Acorn barnacle

Acorn barnacle and acorn shell are vernacular names for certain types of stalkless barnacles, generally excluding stalked or gooseneck barnacles. As adults they are typically cone-shaped, symmetrical, and attached to rocks or other fixed objects in the ocean. Members of the barnacle order Balanomorpha are often called acorn barnacles.^[1][2]

Acorn barnacles, Balanus glandula

Acorn barnacles, Semibalanus balanoides

In addition, several species of barnacles are specifically called by the common name "acorn barnacle", including:^[3][4][5]

• Balanus glandula, common acorn barnacle^[6]
• Balanus nubilus, giant acorn barnacle
• Chthamalus antennatus, acorn barnacle
• Megabalanus coccopoma, titan acorn barnacle
• Megabalanus tintinnabulum, titan acorn barnacle
• Paraconcavus pacificus, red-striped acorn barnacle
• Semibalanus balanoides, northern acorn barnacle^[7][8]