Figure 3. Range of dissolved oxygen concentrations for the thirteen dominant genera of mollusks. Figure 4. Negative binomial generalized linear models lines fitted for mollusk abundance per genus dots in relation to oxygen concentrations.
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Echeverria, L. Pessoa, P. Paiva, R. Paranhos, and J. Mutlu and M. Maas, K. Wishner, and B. Molluscs display a large diversity of nervous systems, from the rudimentary nervous system of the brainless bivalves to the complex systems of the cephalopods, who have well-developed brains and are considered the most intelligent of invertebrates.
Chitons Polyplacophora are basal relative to other extant molluscs Fig. Their soft bodies are covered with a series of eight shell plates. The joints between these shell plates enable to chitons to roll up for protection. Chitons are mobile and contract their muscular foot in waves to move about.
The primarily herbivorous chitons have a well-developed radula. Their nervous system is a series of ladder-like nerves and only a few species have poorly developed ganglia. Chitons are found only marine environments. They are most commonly found in tide pools and rocky intertidal zones. Chitons can tolerate the harsh conditions of these habitats where ocean and land meet. Gastropods are the most diverse group of molluscs Fig. The ones we usually think of are snails and slugs. Most gastropods have a calcareous shell protecting the soft-bodied animal inside.
Some gastropods, such as sea slugs, sea hares, and garden slugs, lack a shell or have a reduced shell buried in the folds of their mantle. Most creep about on a flattened foot, but some swim, using extended folds of their mantle as fins. Most snails and terrestrial slugs are herbivorous. They use their radula to scrape algae from surfaces Fig. For this reason, gardeners consider snails and slugs to be pests. Some gastropods are carnivores, stalking other snails, worms, and fish for food Fig.
The colorful and striking nudibranchs contain many carnivorous specialists. Many nudibranchs feed on only one type of sponge; their body coloration and their eggs are patterned to blend in with their prey. Other gastropods use their radula and acidic secretions to bore holes in shells and prey on other molluscs.
In the Hawaiian Islands, the terrestrial cannibal snail Euglandina rosea ; Fig. The giant East African land snail is considered to be an agricultural pest and is also known to be a carrier of the parasitic nematode rat lungworm Angiostrongylus cantonensis ; Fig. Unfortunately, the cannibal snails also predated native land snails, nearly driving them to extinction.
Marine and freshwater gastropods breathe using ctenidia or gills. In many of these gastropods the ctenidia are protected within the mantle cavity. This distinctive trait makes nudibranchs an easily identifiable group of molluscs.
Terrestrial slugs and snails, by contrast are primarily in a subgroup known as the pulmonates that actually have a mantle cavity that has become connected to the circulatory system vascularized to function as a lung. Gastropods move by contracting their muscular foot in a series of waves to creep forward. These trails also provide chemical communication among gastropods. The cannibal snail, for example, tracks its prey by following the mucus trail left behind.
The gastropod nervous system includes bodily nerves and anterior ganglia with relatively sophisticated sensory systems, including light receptors and well developed chemosensory abilities. The bivalve molluscs get their name from the two door-like valves or shells that make up their exoskeleton Fig. Foot size varies among marine bivalves. Clams have a muscular hatchet-shaped foot for moving about and for burrowing in mud or sand Fig.
They swim in short bursts by jet propulsion, clapping their shells together and forcing water out the rim. Bivalves are more enclosed by their shells than other molluscs. Water enters and leaves a bivalve by way of two tubes called siphons.
One siphon takes in water while the other expels water and waste. The water taken in contains oxygen and food particles. Most bivalve species acquire energy and nutrients through filter feeding. Filter feeding or suspension feeding is the process of ingesting water and filtering out food particles. Invertebrate examples of filter feeders include sponges, corals, and bivalve molluscs.
As water is taken into the body, it flows across the gills. Oxygen O 2 and carbon dioxide CO 2 are exchanged between the circulatory system and the water. Mucus on the gills traps microscopic food particles, and tiny hairlike cilia move the food-laden mucus toward the mouth.
Liplike structures called palps help sort the food and direct it into the mouth. Bivalves do not have a radula Fig. The food suspended in mucus moves through the digestive organs, which break it down and absorb it. Bivalves such as clams, oysters, and scallops are valuable as food. They make up a major share of the marine invertebrate seafood industry. Bivalves should not be eaten when the water in which they grow becomes polluted with chemicals or disease organisms.
At certain times of year, microscopic organisms called dinoflagellates multiply rapidly in nearshore waters. Toxic substances produced by dinoflagellates can concentrate in the clams and oysters that use them as food. Although the bivalves are not harmed, the toxin can attack the nervous system of humans who eat the tainted shellfish. Toxic shellfish poisoning can be fatal to humans. A pearl forms as coats of nacre build up around the foreign particle.
Cultured pearls used in jewelry are produced when farm-raised oysters are intentionally seeded with foreign particles to stimulate the production of nacre. The cost of pearls varies with size, color, and luster. Before plastic came into use, the shells of bivalves were commonly used to make buttons. The material known as mother-of-pearl is harvested from the nacre of mollusc shells. The cephalopods are molluscs with large heads and tentacles. Examples of cephalopod molluscs include squid, octopus, cuttlefish, and nautilus Fig.
Most cephalopods are relatively small. But the giant octopus Enteroctopus sp. The giant squid, the largest invertebrate, reaches lengths of 15 m.
The foot in this group has specialized by dividing into arms that are attached to the head, thus the name cephalopod, meaning head-foot. Land snails have been used extensively in European archaeology to interpret environments of the distant past. They can also be indicators of pollution, as they uptake environmental toxins such as cadmium.
Human use of land snails as food ranges from Native American consumption of Oreohelix species snails in the western states, to fine dining upon Helix species snails served as escargot in restaurants. Medical uses include the production of an anti-agglutinin from the albumin glands of Helix aspersa. Land snails can also have negative interactions with other organisms. Snails are intermediate hosts to a variety of mammalian parasites. The cervid brainworm Parelaphostrongylus tenuis is carried by deer and can severely limit moose and caribou populations.
But the most serious ecosystem and agricultural impacts due to land snails are often related to non-native pest populations. For example, the introduced European white garden snail Theba pisana that can damage ornamental and citrus plants has been the subject of eradication programs in California.
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