Pteria sterna

Pteria sterna is a marine bivalve of the family Pteriidae, accepted in the world register of species under the author Gould and the year 1851 ^[1]. In Peru it is called concha nácar, and the master plan of the Los Manglares de Tumbes National Sanctuary lists it among the bivalves of that protected area ^[2].

It lives attached by a byssus to hard substrates from the low intertidal down to about 23 m depth, and its most important Peruvian populations are in Piura and Tumbes ^[3]. In Mexico its muscle and shell are of fishery importance ^[4], and pearl farming in that country relies mainly on it ^[5].

According to the literature, the species ranges across the tropical and subtropical eastern Pacific, from Baja California in Mexico to Peru ^[5]. For Peru, the IMARPE study, citing earlier literature, states that it reaches Pimentel and that well-identified populations extend south as far as Talara ^[3].

It inhabits the infralittoral zone and mangroves, from the lowest tide level down to 23 m depth, and attaches by its byssus to rock, gorgonian corals, submerged metal structures or coarse sand ^[3].

The most important natural populations in Peru are in Talara province, off Punta Arenas, Negritos and Punta Capullanas, in waters of 16.5 to 17 °C ^[3]. In Tumbes they settle on the oil platforms of La Cruz, Zorritos and Punta Sal, at depths of 3 to 45 m, with densities that, according to an earlier work the study cites, range from 1 to 120 individuals per m² ^[3].

After the 1982-1983 El Niño event, specimens were recorded outside the usual range, in the bays of Ancón and Independencia in Peru and at Mejillones in Chile ^[3].

The sexes are separate and cannot be told apart by eye; they are identified under the microscope from a sample of the gonad ^[3]. At Zorritos, of 522 individuals analysed between April and November 2011, 162 were males, 164 females and 196 could not be determined, so the ratio was one female per male ^[3]. At Bahía de los Ángeles in Mexico the ratio was 1.19 females per male in 376 individuals ^[4].

Spawning is nearly continuous. At Zorritos spawning females were found in six of the seven months studied, all but June, and in November they made up 55.6 % of the females ^[3]. In Baja California the population studied spawned without a pause, with peaks in December, February and August ^[4]. In Acapulco gonad development was continuous throughout the year, with a maximum in February and March at 25-26 °C ^[5].

Growth is fast in culture. In Acapulco Bay juveniles grew 5.3 mm per month in the first six months and about 3 mm per month afterwards, reaching 60 mm shell height in one year ^[5]. Gametogenesis began when males measured 50.9 mm and females 59.5 mm ^[5].

In the Zorritos population valve height ranged from 5 to 94 mm, the mean size rose from 50.6 mm in April to 65.4 mm in November, and five age groups were distinguished ^[3]. Total weight grows more slowly than the cube of height, that is, with negative allometry (exponent 2.629) ^[3].

It is an opportunistic species that settles on artificial structures. At Zorritos, the population studied lived on a purse seine sunk at 4 m depth when a boat was wrecked on 28 January 2010; the individuals were on the part of the net suspended in the water column and none on the part touching the bottom ^[3].

According to works gathered in the IMARPE study, it can survive in very turbid water, at temperatures below 18 °C and above 32 °C, and salinities under 34.5 and over 37 psu. The authors credit these traits with a wide range of strategies for remaining in an area or extending its range ^[3].

Recruitment at Zorritos was continuous, with peaks in April and August ^[3]. In La Paz Bay, Mexico, recruitment onto artificial collectors has been recorded all year, according to a work cited in the IMARPE study ^[3]. In the Gulf of California the species breeds between winter and spring, when water temperature falls and productivity rises ^[5].

In Peru the adductor muscle, the part that is sold and is called talo, is the one used, and its yield was 6.3 % of total weight at Zorritos; earlier works the study cites gave 10 % in Tumbes and in Talara, and 8 to 11 % in Tumbes in another ^[3]. In Mexico the muscle and the shell are also of fishery importance ^[4].

Pearl farming, a growing activity in Mexico, relies mainly on this species, which yields high-quality multicoloured pearls valued on the international market ^[5]. In Acapulco juveniles reached in 15 to 17 months the size of 70 to 80 mm shell height needed for pearl grafting ^[5].

The Peruvian literature places the most important populations in Talara and on the oil platforms of Tumbes, at depths of 3 to 45 m; the Tumbes platforms lie more than 3 nautical miles offshore, so they are not an easy place to reach ^[3].

In 2011 a small population lived on a sunken net at 4 m depth off the Zorritos artisanal landing pier, but local fishers were removing the net, so that site may have disappeared ^[3]. The 2023-2027 master plan of the Los Manglares de Tumbes National Sanctuary includes the species in its list of bivalves, without saying where in the area it is found ^[2].