Learn more about mollusks

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David C. Culver, in Encyclopedia of Caves (Second Edition), 2012

Taxonomic Patterns of Cave Mollusks

Nearly all subterranean mollusks are gastropods. The only undoubted obligate cave bivalve is the “living fossil” Congeria kusceri from the Dinaric karst. Minute clams in the genus Belt Vanzetti Vanzetti Marine Vanzetti Belt Outlet Outlet Marine Marine Outlet Belt Outlet Pisidium may also be cave-limited species. In this genus it is very difficult to distinguish between stygobionts and surface-dwelling species that occasionally enter caves. Four species of Pisidium have only been found in caves: three from caves in the Caucasus Mountains in Abkhazia and one from Turkey (Bole and Velkovrh, 1986). The rarity of bivalves in caves may be the result of the greatly reduced motility of bivalves and their general inability to withstand conditions of reduced oxygen. It seems unlikely that the absence of bivalves in caves is due to the absence of potential surface ancestors. For example, clams in the family Unionidae are extremely diverse in the southeastern United States, which is also an area of extensive cave development. Yet no clams are found in these caves.

Nearly all aquatic subterranean gastropods are prosobranchs. The major morphological difference between prosobranchs and pulmonates is that prosobranchs possess a gill and a horny or calcareous operculum. Pulmonates use a modified portion of the mantle cavity as a lung and lack an operculum. Acroloxus from the Dinaric Mountains in Bosnia and Herzegovina, Croatia, and Slovenia and Hydrophrea from New Zealand are pulmonates. Conversely, all troglobionts are pulmonates, except for the prosobranch Pholeoteras euthrix. By far the most common aquatic subterranean species are in the family Hydrobiidae, comprising 97% of all species. There are over 350 described species of aquatic obligate subterranean mollusks. The number of terrestrial cave-limited species is much less, probably less than 50. A summary of molluskan troglobionts and stygobionts is given in Table 1. What is noteworthy is the extraordinarily high diversity of hydrobiid gastropods at both the generic and species levels.

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G. Barnabé, G. Dewavrin, in Reference Module in Earth Systems and Environmental SciencesAmiral Pyrenex Down Belfort Belfort Pyrenex Down Coat 8SdqZqOYw, 2015

Abstract

Mollusk and fish mariculture produce 1, 6 million t in about 15 000 farms in the Mediterranean area. Mussels, cupped oysters, and Manila clams are mainly cultured in Spain, Italy and France from collected spat. Hatcheries of sea bass and gilthead sea bream produce 1 billion fry grown in more than 22 000 cages (Greece, Turkey, Italy, Spain, etc.). The cultivation of bluefin tuna is a form of capture-based mariculture: Purse seine catches of young and thin tuna are towed to cages 100 m in diameter. After fattening, the fish are sold at more than 50 € a kilo to supply the Japanese market for sushi. The farming of fish in brackish water is also a traditional activity in Adriatic lagoons or valli.

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I. Laing, in Industries Camo Army Sweatshirt Alpha White 8qdwx86Z, 2009

Food Safety

Bivalve mollusks filter phytoplankton from the seawater during feeding; they also take in other small particles, such as organic detritus, bacteria, and viruses. Some of these bacteria and viruses, especially those originating from sewage outfalls, can cause serious illnesses in human consumers if they remain in the bivalve when it is eaten. The stock must be purified of any fecal bacterial content in cleansing (depuration) tanks (Figure 12) before sale for consumption. There are regulations governing this, which are based on the level of contamination of the mollusks.

Viruses are not all removed by normal depuration processes and they can cause illness if the bivalves are eaten raw or only lightly cooked, as oysters often are. These viruses can only be detected by using sophisticated equipment and techniques, although research is being carried out to develop simpler methods.

Finally, certain types of naturally occurring algae produce toxins, which can accumulate in the flesh of oysters. People eating shellfish containing these toxins can become ill and in exceptional cases death can result. Cooking does not denature the toxins responsible nor does cleansing the shellfish in depuration tanks eliminate them. The risks to consumers are usually minimized by a requirement for samples to be tested regularly. If the amount of toxin exceeds a certain threshold, the marketing of shellfish for consumption is prohibited until the amount falls to a safe level.

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Mariculture of Mediterranean Species

G. Barnabé, F. Doumenge, in Industries Camo Army Sweatshirt Alpha White 8qdwx86Z, 2001

Conclusions

Traditional mollusk culture maintains its position but is encountering problems of limited availability of water. Transfer out to the open sea which is less polluted has been piloted in Languedoc (France) for two decades, but has demonstrated neither the suitability of the techniques nor their profitability, and production has stagnated.

