MOLLUSCA OF SOUTHERN CHILE 



111 



San Rafael and Estero Elefantes is situated at a latitude of 46°S, 

 between the Chonos Archipelago and Taitao Peninsula to the west, 

 and the Andean Cordillera to the east. Immediately to the south east 

 lies the extensive San Valentin Icefield, and from this the San Rafael 

 Glacier descends to sea level, calving directly into the Laguna. This 

 is an enclosed body of water up to 1 5 km in diameter, cut off from the 

 open ocean of the Golfo San Esteban by a low-lying marshy 

 moraine. The only outlet from the Laguna is a narrow channel to the 

 north, the Rio Tempanos, that connects with the Golfo Elefantes at 

 the head of the fjord system. The Golfo in turn is delimited at its 

 northern end from the main Estero Elefantes by a series of spits and 

 islets. Beyond these, the main fjord varies in width from 3 to 10 km 

 and is fed by several river systems from the mountains to the east, 

 while to the north and west it eventually connects with the open sea 

 through a complex system of channels between the large islands of 

 the Chonos Archipelago. Rainfall in the region is heavy throughout 

 the year, with up to 4000 mm annually (Viviani, 1979; Davenport et 

 al., 1995), and both freshwater runoff and glacial meltwater have a 

 strong local influence, lowering the seawater salinity in the vicinity 

 of the numerous rivers and streams. Mean monthly temperature 

 varies between about 5 and 15°C (Viviani, 1979). The area covered 

 by our survey extends from the Laguna San Rafael to Isla Traiguen, 

 a distance of 1 20 km. As a result of the physical configuration of the 

 fjord system, three somewhat distinct regions can be distinguished: 

 the Laguna San Rafael, the Golfo Elefantes and the Estero Elefantes 

 (Figure 1). A general description of physical features and marine 

 zonation has been provided by Paterson et al. (in press), and here 

 only a short account of the main habitats and molluscan assemblages 

 will be given. 



The Laguna San Rafael (technically an estuarine ob) is 80-250 m 

 deep and contains icebergs from the glacier. The shoreline is mainly of 

 rocks and gravel (stations 29, 30; Table 1 ), moderately sheltered from 

 wave action except in the immediate vicinity of the glacier where 

 waves are produced by calving icebergs. On the shallower and more 

 sheltered northeastern shore there is development of mudflats and 

 some salt marsh (station 31). The salinity is low (15— 17%o) and the 

 water column is well mixed (Davenport et al., 1995). Diversity of 

 algae and invertebrates is low, and only three mollusc species were 

 found: Mytilus edulis chilensis on boulder shores and in the sublittoral 

 fringe, Malletia tnagellanica in soft sediment to a depth of 10 m and 

 Chilina patagonica on mud among saltmarsh vegetation. 



The Golfo Elefantes is characterized by increasing salinity (15- 

 21%c at surface) and moderate wave action. The shoreline of the 

 investigated area (station 25) consisted of low conglomerate cliffs, 

 boulders and semi-sheltered coves of muddy sand. In the littoral 

 zone the diversity of molluscs was low and just four species were 

 abundant on hard substrates: Perumytilus purpuratus, Mytilus edulis 

 chilensis, Nacella magellanica and Siphonaria lessonii. The chiton 

 Plaxiphora aurata was occasional and Bankia martensi common 

 boring in driftwood. Valves of Mulinia edulis were found on a beach, 

 suggesting its presence in shallow-water sediments, and living 

 Aulacomya atra washed up on the shore indicated a possible subtidal 

 population. Dredging on a bottom of silt and cobbles revealed a 

 community dominated by the brachiopod Magellania venosa, and a 

 sparse but more diverse molluscan fauna of 10 species, including the 

 predatory whelks Trophon plicatus, Xymenopsis muriciformis, 

 Pareuthriapowelli, and occasional examples of Leptochiton medinae, 

 Iothia coppingeri, Margarella violacea, Eumetulapulla, Lamellaria 

 ampla, Malletia magellanica, and Venus antiqua. In the estuary of 

 the Rio Gualas (station 26) a heavily silted boulder shore was 

 encrusted with the barnacle Elminius and the green alga Entero- 

 morpha, and the only living molluscs were Mytilus edulis chilensis, 

 Perumytilus purpuratus and Plaxiphora aurata. 



