2.2.1 Distribution
The intertidal wedge clam Donax hanleyanus1 Fig. 2
Philippi, 1847 (Bivalvia:
Donacidae) ( ), also known under the Spanish synonym ‘berberecho’, is common in the intertidal along the South American Atlantic coast from tropical (17°S Caravelas, Brazil) to temperate regions (37°S Punta Mogotes in Mar del Plata, Province of Buenos Aires, Fig. 5b) (Penchaszadeh and Olivier 1975;
Narchi 1978; Cardoso and Veloso 2003). The eurytopic and superficially burrowing (mainly 3-5 cm deep) D. hanleyanus is capable of inhabiting beaches of all morphodynamic types (sensu Short and Wright 1983), from steep, reflective beaches with coarse sands (Veloso et al. 1997) to flat, dissipative
1 D. hanleyanus reference specimens were deposited in the collection of the ‘Museum für Naturkunde der Humboldt-Universität zu Berlin’ under the reference number ZMB/Moll.104642.
Introduction – The Wedge Clam 17
beaches with fine sands (Penchaszadeh and Olivier 1975; Defeo and de Alava 1995). In the Argentinean sublittoral the wedge clam represents the only Donax species; it co-occurs negatively correlated with the yellow clam M. mactroides (Olivier et al. 1971; Penchaszadeh and Olivier 1975). Negative correlations between the abundance of D. hanleyanus and co-occurring filter feeders M. mactroides and E. brasiliensis, respectively, were also observed at Uruguayan (Defeo and de Alava 1995) and Brazilian sandy beaches (Cardoso and Veloso 2003).
As is characteristic of donacids, this bivalve has adapted to life in exposed swash zones characterized by a fair amount of wave action aerating the sediment. This keeps organic detritus in suspension and allows for tidal migration (Mori 1938; Ansell and Trevallion 1969; Penchaszadeh and Olivier 1975; Narchi 1978; Ansell 1983). Vertical distribution patterns of temperate D. hanleyanus appear to be different from other tropical and subtropical Donax species. Whereas for example D. incarnatus from India (Ansell and Trevallion 1969) and D. trunculus from Algeria (Mouëza 1972) keep their position relative to the swash zone during tides, D. hanleyanus shows an intensive migration to the upper intertidal during high tides (Penchaszadeh and Olivier 1975).
The variability in physical factors is unrelated to temporal abundance fluctuations of the wedge clam but not to its geographical distribution.
Penchaszadeh and Olivier (1975) determined that the constitution of sediments is one of the principle factors affecting the distribution of D. hanleyanus. It is remarkable that fossil findings in South America indicate that D. hanleyanus was a very common species during the Querandinense period of the early Holocene (von Ihering 1907; Camacho 1966; Martinez and del Río 2005), but living specimens of D. hanleyanus were reported not earlier than 1960 from the Argentinean coast (de Castellanos and Fernández 1965; Penchaszadeh and Olivier 1975), from Brazil in 1949 (Lange de Morretes 1949) and from Uruguay in 1951 (Barattini 1951). It is expected that planktonic larvae of D. hanleyanus had to cut across the Río de la Plata with favourable ocean currents and to settle on sandy beaches south of this estuary (de Castellanos and Fernández 1965; Penchaszadeh and Olivier 1975).
2.2.2 Biology and Ecological Role
D. hanleyanus is bisexual, the sex ratio does not differ from 1:1 (Penchaszadeh and Olivier 1975; Gil and Thomé 2004a; Delgado and Defeo 2007b). In common with other intertidal suspension feeding invertebrates D. hanleyanus plays an important role in the food web by linking benthic and planktonic ecosystems (Wade 1967a; McLachlan et al. 1981; McLachlan and Lewin 1981;
McDermott 1983; DeLancey 1989; Heymans and McLachlan 1996; Soares et al. 1997). The wedge clam is the main primary consumer and is in turn subject to predators such as the gastropods Olivancillaria vesica auricularia and Buccinanops duartei (Marcus and Marcus 1959; Gianuca 1985; Rocha-Barreira de Almeida 2002), demersal fish such as the black drum (Pogonias cromis,
‘corvina negra’), the white croaker (Micropogonias furnieri, ‘corvina rubia’), the jewsharp drummer (Menticirrhus martinicensis, ‘corvina de perita’ or
‘burriqueta’) and seabirds such as the American oystercatcher Haematopus ostralegus (Olivier et al. 1971; Penchaszadeh and Olivier 1975;
Cousseau and Perrotta 2000).
