Bivalve

Bivalves are a large class of mollusc phylum and other aquatic invertebrates having a hinged double shell. 

Also, the bivalve in previous centuries was referred to as the Lamellibranchiata and Pelecypoda. Bivalve molluscs species is a class of marine and freshwater molluscs that have sideward compressed bodies enclosed by a shell consisting of two hinged parts.

These aquatic species belong to a kingdom of Animalia and a class of Bivalvia; Linnaeus, 1758. Also, these species have the scientific name of Bivalvia.

Besides all these, they have a particular habitat and distribution. Additionally, they have characteristics like feeding patterns, circulatory systems, excretory systems, and so on. We will discuss all these in detail.

Bivalve Etymology

The taxonomic term Bivalvia was first utilized by Linnaeus in the tenth release of his Systema Naturae in 1758 to allude to creatures having shells made out of two valves. All the more as of late, the class was known as Pelecypoda, signifying "hatchet foot" (given the state of the foot of the creature when expanded). 

The name "bivalve" is gotten from the Latin bis, signifying "two", and valvae, signifying "leaves of a door".

Paired shells have developed freely a few times among creatures that are not bivalves; different creatures with combined valves incorporate certain gastropods (little ocean snails in the family Juliidae), the individuals from the phylum Brachiopoda and the moment scavengers known as ostracods and conchostrachans.

Bivalve Anatomy

Bivalve molluscs vary greatly in overall shape. A few, like the cockles, have shells that are almost globular; cockles can bounce by bowing and fixing their foot. Others, for example, the razor shellfishes, are tunneling experts with prolonged shells and an amazing foot adjusted for fast burrowing. 

Also, the shipworms, in the family Teredinidae, have significantly extended bodies, yet their shell valves are quite diminished and confined to the foremost finish of the body, where they work as scratching organs that license the creature to burrow burrows through the wood.

Bivalve Evolution History

The Cambrian blast occurred around 540 to 520 million years prior (Mya). In this geographically short period, every significant creature phyla separated and incorporated the primary animals with mineralized skeletons. Brachiopods and bivalves showed up now and left their fossilized remaining parts behind in the rocks.

Conceivable early bivalves incorporate Pojetaia and Fordilla; these presumably lie in the stem as opposed to the crown bunch. Watsonville and Anabarella are seen to be (prior) direct relations of these taxa. Just five genera of assumed Cambrian "bivalves" exist, the others being Tuarangia, Camya and Arhouriella, and conceivably Buluniella. Bivalves have additionally been proposed to have developed from the rostroconchs.

Bivalve Fossils

Bivalve fossils can be shaped when the dregs in which the shells are covered solidify into rock. Regularly, the impression made by the valves stays as the fossil instead of the valves. 

During the Early Ordovician, an incredible expansion in the variety of bivalve species happened, and the dysodont, heterodont, and taxodont dentitions advanced. By the Early Silurian, the gills were getting adjusted for filter taking care of, and during the Devonian and Carboniferous periods, siphons previously showed up, which, with the recently evolved solid foot, permitted the creatures to cover themselves somewhere down in the sediment.

Bivalve Facts:

In the middle of the Paleozoic, i.e., around 400 Mya, the brachiopods were among the most abundant filter feeders in the sea, and more than 12,000 fossil species are recognized. 

Also, by the Permian–Triassic termination occasion 250 Mya, bivalves were going through gigantic radiation of variety. The bivalves were hard hit by this occasion, however, restored themselves and flourished during the Triassic time frame that followed. 

Bivalves and Branchiopods

Bivalves are cursorily like brachiopods, however, the development of the shell is totally extraordinary in the two gatherings. Firstly, in brachiopods, the two valves are on the dorsal and ventral surfaces of the body. Secondly, in bivalves,  they are on the left and right sides.

Interestingly, the brachiopods lost 95% of their species diversity. The capacity of certain bivalves to tunnel and accordingly keep away from hunters may have been the main consideration in their prosperity. 

Other new variations inside different families permitted species to involve beforehand unused transformative specialties. These remembered expanding relative lightness for delicate residue by creating spines on the shell, acquiring the capacity to swim, and in a couple of cases, receiving savage habits.

Bivalves vs Branchiopods

For quite a while, bivalves were believed to be preferably adjusted to sea-going life over brachiopods were, outcompeting and consigning them to minor specialties in later ages. These two taxa showed up in reading material to act as an illustration of substitution by contest. 

Almost globular shell proof given for this incorporated how bivalves required less food to remain alive due to their enthusiastically proficient tendon muscle framework for opening and shutting valves. This has been extensively disproven, however; rather, the conspicuousness of present-day bivalves over brachiopods appears to be because of chance incongruities in their reaction to elimination occasions.

Bivalve Morphology

They have a hard calcareous shell made of two sections or 'valves'. The delicate parts are inside the shell. These bivalve shells are typically reciprocally even. 

Bivalves are most effortlessly perceived by their two shells (henceforth "bi-valv-ia"). Their shells are contained a couple of along the side-packed pivoted valves and the pallial pit encompasses the entire body.

In other words, bivalves have two shells or valves associated with a pivot with pivot teeth. They are formed of calcite, calcareous mineral, or aragonite. The valves are covered by a periostracum, which is a natural horny substance. This structures the natural hued layer on the shell. 

The shells are typically held closed by solid adductor muscles. Scallops can utilize their muscles to fold the valves and swim.

Bivalve Shell Morphology

The bivalve shell comprises two valves that are pivoted dorsally, for the most part with shelly interlocking teeth (the pivot), and consistently with a horny tendon that associates the two valves along their dorsal surfaces and acts to drive the valves separated. 

