Do bivalves show cephalization?

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Bivalves, shellfish that dwell in fresh water, belong to the mollusk phylum. Subdivided into more than 30,000 species, the most commonly known mollusks include mussels, scallops and oysters. As the name implies, bivalves are chiefly characterized by the presence of two mirror-image, though not necessarily symmetrical, shells that surround the organism’s soft inner tissue. The composition of each shell is calcium based, having a coarse outer surface texture, or periostracum, combined with a smooth texture on the inner surface, or pterioida. The inner surface is lined with a membrane called the mantle. The mantle situates the soft inner tissue of the organism within the body of the shells.

With the shells connected together by a flexible hinge-like ligament located at a small region somewhere on the perimeter of each shell, the organism is capable of opening and closing both shells using the adductor muscle located at the back of the shells next to the hinge ligament. In addition to the adductor muscles present next to the hinge ligament (posterior adductor muscle), many species also have a set of adductor muscles further up near the opposite end, called the anterior adductor muscles. When the shells are closed, all bivalves are protected by their external shells, opening them in order to capture nourishment or to allow for transportation and movement. Nourishment is commonly obtained through the use of a syphon, a tubular structure that extends out from the mantle. It collects nutrients by taking in and expelling water through a collecting filter. Other bivalves use tentacles to remove plankton and other microorganisms from the sand and rocks on the seafloor. This action typically occurs while the organism is stationary and, consequently, more vulnerable to predators.

Unlike most other bivalves, scallops are able to move from one place to another by rapidly opening and closing (“flapping”) their shells. Their strong adductor muscle aids in this action and is the part of the scallop consumed as food by humans. In the case of bivalves such as mussels, the organism adapts to, navigates and manipulates its environment using what is called an epifaunal appendage. This structure behaves similar to a suction cup, allowing the organism to attach itself to numerous types of surfaces (mostly rock) and also to move from place to place. The most common example of such an appendage is the suction cup used by a snail. Many other bivalves have an appendage called a foot, with which they may push themselves from place to place. The system of nerves and sensory peripherals found in bivalves reflects the absence of a brain. Pockets of neural ganglia found primarily in the digestive tract regulate responses to external stimuli, feeding and responses to predators.

The unusual shape and physiological orientation of bivalves has led many to ponder whether or not they display cephalization similar to other mollusks. Cephalization is the evolutionary tendency in all organisms for primary sensory input organs to aggregate in a single spot, thus giving that organism a definite head and tail. In humans, for instance, cephalization is evident in the close proximity of the eyes, nose, ears and mouth on the face; humans have cephalized as part of their evolution. In mollusks such as snails and squid, cephalization has essentially polarized their forms. In snails, the head area is evidenced by the presence of the antennae and eyes. Similarly, the tentacles and beak of a squid as well as the eyes are located opposite its pointed tail. There have been disagreements, however, about the presence of cephalization in bivalves. The variety of shapes among the several thousand species has led to the consensus that some display greater evidence of cephalization than others. In other words, some species do exhibit cephalization, while other show none at all.

Resource by Will

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