Triangle barnacle
Triangle barnacles found encrusting a shell in Portugal: they are introduced to the Atlantic ocean.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Thecostraca
Subclass:	Cirripedia
Order:	Balanomorpha
Family:	Balanidae
Genus:	Balanus
Species:	B. trigonus
Binomial name
Balanus trigonus Darwin, 1854

Balanus trigonus, the triangle barnacle, is a species of barnacle in the family Balanidae. It is steep-sided, conical barnacle, has six shell plates and is pink in colour. Originally found only in the Indo-Pacific and the east Pacific coast, they have now been introduced to the Atlantic Ocean and are now found worldwide. Usually living on subtidal rocks and shells, they are also by nature foulers of ships and dock infrastructure, and, in areas where they are invasive, may compete with native species for living room. First described in the genus Balanus by Charles Darwin, the triangle barnacle has since had its mitochondrial genome sequenced.

Balanus trigonus was first described in 1854 by Charles Darwin, with its current name.^{[1]: 223 [2]}

The mitochondrial genome of the triangle barnacle has been completely sequenced.^[3] The mitochondrial genome is the DNA found in the mitochondria and making up just a part of the organism's entire genome; it can be used maternal lines of descent.^[4] Triangle barnacles' mitochondrial DNA comprises 15336 base pairs and 37 genes with an order and traits similar to other Balanid species. The mitochondrial genome comprises 22 transfer RNA genes, two ribosomal RNA genes, and 13 protein-coding genes – for comparison, the human genome has about 20,000 of the latter genes.^[3][5]

Triangle barnacles are medium-sized and roughly cone-shaped, growing to tall. The barnacle shells include a shell wall, comprising plates which are connected by sutures to wrap around to form a circular barrier. In this species, the shell wall is made of six dark pink plates which are covered with white rib-like ridges as well as pores. Each plate is made up of a central triangular portion named the parietes, and overlapping side segments known as radii; the latter, in triangle barnacles, are paler than the parietes, being white or pale pink. The shell also includes a calcareous basis, which forms the bottom of the shell and connects to the ground or material the barnacle is growing on. Triangle barnacles' basis is flat-shaped and pore-covered. The opened top part of the barnacle shell, which connects to the outside, is called the orifice. In this species it is relatively smooth and triangular in shape. It can be closed by the operculum, which is composed of two small shells, known as the tergum and the scutum, divided by a small linear opening; this acts as something of a lid for the barnacle. In living triangle barnacles, often only the scutum can be seen, and has up to six lines of small indentations formed by deep furrows crossed perpendicularly by high ridges. The tergum, meanwhile, has a wide but dulled spur which can take up up to half the opercular valve's width.^{[6]: 352 & 359–61}

Triangle barnacles, once native to the Indian and Pacific oceans, have been introduced to the Atlantic Ocean and are found in coastal areas around the world.^[7][8] They are known from both coasts of the Atlantic – though not the British Isles – from Labrador through Central America and parts of the Caribbean to points just south of Río de la Plata in the west, and from Denmark southwards along the European coast, in the Mediterranean Sea, in the Canary islands, the Azores, and parts of the West African coast, and in Southern Africa from Angola through to Cape Town. According to the Global Biodiversity Information Facility (GBIF), they are largely absent from the east African coast, except for in South Africa and one report from the Gulf of Aden.^[8] They are reportedly native to the Red Sea.^[7] Eastwards, triangle barnacles are abundant off the coast of Oman, and have been reported from the southeast coast of Iran. The GBIF asserts they are not found around the Indian subcontinent nor in much of Maritime Southeast Asia, although they are known from the Philippines, Taiwan, and southeastern China; they have been found in Okinawa, Kyushu, Shikoku and the southern half of Honshū. They are known from Jeju Island and are abundant around South Korea. Triangle barnacles live around Australia – and have been reported at Lord Howe Island – and are found in New Zealand, primarily on that country's North Island, where they are fairly abundant. Triangle barnacles also live around Hawaiʻi, and are much found in southern California, where they are a native species. Elsewhere in the eastern Pacific coast, they have been known as far north as Vancouver Island. They also live on coasts through central America down to the Galápagos, as well as in Peru, where they are native, and have been reported as far south as Valparaíso, Chile.^[7][8][9]

Adult triangle barnacles attach themselves to shells, wood, rocks, and the hulls of ships. The barnacles are often considered fouling organisms of ships and dock infrastructure. In parts of the world where they are an invasive species, they may compete with other, native species for living room.^[7] They are usually a subtidal species, living below the water level, but not beyond 60 metres (200 ft) below the surface.^{[6]: 362 [9]}

Barnacles are sessile as adults but have motile larval stages, first the six stages nauplii, and then the cyprids, which don't eat but attach themselves to a substrate and metamorphosize into adults.^[10] Triangle barnacles' larvae are relatively large: laboratory-raised specimens were approximately 570 micrometres (22 mils) long.^[11]: 34 The nauplii are always without spines on their backs and undersides, and the front half is relatively wide and circular, giving the nauplii a "teardrop"-like appearance.^[11]: 18

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