Balanus laevis

Balanus laevis is an acorn barnacle of the family Balanidae, described by Bruguière in 1789 from specimens collected at Cape Horn, where it grew on the mussel Mytilus magellanicus ^[1],[2].

It is a cold-water species of South America: on the Pacific side it reaches north to Peru, and it lives from the shore down to about 20 fathoms, roughly 37 m ^[2]. In Chile and Peru the common form is the one Darwin called variety nitidus, with a smooth, naked shell that has no membrane ^[3].

The shell is conical, sometimes a little bulging, with a smooth surface that has no folds. It may be bare, white or pale purple, or covered by a dirty yellowish-brown membrane. The opening is small and more or less toothed, and seldom exceeds a third of the basal diameter; the radii are very narrow, often scarcely developed, and the sutures form deep, narrow clefts ^[3].

The scutum carries either one very broad, deep longitudinal furrow, two furrows or, less often, a single narrow, inconspicuous one ^[3]. Inside, the articular ridge is not very prominent and ends below in a free point; the adductor ridge is high and, according to Pilsbry, not joined to the articular one; the pit for the lateral depressor muscle is small but deep ^[3],[2]. The tergum has a longitudinal furrow and a spur of moderate length and width whose end is cut at a bevel and rounded ^[3].

The form varies a great deal. Darwin distinguished three varieties, and Pilsbry treated them as subspecies: the typical southern Chilean form with a yellowish epidermis; nitidus, from Concepción to Peru, with a bare shell; and coquimbensis, with a much lengthened base ^[2]. In the two largest series of variety nitidus that Pilsbry examined, 3 specimens from Arica had one furrow on the scutum, 16 had two and 1 had three; on the coast of Peru, 5 had one, 25 had two and 1 had three ^[2].

Individuals growing packed together become elongated, and the base can form a cup occupying up to four fifths of the total length; in some specimens that base is filled with thin calcareous layers ^[3]. In variety nitidus, where the barnacles are crowded the usual diameter is 10 to 12 mm, and conical, unhampered individuals have a basal diameter of about 13 mm ^[2].

Pilsbry placed its range from Cape Horn north to the Río Negro on the Atlantic coast and to Peru on the Pacific coast, and explained that a California record given by Darwin rested on wrongly labelled specimens, so the species is confined to the cold waters of South America ^[2]. In the south-western Atlantic it is cited as far as the continental shelf off Mar del Plata, and a study carried out in 1997-1998 and published in 2001 recorded it for the first time in Comodoro Rivadavia harbour, at 45°52' S, where the water ranged between 9 and 16 °C ^[4].

It lives from the shore down to about 20 fathoms ^[2]. In the Strait of Magellan, at 10 to 20 fathoms, it attaches to shells and often surrounds pebbles until they form globular masses; in Chile and Peru it was frequently found on the barnacle then called Balanus psittacus ^[3]. It is common on the snail Concholepas concholepas, and at Callao some pebbles were completely covered, like balls of barnacles ^[2].

In Peru, the material Pilsbry examined came from San Lorenzo Island, from Callao and from the coast in general ^[2]. Darwin also mentions specimens found with human remains at San Lorenzo, Callao, in a recent deposit ^[3]. The World Register of Marine Species cites, as a context source for Peru, an annotated list of rocky intertidal invertebrates of the department of Lima ^[1].

In a Patagonian harbour, stones were glued at three levels of the shore and recruits were counted for a year, between December 1997 and December 1998, in seasons of 84 to 100 days. Balanus laevis settled only in the middle intertidal and the subtidal, never in the upper intertidal, and its recruits were about 30 times less dense than those of Balanus glandula ^[4].

That pattern comes from one population in Argentine Patagonia and has not been measured in Peru. Recruitment peaked in spring, but differences among seasons were not significant, partly because it varied widely among replicates only 20 m apart ^[4].

On the appendages it uses to filter food, Darwin noted that no segment of the cirri stands out strongly; in the first pair one branch is almost twice as long as the other, and in the posterior pairs each segment bears seven pairs of spines. Pilsbry added that their fine structure had not been studied ^[2].

In Comodoro Rivadavia harbour, Balanus glandula settled only in the intertidal and Balanus laevis mainly in the subtidal, about 4 m below mean low water, with very little overlap. Recruit density of Balanus laevis was higher underwater in spring and autumn, and in the intertidal in winter and summer ^[4].

The authors proposed two explanations: unequal mortality in the earliest stages after an even settlement, or a vertical separation of larvae in the plankton. They saw no sign of mass mortality of recruits and thought the second more likely, though they had no data on larvae off Chubut ^[4]. Balanus glandula is a North Pacific species that was detected in the south-western Atlantic at the beginning of the 1970s ^[4].

As substrate, Pilsbry's material included pebbles, stones, bivalve and Crepidula shells, the snail Concholepas concholepas and the barnacle Balanus psittacus ^[2]. In the Strait of Magellan Darwin found it associated with Verruca laevigata ^[3].