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Venerupis philippinarum, Japanese littleneck clam Robyn Shean FISH 423: Aquatic Invasion Ecology Fall 2011 Diagnostic information Scientific name: Veneroida Veneridae Venerupis philippinarum Also known as Tapes philippinarum, Ruditapes philippinarum, Venerupis japonica Common names: Japanese littleneck clam, manila clam, steamer Figure 2: Varied coloration of shell in Japanese clam, Japanese carpet shell littleneck; shows concentric rings, radiating ridges, and split siphon (USGS 2011). Physical description and physiology Venerupis philippinarum, white in color, Japanese littlenecks are multicommonly colored and usually have a purple hue along the called Japanese littleneck clams, are bivalves umbo and inner rim of the shell (Figure 2). similar in size and physical appearance as the The shells of Venerupis philippinarum have native littleneck clams Leukoma staminea. concentric rings across the surface with ridges Japanese littlenecks have two outer shells radiating across the rings and outward to the (valves) connected by a hinge, with internal edges of the shell. The inside of the shell is ligaments used to open and close the shells. The smooth. Japanese littlenecks can grow to 3-4 umbo (also known as the beak) is the oldest part inches in length as mature adults. All clam of the clam, and is found along the hinged edge. species contain internal organs that make up the They have an oblong shell with a slightly higher body inside the protective shell. Internal length to width ratio than the native species structures include a heart, stomach, gonads, (Figure 1). While native littleneck clams are off- kidney, intestines, mouth, abductor muscles, and gills to aid in respiratory function (Figure 3). As filter feeders, clams have siphons used for the intake of water and food, as well as excretion of waste material. A foot is present as a means of mobility and the body of the clam is secured to the shell by a thin membrane called a mantle. The mantle builds the shell by storing calcium deposits and secreting the calcium to the inside Figure 1: Non-native V. philippinarum (left) and native L. staminea (right) (WDFW 2011). of the shells. Figure 3: Diagram of internal structure of Japanese littleneck clam (Source: www.barnegatshellfish.org). Both native and indigenous littleneck clams can clams can add rings when stressed or disturbed retract their siphon and completely close their (Toba 2005). Japanese littleneck clams can grow shells. This helps the clam retain moisture when to a size of 1-2 inches in shell length that is exposed to the air during low tides and protects suitable for harvest within 2-3 years of age against predation. Non-native littleneck clams (Toba 2005). can survive out of water for longer periods of time compared to native littlenecks (WDFW Life-history and basic ecology 2011). Life cycle Material is added to the shells of Japanese littleneck clams as it grows to help it become larger and thicker. During winter months when temperatures drop and food sources are limited, the clam grows at a slower rate. While some people think the rings on these clams indicate age, they are not a good predictor of age as Bivalves are broadcast spawners. Female and male adults release sperm and eggs into the water column where fertilization occurs. Once fertilized, the eggs develop into freefloating trochophore larvae that are carried in water currents over several weeks. The trochophore than forms ciliated vellum to assist with mobility and feeding – this is the veliger by seawater. Individuals that settle at higher larval stage of development. In approximately 2- intertidal zones will feed less often and usually 4 weeks, the clam develops into a peliveliger be smaller in size. with a formed foot to assist further with swimming (Jones 1993). In addition, small Reproductive strategies threads are formed (byssal threads) to help the clam secure itself onto the seafloor once it finds a suitable substrate to settle on. The substrate is usually a muddy, sandy, or soft surface the clam can burrow into with its foot. Burrowing into the ground allows the animal to find food and be protected from predators. Once settled, it will stay in the substrate and continue to grow into a mature clam. Individuals become sexually mature between 1-3 years of age, and have a life span up to 14 years (WDWF 2011). Japanese littleneck clams are dioecious – there are male and female individuals, each producing either egg or sperm. Individuals become sexually mature in the first to third year of age. This can vary depending on location and size of shell. In Hood Canal (Washington state), research