Venerupis philippinarum, Japanese littleneck

Robyn Shean

FISH 423: Aquatic Invasion Ecology

Fall 2011

Diagnostic information

Scientific name: Veneroida philippinarum Also known as Tapes philippinarum, 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 (USGS 2011). Physical description and physiology white in color, Japanese littlenecks are multi- , commonly colored and usually have a purple hue along the called Japanese littleneck , 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 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. 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 of the shells. L. staminea (right) (WDFW 2011). 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 . 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 Bivalves are broadcast spawners. littleneck clams as it grows to help it become Female and male adults release sperm and eggs larger and thicker. During winter months when into the water column where fertilization occurs. temperatures drop and food sources are limited, Once fertilized, the eggs develop into free- the clam grows at a slower rate. While some floating larvae that are carried in people think the rings on these clams indicate water currents over several weeks. The age, they are not a good predictor of age as 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 Japanese littleneck clams are dioecious clam secure itself onto the seafloor once it finds – there are male and female individuals, each a suitable substrate to settle on. The substrate is producing either egg or sperm. Individuals usually a muddy, sandy, or soft surface the clam become sexually mature in the first to third year can burrow into with its foot. Burrowing into of age. This can vary depending on location and the ground allows the to find food and be size of shell. In Hood Canal (Washington state), protected from predators. Once settled, it will research has shown that clams are sexually stay in the substrate and continue to grow into a mature when shell length is 5-10 mm. However, mature clam. Individuals become sexually most individuals do not spawn until shell length mature between 1-3 years of age, and have a life is at least 20 mm (Holland and Chew 1974). span up to 14 years (WDWF 2011). 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 and overall recruitment for the color, whereas the siphon is not split in native population. Reports indicate Japanese littleneck 5 littlenecks (Jones et al. 1993). clam fecundity ranges from 4.32x10 eggs to 6 Japanese littleneck clams feed on 2.35x10 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). One study produced results indicating some natives easier to reach for harvesting (WDFW Japanese littleneck clam populations in areas of 2011). the Sea of may exhibit sex reversal For Japanese littleneck clams, the area in which (hermaphrodite) due to 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 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 opportunities for predation by terrestrial

Environmental optima and tolerances organisms and mortality caused by fluctuations in temperature. Temperature plays a critical role in 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- Protozoan parasite infections Several native crab species feed on clams. The Protozoan parasites haven’t been red rock crab () 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. (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, sp. found in the along the Pacific coast is the European green digestive gland, and Marteilioides sp. found in crab (), 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 populations of Japanese littleneck affect this species of clam. clams in . Research has shown a 40 percent decrease in Japanese littleneck clam harvest in California since establishment of the Predation European green crab (WDFW 2008). Seagulls and crows feed on both native Moon (Polinices lewisii) predate on and non-native clams when the tide is out. littleneck clams by drilling a hole in the clam’s Migratory birds, including several species of sea shell and then feeding on the meat inside. Older ducks, predate on clams as an added food source clams have harder shells, so they are less likely during the winter months. Caldow and others to be preyed on by these snails. Additionally, investigated the interaction between the non- some species of seastars and bottom-feeding fish native Japanese littleneck clam and a specific feed on smaller clams. seabird, the Eurasian oystercatcher (Haematopus ostralegus ostralegus). These seabirds rely on Current geographic distribution available food sources, including , for 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 seed (FAO 2011). Japanese 1981). Starting in the early 1900s, these clams littleneck clams spread quickly and are now were introduced both accidentally and found along the west coast of 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 , Ireland, England, beds located in Hood Canal. Tahiti, , , and Scotland. In all of

these regions, this clam species was introduced as a commercially harvested crop. Additionally, Distribution throughout the world trial cultures of V. philippinarum have been Japanese littleneck clams are native in Successful in the Virgin Islands, Tunisia, the , South and East Seas, 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, , and Russia

Figure 5: Regions across the world producing (Ponurovsky 1992).

