The Stunning Diversity of Crustacean Eyes
By Stormie Collins, Crusty Eyes- 27August2025
When we think about the ocean’s most mesmerizing creatures, for some people, crustaceans aren’t always the first animals that come to mind.
Yet, hidden within their alien-like forms are some of the most fascinating and beautiful eyes in the animal kingdom. From shallow coral reefs to the super dark deep sea environment, crustaceans have evolved a dazzling array of eye types, each exquisitely tuned to their environment. Like insects, many crustaceans have compound eyes. These beautiful eyes are mosaics of tiny light-sensing units called ommatidia, each capturing light like a single pixel in a larger image, but not all compound eyes are created equal.
Most baby crustaceans have apposition eyes, in which the “window” of the ommatidia each lead directly to the retina. Apposition eyes are common in shallow-water and brightly lit environments. Each ommatidium captures light from a narrow angle, creating a sharp, high-resolution image. The tradeoff? Apposition eyes don’t perform well in dim light, making them ideal for sunlit shallows but less effective in darker habitats. Superposition eyes, on the other hand, dominate in dim and nocturnal environments. Instead of limiting each ommatidium to a narrow light path, they allow light from multiple ommatidia to be pooled together, dramatically increasing sensitivity. The cost is reduced resolution. The images received by the eye are typically brighter but tend to be blurrier. This balance between sensitivity and resolution is one of the central themes in eye evolution: animals living in bright, colorful environments benefit from crisp detail, while those in the dark depths need every photon they can get.
Fig. 1- the elongate eye of the amphipod, Oxycephalus sp. that comprises nearly its entire head.
In the deep sea, where sunlight barely penetrates, many crustaceans have taken the superposition design to an even more extreme level: the superposition reflecting eye. Instead of bending light with a lens, these eyes use tiny biological mirrors to reflect and focus light onto the retina, pooling light from neighboring ommatidia to increase the amount of light (and therefore image) that falls onto the photoreceptors of the retinae. This mirror-based design allows for incredible light capture, perfect for detecting the faintest glimmers in the abyss, whether it’s the glow of bioluminescent plankton or the shimmer of a predator’s movement. Many deep-sea crustaceans also have a tapetum lucidum, a reflective layer behind the retina that bounces unabsorbed light back through the photoreceptors, giving them a second chance to detect it. This same adaptation is what makes cats’ eyes glow at night when caught in headlights. In crustaceans, it’s a key tool for survival in perpetual darkness.
Fig. 2- the superposition reflecting eye and glowing tapetum lucidum of an oplophorid shrimp, Systellaspis pellucida.
What makes crustacean eyes even more remarkable is that their biological mirrors aren’t just reflective, but that they’re tuned to specific wavelengths of light. In the clear blue waters of the open ocean, many species have mirrors that reflect bluish hues, matching the dominant wavelengths that penetrate those depths. In greener coastal waters, their mirrors shift in color to match the environment. This fine-tuning enhances visual performance by optimizing contrast against the background light field. In some cases, these mirrors shimmer with metallic greens, blues, or golds, making crustacean eyes not only functional but breathtakingly beautiful when viewed up close.
Fig. 3- the blue mirror eye or a larval frog crab (Raninidae).
From the sharp-eyed mantis shrimp scanning coral reefs to the deep-sea shrimp straining to catch the faintest glimmer of bioluminescence, crustacean eyes tell a story of adaptation and survival. They embody one of evolution’s greatest balancing acts. Trading resolution for sensitivity, adjusting mirrors to match the seascape, and even recycling stray photons in the eternal night of the deep sea. Next time you peer into the tidepool or see a shrimp dart through the water, remember that behind those gleaming eyes lies a masterpiece of engineering. An adaptation fine-tuned over millions of years to paint a vivid picture of a watery world we’re only beginning to understand.
