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| Shipworm | |
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| This dried specimen ofTeredo navalis, and the calcareous tunnel that originally surrounded it and curled into a circle during preservation, were extracted from the wood of a ship. The two valves of the shell are the white structures at the anterior end; they are used to dig the tunnel in the wood. | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Bivalvia |
| Superorder: | Imparidentia |
| Order: | Myida |
| Superfamily: | Pholadoidea |
| Family: | Teredinidae Rafinesque, 1815 |
| Genera | |
See text | |
Theshipworms, also calledteredo worms or simplyteredo (from Ancient Greek τερηδών (terēdṓn) 'wood-worm', viaLatinterēdō), aremarinebivalvemolluscs in the familyTeredinidae, a group of saltwaterclams with long, soft, naked bodies. They are notorious for boring into (and commonly eventually destroying) wood that is immersed inseawater, including such structures as woodenpiers,docks, and ships; they drill passages by means of a pair of very small shells ("valves") borne at one end, with which they rasp their way through. They are sometimes called "termites of the sea".[1]Carl Linnaeus assigned thecommon nameTeredo to the best-known genus of shipworms in the 10th edition of histaxonomicmagnum opus,Systema Naturæ (1758).
Removed from its burrow, the fully grown teredo ranges from several centimeters to about a meter in length, depending on the species. An average adult shipworm measures 10 to 15 centimetres (4 to 6 in) in length and less than 6 mm (1⁄4 in) in diameter, but some species grow to considerable size.[2]
The body is cylindrical, slender, naked, and superficiallyvermiform (worm-shaped). In spite of their slender, worm-like forms, shipworms possess the characteristicmorphology ofbivalves. Thectinidia lie mainly within the branchialsiphon, through which the animal pumps the water that passes over thegills. The two siphons are very long and protrude from theposterior end of the animal. Where they leave the end of the main part of the body, the siphons pass between a pair of calcareous plates called pallets. If the animal is alarmed, it withdraws the siphons and the pallets protectively block the opening of the tunnel.
The pallets are not to be confused with the two valves of the main shell, which are at theanterior end of the animal. Because they are the organs that the animal applies to boring its tunnel, they generally are located at the tunnel's end. They are borne on the slightly thickened, muscular anterior end of the cylindrical body and they are roughly triangular in shape and markedly concave on their interior surfaces. The outer surfaces are convex and in most species are deeply sculpted into sharp grinding surfaces with which the animals bore their way through the wood or similar medium in which they live and feed. The valves of shipworms are separated and the aperture of themantle lies between them. The small "foot" (corresponding to the foot of a clam) can protrude through the aperture.
When shipworms bore into submerged wood, bacterial symbionts embedded within a sub-organ called the typhlosole in the shipworm gut, aid in the digestion of the wood particles ingested,[3] TheAlteromonas orAlteromonas-sub-group of bacteria identified as the symbiont species in the typhlosole, are known to digest lignin, and wood material in general. The tough molecular layers of lignin surround the cellulose elementary fibrils in the wood particles, and the lignin must be digested initially to allow access by other enzymes into the cellulose for digestion.[4] Another bacterial species (Teredinibacter turnerae), in the gills secrete a variety of cellulose-digesting enzymes which may be secreted into the shipworm gut via a special organ called the gland of Deshayes. These secretions aid the shipworm's own carbohydrate-active enzymes (CAZymes) in digesting the wood particles in combination with the enzymes and potentially other metabolites secreted by the symbiont bacterial in the typhlosole.[3][5]
The excavated burrow is usually lined with acalcareous tube. The valves of theshell ofshipworms are small separate parts located at the anterior end of the worm, used for excavating the burrow. The protective role of the shells is lost because the animal spends all its life surrounded by wood.[6]
Teredo navalis develops from eggs tometamorphosinglarvae in about five weeks. They spend half of this time in the mother's gill chamber before being discharged as free-swimming larvae into the sea. Their sexes alternate, young arehermaphrodites while adults can be either male or female. Typically, organisms are male at first and female subsequently. A second male to female phase may occur, however shipworms rarely live long enough to complete the second phase. They have a lifespan of 1 to 3 years.[7]
Shipworm anatomy reveals the typical organs of a bivalve mollusk, although with dimensional or positional peculiarities due to the thinness and length of the occupied space. Furthermore, some structures find no equivalent in other bivalve groups.
