Bivalves
The huge and extremely varied class of Bivalves (Bivalvia) includes over 15,000 different species of clams, oysters, mussels, mollusks and scallops. The group is diverse, with members being different in terms of coloring and shape just as much as in terms of size, but they all share two very important features, one of which has given the class its name: they all have a hard shell divided into two components called valves and they all have a strong foot that most species use for burrowing.
Bivalve species can be found in waters all over the world, in the seas and oceans just as much as in freshwater habitats. They are extremely hardy creatures and are, therefore, highly successful – they can survive in tropical climate zones and in boreal waters, in sandy areas that seem otherwise uninhabited and in friendlier habitats preferred by other aquatic species as well. Some bivalve species such as the Antarctic scallop are able to survive in subzero temperatures for extended periods of time, while others, such as the giant white clam live in the deepest waters of the Pacific Ocean, at depth between 4,000-6,000 meters deep, in the complete darkness of the ocean bed.
Bivalve species vary largely in terms of physical appearance. Some of them, such as cockles, have shells that are almost perfectly spherical, while others such as razor clams have elongated shells and a short, but very powerful leg that allows the animal to burry itself in the sand if it senses any danger approaching.
The species belonging to the class are different when it comes to body size as well – the largest of them all is probably the giant clam that can reach almost one and a half meters in length and can grow to weigh more than 250 kg during its lifespan of 40 years on average, while the smallest of them all is a freshwater species, the seed shell that does not grow longer than 2 mm.
Whatever shape, size or color the various bivalve species are, they all have a shell made up by two valves hinged together with powerful ligaments. The anterior part of the shell is where the single foot is located. The posterior part of the shell ends in two siphons, two tube-like organs that fulfill multiple vital functions such as feeding, reproduction, even locomotion.
Most species belonging to the bivalve class have sedentary lifestyles, having a very limited range of motion. Others are completely sessile, attaching themselves to a spot in the substrate and spending their entire life in that spot. Most of them are filter-feeders, capturing food particles from the water with the help of their gill, while others feed on the detritus that accumulates in the waterbed.
Most bivalve species are sexually dimorph, with males and females being distinguishable by their appearance. Most of them proliferate by spawning, the females releasing their eggs into the water where they are fertilized externally; then, after only a few hours, the fertilized eggs hatch and the larvae settle on the waterbed to develop into juveniles, then into adult bivalves.
