Weird Jellyfish Eyes: Are They Missing Links?
“Missing Link?” asks the cover of Nature May 12, next to what looks like an alien head with a giant eye staring out. The article by Nilsson et al.1 suggests that the box jellyfish has optical sensors that could represent primitive eyes that evolved before the more advanced eyes of vertebrates. Most of us don’t think of jellyfish with eyes; “In the light of the current interest in early eye evolution,” they say, “the uniquely evolved eyes of box jellyfish have been neglected.” But just how primitive are these strange eyes?
Cubozoans, or box jellyfish, differ from all other cnidarians by an active fish-like behaviour and an elaborate sensory apparatus. Each of the four sides of the animal carries a conspicuous sensory club (the rhopalium), which has evolved into a bizarre cluster of different eyes. Two of the eyes on each rhopalium have long been known to resemble eyes of higher animals, but the function and performance of these eyes have remained unknown. Here we show that box-jellyfish lenses contain a finely tuned refractive index gradient producing nearly aberration-free imaging. This demonstrates that even simple animals have been able to evolve the sophisticated visual optics previously known only from a few advanced bilaterian phyla. (Emphasis added in all quotes.)
Yet these sophisticated eyes do not focus sharply on a retina, they say:
However, the position of the retina does not coincide with the sharp image, leading to very wide and complex receptive fields in individual photoreceptors. We argue that this may be useful in eyes serving a single visual task. The findings indicate that tailoring of complex receptive fields might have been one of the original driving forces in the evolution of animal lenses.
The paper claims that these jellyfish figured this out on their own: “From the unique crystallin proteins we know that at least the lenses have evolved independently in box jellyfish,” they say, noting that “Making good lenses seems to be a demanding task, because only few animal phyla have accomplished it.” Also, they contain a number of eye-like parts: “All major components of a typical camera-type eye are present: a cornea, a lens, a retina, a pigment layer and an iris.” The tiny lenses, about a tenth of a millimeter across, are spherically symmetric; yet by means of a variable index of refraction, they are able to form “good images.” The packing density of the specialized crystallin proteins provides the refractive index gradient. The researchers measured some pretty remarkable optical qualities, but also some aberration:
Tracing rays through the refractive-index gradient of the upper eye reveal nearly perfect focusing for all ray positions (Fig. 2). For such a minute eye it is surprising to find well-corrected, aberration-free imaging, otherwise known only from the much larger eyes of vertebrates and cephalopods. The gradient in the upper-eye lenses comes very close to the ideal solution. The lenses of the lower eye have a less ideal gradient and consequently display some spherical aberration (Fig. 2e, f). It is the homogeneous lens core and steep peripheral gradient that results in positive spherical aberration in the lower eye.
Surprisingly, both kinds of eyes are severely under-focused. Is this due to clumsy eye geometry, or could there be a reason for under-focused eyes for a jellyfish?
Another, more likely, interpretation is that the eyes are ‘purposely’ under-focused to remove high spatial frequencies (fine image details) from the retinal image, much as occurs in insect dorsal ocelli. If the arrangement is indeed a spatial low-pass filter, it would help the animals to detect the large and stationary structures of their visual environment, but would leave unseen the plankton and small particles floating with the current. Assuming that the lens eyes have evolved to allow the jellyfish to remain in nearshore habitats and to avoid swimming into obstacles, a low-pass filtering of image structure would make sense.
It is not known how the visual information is processed. The authors suggest that the data is filtered early in the jellyfish eye, not requiring a complex brain:
In box jellyfish we find these large complex receptive fields at the level of photoreceptors, indicating that the eyes might be specialized for a specific task only and that this allows complex filtering of information much earlier than in more general visual systems. The fact that box jellyfish have four different types of eye gives support to the idea that each eye type is highly specialized.
So how do box jellyfish fit into the story of eye evolution?
The early evolution of animal visual systems is likely to have started out with eyes that were involved only in single visual tasks. In this perspective it is interesting to note that high visual acuity is not necessarily desirable. The lens eyes of box jellyfish indicate that there might be visual tasks best served by a blurred image. Evolution of sophisticated eyes might therefore be a process with discrete stages representing the sequential addition of visual tasks. Our results also indicate that advanced lenses with graded-index optics might have evolved for tailoring complex receptive fields and not just for improving sensitivity or acuity.
Not many science reporters seem to have given this story a glance. Michael Hopkin in News@Nature avoided speculating that these were missing links, titling his review “Box jellyfish show a keen eye.” Yet New Scientist made evolution its centerpiece: “Multi-eyed jellyfish helps with Darwin’s puzzle,” the title states, claiming it represents a “possible path from simple to complicated” eyes. Given the blurry imaging system of the box jellyfish, the article concludes, “From here it would be an easy step to evolve an image-forming eye.” Susan Milius, on the other hand, writing for Science News,2 warned against such speculation. “Biologists need to be careful in working out the evolutionary implications of the new study,” she says, quoting Alan Collins of NOAA. “The eyes of box jellyfish, cephalopods such as the octopus, and vertebrates seem to have arisen independently. So, unraveling the evolution of box-jellyfish eyes may not reveal the particular path of eye evolution for other lineages.” Her article contains a stunning color picture of the box jellyfish, eyes and all.
1Nilsson et al., “Advanced optics in a jellyfish eye,” Nature 435, 201-205 (12 May 2005) | doi: 10.1038/nature03484.
2Susan Milius, “Built for Blurs: Jellyfish have great eyes that can’t focus,” Science News, Week of May 14, 2005; Vol. 167, No. 20, p. 307.
Oh, how the Darwinists would love to find a sequence of complexity in eyes, to ease Chairman Charlie’s stomach pains when contemplating the wonderful designs in nature. But this story can’t help. The jellyfish eyes appear over-designed for their task (see 06/19/2002 entry). These remarkable optically-near-perfect structures are well adapted to the needs of the organism. Perfect focus would be a drawback for the jellyfish. It would create an image crowded with irrelevant details. Instead, it has a “low-pass filter” to help it see what it needs to see: large, stationary objects so that it can avoid obstacles and find prey in its habitat. If an animal has a structure that meets its needs and is well designed, is it not a non-sequitur to say it is evolving? Did the Darwinists find a gradual sequence of intermediates leading from primitive eyes to complex eyes? No. Brittlestars (see 08/23/2001), trilobites (09/18/2003) and even sponges (08/20/2003) exhibit optical perfection, yet none of these are on a phylogenetic line – evolutionists claim all these things evolved independently (and suddenly, too, considering they burst onto the seen during the Cambrian, without ancestors). So instead of helping Charlie sleep better, this story gives him more indigestion: his tale now needs multiplied miracles of chance and natural selection to keep from falling apart.
We need to get rid of the useless Darwinspeak in biological research, and focus instead on the functional information and adaptive excellence of each species. Arranging the tools in your garage into a hypothetical evolutionary sequence does nothing to help you use them better.