Vandetti The Cephalopoda. Telnes European Squid — Loligo vulgaris. Norman Are there any freshwater cephalopods? Parent The Global Decline of Mollusks. Pojeta Czech Geological Survey Morrison Freshwater Mollusk Conservation Society. Consisting of more than 85, extant species, Mollusca is the second biggest phylum in the animal kingdom, second to only Arthropods.
Ten total classes of Molluscs have existed throughout evolutionary history, but only eight of these classes exist today. These terms were actually initially used to describe many soft-bodied invertebrates that do not fall under this Phylum, including brachiopods, bryozoans and tunicates.
One of the coolest things about molluscs is that their range of adaptations is almost limitless. Most Molluscs have a shell Fig. What is so fascinating about these traits is that even though they are for the most part, very similar, they give rise to an extremely diverse array of functions that have allowed molluscs to thrive! They inhabit freshwater, marine, and terrestrial environments, utilize various food sources, and move around in very different ways.
Speaking of moving around, Molluscs locomotion is extremely diverse. Modes of movement vary greatly from slow crawling, zipping around through the water, or simply staying still. Some other interesting characteristics of Molluscs include: bilateral symmetry, a body with more than two cell layers, tissues and organs Nordsieck, They do not have body cavities, but they do posses a gut with a mouth and an anus.
Most have an open-circulatory system with a heart and an aorta, and do gas exchange through organs called ctenidial gills. They reproduce sexually, which can be external or internal, and can even be hermaphroditic! We chose to present the European Squid Loligo vulgaris representing the Cephalopods squids, octopuses, nautiluses, cuttlefish , the Brown Garden Snail Cornu aspersum representing the Gastropods snails, slugs, sea slugs, limpets , and the Blue Mussel Mytilus edulis, seen in the photo on the right representing the Bivalves mussels, clams, oysters, and scallops.
These were selected as they are common enough to be found locally allowing us a chance to observe them first hand and collect media on them. Because they are local molluscs, they are more familiar to the general public and are therefore, great candidates to represent the three classes of molluscs that we will sample from for this blog.
In The Beginning…. Fossil evidence shows us that molluscs appeared early in the Cambrian period about to million years ago as organisms that crawled along the ocean floor.
According to Nordsieck these fossil records help to explain the division of early molluscs body plans into a soft ventral side used for locomotion the foot , and an armored dorsal side exposed to the environment. Originally, the dorsal side was protected by a thick tissue layer instead of a shell in order to protect their organs.
This was to become the mantle, and can be found in all molluscs. This rendered it additionally resistant. Eventually, the tried and true shell developed! While some molluscs developed overlapping shell plates to allow flexibility, others had shell plates that fused together and sacrificed mobility for protection. This one-part shell adaptation proved so successful that molluscs still have it, and it has allowed molluscs to experience a wide range of diversity.
For instance, according to the University of Cambridge museum of Zoology some molluscs use their shells on dry land to protect against desiccation. The phylogeny in molluscs is still being heavily debated between taxonomists. Regarding the structure of the classes, the depicted phylogenetic tree Figure 2 regards the Testarian Hypothesis described by Sigwart and Sutton However in this source and in many others, the exact Divergence of the classes Cephalopoda, Gastropoda, and Bivalvia are unknown.
Because of this, the divergence within the tree is based off information found by Kumar and Hedges As shown below, the phylum mollusca is branching off of the super-group of Lophophores, even though molluscs are trochozoans.
According to the University of California-Berkeley, the group trochozoa is determined by the larval body form that the organism exhibits, and lophophores are determined by the presence of a strange tube like feeding appendage.
There is a lot of debate with the classification of these groups as well, but currently, according to studies done by Passamaneck and Halanych along with data from Louisiana State University and Sonoma State University, it is a very polyphyletic group, meaning that they are not all grouped together, and that Trochozoa groups can evolve from Lophophore groups, which is believed to be the case with molluscs.