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The inside of the valves contains scars of the different muscles joined to it, specifically the (generally two, now and then one) adductor muscles that, on withdrawal, close the valves. The shell can likewise be inside, decreased, or even missing, for example, in "shipworms" (Teredo). 

Bivalves regularly show respective evenness both in-shell and life systems, yet there are significant departures from this theme in taxa such as scallops and oysters.

Bivalve Molluscs

There are more than 30,000 types of bivalves, including fossil species. There are around 9,200 living species in 1, 260 genera and 106 families. Every one of them lives in the water, a large portion of them in the ocean or in harsh water. Some live in new water. All are filter feeders: they lost their radula over the span of development. A couple is rapacious, eating a lot bigger prey than the little microalgae eaten by different bivalves. 

The most popular bivalves examples are shellfishes, mussels, scallops, and oysters.

Bivalve Characteristics

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Below is the list of Bivalve Characteristics:

• Food

• Feeding pattern

• Nervous system

• Defensive secretions

• Digestive system

Now, let us discuss the bivalve characteristics one by one:

Bivalve Diversity

The grown-up greatest size of the living types of bivalve reaches from 0.52 mm (0.02 in) in Condylonucula Maya,[48] a nut shellfish, to a length of 1,532 millimeters (60.3 in) in Kuphus polythalamia, an extended, tunneling shipworm.

For the most part, in any case, the species viewed as the biggest living bivalve is the monster mollusk Tridacna gigas, which can develop to a length of 1,200 mm (47 in) and a load of in excess of 200 kg (441 lb).

The biggest realized terminated bivalve is a type of Platyceramus whose fossils compare 3,000 mm (118 in) long. 

In his 2010 composition, Abridgment of Bivalves, Markus Huber gives the complete number of living bivalve species as around 9,200 consolidated in 106 families.

Bivalve Food 

A bivalve takes in water that has microscopic fish and different things drifting in it. 

A few (yet not all) molluscs have a piece of their mantle known as the siphon (a cylinder). Siphons, on the off chance that they exist, come two by two, one to suck in, one to remove. 

Anything that is adequately little to fit inside the opening of its incurrent siphon enters the bivalve. At the point when the gliding material comes in, it stalls out in disgusting bodily fluid that is on the outside of the bivalve's gills. The food is dropped down to the mouth, which is on the opposite side of the siphon. Food is processed in the bivalve's stomach and digestive tract, and all that isn't processed goes out through the other direction with water.

Bivalve Feeding Types

There are four taking feeding types, characterized by their gill structure: 

1. Protobranchs utilize their ctenidia exclusively for breath, and the labial palps to take care of 

2. Septibranchs have a septum across the mantle hole which siphons in food. 

3. Filibranchs and lamellibranchs trap food with a mucous covering on the ctenidia; the filibranchs and lamellibranchs are separated by the manner in which the ctenidia are joined.

Bivalve Digestive System

Most bivalves are filter feeders, utilizing their gills to catch particulate food like phytoplankton from the water. 

• The protobranchs feed in an alternate manner, scratching waste from the seabed, and this might be the first method of taking care of utilized by all bivalves before the gills got adjusted for filter taking care of. 

• In the Filibranchia and Eulamellibranchia, water is brought into the shell from the back ventral surface of the animal, goes upwards through the gills, and backtracks to be removed simply over the admission. 

• In burrowing species, there might be two extended, retractable siphons coming up to the seabed, one each for the inhalant and exhalant surges of water. The gills of the filter taking care of bivalves are known as ctenidia and have gotten profoundly altered to build their capacity to catch food. 

For instance, the cilia on the gills, which initially served to eliminate undesirable dregs, have gotten adjusted to catch food particles and transport them in a constant flow of bodily fluid to the mouth.

Bivalve Species Digestive System

A couple of bivalves examples, viz: the granular (Poromya granulata), are predatory, and they eat a lot bigger prey than the little microalgae devoured by different bivalves. In these creatures, the gills are generally little and structure a punctured obstruction isolating the fundamental mantle pit from a more modest chamber through which the water is breathed out. 

The uncommon variety, Entovalva, is endosymbiotic, being discovered uniquely in the throat of ocean cucumbers. It has mantle overlays that totally encompass its little valves. At the point when the ocean cucumber sucks in sediment, the bivalve permits the water to disregard its gills and concentrates fine natural particles.

Bivalve Circulatory and Respiratory System

Bivalves have an open circulatory framework that washes the organs in the hemolymph. The heart has three chambers: two auricles getting blood from the gills, and a solitary ventricle. The ventricle is strong and siphons hemolymph into the aorta, and afterward to the remainder of the body. A few bivalves have a solitary aorta, however, most likewise have a second, normally more modest, aorta serving the rear pieces of the creature. 

How Bivalves Inhale?

Oxygen is transported into the hemolymph in the gills that give the required respiratory surface. The gills hang down into the mantle depression, the mass of which gives an auxiliary respiratory surface being all around provided with vessels. In species without any gills, for example, the subclass Anomalodesmata, the mass of the mantle depression is the only organ associated with breath. 

Bivalves adjusted to flowing conditions can get by for a few hours out of the water by shutting their shells firmly. Some freshwater species, when presented to the air, can expand the shell marginally and gas trade can take place. Shellfish, including the Pacific clam (Magallana gigas), are perceived as having fluctuating metabolic reactions to ecological pressure, with changes in breath rate is regularly observed.

Point to Note:

The hemolymph as a rule does not have any respiratory colour, in spite of the fact that individuals from the families Arcidae and Limidae are known to have hemoglobin broken down straightforwardly into the serum. In the flesh-eating sort Poromya, the hemolymph has red amoebocytes containing a hemoglobin colour.