Species Description: The naval shipworm, Teredo navalis, is not a worm at all. It is a highly specialized bivalve mollusc adapted for boring into and living in submerged wood. The genus Teredo is one of several genera of wood-boring bivalve shipworms. Bankia and Lyrodus are two other genera of shipworm that can be found in the U.S. south Atlantic. Though it now exhibits a cosmopolitan distribution, T. navalis is a cryptogenic species in Florida and is believed to be non-native to North America (Carlton and Ruckelshaus 1997).
The body of Teredo navalis is long, wormlike and reddish pale in color. Unlike most bivalves that rely on their shell for protection, T. navalis has a small (up to 2 cm long), helmet-like shell that encloses only a small portion of the animal. The shell is modified for burrowing into wood. Fine ridges on the tri-lobed valves of the shell are used to rasp away wood. Instead of relying on the shell for protection, T. navalis protects its soft elongate body by residing in a secreted calcareous tube lining the excavated burrow. The tube is capped near the mouth of the burrow by a calcareous septum. An incurrent and excurrent siphon located at the anterior of the animal protrude through a small hole in the septum and into the water to facilitate feeding, respiration and excretion/egestion. The siphons can be rapidly contracted and protected underneath a pair of 0.5 cm long calcareous paddle-shaped pallets (NIMPIS 2002, Didžiulis 2007).
Potentially Misidentified Species: At least six other species of Teredo occur in the U.S. south Atlantic, including T. bartschi, T. fulleri and T. furcifera, which have been recorded from the IRL region of Florida, and T. clappi, that has been found on a ship's keel in Key West (Carlton and Ruckelshaus 1997). A number of shipworms from two additional genera, Bankia (B. carinata, B. fimbriatula) and Lyrodus (L. bipartitus,L. medilobatus, L. massa) have also been recorded from Florida. All of these shipworm species are considered either cryptogenic or introduced (Carlton and Ruckelshaus 1997).
Classification and identification of Teredo species is based on the shape of the siphonal pallets (see above). The paddle-like shape of these structures in T. navalis may be useful as a diagnostic key (NIMPIS 2002).
Regional Occurrence: Teredo navalis is believed by several authorities to be native to the European western Atlantic. It is now, however, a cosmopolitan species found both in Atlantic and Pacific oceans, from the tropics and subtropics, to cool temperate waters of both the northern and southern hemisphere (Didžiulis 2007).
IRL Distribution: Details of the distribution of Teredo navalis within the IRL region are not known. The cosmopolitan distribution of the species and its euryhaline nature (see below), however, suggest it may be well established.
Abundance: Modernization of the world's ships away from wooden vessels, and improvement in the chemical treatment of wood pilings and other submerged timbers, has greatly lessened the available human-made substrata available for colonization by wood-boring marine invertebrates.
Historical abundance of Teredo navalis in many harbors was so great that this animal was a key factor limiting the life expectancy of wooden ships. Forbes and Godwin-Austin, in their book, The Natural History of the European Seas (1859), note that T. navalis abundance in the Welsh harbor of Sebastopol was once so great as to cause the destruction of submerged ship timbers in just eight years on average.
Age, Size, Lifespan: The largest Teredo navalis individual recorded from the Baltic Sea was 30 cm in length, and individuals from tropical waters may reach 50 cm (Sordyl et al.1998). T. navalis digs approximately 1 cm wide burrows up to 0.6-1m long. (Lane 1959, Turner 1966, NIMPIS, 2002, Rowley 2005, Didžiulis 2007, ITIS 2007).
T. navalis individuals typically live 1-3 years. (Sordyl et al. 1998).
Reproduction: Reproduction in Teredo navalis is sexual and individuals become reproductive as soon as 6 weeks post-settlement. Spawning is temperature dependent, occurring from April-September in Barnegat Bay, New Jersey, and slightly later (May-October) at Woods Hole, Massachusetts, once the water has warmed somewhat. Salinities of 12 ppt or greater may be required. Male gametes are released to the water column and subsequently taken in through the incurrent siphons of other individuals; fertilization occurs internally within the epibranchial cavity (Grave 1928, 1942, Coe 1941, Lane 1959, Richards et al. 1984, NIMPIS 2002, Didžiulis 2007).
Fecundity in this species is high, with individual worms capable of brooding 1-5 million larvae (Grave 1928). Evidence of hermaphroditism has been recorded in young animals but the sexes appear to be separate as adults (NIMPIS, 2002).