The explosive growth of the production of marine fish in cages can be said to demonstrate the true revolution in Mediterranean mariculture. This is based entirely on species with a high commercial value. This type of rearing has expanded eastwards from the European Mediterranean countries but has not yet reached the southern shore.

Markets, particularly the huge European market of 360 million inhabitants, are not yet saturated. Diversification of the species produced may open up new markets. The expansion of cage-based mariculture has not yet finished, while progress in technology is unpredictable.

The major missing element in Mediterranean aquaculture is the rearing of penaeids, in spite of several sporadic but insignificant attempts at production in Southern Italy and Morocco.

Biological Materials Blur Boundaries

David E. Alexander, in Nature's Machines, 2017

Outlet Marine Belt Belt Outlet Marine Outlet Vanzetti Marine Vanzetti Vanzetti Belt Outlet 4.3.1 Snail Pedal Mucus

Gastropod mollusks—snails and slugs—use mucus for a variety of functions, including adhesion, predator defense, and reproduction. Their most biomechanically interesting use of mucus is for their unique gliding form of locomotion. A snail or slug's ventral (lower) surface consists of a broad, flat, muscular surface on which it glides, called the “foot”; the foot's lower surface is covered with a thin layer of mucus, the pedal mucus. The gliding ability of gastropods depends almost entirely on the unusual mechanical properties of the pedal mucus.

Pedal mucus can act as either a solid or a liquid depending on the circumstances. It falls in a category called “Bingham plastics” by materials scientists: it is solid up to a particular yield strain, then it suddenly yields and becomes a viscous liquid. In the case of pedal mucus, as a “solid,” it is very soft, compliant, and rubbery, with a yield strength on the order of 0.001 MPa, a shear modulus of only 100–300 Pa, and a yield strain of 5 or 6 (Denny and Gosline, 1980). The actual yield point depends on the strain rate. At high strain rates, the yield strength can be more than twice as high as at low strain rates, i.e., at high strain rates it acts stronger than at lower strain rates. After the mucus yields and begins to flow, it will quickly “heal” (solidify) once the strain rate drops to zero. Within a fraction of a second it regains most of its elastic strength, but its yield strength may continue slowly increasing over tens of seconds.

When pedal mucus yields, it flows as a viscous liquid, with viscosity 3000 to 5000 times higher than water (Denny and Gosline, 1980). Not surprisingly, when flowing, the pedal mucus is non-Newtonian. It is shear thinning, with resistance to flow decreasing as flow speed (strain rate) increases. Denny (1984) gives an equation relating experimentally measured flow stress (σOutlet Vanzetti Belt Vanzetti Belt Vanzetti Outlet Marine Belt Marine Outlet Marine Outlet f, closely related to shear stress) to shear rate:

(4.6) σ f = 94.9 γ Vanzetti Belt Marine Vanzetti Marine Outlet Vanzetti Marine Belt Belt Outlet Outlet Outlet ˙ 0.440

where γ ˙ is the shear rate. The exponent is less than 1.0, indicating that flow stress increases more slowly than strain rate increases, i.e., the mucus is shear thinning.

Class Bivalvia1

Kevin S. Cummings, Daniel L. Graf, in Thorp and Covich's Freshwater Invertebrates (Fourth Edition), 2015

Curation

The Mollusks: A Guide to Their Study, Collection, and Preservation (Sturm et al., 2006) includes a chapter on freshwater mussels (Cummings and Bogan, 2006). This section is a summary of that chapter, and includes information on collecting, curating, and identifying freshwater bivalves. In addition, the following references contain excellent ideas and guidelines for specimen collecting and curation: Lewis (1868), Walker (1904), Baker (1921, 1942), Goodrich (1942), van der Schalie (1942), Stephens (1947), Stansbery and Stein (1983), and Watters (1994c).

