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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.
Semibalanus balanoides (common barnacle, common rock barnacle, or northern rock barnacle) is a common and widespread boreo-arctic species of acorn barnacle. It is common on rocks and other substrates in the intertidal zone of north-western Europe and both coasts of North America .
This acorn barnacle is a moderate-sized one with a diameter of up to 22 mm (0.9 in). [1] The shell is formed by overlapping plates and has a calcareous basis. It has more the shape of a cylinder than the shape of a cone. The white operculum has heavily ridged walls. [1] It can live up to ten years. [1]
Most barnacles are encrusters, attaching themselves to a hard substrate such as a rock, the shell of a mollusc, or a ship; or to an animal such as a whale (whale barnacles). The most common form, acorn barnacles, are sessile, growing their shells directly onto the substrate, whereas goose barnacles attach themselves by means of a stalk. [8]
Balanus nubilus, commonly called the giant acorn barnacle, is the world's largest barnacle, reaching a diameter of 15 cm (6 in) and a height of up to 30 cm (12 in), [3] and containing the largest known muscle fibres. [4] [5] Balanus nubilus is a northeast Pacific species that ranges from southern Alaska to Baja California. [6]
Balanus is a genus of barnacles in the family Balanidae of the subphylum Crustacea.. Fossil shells of Balanus from Pliocene. This genus is known in the fossil record from the Jurassic to the Quaternary periods (age range: from 189.6 to 0.0 million years ago.).
In mature individuals (barnacles at least ten millimeters in diameter) the white vesiculae seminales are very much enlarged at this time and filled with spermatozoa, occupying much of the body cavity and the penis is also greatly enlarged. At the same time, a creamy mass of eggs are present in the ovarian tubules. Fertilisation takes place over ...
Semibalanus balanoides (Linnaeus, 1767) (northern acorn barnacle) Semibalanus cariosus (Pallas, 1788) (thatched barnacle) Semibalanus madrasensis (Daniel, 1958)