Lithophaga or its borings in oyster shells along the Argentine Atlantic coast have been common since the late Miocene (Pascual et al ., 2001; Farinati and Zavala, 2002; Casadío et al ., in press). The extant Lithophaga patagonica (d'Orbigny, 1842) is known in South America from the coast of Santa Catarina (Brazil) to Bahía Camarones (Argentina), where it is a usual component of benthic communities associated with rocky shores (Pastorino, 1995).
Because of its size, Lithophaga patagonica is considered a macroborer and because it actively bores the substrate where it lives it is classed as a euendolith (Golubic et al ., 1975). Its body is sub-cylindrically elongated and its shell is thin (Blanco et al ., 1988). According to Blanco et al . (1988), the body shape -with a clear lengthwise growth component and a restricted increase in width and height- can be attributed to the anterior-posterior development of the boring mechanism and the resulting reduction of the cavity due to the carbonatic layer secreted by the organism and that covers the inner surface.
There are many studies on extant species of Lithophaga and their role in boring communities (Kleemann, 1980; Kelly and Bromley, 1984; Scott, 1988; Scott and Risk, 1988; Cantera and Contreras, 1988; Jones and Pemberton, 1988). Most of the studies concerning the role of Lithophaga in hard substrate communities are within a general context and treat these borers together with other bioerosion-producing organisms. In addition, they were all carried out in tropical or subtropical areas and mainly in coral reefs (Hein and Risk, 1975; Scott, 1988; Scott and Risk, 1988; Guzmán and Cortéz, 1989; Cortéz, 1991, 1992; Perry, 1998).
Other studies have focussed on determining the specificity in substrate selection by larvae of Lithophaga settling on corals (Highsmith, 1980; Mokady et al ., 1991, 1992, 1994; Brickner et al ., 1993; Krumm and Jones, 1993). This selection is very important because the postlarvae completely and irreversibly loose their mobility and are unable to shift substrates (Chia fide Mokady et al ., 1991). Physical and biological factors such as the texture and position of the substrate bacterial coverings and the presence of co-specifics may control the active selection of a substratum in larvae of marine invertebrate (Crisp, 1976; Barnett and Crisp, 1979; Weiner et al ., 1989). This may involve complex behavioral patterns, including active swimming and the use of sensory mechanisms such as photothaxis, geothaxis and chemothaxis (Burke, 1983).
According to Mokady et al . (1991, 1992), the selection of substrate and metamorphosis in living corals by Lithophaga larvae may have been controlled by chemical factors such as living tissue of the corals.
In the San Matías Gulf, Lithophaga patagonica is a common member of the community recorded in shells of the extant Ostrea puelchana d'Orbigny, 1842. Together with sponges they are the main boring organisms living within the shells of these oysters (Castellanos, 1957; Pascual et al ., 2001).
In this study we analyze the frequencies and distribution patterns of Lithophaga patagonica on valves of Ostrea puelchana and compare them to those of Lithophaga sp. observed on the fossil species ? O .? patagonica d'Orbigny, 1842, and ? O .? alvarezii d'Orbigny, 1842, from the late Miocene Puerto Madryn Formation.
In Ostrea puelchana the boring frequency was significantly higher for the left valve and, within it, the areas more heavily bored were the umbones and the platform. Similar results were obtained for valves of ? Ostrea ? alvarezii , suggesting that this oyster showed life habits similar to the living one. On the other hand, in ? Ostrea ? patagonica the frequency of Lithophaga borings was the same on either valve. This agrees well with its life habit, which was mainly with the shells placed almost vertically forming nests or concentrations of several specimens cemented to each other.
Results suggest that the life position of oysters is one of the factors influencing the abundance and distribution of Lithophaga borings. Therefore, this kind of information is useful for infering the life position of fossil oysters and also helpful in reconstructing their taphonomic history.
