During the Mesozoic period, burrowing bivalves with siphons underwent some species differentiation that eventually proliferated into other time periods. For example, Nordsieck point out that swimming scallops appeared during the Triassic, reef building Rudist bivalves dominated during the Cretaceous displacing coral and freshwater bivalves appeared in the Devonian. In Gastropods, the shell is very different from other mollusc shells as it is coiled to form its characteristic spiral.
Garden snails for example evolved to have developed a dorsal sack, known as the visceral hump, to contain most of the internal organs. This part remains under the mantle and is always within the shell for maximum protection. Because of the twisting of the digestive tract, the anus in Gastropods is located above their head Nordsieck, Cephalopods are the most derived mollusc group. They reside in the subphylum Conchifera, containing mainly molluscs that have retained their shells.
Which is weird seeing as cephalopods have lost their shells completely. They demonstrate a body plan similar to that of slugs and other unshelled gastropods: a reduction of the shell, at the cost of protection but improving movability. However, Nautilus, an extant species of cephalopods, still bears an external shell.
Nordsieck mentions that squids also have a highly developed nervous system,and contain chromatophores, which can change the color of their skin cells. The unique adaptation gives cephalopods ability to visually communicate with one another, as well to camouflage themselves.
Bivalves are aquatic and are mainly found in marine environments, however, multiple bivalve groups have adapted to freshwater environments several times, resulting in several non-closely related freshwater mussel groups according to Nordsieck Because these freshwater environments sometimes have no water flow, bivalve larvae and are often adapted to attach to mobile inhabitants, as the case with the fingernail clam, in order to disperse itself throughout the environment.
According to Nordsieck gastropods are unique in that they are the only of our three classes to live in marine, fresh-water, and terrestrial environments! How cool!. Their shell plays a large part in their adaptation to terrestrial environments, as it provides protection against desiccation by trapping water and by giving mobile protection against solar radiation, as seen in our model species the Brown Garden Snail.
Cephalopods are restrained to only marine environments. This is because of their large, developed nerve system that is strongly correlated with the salty environment.
Though largely sessile, Bivalves still have the ability to move short distances. The most common method of moving based on studies by Nordsieck is reaching their muscular foot from out of their shell, anchoring it to a nearby substrate, and then contracting the foot pulling the Bivalve towards the tethered end.
Studies done by the Cambridge University Museum of Zoology show that the foot is also used in a similar way to burrow into sediment. This allows it to stay anchored in one place while filtering, so that it does not get dragged away by water currents. Though this is a common form of locomotion, the Blue Mussel is actually quite different.
Holthuis makes the point that gastropods are actually very mobile in comparison to bivalves, as they crawl along on their large, flat, muscular foot. Cephalopods are incredibly mobile, and are by far the most agile taxon in the phylum. As stated by Howard , they use jet propulsion in order to move. This involves pumping water through their bodies and out of a tube known as the siphon. The siphon is a muscular organ it uses to steer itself by directing the water flow in order to control the direction of propulsion.
Some bivalves have adapted a similar method of jet propulsion, allowing them to launch themselves through the water! Bivalves are completely dependent on filter feeding, meaning that they feed by using their muscle to pump water into their body through their siphon, and then sifting out particles for nutrition. Nordsieck states that edible particles are transported to their digestive tract, and inedible particles are excreted through the exhalant siphon.
This ability to improve the quality of water is an incredibly crucial ability that is essential to maintaining water quality around the world. However, it is because of this feeding method that bivalves are so afflicted by sea water pollution, and will accumulate harmful substances in polluted areas.
Gastropods are primarily herbivores, relying on their shell as a protection in order to slowly explore open environments to reach rocks and other hard substrates which may contain food.
Nordsieck says that in aquatic environments, gastropods primarily feed on alga, but in terrestrial environments, snails such as the Brown Garden Snail primarily consume herbs and vegetables. This alga consumption makes them important organism in any aquatic environment, as they play a large part in keeping alga levels from rising too high, which could be harmful towards the ecosystem. They also assist in breaking down dead tissues by using their rasping tongue to scrape away at the substrate, assisting in the process of decomposition.
Cephalopods have a highly derived foot used primarily for hunting, which works well with their carnivorous diet. The foot has been modified into arm-like tentacles, which contain suckers that are used while hunting for grabbing prey figure 7 , and then restraining it as the beak crushes the prey while the radula tears and ingests the tissue.