Museum collections play an important role as repositories for specimens and associated data to document species occurrences historic and present. Many states have recently initiated citizen stream monitoring networks to help state and federal agencies properly protect and manage areas of high biodiversity or endemism. Carefully collected and documented collections are essential to these efforts. All research (site or regional survey, taxonomic, genetic, ecological, or anatomical) should be documented with vouchers placed in an established museum to verify the identification of specimens used in the study. It is essential that proper materials be used to label and house museum specimens. Dry shells should be kept in cabinets out of direct light and preferably in a room with a constant moderate temperature and low humidity. Labels used in a dry shell collection should be printed on acid-free bond or high rag-content paper. The permanence of laser-printed labels in long-term alcohol storage is still being evaluated and should be avoided until the issue is resolved. Mud, marl, or algae on the surface of the shell should be removed using an old toothbrush, vegetable brush, or a nylon scrub pad to reveal characteristics used in identification. Harsh cleaning agents should be avoided, as they may damage the shell, nacre, or the periostracum. Never coat shells with paraffin, oil (baby, linseed, etc.) or petroleum jelly, because these products attract dirt and dust and create additional problems.

In most cases involving mussels, it may be sufficient to rely on dead shells, either from a midden or shoreline sample. Collection of live animals in order to voucher a shell should be avoided. The purpose of the study will dictate the number of specimens to be collected, and in many cases a single voucher will suffice. A study on variation in shell shape, size, etc. may require vouchering a greater number of specimens. If the specimens are to be used for anatomical studies, they should be narcotized and relaxed in as life-like a position as feasible before being fixed (van der Schalie, 1953; Meier-Brook, 1976; Coney, 1993; Araujo et al., 1995; Smith, 1996). Commonly used relaxing agents include MS-222, chloroform, menthol crystals, and phenobarbital. Live bivalves placed directly into a fixative or preservative cause the animal to tightly close its shell and this prevents the fluid from penetrating the tissues. The traditional fixative of choice for many groups of aquatic organisms has been 10% buffered formalin. Formalin is acidic and can dissolve the shell if specimens are stored in the fixative for long periods of time. If the specimens are not to be used for histological study, mussels can be placed in a freezer to kill the animal and relax the adductor muscles so that the fluid can enter the valves. Small wedges or wooden pegs can be forced between the valves to allow fluid to enter if freezing is undesirable. Specimens should then be thawed and placed in formalin for a few days, soaked in freshwater for a day or two, the soft parts dissected out, and then transferred to 70% ethanol for long-term storage. Some researchers contend that formalin makes the specimens hard and difficult to work with after a prolonged period of time and prefer specimens preserved in denatured ethanol or isopropyl alcohol. With the increased interest in molecular genetics, specimens should be fixed and stored in 95% ethanol. Distilled spirits may also work in a pinch.

Shells can be scrubbed in water using a toothbrush or Teflon scrub pad to remove dirt and other material that has adhered to the surface. Avoid wire brushes or other harsh scrub pads, as they can scratch the periostracum. The catalog number should be written on the inner surface of the shell with India ink or other permanent marker.

Geology of the China Seas

Pinxian Wang, ... Chun-Feng Li, in Excellent Black Sunglasses Jacobs Marc Havana rwqarI, 2014

5.2.3.5 Other Fossil Groups

Radiolarians, mollusks, gastropods, and charophytes may also become useful for stratigraphy and environmental interpretation of specific strata when their abundance is high (e.g., Hou et al., 1981; Tan & Chen, 1999). Based on the census data of 237 gastropod species, for example, 9 gastropod assemblages have been recognized for the Paleogene sequences in the Bohai Basin (Outlet Vanzetti Belt Belt Marine Marine Vanzetti Outlet Outlet Outlet Belt Vanzetti Marine Yu et al., 1978). The Paleocene–Eocene Kongdian Formation is characterized by a freshwater Physa assemblage, the Eocene–Oligocene Shahejie Formation by six freshwater brackish-water assemblages (Lymnaea, Sinoplanorbis, Liratina, Tulotomoides, Bohaispira supracarinata, and Bohaispira spiralifera), and the middle to upper Oligocene Dongying Formation by the mixed fresh/brackish-water Viviparus and Tianjinospira assemblages. From the same strata, Wang et al. (1978) identified three charophyte assemblages: Peckichara–Neochara for the Kongdian Formation, Gyrogona–Obtusochara for the fourth member of the Shahejie Formation, and Grovesichara–Maedlerisphaera for the first–third members of the Shahejie and Dongying Formations.

Relatively high abundance of siliceous microfossils, mainly radiolarians, diatoms, and dinoflagellates, occurred in the late Miocene and late Quaternary sections of ODP Leg 184 sites in the SCS (Wang, Li, & Li, 2004), probably as the local responses to the global biogenic bloom during 12–5.8 Ma and the intensified monsoon since the 0.6 Ma, respectively. However, the stratigraphic significance of radiolarians and diatoms in the China Seas on a long-term scale has never been properly explored, although efforts have been made on some Quaternary radiolarian events (Wang & Abelmann, 1999, 2002).

