The all seeing, all knowing, eye of upside down barnacles

Reader Jonathan W. wrote into DSN with this

You guys are some of the most accessible in marine science, so I thought I pose this incredibly specific question that’s nagged at me for years: We all know a nauplius has a compound eye, but I’ve run across passing mention of *adult* barnacles retaining an eye or eye spot somewhere that can sense light and dark…Does a mature barnacle possess an “eye?” If so, where is it?

Well Jonathan W. adult barnacles do have an eyespot. It is a third eye that occurs in the middle of their crustacean foreheads and aligns their arthropods selves with a cosmic energy.

Do barnacles see the cosmic energy that binds us all?

Do barnacles see the cosmic energy that binds us all?

Seriously though, the adult barnacle eyespot is much cooler than a cosmic eye. The larva crawls around until it finds a favorable spot to set up shop.  Usually next to adults already hanging about, because no barnacle wants to be alone.  At this point larva attaches their head to the rock or other hard surface using cement secreted by the first antennae.  This triggers a metamorphosis that turns them into tiny adults. Basically a barnacle spends its entire life doing a keg stand, well without the beer or circle of frat boy cheering it one.

Look at this drawing of a barnacle I spent way too much time working on.

Look at this drawing of a barnacle I spent way too much time working on.

During this metamorphosis the compound eye is lost, shed with the larval exoskeleton.  It is replaced with a group, called ocelli, of 3 large photoreceptors.  These receptors have only an “on” or “off” response and can detect the presence or absence of light.  As experiment try this: barnacles in shallow water will generally withdraw and close up when suddenly placed in a shadow.

Dr. M (1628 Posts)

Craig McClain is the Assistant Director of Science for the National Evolutionary Synthesis Center, created to facilitate research to address fundamental questions in evolutionary science. He has conducted deep-sea research for 11 years and published over 40 papers in the area. He has participated in dozens of expeditions taking him to the Antarctic and the most remote regions of the Pacific and Atlantic. Craig’s research focuses mainly on marine systems and particularly the biology of body size, biodiversity, and energy flow. He focuses often on deep-sea systems as a natural test of the consequences of energy limitation on biological systems. He is the author and chief editor of Deep-Sea News, a popular deep-sea themed blog, rated the number one ocean blog on the web and winner of numerous awards. Craig’s popular writing has been featured in Cosmos, Science Illustrated, American Scientist, Wired, Mental Floss, and the Open Lab: The Best Science Writing on the Web.

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