2.2.3 Population Dynamics
D. hanleyanus has two spawning periods (August-September and January-February) with two corresponding periods of recruitment (October-November and February-March) (Penchaszadeh and Olivier 1975). Furthermore, the authors reported that wedge clams are sexually differentiated at the age of two (males) and four months (females) and no period of complete gonadal inactivity was found. Population dynamic aspects of D. hanleyanus vary somehow between populations within its distribution area (Penchaszadeh and Olivier 1975; Defeo 1996; Cardoso and Veloso 2003). Wedge clams grew faster in Argentina (K = 1.18 yr-1) in the 1970s than presently in Uruguay (K = 0.80 yr-1) and Brazil (K = 0.80 yr-1) and reach the smallest maximum length in the northern limits of their distribution (33.5 mm; 30.0 mm and 26.4 mm, respectively). Previous investigations demonstrated that the growth of D. hanleyanus is subject to moderate seasonality (C = 0.8) with slowest growth rates between March and May (WP = 0.25-0.4). The longevity of this species
Introduction – The Wedge Clam 19
was estimated for the Brazilian populations to be 1.5 yrs (Cardoso and Veloso 2003) and for the Argentinean one 3 yrs (Penchaszadeh and Olivier 1975) with a mortality ranging between 1.55 and 1.70.
2.2.4 Fishery
Fig. 1: Shell free D. hanleyanus (a) and M mactroides (b) offered by a supermarket in Buenos Aires (Argentina) for approximate 9 € (34.90 Argentinean Pesos) and 10 € (39.90 Argentinean Pesos) per kilo, respectively, even though extractions are forbidden, as indicated by large prohibition signs at tourist destinations at the coast of Buenos Aires (c).
Although globally several Donax clams such as D. denticulatus and D. striatus in the Caribbean, D. trunculus in Europe, D. serra in Africa, D. cuneatus and D. faba in Asia, and D. deltoides in Australia are targeted by commercial and artisanal fisheries (McLachlan et al. 1996b), D. hanleyanus is not commercially exploited yet in Argentina, but is used as bait for the recreational fishery. The wedge clam is edible and tasty (Veloso et al. 1953; Penchaszadeh and Olivier 1975) and offered occasionally in grocery stores (Fig. 1a). However, this species is generally not used as a food resource in Argentina, very probably due to the relatively cheap and very high quality beef.
2.2.5 Systematic Classification
Fig. 2: Argentinean wedge clam D. hanleyanus (apSL = 35 mm) collected at Faro Querandí, arrows: exhalant siphon (es), inhalant siphon (is) and foot (f), scale bar:
10 mm.
Table 1: Taxonomic hierarchy of the wedge clam D. hanleyanus.
On a global basis, the family Donacidae (super family Tellinoidea) form by far the most diverse group, inhabiting highly dynamic sandy beach ecosystems, integrating the genera Egeria, Iphigenia and Donax, with the latter group being composed of 64 species (Pearse et al. 1942; Ansell 1983; Brown and McLachlan 1990; Wilson 1999). In some cases, the latter dominate the macrozoobenthic communities in number and biomass (> 95 %: McLachlan et al. 1981; Arntz and Fahrbach 1991; Ieno and Bastida 1998). Most well-known Donax species are included in the geographical distribution map (Fig. 5b).
Introduction – The Yellow Clam 21