As stated by Howard , cephalopods are also the least dependent on a solid substrate to move, and so are able to catch prey unlike the herbivorous and filter-feeding bivalves and gastropods. Figure 7 — An Up-close picture of the suckers on an octopus tentacle. This is used to restrain prey while hunting and feeding. A detailed account by Nordsieck describes the Blue Mussel life cycle. They have separate sexes, and use the water they are so dependent on to reproduce by releasing 5 — 12 million eggs into the water per female!
The males also release sperm, in hopes the two will meet in a chance fertilization. A larval stage will develop from this, and may be transported several hundred kilometres by sea currents. Most about Until they have reached the size of 5 cm, they will move about until they finally settle to a suitable piece of ground. Preferably, this spot is by other mussels, forming a mussel bed. This is preferable because the chance of fertilization is much higher when around more mussels!
Though most bivalves have separate sexes, there are some hermaphroditic groups. This occurs in oysters for example. It occurs when the number of male oysters is too high in comparison to the female population, resulting in some male oysters altering their sex to become female!
Gastropods are also very diverse when it comes to reproductive strategies! Though it varies widely, there are some correlations between method and the environment. Nordsieck states that Aquatic snails often use the water to transfer sperm cells. Terrestrial snails have sperm packets known as spermatophores that are not dependent on water and resistant to desiccation.
In the case of the Brown Garden Snail, individuals are hermaphroditic, meaning that they have the reproductive organs of both sexes. Cephalopods are unique in their phylum because they are only male and female! Male squids have a special arm called a hectocotylus, which is slightly shorter than the rest. These are then hidden where they develop into their larval stage, but unfortunately, most are eaten before they can grow large enough to avoid being such an easy meal!
In all taxon, larvae are a very important part of the food chain. Without the massive amount of larvae that become food for other marine species, many species may not persist! Each class consists of many groups of Molluscs, but many of these groups have gone extinct. Harvard Research Site states that among these were the Rudists, which were ring-shaped bivalvia important to the formation of reefs, and Ammonites, which were heavily shelled, predatory cephalopods that fed on plankton in open ocean water.
Though there once ten classes of Molluscs, two of these classes, Rostroconchia and Helcionelloida, are both extinct. According to Pojeta , though the Rostroconchia were a sort of filter feeding, bivalve-like creature, they did not have many similarities with the bivalves that we know and love. Actually, they are considered to be more related to Tusk Shells, which is a phylum that we did not discuss in this blog.
Despite having over a thousand species they are believed to have gone extinct in the Permian Extinction. Helcionelloida resembled snails, as they had a large muscular foot, as well as cone-like shell with large ridges on it. As molluscs are so diverse, it makes sense that they are found in nearly every ecosystem. Mostly molluscs are marine, inhabiting corals, beaches, shores and other habitats. They can also be found in the deep ocean, with the Giant Squid being the most recognizable example.
In addition, the diverse gastropod class has provided terrestrial mollusc species, which are able to inhabit most terrestrial ecosystems Bunje, Gastropods such as the Garden Snail picture below range second, only behind insects when it comes to the number of named species. Today there are more than 62, living species of gastropods. They are the only molluscs group to have invaded land habitats and thus they occupy the widest ecological niche of all molluscs.
It is well-known that molluscs form a large part of the invertebrate fossil record. Especially bivalves, gastropods, and cephalopods are among the most diverse groups. We are looking for contributions which describe the continuous mollusc radiation and address the question why molluscs have become so important. Squids and octopuses are all predators; they eat fish, crustaceans, and other mollusks. Animal Diversity Web Cybertracker Tools. All rights reserved. Skip directly to main content. Kids' Inquiry of Diverse Species.
Critter Catalog. It is important to learn about the diversity and evolution of organisms in the deep before there is further destruction. We are using molecular methods to build robust phylogenies of deep-sea molluscs to help answer some fundamental questions. Some deep-sea molluscs have developed novel diets. Some have become carnivorous while others survive by only eating wood.
We also show that a global cooling around This appears to be a result of increased nutrients supplied to the ocean by changing erosion, ocean circulation, tectonic events and upwelling. This suggests that food availability may have been a factor limiting the exploitation of deep-sea habitats.
Our scientists are investigating the taxonomy, systematics and biodiversity of groups of invertebrates.
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