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 (1756 Posts)

Craig McClain is the Executive Director of the Lousiana University Marine Consortium. He has conducted deep-sea research for 20 years and published over 50 papers in the area. He has participated in and led dozens of oceanographic expeditions taken him to the Antarctic and the most remote regions of the Pacific and Atlantic. Craig’s research focuses on how energy drives the biology of marine invertebrates from individuals to ecosystems, specifically, seeking to uncover how organisms are adapted to different levels of carbon availability, i.e. food, and how this determines the kinds and number of species in different parts of the oceans. Additionally, Craig is obsessed with the size of things. Sometimes this translated into actually scientific research. Craig’s research has been featured on National Public Radio, Discovery Channel, Fox News, National Geographic and ABC News. In addition to his scientific research, Craig also advocates the need for scientists to connect with the public and is the founder and chief editor of the acclaimed Deep-Sea News (http://deepseanews.com/), a popular ocean-themed blog that has won numerous awards. His 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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