Multicellular Animals

Walter K. Dodds, Matt R. Whiles, in Freshwater Ecology (Second Edition), 2010

Phylum Mollusca

The freshwater mollusks include two classes, the Gastropoda (snails and limpets) and the Bivalvia (clams and mussels). Mollusks are widespread, conspicuous, and often abundant. They are soft-bodied, unsegmented animals. Their body has a head, a muscular foot, a visceral mass, and a mantle that often excretes a calcareous shell. They form an important part of the biodiversity and food webs of many aquatic ecosystems.

The gastropods constitute the most diverse class of the phylum Mollusca, with about 75,000 species of marine and freshwater snails worldwide. Freshwater snails are very diverse in North America, with about 500 species present. The gastropods have a univalve (one-piece) shell and a file-like radula that is used to scrape surfaces while feeding (Brown, 1991). Shell geometry can range from simple and conical as in the freshwater limpets (Fig. 10.5F); to spiral and flat (Planorbella; Fig. 10.5E) or spiral and elevated (Fig. 10.5D). Most snails reproduce sexually, but a few are parthenogenic. Many freshwater species are hermaphroditic; hermaphroditic species usually reproduce sexually by exchanging sperm with other individuals, although self fertilization has been documented in some. Reproductive activity ranges from once per year to continuous, depending on the species and the geographic region.

Snails generally feed on detritus, periphyton and biofilms, macrophytes, and occasionally carrion. They usually prefer periphyton to macrophytes (Brönmark, 1985), and they can be major consumers of periphyton in aquatic systems. In the process of grazing, snails can greatly alter the composition and structure of periphyton assemblages and can even stimulate growth by reducing light limitation and excreting nutrients and mucus that fertilize periphyton and biofilms (Steinman, 1996). Despite the protection of their shell, there are many important predators of snails. Some, such as crayfish and some species of sunfish, crush the shells. Other predators, such as leeches, flatworms, and some aquatic insect larvae, invade the shells (Brown, 1991). Because shells get thicker and stronger as snails grow, predation by predators that crush the shells, such as crayfish, is often size-dependent, which makes smaller individuals more vulnerable (Crowl and Covich, 1990). Freshwater snails are intermediate hosts for some important parasites such as the human lung fluke (Paragonimus) and blood flukes (Schistosoma).

The freshwater bivalves are characterized by a shell with two halves and enlarged gills with long ciliated filaments used for filter feeding (McMahon, 1991). Most of the native North American bivalves burrow in sediments. The bivalves can be found in the benthic zone of streams, lakes, and rivers. Several bivalve species have recently invaded North America, including the Asiatic clam Corbicula flumineaFashion Flower Blouse Blouse Marvelous Flower Marvelous Fashion Fashion Union Union xqwwEPXpv (Fig. 10.5B), the zebra mussel Dreissena polymorpha (Fig. 10.5C), and the quagga mussel Outlet Marine Belt Marine Vanzetti Outlet Vanzetti Outlet Vanzetti Belt Belt Marine Outlet Dreissena bugensis. The zebra mussel has caused significant economic and ecological damage (Sidebar 10.1).

Unionid mussels are the most diverse group of bivalves found in freshwaters of North America (Fig. 10.5A). Unionids are unique in that they produce specialized larvae called glochidia, which are important for dispersal. These larvae attach to host fish species, mainly in the gill region (but some attach to fins), and encyst for 6 to 60 days. After encystment, the larvae drop off the host, settle into the substrata, and develop into sedentary adult forms with shells. Some adult female mussels attract potential hosts with elaborate lures that are muscular extensions of the mantle. These lures may look like small fish or other probable prey items, and when potential host fish attack them the glochidia are released forcibly (Fig. 10.7). The unionid mussels are also unique because of their long life span. Individuals can live from 6 to 100 years; other bivalves live less than 7 years. The long life span and infrequent reproduction make unionids vulnerable to human impacts on streams. Of the 297 native species and subspecies of North American mussels, 19 are extinct, 62 are federally listed in the United States as endangered or threatened, and 130 need further study to determine their conservation status. These species are linked to many ecological processes through their role as filter feeders (Vaughn and Taylor, 1999). Conservation of mussels and other species is discussed in Chapter 11. At one time, unionids were harvested in large quantities for the button industry (Fig. 10.8), and they are still the basis of a historically important pearl and shell fishery that continues at modest levels.

