What is a Bivalve?

Drawn by Muriel Gottrop

Bivalves (class Bivalvia) are a class of mollusks distinguished by their two-valved shell, held together with a tough elastic ligament and controlled by means of the adductor muscles. With over 15,000 species, members of this class are widespread,inhabiting essentially every aquatic environment on the planet, and vary widely in their structures and habits.

Bivalves proper emerged more than 500 million years ago, during the Cambrian period. They are generally sessile, meaning individuals are permanently or semi-permanently attached to a substrate (Georgia Southwestern State University). Consequently, most bivalves are filter feeders and have gills adapted to filter feeding, called ctenidia, first observed in fossils from the Silurian era.

As mollusks, bivalves generally have a siphon and a large, muscular foot, as shown at right. The siphon is a modification of the mantle tissue inside the shell which flushes water through the mantle, aiding in circulation, respiration, and waste removal(Georgia Southwestern State University). Both the siphon and foot are first observed in fossils from the Devonian and Carboniferous periods. These adaptations allow many bivalves to burrow deep in the sediment, an adaptive strategy that has proven remarkably effective.

What is Behavior?

There is no single set definition of a behavior, but when looking at behavior in animals, scientists often frame their inquiries through the lens of four questions originally posed by ethologist Niko Tinbergen.

  • Ontogeny: What role does life experience and development play in shaping behavior?

  • Mechanism: What cues serve to trigger a particular behavior? What chemical pathways and morpho-physiological processes are involved in the execution of a behavior?

  • Phylogeny: What changes occurred in an evolutionary context to lead to the development of a behavior?

  • Survival Value: How does a behavior lead to increased survival value for an ainmal? What selective pressures favored the evolution of a behavior?

Tinbergen's questions are not the only way to approach behaviors, and at times behavioral research does not fit well into this framework. Other scientists, such as Jörg Peter-Ewert, a pioneer of the field of neuroethology in the 1980s, have developed alternative questions to discuss behaviors. Peter-Ewert combines mechanistic questions with questions of ontogeny and adaptive value. This website attempts to discuss bivalve behaviors within the framework of Tinbergen's questions when applicable and works to integrate Tinbergen's four areas of research as necessary to understand the behaviors discussed.

Do Bivalves have Behaviors?

As sessile animals, bivalve behaviors are often overlooked, yet, though sometimes underestimated, bivalves have complicated behaviors, which can be analyzed and discussed through the field of animal behavior. They respond to changes in their environment thoughout their life cycle. The three behaviors discussed here, among the more interesting of those exhibited by bivalves, are escape behaviors to avoid predation, larval dispersion behaviors, and a wide variety of feeding behaviors.