Mollusc Classification Taxonomy Class Polyplacophora: chitons Class Scaphopoda: tusk shells or tooth shells Class Bivalvia: clams, oysters, mussels Class Gastropoda: snails, slugs, limpets, whelks Class Cephalopoda: Octopus, squid, nautilus Class Polyplacophora: chitons Exclusively marine, chitons are somewhat dorsoventrally flattened molluscs with a dorsal surface covered with a series of usually 8 (rarely 7) overlapping, articulated, calcareous plates. Hence, the name Polyplacophora (“bearing many plates”). Figure 16.09a 10.8 Class Polyplacophora: chitons Most chitons are small (<5cm), the largest reaching 30cm. About 650-800 living species. Most often found on rocky surfaces in the intertidal zone where they graze on algae, which they scrape off using their radula. Chitons grip their substrate firmly and are very hard to dislodge. If displaced, they curl into a ball for protection like an armadillo. Class Scaphopoda: tusk shells Scaphopoda (‘shovel foot’) are marine molluscs that have a long tusk-like shell open at both ends. Most of the 350 or so species are found in the deep sea, but some occur in shallower water. Class Scaphopoda: tusk shells Burrow into sediments using their foot and the narrow end of the shell protrudes into the water above. They lack gills and gas exchange occurs across the mantle itself. Class Scaphopoda: tusk shells Most food is detritus and protozoa and other small organisms found in the sediment. Food is captured using threadlike mucuscovered tentacles called captacula which have adhesive knobs on their end. Figure 16.11 Fig 10.10 Tusk Shell Hermit Crab Tusk Shell Hermit Crab Class Bivalvia The Bivalvia (two valves/shells) sometimes referred to as the Pelecypoda (Greek: hatchet foot). These are the two-shelled molluscs and include the mussels, scallops, oysters, clams, and shipworms. Class Bivalvia The Bivalvia are the second largest class of molluscs (gastropoda 1st) with approximately 15,000 species. Characteristics of Class Bivalvia Bivalves are laterally compressed. Two shells (valves) are held together by a hinge ligament. Characteristics of Class Bivalvia Adductor muscles and hinge open/close shells. Figure 16.26c Figure 16.26d Figure 16.31 Fig 10.22 show adductor muscles 10.23 c adductors open and shut. Gas Exchange and Filter Feeding In some marine bivalves the incurrent and excurrent openings are extended into long siphons formed by the mantle being extended into tubes. The siphons allow a bivalve to burrow beneath the sediment, yet still respire and feed. Figure 16.28bcd Figure 16.30 10.27 Gas Exchange and Filter Feeding In addition to gas exchange, the gills are involved in filter feeding. Gland cells secrete large quantities of mucus. This mucus traps particles suspended in the water passing through the gill pores. Gas Exchange and Filter Feeding The particles trapped in the mucus move along ciliary tracts to the mouth where they are consumed; particles not ingested = pseudofeces. Gas Exchange and Filter Feeding The particles trapped in the mucus move along ciliary tracts to the mouth where they are consumed; particles not ingested = pseudofeces. “false poop” Gas Exchange and Filter Feeding Filter feeding in bivalves is very efficient and extremely small particles are captured. Mussels, for example, filter out essentially all particles 3-5 microns in diameter and 50% of particles 1-2 microns in diameter. Class Bivalvia The foot like the rest of the body is also laterally compressed and this helps the bivalve work its way into soft sediments. To burrow, a bivalve extends its foot and pumps blood into it, which causes the foot to swell. Class Bivalvia: reproduction In bivalves the sexes are separate and fertilization is usually external. Marine larvae typically go through three free-swimming stages before settling: trochophore, veliger and spat. Class Bivalvia: reproduction In freshwater clams fertilization usually occurs internally and the larvae go through early development in a brood chamber. The larvae develop into specialized veligers called glochidia. These are released into the water and attach to passing fish’s gills where they feed and hitchhike as parasites for several weeks before dropping off and settling. Glochidium larvae Figure 16.35a Glochidium attached to fish gills Class Gastropoda The Gastropoda are by far the largest class of molluscs. There are about 40,000 living species and 15,000 known fossil species. They include: snails, slugs, whelks, limpets, conchs, sea slugs, sea hares and periwinkles. Class Gastropoda Most gastropods are slow moving as most have heavy shells and depend on using their single slow moving foot to get around. The shell, if present, is almost always a one-piece shell and is often coiled. Class Gastropoda Basic body form Class Gastropoda Basic body form Class Gastropoda Some snails possess an operculum a proteinaceous lid that can be used to close the shell’s opening. Closing the operculum reduces water loss. Class Gastropoda Gastropods are basically bilaterally symmetrical. However, because of the process of torsion, a twisting process that occurs during the gastropod’s embryonic/larval development, the visceral mass has become asymmetrical. 