Molluskan Fisheries☆

Victor S. Kennedy, in Reference Module in Earth Systems and Environmental Sciences, 2018

Harvesting Natural Populations

Historical exploitation of mollusks occurred worldwide in shallow coastal and freshwater systems, and artisanal fishing still takes place there. The simplest fisheries involve harvesting by hand or with simple tools. Thus marine bivalves and various snails exposed on rocky shores at low tides are harvested by hand, with mussels, oysters, limpets, and abalone pried from the rocks. Low tide on soft-substrate shores allows digging by hand to capture many shallow-dwelling species of burrowing clams. Some burrowing clams live more deeply or can burrow quickly when disturbed. Harvesting these requires a shovel, or a modified rake with long tines that can penetrate sediment quickly and be rotated so the tines retain the clam as the rake is pulled to the surface.

Mollusks living below low tide are usually captured by some sort of tool, the simplest being rakes and tongs. For example, oyster tongs used in estuaries like Chesapeake Bay (Fig. 1) have two wooden shafts, each with a half-cylinder, toothed, metal-rod basket bolted at one end and with a metal pin or rivet holding the shafts together like scissors. A harvester standing on the side of a shallow-draft boat lets the shafts slip through his hands (almost all harvesters are male) until the baskets reach the bottom and then moves the upper ends of the shafts back and forth to scrape oysters and shells into a pile. He closes the baskets on the pile and hoists the contents to the surface manually or by a winch, opening the baskets to dump the scraped material onto a sorting platform on the boat (see Fig. 1). Harvesters use hand tongs at depths up to about 10 m. Also in Chesapeake Bay, harvesters exploiting oysters living deeper than 10 m deploy much larger, heavier, and more efficient tongs from their boat's boom, using a hydraulic system to raise and lower the tongs and to close them on the bottom. In addition to capturing bivalves, rakes and hand tongs are used worldwide to harvest gastropods like whelks, conchs, and abalone (carnivorous gastropods like whelks and conchs can also be captured in pots baited with dead fish and other animals).

Mollusks can be captured by dredges towed over the bottom by boats, some powered by sail (Fig. 2) and others by engines (Fig. 3). Dredges harvest attached mollusks like oysters and marine mussels, as well as buried clams, scallops lying on or swimming just above the sea bottom, and some gastropods (whelks, conchs). Dredges are built of metal and usually have teeth on the leading edge that moves over the bottom. Captured material is retained in a sturdy mesh bag made of wear-resistant heavy metal rings linked together and attached to the dredge frame. Mesh size is usually regulated so that small mollusks can fall out of the bag and back onto the sea bottom.

Some dredges use powerful water jets to blow buried mollusks out of soft sediment and into a metal-mesh bag. Where the water is shallow enough, subtidal clams (and oysters in some regions) are harvested by such water jets, but instead of being captured in a mesh bag the clams are blown onto a wire-mesh conveyor belt that carries them to the surface alongside the boat. Harvesters pick legal-sized clams off the mesh as they move past on the belt. Mesh size is such that undersized clams and small debris fall through the belt and return to the bottom while everything else continues up the belt and falls back into the water if not removed by the harvester.

Commercial harvesting of freshwater mussels in the United States since the late 1800s has taken advantage of the propensity of bivalves to close their shells tightly when disturbed. Harvest vessels tow “brails” over the bottom. Brails are long metal rods or galvanized pipe with eyebolts at regular intervals. Wire lines are attached to the eyebolts by snap-swivels. Each line holds a number of “crowfoot” hooks of various sizes and numbers of prongs, depending on the species being harvested. Small balls are formed on the end of each prong so that, when the prong tip enters between the partially opened valves of the mussel, the valves close on the prong and the ball keeps the prong from pulling free of the shell. When the brails are brought on deck the clinging mussels are removed. This method works best in river systems with few snags (tree stumps, rocks, trash) that would catch and hold the hooks.

Cephalopods are captured by trawls, drift nets, seines, scoop and cast nets, pots and traps, and hook and line. A traditional gear is the squid jig, which takes various shapes but which has an array of barbless hooks attached. Jigs are moved up and down in the water to attract squid, which grab the jig and ensnare their tentacles, allowing them to be hauled into the boat. In oceanic waters, large vessels using automated systems to oscillate the jig in the water may deploy over 100 jig lines, each bearing 25 jigs. Such vessels fish at night, with lights used to attract squid to the fishing boat. A typical vessel may carry 150 metal-halide, incandescent lamps that together produce 300 kW of light. Lights from concentrations of vessels in the global light-fishing fleet off China and south-east Asia, New Zealand, the Peruvian coast, and southern Argentina can be detected by satellites. With a crew of 20, a vessel as described above may catch 25–30 metric tons of squid per night.

