(BG) Acorn Barnacle (Semibalanus balanoides) with notes

http://bugguide.net/node/view/1057885/bgimage
2013-7-7 - near Seawall Pond, on Seawall Road, Southwest Harbor, Acadia National Park, Hancock County, Maine

While trying to ID these for Karen, I was surprised to learn first, that these are Barnacles(!) and second, that they're Arthropods(!) and their legs are called "cirri". Whether they are or are not S. balanoides, it's been fascinating learning about Acorn Barnacles.

Semibalanus balanaoides has several identifiable characteristics: 6 wall plates, a diamond-shaped top that doesn't look recessed like some others, the inside pale color looks about the same as the outside, the suture is sinuous, and it's found in the intertidal zones of northern oceans the world over. (Info probably from a link, don't have it, not unless it was from here, http://www.pznow.co.uk/marine/barnacles.html, but that link has since died.)

A key to Acorn Barnacles
http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Arthropoda/Crustacea/Maxillopoda/Cirripedia/Suborder_Balanomorpha_key.html

Info. here for various levels of interest.
http://eol.org/pages/336009/details

A similar species, Balanus glandula, appears restricted to Pacific coast of US
http://www.wallawalla.edu/academics/departments/biology/rosario/inverts/Arthropoda/Crustacea/Maxillopoda/Cirripedia/Balanus_glandula.html
As a point of comparison, they also state that "Semibalanus balanoides has a sinuous line of contact but also has no centripetal ridges at the interior base of the shell." "Centripetal ridges" - hmmm, yes, well, brain fried - won't be looking that one up! :oD

My favorite link is to a University student's Barnacle project:
http://courses.washington.edu/mareco07/students/nina/barnacleshome.html
An easier read and more fun than the others, it has lots of pictures and pretty colors. :o)