Embryology: Teredo navalis larvae are brooded within the gills until a velum and a straight-hinged shell have formed at which time they are released to the water column. Duration of the planktonic phase appears in debate, with various authors citing a period ranging from less than 4 days to as much as 2-4 weeks. While residing in the plankton the larva develop siphons, gills and a prominent foot (Costello and Henley 1971, NIMPIS 2002, Didžiulis 2007).
At settlement, T. navalis individuals undergo a rapid metamorphosis during which the larval velum is shed and consumed (Lane 1959, Didžiulis 2007).
Temperature: Teredo navalis individuals survive water temperatures as high as 30°C, although growth may cease above 25°C. Minimum reproductive temperature is reported as approximately 11-15°C (NIMPIS 2002). The species has recently been reported from Weser Estuary, northern Germany, where winter water temperatures of 0.7°C were recorded (Tuente et al. 2002).
Salinity: The NEMESIS database provides a salinity range of 5-45 ppt for the euryhaline Teredo navalis. A lethal low salinity limit of 5 ppt has been suggested for larval individuals (Tuente et al. 2002).
Oxygen: Teredo navalis can survive extended periods of anoxia (up to 6 weeks) by suspending feeding activities and remaining sealed in burrows, metabolizing stored glycogen reserves for energy (Richards et al. 1984).
Trophic Mode: Teredo navalis is primarily xylophagous, feeding directly on wood. Some authors suggest a limited degree of filter feeder on water column plankton as well, although Mann and Gallager (1985) report experimental results indicating non-significant growth enhancement in T. navalis when phytoplankton was provided in addition to wood.
Genus Teredo is unique even among wood-boring bivalves in its ability to feed solely on wood (Gallager et al. 1981). It does so with the aid of symbiotic cellulolytic, nitrogen-fixing bacteria contained in specialised epithelial cells on the gills (Popham and Dikson 1973, Distel et al. 2002). T. navalis uses its sculpted shell to rasp wood particles, which are then moved via cilia to the mouth for ingestion. Water is obtained through the incurrent siphons and is used for feeding as well as respiration and excretion/egestion (Didžiulis 2007).
Associated Species: Teredo navalis is likely to occur in association with other shipworm species within submerged timbers.
Invasion History: Teredo navalis has been transported by ships for so many centuries that its historic native distribution cannot be known with certainty. Its center of endemism is believed to be European, however; there exists substantial evidence supporting the assertion that the species is correctly considered exotic and introduced on both coasts of the Americas (Ruiz et al. 2000, NIMPIS 2002).
In 1839, T. navalis was first reported in Massachusetts Bay in the sheathing of foreign wooden vessels. A century later the species was abundant in samples taken fromm Nova Scotia to Massachusetts. The species was first collected from Long Island Sound in 1869, again from the timbers of a sailing vessel. Within several decades the species was collected in abundance from test boards from all around New York Harbor (Brown 1953).
This shipworm occurred at low densities around Chesapeake Bay as early as 1878. As recently as the mid 1950s, the species was reported as rare in Chesapeake Bay (Andrews 1956). Subsequently, T. navalis has been collected from North Carolina and southward to Florida, Texas, the Bahamas, and Puerto Rico (Brown 1953).
On the U.S. west coast, introduction of T. navalis into San Francisco Bay in 1913 led to a serious invasion and substantial negative economic impact (Cohen 2004).
Potential to Compete with Natives: Where Teredo navalis co-occurs with native shipworm species, some degree of resource competition is likely.
Possible Economic Consequences of Invasion: Turner (1966) proposed Teredo navalis as likely the most widespread marine wood borer in the world. The historic negative economic impacts of T. navalis invasion may rival those of any other introduced marine species. It is the species believed responsible for a massive infestation of Dutch dikes in the 17th century. It was also described by a Dutch commission in 1731 as a "horrible plague" threatening to destroy the dikes protecting the lowlands of Holland (Cohen and Carlton 1995, Reise et al. 1999). The damage inflicted by the shipworms prompted replacement of wooden dikes with stone.
Massive T. navalis infestation was also responsible for the destruction of an unknown number of wharves, piers, ferry slips and other wooden harbor structures at a rate of a major structure a week for a period of two years in San Francisco Bay from 1919-1921. Cohen (2004) notes that in current dollars this would have equated to between $2 billion and $20 billion in damage.
In general T. navalis has a centuries-long history of causing damage to sailing vessels, piers, pilings, marinas, and any other submerged wooden structures. In 1946, shipworms were estimated to cause $55 million/year of damage to waterfront structures in U.S. (Scheltema and Truitt 1954).
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