10.11 Class Gastropoda Torsion moves the mantle cavity to the front. Because of the twisting of the viscera the anus now opens anteriorly. As a result, there is a problem with fouling as waste tends to wash back over the gills. Why? Sense organs up front and/or more space in shell for foot to fit. Gastropoda: feeding habits Being such a large group it is not surprising that gastropods feed on a wide variety of different foods. All of them however make some use of the radula in foraging. Many are herbivores and feed by rasping algae or vegetation with their radula. Others are scavengers or predators and use their radular teeth to tear apart their prey. Gastropoda reproduction There are both dioecious and hermaphroditic gastropods. Gastropoda: Classification Major groups of gastropods. Three subclasses are traditionally recognized: Prosobranchia: periwinkles, whelks, oyster borers, and limpets Opisthobranchia: sea slugs, sea butterflies, sea hares Pulmonata: land snails (some marine) and slugs Prosobranchia: periwinkles, whelks, oyster borers, and limpets Opisthobranchia: sea slugs, sea butterflies, sea hares Pulmonata: land snails (some marine) and slugs Cephalopods The cephalopods (“head foot”) are the most complex of the molluscs and are among the most complex of all invertebrates All are marine and the group includes squids, octopuses, cuttlefishes and nautiluses. Cephalopods All of the cephalopods have tentacles that they use to grasp and manipulate objects. Octopuses have 8, cuttlefish and squids have 10 including (in squids) two extended arms that are flattened at the end. Nautiluses have 38 suckerless tentacles. Cephalopods Tentacles are equipped with suckers, and in squids many of them also have hooks on them. Cephalopods possess a radula, but more importantly, have a horny parrotlike beak which they can use to bite and tear tissue. They also inject poison when they bite. Cephalopods The brain in cephalopods is well developed and octopuses in particular are quite intelligent. They can solve problems and have a memory. The enlarged brain also equips them for an active predatory lifestyle. Cephalopods: vision The cephalopods possess eyes that are comparable in structure to those of vertebrates, possessing a retina, lens, cornea, and iris. There is some debate as to how well they see, but in certain aspects their eye is better designed than that of humans. Cephalopods: locomtion Crawling one bottom with tentacles Using tentacles to “swim” Jet propulsion (suck water into mantle, direct flow OUT through a siphon or through mantle opening) Crawl/walk swim Jet propulsion Cephalopods: chromatophores The skin of most cephalopods contains chromatophores, which are pigment-containing cells that can be expanded or contracted to change the animals color. Tiny muscles attached to the periphery of the chromatophores contract to expand the cell into a plate and make the color more obvious or relax to concentrate the pigment into a spot and make it less obvious. Reproduction in Cephalopods Almost all cephalopods are dioecious. Sperm are encased within a spermatophore and are kept until needed in a storage gland. Mating involves copulation, but males do not possess a penis. Instead, one of the arms is modified tentacle used to transfer spermatophore. Reproduction in Cephalopods There are no larval stages in cephalopod development as development occurs directly within the egg. Cephalopods: Giant squid The cephalopods include the largest of all invertebrates the giant squid and the colossal squid. These deep sea species can measure (including tentacles) as much as 60 feet and weigh up to 1,000 lbs. The mantle length is 15-18 feet with the head about 3 feet. Giant squid washed ashore in New Zealand in 1996 Cephalopods: Giant squid Giant squid are known mainly from stranded specimens or those trapped in fishing nets and live specimens have not been studied, although a few photos of live ones have been taken. Washed up off coast of Chile In water off coast of Japan Cephalopods: Giant squid Giant squid are predators and appear to have excellent vision (they have the biggest eyes of any known organisms [up to 10 inches in diameter]). They are believed to prey mainly on fish and other squid. Cephalopods: Giant squid For many years giant squid were known only from the sucker marks they left on the skin of sperm whales and their beaks taken from sperm whale stomachs. Based on how often they have turned up in sperm whale stomachs, giant squid appear to be an important component of their diet.
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