FACULTATIVE MONOGAMY IN OSTREA SP., A BROODING OYSTER FROM THE LOWER EOCENE OF CAMBAY BASIN, WESTERN INDIA AND ITS POSSIBLE RELATION TO GLOBAL WARMING

A curious reproductive behavior is known in an extant oyster Ostrea puelchana from Patagonia, Argentina. The large females of this species carry dwarf males at the interior of anterior edge of their shells (Pascual et al. 1989). Similar carriage phenomenon was reported from an upper Pliocene (2.5-3 Ma) fossil oyster O. alvarezi, also from Patagonia (Iribarne et al. 1990). This phenomenon has been interpreted to have evolved in response to advantages such as (a) increased success in fertilization of eggs, (b) increased male fitness by reduction of sperm loss, (c) protection of small settling males, etc. (Pascual et al. 1989, Pascual 1997).

An identical association of dwarf males and large females of a fossil species of Ostrea is reported here from the lower Eocene (~54 Ma) deposits of Cambay basin, western India. The specimens were collected from a lignite mine in the district of Surat, Gujarat. The phenomenon is christened as facultative monogamy. This report pushes back the evolution of this phenomenon in the brooding oyster subfamily Ostreinae by more than 50 m.y. Rarity of occurrence of this phenomenon in the long history of this lineage, in spite of its purported advantages, is noteworthy. Analysis of stable carbon isotopic ratios (δ¹³C) of the lower Eocene rock succession from the mine reveals the presence of a hyperthermal event enveloping the layer that yielded the oyster. It is argued here that facultative monogamy was a response to the environmental stresses associated with the hyperthermal event. The two previous reports of the occurrence of this phenomenon in Ostreinae are from times that are also characterized by elevated global temperatures and associated other environmental changes.

References

Pascual MS et al. (1989). J. Exp. Mar. Biol. Ecol., 132: 209-219.

Pascual MS (1997). J. Exp. Mar. Biol. Ecol., 212: 173-185.

Iribarne OO et al. (1990). Lethaia, 23: 153-156.