Uncommon Descent Serving The Intelligent Design Community

An attempt to answer the question, “What good is half an eye?”, using hydras

arroba Email
hydra, and stinging cells shown in red/David Plachetzki

It is a question often posed by those who doubt Darwin. From “Genetic Link Between Visual Pathways of Hydras and Humans Discovered” (ScienceDaily, Mar. 5, 2012), we learn,

The research found that the light-sensitive protein opsin in sensory cells regulates the firing of the hydra’s harpoon-like cnidocytes. These same cells are found in the mechanisms hydra use to grasp prey, and to summersault through the water.

The linking of opsin to the stinging cells helps explain how hydra can respond to light despite the absence of eyes, the scientists said, because the sensory neurons also contain the ion channels and additional proteins required for phototransduction — the process by which light is converted to electric signals. Phototransduction in humans occurs in the retina.

“I wouldn’t call this vision, because as far as we know the hydra are not processing information beyond what’s light and what’s dark, and vision is much more complicated than that. But these genes that we’re studying are the keystones of vision,” Oakley said. “For us, as evolutionists, the message is that photoreception can do other things besides just facilitate vision. It can do unexpected things. What good is half an eye? Even without eyes there are other functions for light sensitivity that we may not be thinking of.”

The problem is that opsin isn’t anything like half of an eye, nor on the way to becoming one, so far as we can see. It can help trigger stinging cells, and that is very interesting, but doesn’t answer questions about eyes.

Hunting strategy of the freshwater hydra:

And I can't help wondering how cnidocytes being triggered by a burst (?) of light alone could possibly help a hydra actually catch anything. Querius
Even from an evolutionary point of view, light sensitivity is just that, nothing more. Plants are light sensitive, but how many of them have evolved eyes? I know. Let's do an experiment. Find some kind of motile bacteria and culture it in a half lit, half dark area. After a few days and tons of radiation, kill all the ones in the dark area. Wait a few more days and repeat. Do this for 20 years. After billions of generations, the bacteria left should not only be able to see, but should have evolved the ability to read the lab schedule! Querius
I suspect, and hope, the eye is all about just segregating light. Its difficult to see how eyes could evolve from non seeing operations. I can imagine light sensitivity as raw material to work with but this itself is impossible to evolve from non seeing bits and anyways its the mechanism for change that makes evolutionary biology false. The creator probably only has one idea for seeing and types of eyes are just manifestations of this idea. Robert Byers
Oops. Observation 3 is followed by 4. Heh. Querius
You might enjoy this . . . Many years ago when I was in high school, I raised and observed hydra in Biology class. My initial attempts to raise them in Petri dishes filled with distilled water, trying to feed them daphnia following the instructions provided, were a total failure. The instructions suggested that hydra sometimes went into "depression" and refused to eat. I thought that this was an unlikely explanation so I decided to try to simulate a more natural environment. I got a large jug, cut off the top, and stocked it with pond water, elodea, daphnia, and a few hydra and located it in a shaded corner of the classroom. The results were dramatic! In a few weeks, I had a population explosion of hydra. Here were my observations: 1. I never observed hydra capturing any daphnia. In fact, the hydra seemed to reject the ones that blundered into their tentacles. 2. The hydra tentacles extended all the way across the tank in a criss-cross of extremely thin tentacles that were roughly 10 cm long--this was stunning! I could see the tentacles when I shined a flashlight through the tank against a piece of black paper. 3. I never observed the tentacles retract unless a daphia crashed into it (and swam away apparently unharmed). 3. I saw evidence of both sexual and asexual reproduction (mostly asexual). My tentative conclusion was that the species of hydra that I was raising preferred to eat protozoans instead, and that the "depression" in my original batch of hydras was due to starvation. So how would the researchers explain the function of opsin in my environment? Did the researchers actually observe the cnidocytes being triggered by changes in light or does opsin have a different function? Just curious. Querius
"Even without eyes there are other functions for light sensitivity that we may not be thinking of.” And that proves what? That it was designed that way originally? Or that it was an in between step in the evolution of the eye? Which is more likely? The evidence is the same. Just depends on what lens you look at it and interpret it with. "The problem is that opsin isn’t anything like half of an eye, nor on the way to becoming one, so far as we can see." Exactly. Design still seems the most logical conclusion. tjguy

Leave a Reply