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D.G. REID AND C. OSORIO 



ridge adjacent to ligament not pitted by pores; posterior adductor 

 and retractor scars continuous; radial sculpture absent; periostracum 

 very thick, glossy; colour blue to purple, overlaid by black 

 periostracum, often eroded at umbos; interior nacreous, silvery blue. 



Habitat. No living specimens were found during the present 

 study. Elsewhere, this mussel has been recorded in a variety of 

 habitats; on the exposed rocky coasts of southern Chile it occurs in 

 the sublittoral, and also in the low eulittoral in the absence of 

 collection by humans (Moreno, 1995); it is also frequent in estuar- 

 ies, sometimes on sedimentary substrates, and at reduced salinities 

 (monthly average high tide salinity of 20%o, Navarro, 1988). A 

 depth range of 4-20 m was given in a study of growth and reproduc- 

 tion by Lozada, Rolleri & Yanez (1971). 



Records. Stations 9 (midden material), 10 (fresh valves). Else- 

 where Pacasmayo (Peru) to Tierra del Fuego (Soot-Ryen, 1955). 

 Range: 7-56°S. 



Remarks. Superficially similar in shape to the smaller Mytilus 

 edulis chilensis, this species is distinguished by its single hinge tooth 

 and resilial ridge without pits. It is the largest and most esteemed of 

 the edible mussels of the Chilean coast, and consequently is exten- 

 sively collected; in 1997 the catch was 527 tonnes. Despite a 

 considerable literature on the culture and laboratory behaviour of 

 this species, there is little information about its natural ecology. At 

 present, populations are almost entirely sublittoral throughout the 

 region, although where shores are protected from human activity 

 large numbers appear in the lower eulittoral (Moreno, 1995). This 

 mussel is a major component of the large shell middens on Isla 

 Traiguen; the age of these deposits is not known, but intact shells 

 occurred at depths of up to 5 m in the eroding face of the midden at 

 Station 9. In addition, fresh shells were found at Station 10, presum- 

 ably recently discarded by fishermen. The species was an important 

 item of food for the indigenous people of the Chonos Archipelago, 

 who collected it by diving, and it is possible that the present 

 restricted distribution is a consequence of exploitation over a long 

 period of time (P.J. Curry, pers. comm.). In the north of Chile the 

 species is likewise common in middens, but is now locally extinct, 

 although whether this is a result of human activity is unclear 

 (Viviani, 1979). 



Aulacomya atra (Molina, 1782) 



(Figure 5A, B) 



Mytulus ater Molina, 1782: 202-203. 



Aulacomya ater- Soot-Ryen, 1955: 33-34, pi. 1, fig. 6, textfigs 17, 



18 (synonymy). Soot-Ryen, 1959: 26-27. Olsson, 1961: 116, pi. 



14, fig. 9 (synonymy). Osorio & Bahamonde, 1970: 192. 



Marincovich, 1973: 8, fig. 3. Osorio, Atria & Mann, 1979: 22-24, 



fig. 24. Bernard, 1983: 17-18 (synonymy). 

 Aulacomya ater ater - Dell, 1964: 175-177 (synonymy). Dell, 



1971: 171. 

 Aulacomya atra - Cazzaniga, 1994: 110. 



Description. Shell to 154 mm (to 200 mm, Suchanek, 1985); 

 beaks terminal, pointed; shell solid; single broad tooth-like fold 

 within umbo of left valve, and corresponding groove in right valve; 



resilial ridge adjacent to ligament not pitted by pores; posterior 

 adductor and retractor scars broadly continuous; strong radial ribs 

 present; periostracum thick, glossy; shells less than 40 mm white, 

 overlaid by yellow to brown periostracum; larger shells purple to 

 dark blue, overlaid by black periostracum, eroded at umbos; interior 

 nacreous, silvery to purple. 



Habitat. Adults of this species were only found commonly on 

 the shore at two relatively inaccessible sites on an exposed rocky 

 shore and a cliff (Stations 13, 17), where they formed dense clumps 

 at the top of the calcareous red algal zone in the sublittoral fringe. 