has shown that clams are sexually mature when shell length is 5-10 mm. However, most individuals do not spawn until shell length is at least 20 mm (Holland and Chew 1974). Spawning can occur either once or twice every year depending on location and environmental Feeding habits factors (Ponurovsky 1992). Spawning usually All clam species are filter feeders and occurs between April and October when water have siphons that protrude out of the shell for temperatures are warmer. In Hood Canal, feeding. They take water in through the spawning in some populations happened during incurrent siphon, filter and extract food items, this entire period of time (Holland and Chew and then excrete the filtered water and waste out 1974). the excurrent siphon. Japanese littleneck clams As broadcast spawners, clams generate high have shorter siphons compared to other clam numbers of gametes that are released once an species; this is why these clams are referred to as individual is sexually mature. High fecundity littlenecks (Cohen 2005). The siphon tip in increases the chances for large numbers of Japanese littleneck clams is split and dark in larvae color, whereas the siphon is not split in native population. Reports indicate Japanese littleneck littlenecks (Jones et al. 1993). clam fecundity ranges from 4.32x105 eggs to Japanese littleneck clams feed on phytoplankton 2.35x106 eggs depending on shell length (Yap throughout their life cycle. Some species can 1977). In addition to temperature, another cue filter up to 65 gallons of seawater each day that initiates spawning is the presence of eggs or (Toba 2005). Clams can only feed while covered sperm in the water column (Jones et al. 1993). and overall recruitment for the One study produced results indicating some natives easier to reach for harvesting (WDFW Japanese littleneck clam populations in areas of 2011). the Sea of Japan may exhibit sex reversal For Japanese littleneck clams, the area in which (hermaphrodite) environmental they can successful settle is dictated by two conditions (Ponurovsky 1992). In Vostok and factors – competition and predation at lower Possjet Bays, all individuals with a shell length limits and exposure at upper limits (Toba 2005). less than 15 mm produced sperm during their Non-native Japanese littleneck clams settle first breeding cycle. However in subsequent higher in the intertidal zone and in shallower spawnings, the number of males and females depths than native littleneck clams. This can were equal but there was a greater number of limit feeding times (they only feed when tide is females in older clam populations. in and substrate is submerged) and create greater due to opportunities Environmental optima and tolerances Temperature plays a critical role in for predation by terrestrial organisms and mortality caused by fluctuations in temperature. spawning, growth, and survival of Japanese littleneck clams. The water must be a certain Biotic associations temperature for the males and females to release Brown ring disease egg and sperm. Spawning usually occurs While not seen in Japanese littleneck between 20-25 degrees C (FAO 2011). While clam populations in the Pacific Northwest, clams spawning can be initiated by lower temperature, in Europe have been regularly affected by a 12 degrees C is the minimum threshold for bacterium Vibrio tapetis leading to a condition which this species cannot spawn. The optimal called brown ring disease (Trinkler et al. 2011). range for growth is 15-28 degrees C, but Once infected, a brown deposit forms on the individuals can survive from 0-35 degrees C for inner surface of the shells – the clam secretes short durations. Salinity is another factor that this substance as a defensive response to the impacts spawning and survival of this species. presence of the bacteria. Brown ring disease can For spawning, optimal salinity levels are cause problems with shell formation and repair between 24-25 ppt. This species can survive in (biomineralisation) and have negative effects on salinity ranging from 13.5-35 ppt (FAO 2011). the circulatory system of the clam. High Japanese littlenecks are shallow burrowers mortality rates caused by this disease were usually found 2-4 inches under the ground reported in France in the 1987 (Trinkler et al. surface within the high intertidal range. This is 2011). higher than the zone where native littleneck and butter clams are found; this can make non- Several native crab species feed on clams. The Protozoan parasite infections been red rock crab (Cancer productus) is found detected in Japanese littleneck clam populations throughout central and southern Puget Sound in the Pacific Northwest, but