Normally, the shipworm's body fills the entire length of the gallery, but the anterior region can retract itself slightly with respect to the latter's extremity. Without the gills, the viscera only cover one-fourth of the total length and only their anterior part is partially covered by the shell.[9][10]
Shipworms are marine animals in the phylumMollusca,orderBivalvia,familyTeredinidae. They were included in the now obsolete orderEulamellibranchiata,[11] in which many documents still place them.
Ruth Turner ofHarvard University was the leading 20th century expert on the Teredinidae; she published a detailed monograph on the family, the 1966 volumeA Survey and Illustrated Catalogue of the Teredinidae published by theMuseum of Comparative Zoology. More recently, theendosymbionts that are found in the gills have been subject to study thebioconversion of cellulose for fuel energy research.[12]
Shipworm species comprise severalgenera, of whichTeredo is the most commonly mentioned. The best known species isTeredo navalis. Historically,Teredo concentrations in theCaribbean Sea have been substantially higher than in most other salt water bodies.
Genera within the family Teridinidae include:[13]
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TheTeredogenus has approximately 20species that live in wooden materials such as logs, pilings, ship, and practically any other submerged wooden construction from temperate totropicalocean zones. The species is thought to be native to theAtlantic Ocean and was once known as the Atlantic shipworm, although its exact origin is unknown.[14] The longest marine bivalve,Kuphus polythalamia, was found from a lagoon nearMindanao island in the southeastern part of thePhilippines, which belongs to the same group of mussels and clams. The existence of huge mollusks was established for centuries and studied by the scientists, based on the shells they left behind that were the size ofbaseball bats (length 1.5 m or 4 ft 11 in, diameter6 cm or2+1⁄2 in).[15][16] Thebivalve is a rare creature that spends its life inside anelephant tusk-like hard shell made ofcalcium carbonate. It has a protective cap over its head which it reabsorbs to burrow into the mud for food. The case of the shipworm is not just the home of the black slimy worm. Instead, it acts as the primary source of nourishment in a non-traditional way.K. polythalamia sifts mud and sediment with its gills. Most shipworms are relatively smaller and feed on rotten wood. This shipworm instead relies on a beneficialsymbiotic bacteria living in its gills. The bacteria use thehydrogen sulfide for energy to produceorganic compounds that in turn feed the shipworms, similar to the process ofphotosynthesis used bygreen plants to convert thecarbon dioxide in the air into simplecarbon compounds. Scientists found thatK. polythalamia cooperates with different bacteria than other shipworms, which could be the reason why it evolved from consuming rotten wood to living on hydrogen sulfide in the mud. The internal organs of the shipworm have shrunk from lack of use over the course of its evolution.[citation needed] The scientists are planning to study the microbes found in the single gill ofK. polythalamia to find a new possible antimicrobial substance.[citation needed]
Teredo navalis are a cosmopolitan species that can be found both in theAtlantic andPacific oceans.[6] Since they occupy woodenflotsam and naturaldriftwood such as dead tree trunks, they are spread as the wood is carried by currents. They also travel inside the wooden-hulled vessels that help increase their spread worldwide.[17] However, the origin ofT. navalis remains uncertain due to the widespread usage ofships inglobal trade and the resulting spreading of shipworms.[18]
During the free-livinglarva stage, the species colonizes new habitats and spreads. Larvae are extremely sensitive to the presence of wood and will take advantage of any opportunity to attach to and penetrate wooden structures. In theBaltic Sea, free-floating piles carved by shipworms can be observed floating hundreds of kilometers away from the original wooden structures. The limiting element for propagation issalinity, which must be greater than 8% for successful reproduction. Consequently,freshwater is deadly to theseinvertebrates.[6] Reproduction occurs during warmsummer months, and the larvaemature for production in just eight weeks. Each year, severalgenerations can be produced. Their ideal temperature range is 15 to 25 °C (60 to 75 °F) and thereforeT. navalis can be found in temperate and tropical zones.[18]
