Mesodesma donacium

Mesodesma donacium is a bivalve of the family Mesodesmatidae, in the order Venerida ^[1]. It is a beach clam native to the Southeast Pacific coast, harvested by artisanal fishers on sandy beaches of Peru and Chile ^[2]. In Peru it is called «macha» ^[3]. It lives buried in the sand of the intertidal zone and is gathered by hand at low tide ^[4]. Its fishery boomed between 1975 and 1980 and collapsed afterwards, partly because of the El Niño events of 1982-83 and 1997-98 ^[3].

It is a bivalve with triangular, elongated valves and a short, truncated rear margin; each valve has an umbo, the outer face is multicoloured and the inner face is pearly, porcelain-coloured ^[4]. The hinge carries a pair of diverging lateral teeth on either side of the triangular pit that holds the ligament, and the shell has 3 layers: the outer periostracum, an intermediate layer and the inner nacre ^[4].

In the Pozo de Lizas bed, at Ilo, the maximum size found among 1800 specimens obtained was 92.5 mm ^[5]. At Santa Rosa beach, in Tacna, 575 clams measured in 2009 ranged from 59 to 89 mm, with a mode of 74 mm ^[6]. Virginal individuals, which have never been reproductively active, are usually juveniles of 35 mm or less in total valve length ^[3].

It is endemic to the Southeast Pacific coast; a phylogeographic study sampled 7 natural beds between 17 and 42° S ^[2]. Its distribution has been described from Sechura, in Piura, to Chiloé, in Chile, and the most productive beaches in Peru have been those of Tacna, Mollendo, Camaná and Ocoña ^[4].

In southern Peru it is found on the sandy beaches of Tacna, Moquegua and Arequipa; according to the Pozo de Lizas study, in Tacna it runs from La Concordia, on the border with Chile, to Vila Vila beach, and further north it appears at Pozo de Lizas, near Ilo, and the authors that the source cites record it northwards to Sechura or even Trujillo ^[5].

It lives on sandy and sandy-muddy beaches exposed to heavy surf ^[3]. It seeks the churned water of the intertidal zone, buries itself in the sand at 5 to 20 cm depth and is found at temperatures of 14 to 21 °C ^[4]. At Santa Rosa the resource formed a belt 86 m wide along 1 km of beach, with a very dense patch of 20 to 30 individuals per m2 and cores of 30 to 50 individuals per m2 ^[6].

The sexes are separate and there is no sexual dimorphism; fertilisation is external and the larvae, trochophore and veliger, are planktonic for 30 to 45 days ^[4]. The minimum spawning size falls between 47 and 57 mm in length ^[4].

At Pozo de Lizas, between April 1992 and March 1993, 1800 specimens were studied and the gonads of 600 were examined in histological sections; the sex ratio was 1:1, first sexual maturity came at 60.0 mm and mature individuals averaged 77.2 mm ^[5]. There the immature state predominated from March to July, the mature state from August to October and spawning from November to February, with a peak of 92 % of specimens spawning in December, although minor spawning was detected through most of the year ^[5].

With 2503 samples gathered by the Peruvian Sea Institute between 2006 and 2014 between Cerro Cortado and Santa Rosa, in Tacna, 6 stages of gonadal maturity were defined, from virginal to recovery after spawning, and maturation was found to be of the cortical type ^[3].

It is a filter-feeding bivalve with a microphagous diet: it takes phytoplankton and zooplankton, such as dinoflagellates, diatoms and tintinnids, and organic particles filtered from the interstitial fauna of the sandy bottom ^[4]. It lives alongside crabs such as Emerita analoga, crabs of the genus Hemigrapsus and hermit crabs of the genus Pagurus ^[4].

At Santa Rosa beach, of 6315 individuals collected in the intertidal zone, 9 species from 3 groups were identified; 89.98 % were crustaceans, 9.94 % molluscs and 0.08 % polychaetes, and Emerita analoga made up 88.57 % of abundance, Mesodesma donacium 5.92 %, the small clam Mulinia edulis 4.02 % and the hermit crab Isocheles pacificus 1.22 % ^[6].