these parasites and Hood Canal and feeds on native and non- have been seen in clam populations in Japan native clams in the region. Dungeness crab (Itoh et al. 2004). Three specific protozoan (Cancer magister) and the graceful crab (Cancer parasites detected in Japanese littlenecks in gracilis) also predate on Japanese littleneck Japan include Haplosporidium sp. found in the clams. A newly introduced non-native crab connective tissues, Marteilia sp. found in the along the Pacific coast is the European green digestive gland, and Marteilioides sp. found in crab (Carcinus maenas), which feeds on clam the oocytes. These parasites haven’t been shown species. While effects of this predator on clam to have negative impacts on the clams, but populations have yet to be seen in Washington further research is needed to determine if and state, there have been significant impacts on how these parasite infections could negatively aquaculture populations of Japanese littleneck affect this species of clam. clams in California. Research has shown a 40 Protozoan parasites haven’t percent decrease in Japanese littleneck clam Predation Seagulls and crows feed on both native and non-native clams when the tide is out. Migratory birds, including several species of sea ducks, predate on clams as an added food source during the winter months. Caldow and others investigated the interaction between the nonnative Japanese littleneck clam and a specific seabird, the Eurasian oystercatcher (Haematopus harvest in California since establishment of the European green crab (WDFW 2008). Moon snails (Polinices lewisii) predate on littleneck clams by drilling a hole in the clam’s shell and then feeding on the meat inside. Older clams have harder shells, so they are less likely to be preyed on by these snails. Additionally, some species of seastars and bottom-feeding fish feed on smaller clams. ostralegus ostralegus). These seabirds rely on available food sources, including shellfish, for Current geographic distribution survival through the winter and in preparation for migration in the spring. Modeling of Distribution in PNW and U.S. shorebird feeding activities was used to show Japanese littleneck clams were imported that Japanese littleneck clams in one invaded to the Hawaiian Islands from Japan in the early area reduced over-winter mortality rates of the part of the century. In the mid-1930s, these non- oystercatcher (Caldow et al. 2007). Figure 4: Distribution of Venerupis philippinarum in U.S. and Pacific Northwest (USGS 2011). native clams were then introduced into the Yellow Sea, Sea of Japan, the Sea of Okhotsk Pacific Northwest accidentally in shipments of and around the South Kurile Islands (Scarlato Pacific oyster seed (FAO 2011). Japanese 1981). Starting in the early 1900s, these clams littleneck clams spread quickly and are now were found along the west coast of North America intentionally throughout the world (Figure 5). In from British Columbia down to the California addition to being introduced in North America coast (Figure 4). In British Columbia, Japanese and Canada, this clam species was introduced in littlenecks are found in bays and inlets the Mediterranean and West Atlantic coast throughout the Strait of Georgia. In Washington waters in the latter half of the 19th century. state, these clams are found throughout the Japanese littleneck clams are now present in Puget Sound region with an abundance of clam France, the United Kingdom, Ireland, England, beds located in Hood Canal. Tahiti, Italy, Germany, and Scotland. In all of introduced both accidentally and these regions, this clam species was introduced Distribution throughout the world Japanese littleneck clams are native in the Philippines, South and East China Seas, as a commercially harvested crop. Additionally, trial cultures of V. philippinarum have been Successful in the Virgin Islands, Tunisia, purposes. This clam species was introduced in the Mediterranean and West Atlantic coast waters in the latter half of the 19th century. Japanese littleneck clams are now present in France, the United Kingdom, Ireland, England, Tahiti, Italy, Germany, and Scotland. Additionally, trial cultures of V. philippinarum have been successful in the Virgin Islands, Tunisia, Belgium, Israel, and Russia (Ponurovsky 1992). Figure 5: Regions across the world producing Venerupis philippinarum (FAO 2011). Invasion process Belgium, Israel, and Russia (Ponurovsky 1992). Pathways, vectors and routes of introduction Live shellfish import is the primary History of invasiveness pathway Japanese unintentionally littleneck introduced clams to the were Pacific Northwest as ‘hitchhikers’ in shipments of