Horseshoe crabs are real living fossils that have changed little over the past few hundred million years. Among their unique features are eyes protruding from the outside of a large carapace while the rest of the animal is hidden inside. The new article describes the horseshoe crab’s unique eye structure, which greatly distinguishes it from other animals.
Horseshoe crabs are a group of large marine invertebrates, represented by only a few living species. They have a massive shell (exoskeleton) and a threatening tail spike, especially when they come ashore in large clusters during the breeding season. In fact, for humans, these creatures are safe and even beneficial, since horseshoe crabs are collected on the shore in tons and used as livestock feed, fertilizer and in scientific research.
Horseshoe crabs are vaguely reminiscent of crabs: in English they are called horseshoe crab. However, in reality, these outlandish animals are much closer to spiders and scorpions, with which they are combined into one type – chelicerae.
Horseshoe crabs are classified as living fossils because their appearance has changed little since the Paleozoic era, when they first appeared on Earth. One of the unique features of these invertebrates is the eyes, which have repeatedly attracted the attention of biologists.
Now the authors of a new article in the journal Advanced Science have figured out the device of the organ of vision of the horseshoe crab Limulus polyphemus in full detail. As befits an arthropod (especially one leading an active lifestyle), the horseshoe crab has compound eyes. In them, visual acuity is achieved by combining a series of simple eyes that work like lenses and collect light from various angles and transmit it to light-sensitive cells.
“Compound eyes have become nature’s answer to the need to expand the field of view based on a small eye. The same problem is faced by the creators of modern cameras, who strive to make them smaller and smaller, while at the same time maintaining a viewing angle of more than 90 degrees, ”said Professor Yael Politi, research leader from the Technical University of Dresden (Germany) .
The complex eyes of horseshoe crabs are quite primitive, but they are among the largest. But most importantly: they are very different from the eyes of flies, spiders or shrimp. The fact is that in “ordinary” arthropods, simple eyes (from which the facets of the eye are assembled) consist of glassy proteins. However, the evolution of horseshoe crabs went the other way, as a result, these marine animals look at the world through the cuticle – the outer shell that also covers their shell and legs.
The arthropod cuticle is a strong and flexible biological composite consisting of proteins and chitin, a large carbohydrate polymer. The cuticle has a number of unique physical characteristics and can vary in chemical composition, so that different invertebrates use it differently.
A striking example of the use of the cuticle “for other purposes” is just the unique eyes of horseshoe crabs. The outer transparent shell of their eye (cornea) is composed of hundreds of elongated cuticle cones directed inward. These are real lenses that concentrate light on light-sensitive cells.
The authors of the study used X-ray diffraction analysis and modeling of the horseshoe crab cornea, and also established the refractive index of light in it with high resolution. It turned out that the cuticle here has a special architecture: it is formed by a combination of chitin and proteins with bromine inclusions, which creates an optimal gradient of optical properties. The result is a matrix in which the molecules are cross-linked, some areas contain more water than others, and there are many pores.
Surprisingly, the seemingly primitive eye of the horseshoe crab works differently in direct light and when rays fall at an acute angle. The unique optical properties of his cornea are determined by a hierarchy of levels of organization and corresponding physical effects.
“At the end of the work, we were particularly surprised that the cuticular lenses seem to work so well that the animal even had to develop a special pigment that reduces the amount of light collected by them,” concluded Professor Politi.