No specimen of the fossil oysters showed borings of Lithophaga sp. on the interior surface of the valves. This suggests that they were produced while the oysters were still living and, at the same time, that the oyster shells were rapidly buried after death.
In ? Ostrea ? patagonica the presence of Lithophaga sp. borings and the average number of them for both valves are higher than for the other two species considered. The reason for this could be either a preference of the larva for a particular substrate or else differential survival of the larvae.
The more rugose surface of the valves of ? Ostrea ? patagonica and of the left valves of O . puelchana and ? O .? alvarezii could possibly trigger an active selection by the larvae (rugophilic behavior according to Bien et al ., 1999 and Bottjer, 1982). The larvae settling on rugose surfaces would be better protected from grazers, increasing their survival rate. Pascual (1997) recorded that the effect of chitonids on the shells of Ostrea puelchana was one of the main epibiont mortality causes.
Likewise, assuming similar life habits for Ostrea puelchana and ? O .? alvarezii , i.e ., with the right valve preferably lying against the bottom, the larvae of Lithophaga sp. would have a higher survival chance on the left valve.
The beds with ? Ostrea ? patagonica reflect an environment of mainly low energy and low sedimentation rate. Therefore the oysters would have had a longer residence period on the bottom with the consequent exposure to larvae settling. Contrarily, ? Ostrea ? alvarezii is found in rocks representing tidal channels. Individuals here would have been covered by sediment for variable periods of time, in a manner similar to that observed in the extant population of Ostrea puelchana at Las Grutas.
Substrate hardness is another factor affecting larvae settling and subsequent boring rate (Kleemann, 1973, 1980). Ostrea puelchana and ? O .? alvarezii have more compact and smaller shells than ? O .? patagonica , in which larvae of Lithophaga sp. could have been thus more successful.
Left and right valves of Ostrea puelchana and ? O .? alvarezii show preferential distribution of the borings they carry. In both Ostrea puelchana and ? O .? Alvarezii the preferred sectors in the left valve are the platform and the apex and in the right valve are the apex and anterior margin. This means that borings occur more frequently in the thicker and more compact umbonal area, while along the thinner margins of the valves the number of borings is comparatively lower. Likewise, the higher number of borings on the apex of ? Ostrea ? alvarezii could be caused by the fact that ?as in O . puelchana ? this area remains exposed even when the rest of the shell has been covered by sediment after storm events. Tidal currents subsequently reexpose then (personal observations).
In the right valves of ? Ostrea ? patagonica the posterior and anterior margins show perforation values that are higher than expected. This could be attributed to the fact that the clustered ecophenotype oysters lived with their commissure perpendicular to the bottom, although most of them show a slight tilting towards the posterior or anterior margin. This tilt led to the exposure of the posterior and anterior margins, which thereby became more likely to be colonized by Lithophaga sp. larvae.
Acknowledgments
Answers to all your Biology Questions