Pacific oysters in the 1930s. The non-natives were packed in containers of oyster seed shipped from Japan to Washington state where they then established and spread along the Washington, Oregon, and California coasts, and throughout the Puget Sound. This non-native clam appears to fill an ecological niche that compliments that of the native littleneck clam (Becker et al. 2008). Starting in the early 1900s, Japanese littleneck clams were introduced to coastal and aquatic regions throughout the world (Figure 5). Most regions with Japanese littleneck for philippinarum; the introduction specifically, of V. import of commercial oysters. Individuals were included with shipments of Pacific oyster spat from Japan. Another source of possible introduction may be through ballast water of ships departing from regions with populations of this clam species. As broadcast spawners with a larval stage beginning the life cycle, individuals are free-floating and able to drift in water currents over large areas and be transported through water brought into a ship’s hull area. Transport of ships from the Eastern Pacific to the Western Pacific can occur within a short time, increasing the likelihood of survival of larvae. clam populations are within a similar latitudinal range Factors influencing establishment and spread as the U.S (FAO 2011). In many cases, Because of similarities in environmental introductions were intentional for aquaculture conditions between Japanese waters and aquatic systems in the Pacific Northwest, it was expel water from their siphons. Harvesting relatively easy for the non-native clam species to activities in aquaculture settings, such as become established quickly after introduction. dredging, affect ecosystems as well. Sediment is Additionally, as broadcast spawners, fertilized released into the water column as clams are eggs can travel through water currents to pulled from the substrate. In the Pacific locations outside of documented established Northwest, Japanese littleneck clams activities areas. are haven’t been documented as having negative appropriate (suitable temperature and sandy, impacts on ecosystem function. However, in loose substrate), settlement and spread of this other regions where this species is established, non-native clam is easily facilitated. Once a research has shown that this clam has ecological small population is established, spawning each affects. year generates large numbers of potential This species was introduced into the Venice recruits due to the high fecundity of this species. Lagoon in 1983 as an aquaculture crop to boost If environmental conditions the economy of the region (Pranovi et al. 2006). Economic impacts Invasion of the Japanese littleneck clam into the Pacific Northwest created an economic boom in the shellfish industry. Washington state is the leading producer of farmed bivalve shellfish in the U.S. In 2000, farmed shellfish harvests generated just under $77 million in revenue (Puget Sound Partnership 2003). Of these sales, clams accounted for approximately $14 million. Japanese littleneck clams are the leading commercial clam species harvested in the state, with approximately 4,500 tons harvested annually (WDFW 2011). While boosting the economy and generating revenue, harvest activities have caused severe stress on the benthic communities and the entire lagoon ecosystem. The non-native clam spread throughout the lagoon in which it was introduced and expanded into other coastal areas. Because of the rapid growth of this species, it has become one of the dominant species in the lagoon in biomass and abundance, displacing other native species (Pranovi et al. 2006). With large numbers of Japanese littleneck clams, harvest activities have increased and affected the remaining native clam species that are more sensitive to dredging and sediment disturbance. Ecological impacts Rapid changes to the environment caused by Burrowing aquatic species affect the increased harvesting activities can make well- ecosystem in which they live. Clams disturb adapted native species more vulnerable to sediment as they move through the substrate and pressures release particles into the water column as they littleneck clams. caused by non-native Japanese As filter feeders, Japanese littleneck clams organisms. Paralytic shellfish poisoning and consume phytoplankton and excrete waste amnesic shellfish poisoning can infect shellfish (feces, pseudofeces) into the water column. With and in turn, cause illness in organisms that extensive populations of these organisms, these utilize clams as a food source, including humans activities can affect the chemistry and turbidity (WDFW 1998). Monitoring