Ostrea puelchana d'Orbigny, 1842
Ostrea puelchana ranges from Río Grande do Sul (Brazil) to the San Matías Gulf (Argentina), where it forms large oyster grounds (Castellanos, 1957). It has a solid shell, with a flat right valve and a larger and convex left valve. The left valve can reach up to 11.4 cm high, 8.8 cm long, 2.5 cm thick and weigh as much as 0.104 kg. The right valve up to 9.2 cm high, 7.2 cm long, 2.0 cm thick, and may weigh up to 0.046 kg.
The demography of this species within the San Matías Gulf is very peculiar. It shows a bimodal population structure reflecting the existing size difference between sexes. Specimens smaller than 5.5 cm are predominantly epibiontic males, while larger specimens are mainly females (Pascual et al ., 1989; Pascual et al ., 2001).
Ostrea puelchana is larviparous and the dispersal period occurs during the veliger stage lasting up to 20 days (Pascual et al ., 1989; Pascual and Zampatti, 1995). After this period, if the pediveliger larva finds an adequate substrate, it attaches to it.
The attachment process is very fast and is produced on the mid-ventral margin of the left valve. After fixation, the larva undergoes metamorphosis and becomes a juvenile oyster (Pascual, 1993). Shortly after fixation the oyster may reach a size larger than the substrate to which it became attached. Therefore, from then on it lies free on the bottom and may be affected by currents.
According to Pascual et al . (2001) the life position that the oyster acquires is a characteristic that can change along its life and affect its stability in the water current.
Sixty one percent of oysters from the Las Grutas ground in the San Matías Gulf live with the right valves downwards. This position is more stable because it creates a suction effect that prevents the oyster from being dragged by the current (Pascual et al ., 2001). When analyzing the life position adopted by oysters according to their size class, a decreasing trend in the percentage of shells lying on the right valve can be noticed as size increases. The values recorded by Pascual et al . (2001) were 99% for oysters less than 3 cm wide (n=67), 78% for those between 3 and 7 cm wide (n=225) and 59% for sizes larger than 7 cm wide (n=1,487).
Ostrea puelchana has a unique characteristic among living oysters, i.e ., ?carriage?. This consists in the development of a flat platform from the anterior margin of the left valve in females, used for settling of the larvae. Growth rate of the oysters in this platform is very slow because of an inhibitory effect of the female which maintains them in their protandric phase - i.e ., male-. This process assures female fertilization (Pascual et al ., 1989; Pascual, 1993, 1997).
Although there are no studies concerning space competition between the epibiontic males of Ostrea puelchana and specimens of Lithophaga patagonica , it seems likely that the development of perforations on the platform of the left valve of females may influence the reproductive success as it hampers the settling or permanence of the males.
Description of the Las Grutas oyster ground
The Las Grutas oyster ground lies in open areas of the San Matías Gulf (figure 1). During low tide it ranges from 2.5 to 6 m deep and the speed of the tidal currents range from 20 to 30 cm.s- 1 (Servicio de Hidrografía Naval, 1969). According to Fernández (1989), the average water temperature is lowest in August (8ºC) and highest in January (21ºC).
The area of the oyster ground that was included in the study by Pascual et al. (2001) was 2 km 2 . The bottom consists of silty to clayish rocks with limestone concretions. They are crossed by channels filled with sand where, besides the oysters, there are small areas colonized by algae. The density of oysters, maximum and average, was respectively 22 individuals/ m 2 and 8 individuals/m 2 (Pascual et al ., 2001).
?Ostrea? patagonica d'Orbigny, 1842
? Ostrea ? patagonica is a large, heavy and thickshelled oyster, in which the left valve of the adult specimens may reach more than 30 cm high, 20 cm long, 5 cm thick and weigh as much as 3 kg. The right valve is nearly flat, up to 20 cm high, 15 cm long, 5 cm thick, and may weigh up to 2.5 kg. Valves show very strong commarginal growth lines, many of which rise to form ridges or crests that render a very rugose aspect to the external surface. In the Puerto Pirámide section, ? Ostrea ? patagonica shows two ecophenotypes, i.e., clustered and gryphaeate. Both types inhabited soft sandy and muddy bottoms in a shoreface to offshore environment. Bunched forms consist of nearly vertical assemblages several specimens thick. The specimens are cemented to each other. These clusters can be associated to form reefs of several hundred square meters and reach more than five meters thick. On the other hand, the gryphaeate forms grew isolated and the shells lay on the left convex valve with the ventral commissure elevated above the bottom surface.