As mentioned above, fossil evidence, shows that molluscs are believed to have appeared around to million years ago, if not sooner. Though according to Kumar and Hedges molecular evidence places the date of divergence from Annelids to be between ato million years ago. Fossil evidence shown below Fig 3 shows long shell imprints associated with ancient cephalopods. The fossil is dated to be approximately million years old.
The shells resembling bivalvia in this figure are actually brachiopoda, and based on the information from Nordsieck , Bivalvia existed, but were heavily out competed until the permian extinction. According to Kumar and Hedges , cephalopoda split off from bivalvia and gastropoda approximately million years ago, while bivalves and gastropods split off from one another million years ago.
This long time period between classes can help explain why the separate classes are so diverse and specialized from one another. Although they are highly derived from one another, molluscs share a few important traits that place them under the same phylum while excluding other soft bodied vertebrates. These traits are called synapomorphies, which include the foot, the shell, the radula and the mantle. But keep in mind, although these are key attributes of all molluscs, they appear very different between classes, and even within families, allowing for greater diversity within the phylum.
We will start with the foot. The foot is the muscular part of the mollusc which is in contact with the substrate. The muscles that are mainly responsible for movement of the bivalve foot are the posterior and anterior pedal retractors.
They effect back and forth movement by retracting the foot. It is compressed and blade-like and it is pointed for digging.
Staying rooted in a single spot is especially helpful if the bivalve is in an ideal location away from predators, or in a nutrient-rich spot. In cephalopods such as the European Squid, the foot derived to be the arm-like tentacles used in hunting Fig 4. According to Howard , they can use their arms for a wide variety of things, such as movement, capturing prey, or fending off predators.
Some molluscs groups had foot divided into left and right halves and separate waves moving on each side. The foot is the organ of locomotion for gastropods such as the Garden Snail. According to Myers and Burch the movement of the Garden Snail is orchestrated by the contraction of muscular waves starting from the posterior end and moving to the anterior end of the of the foot. The shell mainly serves as a surface for muscle attachment but it also serves other purposes. It acts to protect against predators and also from mechanical damage.
According to Nielsen in freshwater snails, land snails and other species, the shell further serves as protection against the sun and also against drying out. The shell is especially visible in both the Garden Snail and in the Blue Mussel, but is not externally visible in the Cephalopod. The radula, also known as the rasping tongue, consists of an elastic band and contains chitin teeth.
It has a bow-shaped jaw used to cut off food particles before it is transported to the gut. According to the Missouri Botanical Gardens it is suited for different kinds of nutrition based on the different habitats molluscs inhabit. The radula is absent in Bivalves because they are filter feeders, but present in gastropods which is shaped to scrape algae off of substrate, such as in this video below.
Fig 6. Another key feature is the mantle. The mantle of the European squid is the long, slender layer that encases everything posterior to the eyes. The mantle of the mussel simply lines inside of the shell. In the garden snail The mantle in the snail is usually fully or partially hidden inside the gastropod shell. Cephalopods are the most mobile of all Molluscs. According to Howard cephalopods are jet setters.
With the exception of the octopus, most spend much of their lives swimming above the bottom. Cephalopod swimming is quite different from that of fish. Cephalopods use jet propulsion, pumping water into their bodies, over their gills and out through a tube called the siphon or funnel. This siphon is a muscular and mobile organ that the animal can use to direct the water jet in almost any direction to steer itself. Holthuis makes the point that gastropods snails, whelks, conchs, are also quite mobile and crawl along on their large foot.
Some bivalves clams can even jet surprising distances by pumping water through their siphons with rapid opening and closing movement of their mantles. They live in different habitats and are extremely diverse in size, body and shell morphology. Gastropod larvae undergo torsion or a twisting which brings the rear of the body the mantle cavity, gills, and anus to a position near the head, which results in the twisting of internal organ systems.
In many species, this twisted form is retained by the adult; while in others it is partially lost. It has originated from western europe, Britain and along the borders of the mediterranean, but today it is one of the most widely spread land snail species in the world.