toxin levels in of water in shallow habitats and ‘generate high shellfish, including clams, can aid fishery microbial respiration leading to sediment anoxia agencies in assessing the health of aquatic that inhibits nitrification and kills benthic fauna’ systems and assist in recovery efforts. Most of (Pranovi et al. 2006). Another study performed the time, this involves closure of shellfish in the Sacca di Goro (Italy) investigated effects harvesting in affected areas until the algal bloom of Japanese littleneck clam farming on nutrient is over. and respiration levels. Results indicate that farming Japanese littleneck clams has a strong Management strategies and control methods affect on oxygen and carbon dioxide levels and increased densities of certain bacteria (Bartoli et al. 2001). These effects could increase the risk for anoxic conditions in the aquatic environment and cause harm to benthic organisms. The presence of clams also stimulates inorganic nitrogen and phosphorus from sediment into the water column. Increased circulation of these nutrients can promote production of phytoplankton, but also encourage growth of some algae species leading to potentially harmful algal blooms (Bartoli 2001). Management of the non-native Japanese littleneck clam is minimal, as this is one of the top commercial shellfish harvest industries in Washington state (Puget Sound Partnership 2003). Aquaculture of this species is a major source of revenue for the region. Harvest of Japanese littleneck clams in naturalized beds by private citizens is another source of income for the state due to revenue generated by the cost of shellfish licenses. In addition to providing revenue as a commercial aquaculture crop, these clams act as an additional food source for Indicators of ecological changes in aquatic migrating seabirds – another benefit of having systems this nonindigenous species established within As filter feeders, clams take in water and Pacific Northwest aquatic regions. extract phytoplankton. Any bacteria or toxins are One problematic area of these non-native clams also filtered through their system and can involves harvesting naturalized and cultured accumulate in the internal organs. This happens beds. Harvesting clam beds can disrupt the most often during algal blooms, when certain ecosystem and create challenges for native conditions species. Mechanical and manual harvesting promote production of these techniques are used, both of which disturb the substrate, displace native organisms, Literature cited and increase sediment and nutrient loads in the water Becker P, Barringer C, Marelli D (2008) Thirty column. years of sea ranching Japanese littleneck Manual harvesting involves raking the clams out clams of the substrate and bringing them to the surface Successful techniques and lessons learned. where they can be gathered. Mechanical Reviews in Fisheries Science 16: 44-50 (Venerupis philippinarum): harvesting involves dredging by a tractor (Figure 6). The tractor is equipped with a lateral belt that Caldow RWG, Stillman RA, le V dit Durell, digs and grades the sandy bottom areas. In Sarah EA, West AD, McGrorty S, Goss- dredging the surface floor, whether by manual or Custard JD, Wood PJ, Humphreys J (2007) mechanical methods, other organisms are pulled Benefits to shorebirds of invasion of a non- from the substrate and nutrients are released native shellfish. Proceedings of the Royal from the sediment. Society B 274: 1449-1455 Cohen, Andrew N (2005) Guide to the Exotic Species of San Francisco Bay. San Francisco Estuary Institute, Oakland, CA de Moura Queiros A, Hiddink JG, Johnson G, Kaiser MJ (2011) Context dependence of marine ecosystem engineer invasion impacts on benthic ecosystem functioning. Biological Invasions 13: 1059-1075 Fisheries and Aquaculture Department (2011) Database on Introductions of Aquatic Species (DIAS). Fishery Records Collections. FIGIS Data Collection. FAO Fisheries and Aquaculture Department. Figure 6: Mechanical methods of harvesting Accessed 29 November 2011: Japanese littleneck clams with small and large http://www.fao.org/fishery/culturedspecies/ scale tractors (FAO 2011). Ruditapes_philippinarum/en Holland DA, Chew KK (1974) Reproductive Scarlato OA (1981) Bivalves of temperate cycle of the Japanese littleneck Clam waters of the northwestern part of the (Venerupis japonica) from Hood Canal, Pacific Ocean. Nauka Press, Leningrad Washington). Proceedings of the National Shellfish Association 64: 53:58 Tinkler N, Bardeau JF, Marin F, Labonne M, Jolivet A, Crassous P, Paillare C (2011) Itoh N, Momoyama K, Ogawa K (2004) First Mineral phase in shell repair of Japanese report of three protozoan