?Ostrea? alvarezii d'Orbigny, 1842
Adult specimens of ? Ostrea ? alvarezii show, in general, shapes and sizes similar to those of O. puelchana . The shell is solid and the right valve is flat, while the left one is larger, convex and has strong radial ribs. The left valve can reach up to 10 cm high, 8 cm long, 1.5 cm thick and can weigh as much as 0.180 kg. Right valves can reach 8 cm high, 6 cm long, 0.5 cm thick, and may reach 0.080 kg. At the Puerto Pirámide section, ? Ostrea ? alvarezii grew isolated, free, mainly lying on the right valve and living in tidal channels with shelly-sandy bottoms.
Iribarne et al. (1990) mentioned the presence of small epibiontic oysters attached to the anterior margin of specimens of ? Ostrea ? alvarezii collected in the late Miocene at Gran Bajo del Gualicho (Río Negro). They suggested for this oyster a reproductive strategy similar to the one recorded for Ostrea puelchana .
Description of the beds with ? Ostrea ? patagonica and ? Ostrea ? alvarezii
Fossil ? Ostrea ? patagonica and ? O .? alvarezii come from the Puerto Madryn Formation, which is very well exposed at Puerto Pirámide, in the province of Chubut (figure 1). The section exposed at Puerto Pirámide belongs, as suggested by del Río et al . (2001) and Casadío et al . (in press), to the upper part of a depositional sequence including a transgressive system tract (TST) and a highstand system tract (HST). The TST interval is represented by shelf sediments deposited below wave base, while those of the HST represent tidal channel and tidal flat deposits. Casadío et al. (in press) described 3 facies associations (figure 2).
Facies association 1 comprises a group of lithofacies representing a fining-upwards cycle. It includes sandstone and massive bioturbated mudstones facies. The association includes trace fossils of the Cruziana ichnofacies. Invertebrates in life position are common. ? Ostrea ? patagonica is grouped in clusters of up to 30 specimens. Other taxa include isolated specimens of ? Ostrea ? alvarezii , Aequipecten paranensis , Amusium paris , Mytilus sp. and Pachymagas piramidesia in life position as well as several species of crabs. The facies grade upwards into a laminated mudstone facies with trace fossils belonging to the Zoophycos ichnofacies. The grain-size, trace fossil assemblages, and fossil preservation suggest a relatively low sedimentation rate in a middle shelf, low-energy environment below fair weather wave base. However, according to taphonomic information, sudden increases in sedimentation rate possibly related to storms can be noticed at the base of this association.
Facies association 2 predominates in the middle and upper part of the succession and includes facies of sandstone and mudstone intercalations. Flaser, wavy and lenticular bedding are common. The facies contains few invertebrate remains, but there are beds containing abundant Skolithos and Ophiomorpha . The grain size and sedimentary structures suggest relatively low energy, possibly deposited in a sandy to muddy-sandy tidal flat. This facies association is always related to facies association 3.
Facies association 3 comprises a group of lithofacies clustered to form fining-upwards sequences. The geometry of these beds is lenticular with an erosive base. At the base of these sets there are shell concentrations, within a sandy matrix. In these beds there are common specimens of ? Ostrea ? alvarezii . The beds may show medium to large scale trough cross-stratification. These deposits grade upwards into fine to medium sandstone and mudstone, showing heterolithic bedding.
The geometry of the beds, the grain-size and sedimentary structures suggest that these deposits are the infilling of tidal channels and tidal sand waves. Shelly-sandy dunes and waves migrated along the deepest part of these channels, while in other areas with weaker currents small ripples migrated on sandy tidal flats. The presence of mudstone intraclasts suggests that the channels were flanked by fine tidal flat sediments (facies association 2).
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