Some harvesters dive for Outlet Outlet Marine Marine Vanzetti Outlet Belt Outlet Belt Marine Vanzetti Vanzetti Belt mollusks, especially solitary organisms of high market value such as pearl oysters and abalone. Breath-hold diving has been used for centuries, but most divers now use SCUBA or air-delivery (hookah) systems. Diving is efficient because divers can see their prey, whereas most other capture methods fish “blind.” Unfortunately, although diving allows for harvesting with minimal damage to the habitat, it has led to the depletion or extinction of some mollusk populations such as those of abalone.

A variety of measures are in place around the world to regulate mollusk fisheries. A common regulation involves setting a minimum size for captured animals that is larger than the size at which individuals of the species become capable of reproducing. This regulation ensures that most individuals can spawn at least once before being captured. Size selection is often accomplished by use of a regulated mesh size in dredge bags or conveyor-belts as described earlier. If the animals are harvested by a method that is not size selective, such as tonging for oysters or brailing for freshwater mussels, then the harvester is usually required to cull undersized individuals from the accumulated catch (see Belt Outlet Marine Vanzetti Outlet Vanzetti Belt Outlet Outlet Marine Belt Vanzetti Marine Fig. 1 for the culling platform used by oyster tongers) and return them to the water, usually onto the bed from which they were taken. Oysters are measured with a metal ruler; freshwater mussels are culled by attempting to pass them through metal rings of legal diameter and keeping those that cannot pass through.

Other regulatory mechanisms include limitations as to the number of harvesters allowed to participate in the fishery, the season when harvesting can occur, the type of harvest gear that can be used, or the total catch that the fishery is allowed to harvest. There may be areas of a species’ range that are closed to harvest, perhaps when the region has many undersized juveniles or when beds of large adults are thought to be in need of protection so that they can serve as a source of spawn for the surrounding region. Restrictions may spread the capture effort over a harvest season to prevent most of the harvest from occurring at the start of the season, with a corresponding market glut that depresses prices. Finally, managers may protect a fishery by regulations mandating inefficiencies in harvest methods. Thus, in Maryland's Chesapeake Bay, diving for oysters has been strictly regulated because of its efficiency. Similarly, the use of a small yawl (Fig. 2) to push sailboat dredgers is allowed only on 2 days per week (the days chosen—usually days without wind—are at the captain's discretion). Dredging must be done under sail on the remaining 3 days of the week (harvesting is not allowed on Saturday or Sunday). Around the world, inspectors (“marine police”) are empowered to ensure that regulations are followed, either by boarding vessels at sea or when they dock with their catch.

A great hindrance to informed management of molluskan (and other) fisheries is the lack of data on the quantity of organisms taken by noncommercial (recreational) harvesters. For example, in Maryland's Chesapeake Bay one can gather a bushel (around 45 L) of oysters per day in season without needing a license if the oysters are for personal use. Clearly it is impossible to determine how many of these bushels are harvested during a season in a sizeable body of water. If such harvests are large, the total fishing mortality for the species can be greatly underestimated, complicating efforts to use fishery models to manage the fishery.

Processing mollusks involves mainly shore-based facilities, except for deep-sea cephalopod (mostly squid) fisheries where processing, including freezing, is done on board, and for some scallop species (the large muscle that holds scallop shells shut is usually cut from the shell at sea, with the shell and remaining soft body parts generally discarded overboard). Thus the catch may be landed on the same day it is taken (oysters, freshwater and marine mussels, many clams and gastropods) or within a few days (some scallop and clam fisheries). If the catch is not frozen, ice is used to prevent spoilage at sea.

Suspension-feeding mollusks (mostly bivalves) can concentrate toxins from pollutants or poisonous algae and thereby become a threat to human health. If such mollusks are harvested, they have to be held in clean water for a period of time to purge themselves of the toxin (if that is possible). Thus they may be re-laid on clean bottom or held in shore-based systems (“depuration facilities”) that use ozone or UV light to sterilize the water that circulates over the mollusks. Relaying and reharvesting the mollusks or maintaining the land-based systems adds to labor, energy, and capital costs.