Giant barnacle

Austromegabalanus psittacus is a large acorn barnacle that lives attached to submerged surfaces on the South American Pacific coast; it is reported from southern Peru to southern Argentina ^[1]. It is a simultaneous hermaphrodite, sessile and gregarious, taken by small-scale fishing in Chile and assessed as a candidate for aquaculture ^[2],[1].

It is a cirripede crustacean with a calcareous shell, large for a balanomorph. In a sample of 100 specimens bought in southern Chilean markets in 2007, mean carino-rostral length was about 36 mm, with a maximum of 50 mm, and carinal height ranged from 67 to 140 mm ^[1]. The wall plates are built of calcite grains 0.5 to 10 µm across ^[3].

The genus includes another species, Austromegabalanus nigrescens; a 2008 paper reports both for the first time from New Zealand waters ^[4]. In northern Chile Austromegabalanus psittacus occurs together with another barnacle, Megabalanus concinnus ^[5]. The species was originally described as Lepas psittacus by Molina, and the exact year of that publication has caused confusion ^[6]; the sources consulted write the authority as Molina, 1782 ^[1] or Molina, 1788 ^[7].

It occurs along the whole Chilean coast, from southern Peru to southern Argentina ^[1]. The World Register of Marine Species (WoRMS) accepts it as a marine species; its only distribution record is New Zealand, and its sources include a list of rocky-intertidal invertebrates of Lima department ^[7].

The cyprid larvae, about 0.9 mm, settle on a wide range of submerged surfaces. In trials from October 2009 to July 2010 they settled on polyethylene tubes, plates and tarred fabric hung at depths of 4 to 6 m, at three Chilean sites between 29° and 41° south latitude ^[1].

An analysis of mitochondrial COI sequences along much of the Chilean coast found genetic structure among sites, stronger when populations were split north and south of 33° S; the authors attribute it to oceanographic transitions that affect species with pelagic larvae ^[5].

Outside its native range, a single joined group of specimens was collected in Port Wellington, New Zealand, and is reported among the first known introductions of the genus ^[4].

It is a simultaneous hermaphrodite: each individual has testes and an ovary and passes sperm to another through a penis. The testes are grouped in branching acini and two sperm ducts merge before reaching the penis, while the ovary is a sac surrounding the body. In September the ovaries were yellowish and held ovigerous lamellae with fertilised oocytes, embryos at several stages and free nauplii in the body cavity; in October they were whitish, thick-walled and filled with a milky fluid ^[2].

High fecundity and early sexual maturity are attributed to it. The main breeding period falls in spring and summer, and competent larvae are collected seasonally: from late spring to early summer, with a second period in autumn ^[1].

On collectors suspended in Metri Bay (41°36' S), individuals that settled in spring 2006 reached the average commercial size, about 36 mm carino-rostral length, after 20 months, and the minimum selling size, 21 mm, after 7 months. Harvest was possible between 18 and 24 months, with a yield of 20 to 30 g per commercial-sized individual ^[1].

It is a sessile, gregarious, omnivorous species that feeds on seston filtered from the water. Its gregarious habit causes no density-dependent effects on growth, and it tolerates environmental change well ^[1]. Juvenile collection on collectors was higher at North Totoralillo (29°29' S) than in Tongoy and Metri bays ^[1].

Under controlled conditions, the authors cite an earlier study with faster growth at 10 °C than at 16 °C, and at 4–6 m depth than at 1–2 m ^[1]. In northern Chile it occurs together with Megabalanus concinnus, and its size structure varies across its range ^[5].

In Chile, annual extractions declined progressively, between 685 and 159 t per year over the 10 years before the 2009 data, which suggests, though it is unproven, overexploitation of natural populations; the authors add that there is no information on overexploitation. Small-scale extraction was not subject to specific regulation, and no bans or restrictions had been imposed ^[1].

The period of greatest risk is spring and summer, when specimens that brood the whole embryonic development up to the first larva are removed; for that reason culture is also proposed as a way to repopulate natural beds ^[1].

In Chile it is a traditional resource of local small-scale fishing, aimed at the national market ^[1].

An assessment of culture in southern Chile estimated that one long-line can produce 7 to 10 t gross over the culture cycle and that producing frozen meat for the Japanese market would give a net present value of 490000 dollars, an internal rate of return of 32% and a payback of 4 years ^[1].

Its calcite shell has also been studied: the authors propose that its rough surface reflects crystallisation from an amorphous precursor ^[3].