Adaptation of oysters to life on soft substrates

Abstract

In spite of their sessile nature, oysters of the genera Crassostrea, Ostrea and their allies are adapted to live on soft muddy bottoms. To provide them with secure attachment and to keep the animal above the surface of the mud, these soft-bottom oysters employ two basic strategies. They tend to be gregarious in their settlement, thereby providing one another with surfaces for attachment, and they form lightweight shells. In addition to these common characteristics, these oysters have followed different morphological pathways to live successfully on soft muddy bottoms. There are at least four types of morphologies: (1) relay-type succession (shells pile up on one another over many generations); (2) konbo-type elongation (both valves grow vertically upwards, the dorsal hinge remaining at the base, near the umbo); (3) cone-type elongation (the attached valve grows upwards and the free valve acts as a lid); and (4) lightweight recliners (bulky shells with lightweight fabrics and cavities within them). Judging from the variety of taxa that have adopted it and the sizes of their populations, the relay-type is the most successful of these strategies. Recliners were particularly common during the Mesozoic. The relay-type appeared in the Late Jurassic, becoming the dominant adaptive strategy during the Cenozoic.

Keywords:

• oyster
• adaptive morphology
• lightweight shell
• gregariousness Crassostrea
• soft-bottom oyster

Acknowledgements

Key ideas relating to the adaptation of oysters to muddy bottoms took form through discussion with Prof. Adolf Seilacher and Prof. Wolf-Ernst Reif when I worked in Tübingen. I am much indebted to them for their penetrating observations and constructive advice. Prof. Roger D.K. Thomas thoroughly reviewed drafts of the text and gave valuable suggestions. Prof. Haruyoshi Maeda and Dr Yoshiaki Matsushima helped me in the preparation of manuscript.