Researchers have determined that the broad range of eye forms in hyperiid amphipods, a small group of microscopic crustaceans, is accompanied by significant variations in the organization of their brains.
Their study, published April 7 in Proceedings of the Royal Society B, flies in the face of the prevailing understanding that nervous systems within specific animal groups tend to share a common, highly conserved blueprint, regardless of diversity in external anatomy.
"We know the brain is very conservative. If you compare all the mammals, all the major brain regions are the same, and all [the] relative positions of these brain regions and how they connected, basically the main tracks of neurons, are the same," said first author Chan Lin, who explained further that this conservation seen in mammals is also seen in arthropod groups like crustaceans.
Lin is a lecturer and course director at the University of Maryland, but he completed this study as a postdoctoral researcher with the Smithsonian National Museum of Natural History, where he and his colleagues had access to a huge collection of invertebrates, including hyperiid amphipods.
Hyperiids live in the midwater, a region of the ocean between the surface and the deep sea with little sunlight and slow-moving currents. Like many animals in the midwater, they are microscopic. They also exhibit a startling array of diversity after millions of years of competition for limited light and food.
"So far, we have found at least 11 distinct type of eyes, and many of those are super-weird [types that] we never see in other arthropods," Lin said. "Cystisoma, for example — the eyes are almost half the size of the body, so [they] cover the entire head."
While diversity in eye forms, especially among arthropods, is not unusual, the researchers were curious about how this diversity may be linked to the structure of the crustaceans' nervous system.
"So now we have a small group, only 350 species with such a diversity of eyes; then, does the brain still conserve?" Lin asked. "Our original hypothesis was that even though there is a very wide diversity of eyes, we expect to see the same organization in the brain."
The team tested the hypothesis by studying both preserved amphipod specimens from the Smithsonian and live animals they collected off the coast of Monterey Bay in California. Using a combination of 3D imaging, cell staining and immunochemistry, the researchers were able to determine whether their optic lobes were as conserved as expected.
"The surprise is, they [were] not," Lin said. "Their brains [were] as diverse as their eyes."
The hyperiids' brains were not only different from other crustaceans, they showed a remarkable amount of diversity within the group. Size of the optic lobe varied from 6% to 73% of total brain volume. Some species were also missing entire structures within the lobe.
Lin explained that it is highly unusual to see this level of diversity in a single brain system of a group, though some have proposed it is possible for brain systems to evolve independently.
"There's a hypothesis called mosaic brain evolution hypothesis, [which] proposes that if you just look at different brain systems, they can actually evolve kind of independently," he said. "And now what we found is that even within the visual system, you can break the rule."
The researchers are next interested in investigating precisely how this type of brain and eye diversity emerged in hyperiids through natural selection. They hypothesize that extreme evolutionary pressure in the environment due to lack of sunlight and hiding places triggered such changes.
Meanwhile, Lin explained that the findings demonstrate that studies comparing little-known animals still have value for examining the big picture of evolutionary biology, despite the tendency of modern research to use well-known model organisms or genetic techniques.
"Nature, especially the marine biome, that's the best environment. Best laboratory," he said. "And we really like to connect how ecology and evolution, and neuroscience, how they interplay with each other."
The study, "Strange eyes, stranger brains: exceptional diversity of optic lobe organization in midwater crustaceans," published April 7 in Proceedings of the Royal Society B, was authored by Chan Lin and Thomas W. Cronin, University of Maryland Baltimore County; Henk-Jan T. Hoving, GEOMAR Helmholtz Centre for Ocean Research Kiel; and Karen J. Osborn, Smithsonian National Museum of Natural History.