 Elsewhere, juveniles (to 40 mm) were found in small numbers 

 among the byssus of dense beds of Mytilus edulis chilensis, and 

 under stones in the mid to lower eulittoral on moderately exposed 

 and sheltered shores, but these individuals evidently died before 

 reaching large size. On the sheltered shores of the southernmost 

 Estero Elefantes and Golfo Elefantes, adults were occasionally 

 found on the shore; some were freshly dead, with the byssus 

 detached from the substrate, whereas others were only superficially 

 attached; empty valves were common on the beaches in this region. 

 It appears therefore that there may be considerable sublittoral 

 populations in this area, from which living and dead mussels are 

 washed up on the shore. The presence of beds of Macrocystis near 

 the shore made dredging here difficult, but a living adult was 

 dredged from 7-9 m (station 22), and a few others were brought up 

 in the holdfasts of the kelp from 5-15 m. Salinities of 15-20%c 

 were recorded at the southern sites. Elsewhere, occasional living 

 specimens have been recorded from 40 m at I. Chiloe (Soot-Ryen, 

 1959) and from 79 m in the Falkland Islands (Dell, 1964). It was 

 common in the lower intertidal and subtidal at Iquique 

 (Marincovich, 1973) and found in the mid to lower intertidal at Isla 

 Chiloe (Brattstrom, 1990). This species does occur also in more 

 sheltered situations, as in the Estero Castro, where a natural mussel 

 bank was studied between depths of 4-9 m, but accumulation of 

 mud caused mortality (Lozada, 1968). 



Records. Stations 1, 5, 13, 14, 17, 20, 22, 25. Elsewhere Callao 

 (Peru) to Magellan Strait (Soot-Ryen, 1955). Range: 12-56°S. 



Remarks. Although almost universally referred to as Aulacomya 

 ater in the recent literature, the name should be corrected to A. atra 

 since, as pointed out by Cazzaniga (1994), the adjectival specific 

 name should agree in gender with the feminine generic name. This 

 species is easily distinguished from all other large mussels in the 

 region by its radially ribbed shell; specimens less than 40 mm in 

 length could be confused with Perumytilus purpuratus, but at this 

 size the shells are yellow to pale brown, with less than 17 ribs, 

 whereas those of P. purpuratus are purplish with a brown to black 

 periostracum and have twice as many ribs and a more rounded 

 outline. Like the preceding species, A. atra is a large mussel that is 

 collected commercially in large quantities; the catch in 1997 was 

 6597 tonnes (SERNAP, 1998). It is unclear to what extent the 

 present scarcity of intertidal populations is a result of 

 over-exploitation. This species is the major component of the large 

 shell middens on Isla Traiguen; these are of unknown age, but some 

 are up to 5 m in height. Fresh valves at some sites suggests that 

 collection continues. 



Fig. 5 A, B. Aulacomya atra (juvenile 26 mm; adult 80 mm). C. Perumytilus purpuratus (39 mm). D. Mytilus edulis chilensis (thin-shelled form from low 

 salinity habitat, station 29; 45 mm). E. Mytilus edulis chilensis (thick-shelled form from higher salinity habitat, station 22; 71 mm). F. Choromytilus 

 chorus (186 mm). G. Cumingia mutica (25 mm). H. Tawera gayi (18 mm; Portland Bay and Port Rosario, Patagonia, 'Alert' Expedition, NHM 

 1 879. 1 0. 1 5.60). I. Venus antiqua (67 mm). J. Retrotapes exalbidus (86 mm). K. Tagelus (Tagelus) dombeii (82 mm). L. Hiatella solida ( 1 1 mm). M. 

 Ensis macha (129 mm; Punta Arenas, Magellan Strait, NHM 1868.7.1.22). N-P. Bankia (Bankia) martensi (N, shell 7.2 mm; O, pallet 30 mm; P, whole 

 animal, length 50 mm approx.). (All specimens from study area except H and M; all NHM collection). 



