Leukoma thaca

Leukoma thaca is a clam of the family Veneridae, order Venerida, described by Molina in 1782 ^[1]. It lives buried in shallow sandy bottoms, down to about 20 m, in the south-eastern Pacific between 12° and 45° S ^[2],[3]. It is fished commercially in Peru and in Chile ^[3]. It tolerates high temperatures, which is why its shells can be used to reconstruct El Niño episodes ^[2],[4].

Molina described it in 1782 as Chama thaca, in the family Chamidae; the accepted name today is Leukoma thaca, in Veneridae ^[1]. Much of the scientific literature uses Protothaca thaca, a name WoRMS treats as unaccepted, as it does Venus chilensis, Venus dombeii and Venus ignobilis ^[1].

The shell has two aragonite layers: an inner homogeneous one and an outer prismatic one, where growth increments are easiest to see ^[2]. Unlike other venerids studied, its shell has no conspicuous growth patterns ^[2]. In Dichato Bay (Chile, 36° S), the theoretical maximum length was 82.2 mm, and the adult size distribution peaked between 50 and 65 mm ^[3].

A Chilean study describes it as very similar in shape to Venus antiqua, another shallow-water infaunal clam related to it, so much so that local fishers do not tell the two apart ^[3]. That source gives no character to separate them, and neither does this account ^[3].

Its range runs from 12° to 45° S along the south-eastern Pacific coast ^[2],[3]. Across that range it forms local populations ^[3].

It is an infaunal bivalve: it burrows just below the surface of sandy substrates, from the intertidal to the subtidal, down to about 20 m depth ^[3],[2]. It often shares its habitat with Gari solida and Semele solida ^[3]. In Dichato it was sampled at 2 to 5 m depth, mostly near the mouth of the bay ^[3].

It lives in waters of 5 to 26 °C ^[2],[4].

In Dichato, body weight followed an annual cycle tied to sea temperature: it fell in winter and rose in spring and summer ^[3]. From this the authors infer one breeding cycle per year, with a brief summer spawn followed by a longer winter rest, but they note that gonads were examined under the microscope only in Gari solida, so for Leukoma thaca this is an inference ^[3]. An earlier study in northern Chile, at 23° S, cited in the same paper, found continuous gonad activity with spawning in August, February, March and May ^[3].

With the growth parameters from Dichato, the asymptotic length was 82.2 mm and the growth constant K was 0.174 per year; total mortality was 0.584 per year ^[3]. It can live up to about 7 years ^[2].

In the shells studied (four from Pocoma), the thinnest growth increments are 10 to 120 µm wide and are separated by lines of organic matter; they are interpreted as daily increments. They are thinner late in life, and their width varies in cycles of 6 to 14 increments thought to coincide with the tides ^[2].

In Dichato, annual production of body tissues was 16.9 g ash-free dry weight per m² and gonad production was 14.5 g, with a mean biomass of 62.9 g; the production to biomass ratio was 0.269 ^[3]. The population consisted mostly of old adults, with few recruits ^[3].

It withstands heat well: the temperature lethal to half the individuals after 24 hours was 27.9 °C in Chilean specimens, the highest among the eight Chilean species tested ^[4]. According to Lazareth and colleagues, this tolerance lets it survive El Niño episodes ^[2]. In an experiment with northern Chilean specimens, oxygen consumption, daily growth and body condition of animals acclimated to 12 °C were lower than at 16 and 20 °C, and no difference was seen between those two warmer temperatures ^[5].

When sea temperature exceeds 18 °C for more than 3 weeks, the shell forms a reduced-growth mark called a summer check; in a cool La Niña summer the mark may be missing ^[2]. During the strong 1997-1998 El Niño, the mark was wider than 520 µm ^[2].

It is fished commercially throughout its range ^[3]. The clams of this region, Leukoma thaca among them, are exploited commercially in both Peru and Chile ^[4].

In Dichato Bay, during the study, only a few local fishers gathered specimens, irregularly, to eat in the nearby village; the heavier fishing pressure fell on a nearby population of Venus antiqua ^[3].

Its shells occur in Holocene archaeological deposits in Peru and in Pleistocene marine terraces of southern Peru and Chile, which is why they are studied as an archive of past El Niño events ^[2].

The literature places a Peruvian population at Pocoma (17° 25' S), where the growth of individuals kept in net bags on the seabed was followed from May to September 2003 ^[2]. The source says nothing about access to that place ^[2].