrowhead). (c) Hyperia galba, one pair, large, dome-like 288 compound eyes with dorsally and laterally directed regions. (d) Phronima sedentaria, two pairs compound eyes (dashed line indicates dorsal eye, dotted indicates : 20210216 the lateral eye). Body lengths approximately: 8 cm C. magna (a), 1 cm L. sayana (b), 0.8 cm H. galba (c) and 1.5 cm P. sedentaria (d). (Online version in colour.) (crossed axons) [10–12]. These optic neuropils are, from distal 2. Methods to proximal (from the eyes in), the lamina, medulla and lobula. A fourth optic neuropil, the satellite lobula plate, which is (a) Animals linked through uncrossed axons from the medulla and Specimens of Cystisoma magna and Lanceola sayana were collected lobula, has also been identified in various groups of malacos- with the Monterey Bay Aquarium Research Institute’s remotely tracans [11–13]. Three putative optic lobe neuropils have also operated vehicle Doc Ricketts operated from the Research Vessel been identified in several stem-group arthropod fossils from Western Flyer between December 2016 and September 2018. the lower and middle Cambrian [14–16], suggesting that this Specimens of Hyperia galba and Phronima sedentaria were collected ground pattern arrangement may have been evolutionarily over the same time period, from the R/V Western Flyer using a modified midwater tucker trawl (1.5 m × 1.5 m opening, 1000– stable for more than 500 Myr [14–16]. It is worth noting that 200 µm mesh). ROV dives and trawls were completed over the the names used here for the malacostracan optic neuropils Monterey Submarine Canyon between the surface and 1500 m are adopted from, but may not be homologous to, those depth (36° 320 N, 122° 300 W) from 06.00 to 00.00 h. Additional neuropils with the same names in insects. specimens of C. magna, P. sedentaria and H. galba were collected The functions of malacostracan optic neuropils have been in February 2018 (POS520) and 2019 (POS532) from the submers- studied with electrophysiology and optical recording in the ible Jago and multinet hauls using a Hydrobios Maxi multinet brachyuran crabs Neohelice granulata and Carcinus maenas.It (0.5 m2 in aperture, 2 mm mesh size, nine nets) aboard the was shown that neurons in the lobula are essential for com- R/V Poseidon operated by GEOMAR Helmholtz-Zentrum für puting object features [17], object motion [18–22] and Ozeanforschung. Specimens of Procambarus clarkii were obtained encoding certain flow field information [23], while those in from Carolina Biological Supply, Burlington, NC. Specimens of the lobula plate are implicated in computing wide-field Alima pacifica were collected at Lizard Island Research Station near Australia’s Great Barrier Reef (Great Barrier Reef Marine motion and processing optic flow information that mediates Park Authority Permit no. G12/35005.1, Fisheries Act no. optomotor responses [12,13]. Although the experimental evi- 140763), Neogonodactylus oerstedii in the Florida Keys, USA and dence came only from a few model species, those functional Downloaded from https://royalsocietypublishing.org/ on 28 April 2021 Gammarus mucronatus, in Gloucester Point, VA, USA. attributes are generally assumed to apply across malacostra- cans based on the overwhelming structural conservation and anatomical similarity of those constituent neurons in (b) Osmium-ethyl gallate staining the optic lobes [12,24]. The staining was described in a previous study [25]. In brief, Given the broad diversity of eye morphologies within heads from live animals were detached and fixed in cacodylate hyperiids, how are their optic lobes organized to serve fixative (2% glutaraldehyde, 1% paraformaldehyde in 0.16 M those unique eyes? Because the central nervous system is sodium cacodylate buffer) with 10% sucrose at 4°C overnight. one of the most energetically expensive tissues, is an expan- After several washes in cacodylate buffer, brain tissue was sion in the optic lobes accompanied by a reduction in other dissected and immersed in 1% osmium tetroxide in the dark sensory processing centres, such as the olfactory lobes? In with continuous agitation for 2.5 h at 4°C and an additional 1 h at room temperature. After several washes in buffer, tissue this study, we investigate the brain organization and neural was put in a second immersion with supersaturated ethyl gallate circuits that lie beneath the various hyperiid eye types. (approx. 1% in distilled water) in the dark with continuous Specifically, we address whether or not the malacostracan agitation for 1.5 h at 4°C and an additional 30 min at room optic lobe ground pattern remains conserved across hyperiids temperature. After several washes in distilled water, tissue with different eye forms, and how the various sensory adap- was dehydrated, transferred into Durcupan plastic (Sigma, tations in the eyes relate to the structure and complexity of St.