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 Bivalvia

Scallops, Clams, Mussels

Image of Bivalvia, courtesy of Leal 2002, from Gulf of Mexico Origina Waters and Biota, Chapter 35 page 711
Dinocardium spp. [Image: Leal, 2002, Gulf of Mexico Origin, Waters and Biota, Chapter 35, Page 711]

Bivalvia consists of 528 documented species in the Gulf of Mexico. Bivalvia is so named because of the hinged, two-shelled calcareous valves that enclose the soft parts of the individual. Bivalves live within and on the sea floor in the Gulf of Mexico from the shallowest intertidal areas to the deep ocean.

While some bivalves live attached to hard surfaces (oysters), many other bivalves burrow in the Gulf's substrate using a muscular foot. These burrowers perform important services such as aerating the upper substratum and breaking down wood and coral rock. Bivalves are filter feeders, filtering water through large gills. This filtration helps to clean the Gulf's waters and concentrates toxins in the bivalve body cavity. In general, bivalves reproduce sexually through external fertilization. Bivalves are important for human consumption, as well as jewelry. Bivalves are also important in the food web.


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 Phylum: Mollusca (Mollusks, Bivalves, Squids, etc)

Image of Queen Conch (Strombus gigas), courtesy of E. Hickerson, Flower Garden Banks National Marine Sanctuary
Queen Conch (Strombus gigas) [Photo: E. Hickerson, Flower Garden Banks National Marine Sanctuary].

Mollusca is one of the most diverse phylla on the planet, and includes organisms such as mollusks, bivalves, gastropods, squid, octopus.  Mollusca has 50,000 identified species, but there are estimates of over 200,000 living species worldwide.

In general, mollusks have a soft body with a discernible head and foot region, and a hard exoskeleton in the form of a shell or plate.  Mollusks are ubiquitous in both marine and terrestrial environments, and range in size from smaller than one millimeter to twenty meters long. 

There are eight classes in the phylum Mollusca.  The focus of this discussion is on three of the five classes in the subphylum Conchifera, or shell-bearing mollusks. Three classes in Conchifera with an important presence in the Gulf of Mexico are Gastropoda, Cephalopoda, and Bivalvia.

(Information from Gulf of Mexico Origin, Waters, and Biota: Volume 1, Biodiversity, Texas A&M University Press 2009).

Other Resources Used on This Page:

UC-Berkeley, Museum of Paleontology, Mollusca
 
UC-Berkeley, Museum of Paleontology, Bivalvia

UC-Berkeley, Museum of Paleontology, Gastropoda

UC-Berkeley, Museum of Paleontology, Cephalopoda


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 Cephalopoda

Image of Cephalopoda, courtesy of Vecchione, 2002, from Gulf of Mexico Origin, Waters, and Biota, Chapter 34, page 701
Ommastrephes spp. [Image: Vecchione, 2002, Gulf of Mexico Origin, Waters, and Biota, Chapter 34, Page 701]

Squids, Octopuses, Nautilus

Cephalopoda include organisms such as squid, octopuses, cuttlefish and the chambered nautilus; there are 93 species identified from the Gulf of Mexico. This class contains the largest and most intelligent species of Mollusca.

The physical characteristics that distinguish Cephalopods from other classes of Mollusca are the lack of external shell (with the exception of nautilus), extremely large and complex eye structure, and arms or tentacles. Cephalopods have a central nervous system, with highly developed sense organs. Cephalopods range in size from 6 mm to 14 meters long. Cephalopods have separate sexes and undergo internal fertilization. They then lay eggs on the ocean floor which develop directly into juveniles, with no larval stage.


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 Gastropoda

Snails, Slugs, Limpets, Sea Hares

Image of Gastropoda, courtesy of Leal, 2002 from Gulf of Mexico Origin, Waters, and Biota, Chapter 33, page 579
Melongena spp. [Image: Leal, 2002, Gulf of Mexico Origin, Waters, and Biota, Chapter 33, Page 579]

Gastropoda is the largest class in the phylum Mollusca, consisting of over 62,000 described species including snails, conchs, whelks, abalones, limpets, and slugs. The Gulf of Mexico contains 1742 documented species of gastropods.  Marine gastropods are both benthic and pelagic, and inhabit shorelines down to the deep ocean.

Gastropods range in size from 0.5 millimeters up to 0.6 meters. Gastropod bodies are asymmetrical, with twisted internal organs and a single shell, which coils to the left or right. Gastropods have a well-developed head with eyes, a sensory tentacle, and a large, muscular foot typically used for crawling. Gastropods most commonly have separate sexes (some species are hermaphroditic) and reproduce sexually, through both internal and external fertilization. Gastropoda displays a wide range of feeding habits.


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