Humans are its main predator; an eagle ray and other species of the surf zone also eat it and, on the landward side, so do seabirds such as the gull Larus dominicanus ^[4].

Its Peruvian fishery peaked between 1975 and 1980, and landings fluctuated and fell from 1980 to 2000; the rise in sea surface temperature during the El Niño events of 1982-83 and 1997-98 contributed to the decline of the resource in Chile and Peru, up to the collapse of the fishery ^[3]. According to a genetic study, the presence and abundance of the species have varied widely over time, and its authors attribute this to overfishing and environmental events ^[2].

On the Tacna coast the resource disappeared with El Niño in 1983 and returned in 1986; since 1998 the natural banks were depleted until they vanished and, after 6 years of absence, juveniles began to appear on some beaches ^[4]. El Niño of 1997-98 caused a massive, gradual die-off in the summer of 1998 in the banks of southern Peru, among them Lomas, Camaná, Mejía, Boquerón, La Punta, Corro, Pocoma and Pozo de Lizas, after heavy extraction pressure since 1992 ^[6].

In 2009 the Santa Rosa population consisted only of adults, 59 to 89 mm, with no recruits to ensure its continuity; 89.7 % exceeded the commercial size and the beach was unprotected and easy to reach ^[6]. Ministerial Resolution 108-84-PE of 1984 set a minimum extraction size of 70 mm ^[4]. The authors of the genetic study recommend that genetic information be taken into account in restocking and management work ^[2].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

As a filter-feeding bivalve, the macha can accumulate phytoplankton toxins. In a study in Coquimbo Bay, in Chile (29°51' S), with sampling between May 2009 and April 2010, the toxins it accumulated while feeding during blooms of the dinoflagellate Dinophysis acuminata were measured ^[7].

Pectenotoxin 2 and some products derived from it were detected in the clams, and no toxin of the okadaic acid group was found; the seco-acid form of pectenotoxin 2 reached concentrations more than 10 times higher than pectenotoxin 2 itself and appeared bound to fatty acids ^[7]. The clams, collected by diving at 10 to 15 m depth, cleared the toxins from the digestive gland quickly, at a rate above 0.2 per day ^[7]. The authors recall that there is no evidence that pectenotoxins are toxic to people by the oral route, although some control systems, such as the European one, still keep quarantine levels for them ^[7].

It is traded worldwide as a surf clam, in the United States, Europe and Japan, and the colour of its flesh places it among the pink molluscs, the most prized ^[4]. In southern Peru it has been a resource of socio-economic importance: in Tacna, during the summer, whole families from Cusco, Arequipa, Puno and Tacna come to the beaches to gather it, in rudimentary camps ^[4].

Gathering is by hand at low tide, with a hand net, a net bag, rakes or shovels; the gatherers process most of the catch by traditional means and obtain a smoked, dried product, which is sold in southern markets such as Cusco, Puno, Arequipa, Moquegua and Madre de Dios, and in Lima ^[4].

In Peru the fishery was run mainly by artisanal fishers who dug into the sand until they found the resource; it has been cited as a classic example of boom and bust, in which better extraction techniques led to overexploitation ^[3].

A 2015 article, citing works from 1998 and 2010, indicates that after the El Niño collapses the resource was reduced to a few beaches of Arequipa (La Mansa, Tanaka-Yauka, Quilca, Camaná and El Molle) and of Tacna (Cerro Cortado, El Chasqui, Rancho Chico, Kulauta, Cenizales and Santa Rosa); the Peruvian Sea Institute samples of 2006 to 2014 were taken from Cerro Cortado to Santa Rosa, in Tacna ^[3]. Those data do not say where it is found today ^[3].

In 2009 the bank at Santa Rosa beach, 1 km long, was assessed, and about 1.5 million specimens were estimated; the 2006 surveys had identified 3 natural-bank cores between Santa Rosa and Los Palos ^[6]. The beach had no protection and was easy to reach; sampling was done with water at knee, waist and chest height ^[6].