''Semibalanus cariosus'', commonly known as the thatched barnacle, rock barnacle or horse barnacle, is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

acorn barnacle Acorn barnacle and acorn shell are Common name, vernacular names for certain types of stalkless barnacles, generally excluding goose barnacles, stalked or gooseneck barnacles. As adults they are typically cone-shaped, symmetrical, and attached to r ...

occurring in the northern Pacific Ocean.

Description

This barnacle has six wall plates which are sculpted with vertical tube-like ribs which extend downwards onto the rock in projections rather like the edge of a thatched roof. The rostrum overlaps the wall plates and the

terga A ''tergum'' (Latin for "the back"; plural ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'ma ...

form a beak when the barnacle is closed. There is a sinuous line at the junction between the tergum and the scutum. This is a large species of barnacle and can grow to a diameter of , but when densely packed together, individuals may be much taller than they are wide. The wall plates are white, greenish, brownish or gray. The base of the barnacle is little calcified so that when it is broken from the rock, a membrane and some soft tissues may be left behind.

Distribution and habitat

''Semibalanus cariosus'' is native to the northern Pacific Ocean including Japan and the

Pacific Northwest The Pacific Northwest (sometimes Cascadia, or simply abbreviated as PNW) is a geographic region in western North America bounded by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though ...

coast of North America, where its range extends from the

Bering Sea The Bering Sea (, ; rus, Бе́рингово мо́ре, r=Béringovo móre) is a marginal sea of the Northern Pacific Ocean. It forms, along with the Bering Strait, the divide between the two largest landmasses on Earth: Eurasia and The Ameri ...

to Morro Bay, California. It is found on rocks, pilings and other man-made structures from the mid-

intertidal zone The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species o ...

down to the shallow

subtidal zone The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth. From the point of view of marine biology it forms a relatively stable and well-illuminated ...

. In general, it occupies a belt below the zone occupied by ''

Balanus glandula ''Balanus glandula'' (North American Acorn Barnacle, Common Acorn Barnacle) is one of the most common barnacle species on the Pacific coast of North America, distributed from the U.S. state of Alaska to Bahía de San Quintín near San Quintín, ...

'', often near beds of the

California mussel The California mussel (''Mytilus californianus'') is a large edible mussel, a marine bivalve mollusk in the family Mytilidae. This species is native to the west coast of North America, occurring from northern Mexico to the Aleutian Islands ...

(''Mytilus californianus'') and the bay mussel (''Mytilus trossulus''); it may be very numerous and is often the commonest organism in this belt.

Ecology

Like other acorn barnacles, ''S. cariosus'' is a

filter feeder Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

; when it is under water, the moveable terga at the apex part, and the

cirri Giovanni Battista Cirri (1 October 1724 – 11 June 1808) was an Italian cellist and composer in the 18th century. Biography Cirri was born in Forlì in the Emilia-Romagna Region of Italy. He had his first musical training with his brother ...

are extended to feed. When above water, the terga shut tightly for protection and to prevent desiccation. Small barnacles are sometimes "bulldozed" off the rock by the

limpet Limpets are a group of aquatic snails that exhibit a conical shell shape (patelliform) and a strong, muscular foot. Limpets are members of the class Gastropoda, but are polyphyletic, meaning the various groups called "limpets" descended indep ...

'' Lottia digitalis'' while it is grazing. Predatory gastropod mollusks, such as the channeled dog winkle (''Nucella canaliculata''), drill into the barnacle shell and then inject a

toxin A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849– ...

which causes the muscles to relax, enabling the winkle to consume the soft parts. The winkles were more successful at gaining entry if they drilled between the wall plates of the barnacle rather than through them. Larger barnacles may be too big for the frilled dogwinkle (''Nucella lamellosa''), the purple seastar (''Pisaster ochraceus'') or the sunflower sea star (''Pycnopodia helianthoides'') to tackle.

Human use

Native Americans such as the

Quinault people The Quinault ( or ) are a group of Native American peoples from western Washington in the United States. They are a Southwestern Coast Salish people and are enrolled in the federally recognized Quinault Tribe of the Quinault Reservation. The na ...

of Washington State used barnacles as a food resource, the staples of the diet in spring and summer being "clams, oysters, mussels, barnacles, roots, berries and fish". The shells of ''Semibalanus cariosus'' are found in their

midden A midden (also kitchen midden or shell heap) is an old dump for domestic waste which may consist of animal bone, human excrement, botanical material, mollusc shells, potsherds, lithics (especially debitage), and other artifacts and ecofact ...

References

{{Taxonbar, from=Q4295445 Archaeobalanidae Crustaceans of the eastern Pacific Ocean Crustaceans described in 1788 Taxa named by Peter Simon Pallas