The shipworm lives in waters withoceanic salinity. Accordingly, it is rare in thebrackish Baltic Sea, where woodenshipwrecks are preserved for much longer than in the oceans.[19]
The range of various species has changed over time based on human activity. Many waters in developed countries that had been plagued by shipworms were cleared of them bypollution from theIndustrial Revolution and the modern era; as environmental regulation led to cleaner waters, shipworms have returned.[20]Climate change has also changed the range of species; some once found only in warmer and more salty waters like theCaribbean have established habitats in theMediterranean.[20]
Shipworms greatly damage wooden hulls and marinepiling, and have been the subject of much study to find methods to avoid their attacks.[20]Copper sheathing was used on wooden ships in the latter 18th century and afterwards, as a method of preventing damage by teredo worms. The first historically documented use of copper sheathing was experiments held by the British Royal Navy withHMS Alarm, which was coppered in 1761 and thoroughly inspected after a two-year cruise. In a letter from the Navy Board to the Admiralty dated 31 August 1763 it was written "that so long as copper plates can be kept upon the bottom, the planks will be thereby entirely secured from the effects of the worm."
In theNetherlands the shipworm caused a crisis in the 18th century by attacking the timber that faced thesea dike.[citation needed] After that the dikes had to be faced with stones. In 2009,Teredo caused several minor collapses along theHudson River waterfront inHoboken, New Jersey, due to damage to underwater pilings.[21]
In the early 19th century,engineerMarc Brunel observed that the shipworm's valves simultaneously enabled it to tunnel through wood and protected it from being crushed by the swelling timber. With that idea, he designed the firsttunnelling shield, a modular iron tunnelling framework which enabled workers to tunnel through the unstable riverbed beneath the Thames. TheThames Tunnel was the first successful large tunnel built under a navigable river.[20][22]
Henry David Thoreau's poem "Though All the Fates" pays homage to "New England's worm" which, in the poem, infests the hull of "[t]he vessel, though her masts be firm". In time, no matter what the ship carries or where she sails, the shipworm "her hulk shall bore, / [a]nd sink her in the Indian seas".[23] The hull of the ship wrecked by a whale, inspiringMoby Dick, had been weakened by shipworms.[20] In the NorseSaga of Erik the Red, Bjarni Grimolfsson, an Icelander,[24] had his ship drift into the Irish Sea where it was eaten up by shipworms. He allowed half the crew to escape in a smaller boat covered in seal tar, while he stayed behind to drown with his men.
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InPalawan andAklan in thePhilippines, the shipworm is calledtamilok and is eaten as a delicacy. It is prepared askinilaw—that is, raw (cleaned) butmarinated with vinegar orlime juice, choppedchili peppers and onions, a process very similar to shrimpceviche. Similarly,T. navalis can be found inside the dead and rotten trunk ofmangroves inWest Papua, Indonesia. To the locals, theKamoro[25] tribe, it is referred to astambelo and is considered as a delicacy in daily meals. It can be eaten fresh and raw (cleaned) or cooked (cleaned and boiled) as well and usually marinated with lime juice and chili peppers. SinceT. navalis are related toclams,mussels, andoysters,[26] the taste of the flesh has been compared to a wide variety of foods, frommilk tooysters.[27] Similarly, the delicacy is harvested, sold, and eaten from those taken by local natives in the mangrove forests ofWest Papua and some part ofBorneo Island,Indonesia, and the central coastal peninsular regions of Thailand nearKo Phra Thong.
InBrazil, particularly in the states ofPará andMaranhão, species from the genusTeredo andNeoteredo are considered adelicacy. Considered anaphrodisiac, it is mostly eaten raw or in stews.[28]
T. navalis grow faster than any other bivalve because it does not require much energy to create its small shell. They can grow to be about 30 cm (12 in) long in just six months.Mussels andoysters, on the other hand, with their much bigger shells, can take up to two years to reach harvestablesize.[26]
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