parasites (a littleneck clam Venerupis philippinarum haplosporidian, affected b brown ring disease. Diseases of Marteilioides Marteilia sp.) from sp. the and Japanese Aquatic Organisms 93: 149-162 littleneck clam, Venerupis philippinarum in Japan. Journal of Invertebrate Pathology 88: 201-206 Toba D, Dewey B, King T (2005) Small-scale clam farming for pleasure and profit in Washington. Jones G, Sanford C, Jones B (1993) Japanese littleneck clams, hatchery and nursery Program Washington Publication Sea Grant WSG-AS 03-02, Seattle methods. BC Ministry of Agriculture and Fisheries United States Geological Survey (USGS) (2011) Venerupis philippinarum Fact Sheet. USGS Melia P, De Leo GA, Gatto M (2004) Density and temperature-dependence of vital rates Nonindigenous Aquatic Species Database, Florida in the Japanese littleneck clam Tapes philippinarum: a stochastic demographic Washington Department of Fish & Wildlife model. Marine Ecology Progress Series (2011) Fishing and Shellfishing: Japanese 272: 153-164 littleneck clams. Ponurovsky SK, Yakovev YM (1992) The reproductive biology of the Japanese Washington Department of Fish & Wildlife littleneck, Tapes philippinarum (A. Adams (1998) Shellfish of Washington, publication and Reeve, 1850) (Bivalvia: Veneridea). FM96-03 Journal of Shellfish Research 11(2): 265277 Washington Department of Fish & Wildlife (2008) Invasive Species Fact Sheets: Puget Sound Partnership (2003) Shellfish Economy: Treasures of the Tidelands Carcinus maenas (European green crab). Aquatic Nuisance Species, Seattle Yap WG (1977) Population biology of the Japanese littleneck clam, Tapes Background in shellfish aquaculture; experience as a shellfish biologist. philippinarum, in Kaneoche Bay, Oahu, Hawaiian Islands. Pacific Science 31(3): Laura Hoberecht, PhD 223:244 Northwest Regional Aquaculture Coordinator NOAA’s National Marine Fisheries Service Phone: 206.526.4453 Other key sources of information Email: [email protected] Washington Department of Fish & Wildlife www.wdfw.wa.gov/fishing/shellfish/clams/Japa nese littleneck_clams.html National Oceanic Focus on marine aquaculture species, including shellfish and other invertebrates, finfish and plants. and Atmospheric Administration (NOAA) Aquaculture Program http://aquaculture.noaa.gov/ Graham Gillespie, BS Head of the Intertidal Bivalve, Cephalopod and Crab Programs Fisheries and Oceans Canada (DFO) Washington State Department of Health Shellfish and Water Protection Programs and Services Pacific Biological Station, Nanaimo, British Columbia Phone: 250-756-7215 http://www.doh.wa.gov/ehp/sf/default.htm Email: [email protected] Leads aquatic invasive species project The National Shellfisheries Association (shellfish research) examining distribution and impacts of intertidal non-native species on the http://shellfish.org/ Pacific Coast of Canada. Washington Sea Grant Current research and management efforts http://www.wsg.washington.edu/about.html Management efforts for the Japanese littleneck Expert contact information in PNW Derrick Toba, BS, MS Fisheries Assistant Region Manager, Department of Natural Resources Aquatic Region, Shoreline District Email: [email protected] clam are not necessary for controlling spread of this species, as naturalized clam beds are Washington harvested regularly to maintain stable populations. Hatcheries and nursery systems for generating seed, larvae, and juvenile clams are well regulated. To date, fishery agencies are not engaging in management or control efforts of this non-native species due to lack of harm or negative impacts within established communities. Because this is one of the leading commercial aquaculture crops in this region, there is a strong emphasis on maintaining healthy hatcheries, aquaculture sites, and naturalized beds. The Fisheries and Aquaculture Department indicates that aquaculture of Japanese littleneck clams will likely increase either through expansions in current farmed sites or by new introductions into suitable recipient areas (FAO 2011). Aquaculture practices include use of hatcheries to cultivate seed, nursery beds, trays, cages, bags, and nets (Jones et al. 1993). Developing methods to reduce the impacts of aquaculture within an ecosystem is one focus area at managing effects of this non-native clam. One possible way to minimize environmental impacts of aquaculture activities is polyculture, where multiple species are farmed in one region. Japanese littleneck clams have become recent participants in polyculture farms, being raised with various species of shrimp and fish (FAO 2011). Further research is needed to see if this method of culturing shellfish can reduce the impacts on ecosystems.