Bivalve or Pelecypod Shells (109):

The Ribbed Ark, Arca gradata.  A representative Bivalve or Pelecypod of the Ark Shells of the Arcidae Family.

The Bivalvia class of bivalves is one of ten classes within the superclass Aplacophora, and this superclass is the only superclass within the phylum Mollusca. The Bivalvia class branches into two subclasses, seventeen orders, forty-six superfamilies, one hundred six families, 1,260 genera, and around 9,200 species. Of those species, about 1,192 are found in freshwater, and the rest are marine or brackish. These numbers are somewhat dated (2010)  but they are concise and clearly defined compared to other sources. Sources differ greatly in the taxonomy of these animals, some sources go as high as 20,000 species in this class. Older sources will refer to this class as Pelecypoda. Included in this class are clams, cockles, oysters, mussels, and scallops. Bivalves are known from fossils from the early Cambrian Period, more than 500 million years ago.

Bivalves have laterally compressed bodies that are enclosed by a calcareous shell, which consists of two hinged parts (valves).  These shells are secreted by a mantle that extends in a sheet on either side of the body. The two valves are held together by a hinge ligament, and are closed by contacting the adductor muscles. They do not have heads and lack organs, like the radula, found in other mollusks. They do have labial palps, and may have sensory tentacles or photoreceptors along their mantle. In many species the ends on the mantle fuse together to form a siphon, which is a flexible snorkel-like tube that allows water to be pumped through the shell for respiration. Their gills have evolved into ctenidia, which are  specialized organs for feeding and breathing. Bivalve shells may be circular, ovate, trigonal, quadrate or other shapes in outline. The shells differ also in profile with some being more inflated than others. External sculpting can go from smooth to spiny, and from axial (running lengthwise), commarginal (concentric), or cancellate (crisscross). The interior is usually smooth and may be porcelaneous or nacreous. They range in size from very tiny (less then one millimeter) to over one meter (39 inches).

Most bivalves bury themselves in sediment to avoid predation. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. They may attach themselves by cementing one of their valves to the substrate, by attaching byssal threads (byssus) to the substrate. The byssal threads act like anchor lines, holding the shell in place.  Some bivalves, such as file shells and scallops can swim. Bivalves are found from the intertidal zone to abyssal depths. They live in all the world’s oceans from the tropics to the polar regions. The majority of bivalves are suspension feeders, and feed by capturing plankton and suspended organic matter from the surrounding water. A small percentage of bivalves are predators or scavengers.

Bivalve mollusks have long been a part of the diet of coastal and riparian human populations. Oysters were cultured in ponds by the Romans, and mariculture has more recently become an important source of bivalves for food. Modern knowledge of molluscan reproductive cycles has led to the development of hatcheries and new culture techniques.  Pearl Oysters are the most common source of natural pearls. The shells of bivalves are used in craftwork, and for the manufacture of jewelry and buttons. Bivalves have also been used in the biocontrol of pollution.

This Website, Mexican-shells.org, currently has 110 Bivalves from 30 families found in Mexican waters. Each individual shell has its own stand-alone page which can be found via the links listed below within the alphabetical indexes presented by Common Name, Genus and Species and Family provided immediately below. For family presentations each shell found in this website has been placed in its specific family, along with its fellow family members, with support information provided for its family as well as a link in the photo caption that will take you to the shell page that has additional photographs and shell specific information.

Bivalve or Pelecypod Shells (110) – Alphabetical Index by Common Name
Bivalve or Pelecypod Shells (110) – Alphabetical Index by Genus and Species
Bivalve or Pelecypod Shells – Alphabetical Index by Family (30) by Common Name
Bivalve or Pelecypod Shells – Alphabetical Index by Family (30) by Scientific Name