Bivalvia diverged from their mobile ancestors in order to live a sessile life. Though present in the Paleozoic, bivalves were outcompeted by brachiopods which are arthropods that resemble bivalves , crinoids and corals. After the decline of brachiopods during the Permian Extinction, bivalves established their dominance in the marine environment, essentially replacing brachiopods.
Eventually, around the Devonian period, bivalves with siphons appeared. With the addition of siphons along with the bipartite shell development, bivalves were able to exhibit extraordinary protection which allows the animal to only need to extend its siphon in order to breathe, to feed, and to reproduce, without having to expose the rest of its body. For example, swimming scallops appeared during the Triassic, reef building Rudist bivalves dominated during the Cretaceous displacing coral and freshwater bivalves appeared in the Devonian.
Snails evolved to have developed a dorsal sack, known as the visceral hump, to contain most of the internal organs. Because of the twisting of the digestive tract, the anus in Gastropods is located above their head. They primarily herbivores, relying on their shell as a protection in order to slowly explore environments to intake algae from rocks and other hard substrates with their rasping radula tongue. Even though they reside in the subphylum conchifera, containing only molluscs retaining shells, the shell in cephalopods is highly diminished.
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. They have the ability to hunt and developed long arms with suckers, along with sharp muscular chitin beaks in order to catch and process food. One role that molluscs serve in the environment is actually an indirect role; the shell, used as a barrier to the outside environment, can actually serve as a home to many other organisms, according to the Virginia Department of Game and Inland Fisheries.
For example, many aquatic insects, plants and algae live on the outside of a live mussel and use it as a food source. The most desirable pearls are natural pearls, which are formed when a small, foreign object gets stuck inside an oyster. Learn about our collection and study of molluscs; including chitons, clams, mussels, snails, nudibranchs sea slugs , tusk shells, octopus and squid. AMRI brings together scientific expertise and world-class research infrastructure to increase our knowledge of the world around us and inform environmental decision-making for a better future.
Since we have published the results of studies derived from our collections or research that lead to a better understanding of nature and cultures in the Australian Region. Read about our latest discoveries, research expeditions or discover our interesting archive of blog posts. Use our online enquiry form for help with Australian animal identification, natural history and cultural object enquiries.
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The Australian Museum respects and acknowledges the Gadigal people as the First Peoples and Traditional Custodians of the land and waterways on which the Museum stands. Image credit: gadigal yilimung shield made by Uncle Charles Chicka Madden. This website uses cookies to ensure you get the best experience on our website. Learn more. Skip to main content Skip to acknowledgement of country Skip to footer Malacology, the study of molluscs, include chitons, clams, mussels, snails, sea slugs, tusk shells, octopus, and squid.
Generally molluscs have: an unsegmented, soft body a muscular foot or tentacles a mantle that can secrete a shell Most, but not all, molluscs have: an internal or external shell a radula tongue with teeth Molluscs are one of the largest animal groups with about , species worldwide.
Another type of gastropod, the murex, was cultured by the ancient Greeks for its dye, known as "imperial purple," and the cloaks of some rulers were woven from long threads secreted by the bivalve species Pinna nobilis. One, the Ohridohauffenia drimica was last seen in in springs feeding the River Drim in Macedonia, Greece and was listed as extinct in Additional surveys have failed to find it again.
The vast majority of mollusks live in the deep ocean and are relatively safe from the destruction of their habitat and depredation by humans, but that's not the case for freshwater mollusks i.
Perhaps not surprisingly from the perspective of human gardeners, snails and slugs are most vulnerable to extinction today, as they are systematically eradicated by agriculture concerns and picked off by invasive species carelessly introduced into their habitats. Just imagine how easily the average house cat, used to picking off skittering mice, can devastate a near-motionless colony of snails.
Lakes and rivers are also prone to the introduction of invasive species, particularly mollusks which travel attached to international seagoing ships. Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads.
Apply market research to generate audience insights. Measure content performance. Develop and improve products. List of Partners vendors. Share Flipboard Email. Table of Contents Expand. Octopuses, Squids, and Cuttlefish. Gastropods or Bivalves. Evolutionary History. Conservation Status. Reproduction and Offspring.
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