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 for the introduction of V.

philippinarum; specifically, import of Japanese littleneck clams were commercial . Individuals were included unintentionally introduced to the Pacific with shipments of spat from Northwest as ‘hitchhikers’ in shipments of Japan. Another source of possible introduction Pacific oysters in the 1930s. The non-natives may be through ballast water of ships departing were packed in containers of oyster seed shipped from regions with populations of this clam from Japan to Washington state where they then species. As broadcast spawners with a larval established and spread along the Washington, stage beginning the life cycle, individuals are Oregon, and California coasts, and throughout free-floating and able to drift in water currents the Puget Sound. This non-native clam appears over large areas and be transported through to fill an ecological niche that compliments that water brought into a ship’s hull area. Transport of the native littleneck clam (Becker et al. 2008). of ships from the Eastern Pacific to the Western Starting in the early 1900s, Japanese littleneck Pacific can occur within a short time, increasing clams were introduced to coastal and aquatic the likelihood of survival of larvae. regions throughout the world (Figure 5). Most regions with Japanese littleneck 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. If environmental conditions 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

the economy of the region (Pranovi et al. 2006). While boosting the economy and generating Economic impacts revenue, harvest activities have caused severe Invasion of the Japanese littleneck clam stress on the benthic communities and the entire into the Pacific Northwest created an economic lagoon ecosystem. The non-native clam spread boom in the shellfish industry. Washington state throughout the lagoon in which it was is the leading producer of farmed bivalve introduced and expanded into other coastal shellfish in the U.S. In 2000, farmed shellfish areas. Because of the rapid growth of this harvests generated just under $77 million in species, it has become one of the dominant revenue (Puget Sound Partnership 2003). Of species in the lagoon in biomass and abundance, these sales, clams accounted for approximately displacing other native species (Pranovi et al. $14 million. Japanese littleneck clams are the 2006). leading commercial clam species harvested in With large numbers of Japanese littleneck clams, the state, with approximately 4,500 tons harvest activities have increased and affected the harvested annually (WDFW 2011). 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 caused by non-native Japanese release particles into the water column as they littleneck clams. 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 Management of the non-native Japanese al. 2001). These effects could increase the risk littleneck clam is minimal, as this is one of the for anoxic conditions in the aquatic environment top commercial shellfish harvest industries in and cause harm to benthic organisms. The Washington state (Puget Sound Partnership presence of clams also stimulates inorganic 2003). Aquaculture of this species is a major nitrogen and phosphorus from sediment into the source of revenue for the region. Harvest of water column. Increased circulation of these Japanese littleneck clams in naturalized beds by nutrients can promote production of private citizens is another source of income for phytoplankton, but also encourage growth of the state due to revenue generated by the cost of some algae species leading to potentially shellfish licenses. In addition to providing harmful algal blooms (Bartoli 2001). 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 promote production of these species. Mechanical and manual harvesting techniques are used, both of which disturb the Literature cited substrate, displace native organisms, 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 (Venerupis philippinarum): 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 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 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, Marteilia sp. and affected b brown ring disease. Diseases of Marteilioides sp.) from the Japanese Aquatic Organisms 93: 149-162 littleneck clam, Venerupis philippinarum in Japan. Journal of Invertebrate Pathology Toba D, Dewey B, King T (2005) Small-scale 88: 201-206 clam farming for pleasure and profit in Washington. Washington Sea Grant Jones G, Sanford C, Jones B (1993) Japanese Program Publication WSG-AS 03-02, littleneck clams, hatchery and nursery 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 Nonindigenous Aquatic Species Database, and temperature-dependence of vital rates 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) (: Veneridea). FM96-03 Journal of Shellfish Research 11(2): 265- 277 Washington Department of Fish & Wildlife (2008) Invasive Species Fact Sheets: Puget Sound Partnership (2003) Shellfish Carcinus maenas (European green crab). Economy: Treasures of the Tidelands Aquatic Nuisance Species, Seattle Yap WG (1977) Population biology of the Background in shellfish aquaculture; experience Japanese littleneck clam, Tapes 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 Focus on marine aquaculture species, www.wdfw.wa.gov/fishing/shellfish/clams/Japa including shellfish and other nese littleneck_clams.html invertebrates, finfish and plants.

National Oceanic and Atmospheric Graham Gillespie, BS Administration (NOAA) Aquaculture Program Head of the Intertidal Bivalve, and http://aquaculture.noaa.gov/ Crab Programs

Fisheries and Oceans Canada (DFO) Washington State Department of Health Pacific Biological Station, Nanaimo, British Shellfish and Water Protection Programs and Columbia Services 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 examining distribution and impacts of (shellfish research) 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 clam are not necessary for controlling spread of Derrick Toba, BS, MS Fisheries this species, as naturalized clam beds are Assistant Region Manager, Washington harvested regularly to maintain stable Department of Natural Resources populations. Hatcheries and nursery systems for Aquatic Region, Shoreline District generating seed, larvae, and juvenile clams are Email: [email protected] 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.