Acorn barnacles (Balanus glandula) are crustaceans, related to shrimp, crabs, snails and lobsters. Their habitat is the intertidal zone, the area between the high and low tides. A sessile animal, once attached to rocks along the shore, wharfs, pilings or any other solid surface barnacles spend the remainder of their lives cemented in the same place. Once established, acorn barnacles secrete compounds that attract larvae to populated areas resulting in mats of crowded organisms.
Adult barnacles are surrounded by a strong shell composed of six plates or valves – four on the sides and two that form a little “trap door” on top that can be closed tightly and helps provide protection from predation. When the tide is in the trap door opens and the animal filter feeds on small plankton and other particles from the water using modified legs that look like feathery gills. These appendages are also involved in oxygen exchange. At low tide the valves close tightly trapping water inside and prevent the animal from drying out.
Reproduction in acorn barnacles is very interesting. Although they are hermaphroditic (having both male and female sex organs), each individual operates as either a male or a female. Since they cannot self-fertilize and cannot move, male-functioning barnacles have extremely long penises – the longest relative to body size in the animal world, up to six times the body length. The penis can be extended to a nearby female and thus sperm is passed and received from nearby neighbors. Once the mating season is over, the penis dissolves and grows back the following year.
Females brood fertilized eggs within their shells before releasing the free-swimming larvae into the water. After up to three molts with very slight changes in form, a final molt results in a larva that has a bivalve shell and additional appendages containing fats for buoyancy. At the completion of this stage, the larva settles to the bottom and searches for a place of attachment to which it adheres by its anntennules on the head and the assistance of a secretion from a cement gland. Once attached, the larva undergoes a complete metamorphosis where the bivalve shell is lost, the body mass alters and the adult shell appears.
These acorn barnacles were photographed in the tidepools at Endert’s beach south of Crescent City CA (Del Norte County).
What interesting creatures!!