Along the Estero Elefantes (including the southern shores of Isla 

 Traiguen at its northern end) there were a variety of shore types and 

 a wide range of wave-exposures. Surface salinity readings ranged 

 between 5-15%c in estuaries, 15-22%c in the southeastern corner of 

 the Estero, and elsewhere approached fully marine conditions (27- 

 33%o). The depth of the Estero varies between 1 85 and 440 m (with 

 a sill of 150 m at its northern end), with a halocline at 7-8 m and 

 deep salinity of 32%c (Pickard, 1971). Only here were strongly 

 wave-exposed (stations 16, 17) and moderately wave-exposed 

 (stations 5, 12, 13, 14) rocky shores encountered. A characteristic 

 zonation was developed on such shores which, from the perspective 

 of molluscan distributions, can be summarized as follows. At the 

 upper limit of the eulittoral zone the only mollusc was Nodilittorina 

 araucana, abundant on rocks partly encrusted with the red alga 

 Hildenbrandia. Below, in the upper mid eulittoral, a band of small 

 barnacles (Notochthamalus) was present, sometimes with the algae 

 Iridaea and Porphyra, and associated molluscs were Nodilittorina 

 araucana, Siphonaria lessonii and, in the interstices of the barna- 

 cles, the minute bivalve Lasaea miliaris. The mid and lower 

 eulittoral was generally dominated by dense beds of Mytilus edulis 

 chilensis, sometimes together with Perumytilus purpuratus in the 

 upper part (especially in more sheltered areas or where freshwater 

 runoff occurred), and (only in the most exposed sites, stations 13, 

 17) replaced by Aulacomya atra in the lowest eulittoral and 

 sublittoral fringe. Other characteristic molluscs of the Mytilus belt 

 included Scurria parasitica attached to the mussels, the predatory 

 Acanthina monodon and the herbivores Nacella magellanica, 

 Tegula atra, Fissurella picta and Chiton granosus. On some shores 

 it appeared that density of these grazers was sufficient to produce a 

 narrow bare zone devoid of macroalgal cover in the lowest 

 eulittoral. On exposed shores the rock surfaces in the sublittoral 

 fringe were encrusted by calcareous red algae, sea urchins were 

 abundant, and at the top of this zone clumps of the large barnacle 

 Austromegabalanus were frequent. Mollusca of the sublittoral 

 fringe included Tegula atra, Argobuccinum pustulosum ranelli- 

 forme and, in one strongly exposed site (station 16), Concholepas 

 concholepas. Sheltered microhabitats on these exposed shores, 

 such as large rock pools and among boulders, harboured a more 

 diverse range of molluscs including the gastropods Crepidula 

 dilatata, Trochita trochiformis, Trophon plicatus, Pareuthria 

 fuscata, P. powelli, Nassarius gayii and Tritonia challengeriana, 

 and the chitons Chaetopleura peruviana, Ischnochiton stramineus, 

 Chiton magnificus, Tonicia atrata and T chilensis. 



In bays and inlets of the Estero Elefantes, relatively sheltered 

 from wave action, both bedrock and boulder shores were dominated 

 by algae (stations 1,7,8, 20). In the upper eulittoral zone filamentous 

 algae such as the red Bostrychia and green Enteromorpha were 

 common, together with barnacles, but replaced in the mid and low 

 eulittoral by large fleshy algae among which the red Mazzaella and 

 Nothogenia were most abundant. On sheltered shores of this type the 

 limpets Nacella magellanica, Scurria ceciliana and Siphonaria 

 lessonii were common on rock surfaces on the upper shore, and 

 Diloma nigerrima beneath boulders. Lower on these shores a di- 

 verse assemblage of molluscs occurred among boulders, including 

 Chiton magnificus, Tonicia atrata, Plaxiphora aurata, Fissurella 

 nigra, Tegula atra and Acanthina monodon. In areas of low salinity 

 diversity was reduced (stations 6, 1 1, 21, 22), and on such shores the 

 molluscan fauna resembled that of corresponding areas of the Golfo 

 Elefantes. As in the Laguna San Rafael, the only molluscs on 

 muddy, estuarine shores of lowest salinity (5%o, station 28) were 

 Mytilus edulis chilensis and Chilina patagonica. Sedimentary shores 

 with normal salinity were scarce in the Estero Elefantes; only one 

 sheltered sandy bay was examined (station 1). Here the bivalves 



