|This dried specimen of Teredo navalis was extracted from the wood and the calcareous tunnel that originally surrounded it and curled into a circle artificially. The two valves of the shell are the white structures at the anterior end; they are used to dig the tunnel in the wood.|
The shipworms are marine bivalve molluscs in the family Teredinidae: a group of saltwater clams with long, soft, naked bodies. They are notorious for boring into (and commonly eventually destroying) wood that is immersed in sea water, including such structures as wooden piers, docks and ships; they drill passages by means of a pair of very small shells borne at one end, with which they rasp their way through. Sometimes called "termites of the sea", they also are known as "Teredo worms" or simply Teredo, from the Greek τερηδών teredōn, via Latin. Eventually biologists adopted the common name Teredo as the name for the best-known genus. On April 17, 2017, it was reported that live giant tube shipworms Kuphus polythamia had been discovered in the Philippines.
Removed from its burrow, the fully grown teredo ranges from several centimetres to about a metre in length, depending on the species. The body is cylindrical, slender, naked and superficially vermiform, meaning "worm-shaped". In spite of their slender, worm-like forms shipworms nonetheless possess the characteristic morphology of bivalves. The ctinidia lie mainly within the branchial siphon, through which the animal pumps the water that passes over the gills.
The two siphons are very long and protrude from the posterior 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 the anterior 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 the mantle lies between them. The small "foot" (corresponding to the foot of a clam) can protrude through the aperture.
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 by pollution as the industrial revolution and into the modern era; as environmental regulation led to cleaner waters, shipworms returned and became a problem. Climate change has also changed the range of species; some once found only in warmer and more salty waters like the Caribbean have established habitats in the Mediterranean.
When shipworms bore into submerged wood, bacteria (Teredinibacter turnerae strain ATCC 39867 / T7901) in a special organ called the gland of Deshayes digest the cellulose exposed in the fine particles created by the excavation. The excavated burrow is usually lined with a calcareous tube. The valves of the shell of shipworms are small separate parts located at the anterior end of the worm, used for excavating the burrow.
Ruth Turner of Harvard University was the leading 20th century expert on the Teredinidae; she published a detailed monograph on the family, the 1966 volume "A Survey and Illustrated Catalogue of the Teredinidae" published by the Museum of Comparative Zoology. More recently, the endosymbionts that are found in the gills have been subject to study the bioconversion of cellulose for fuel energy research.
Shipworm species comprise several genera, of which Teredo is the most commonly mentioned. The best known species is Teredo navalis. Historically, Teredo concentrations in the Caribbean Sea have been substantially higher than in most other salt water bodies.
The longest marine bivalve Kuphus polythalamia was found from a lagoon near Mindanao island in the southeastern part of the Philippines, 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've left behind that were the size of baseball bats. The bivalve animal is a rare creature that spends its life inside an elephant tusk-like hard shell made of calcium 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. The animal can reach a length of 1.5 meters (5 ft.) and a diameter of 6 cm (2.3 in.). It has the ability to reabsorb the shell when it needs to grow and burrow deeper into the mud. The K.polythalamia sifts mud and sediment with its gills. Most shipworms are relatively smaller and feed on rotten wood. Instead, the shipworm does not eat, they rely on a beneficial symbiotic bacteria living in its gills. The bacteria use the hydrogen sulfide as energy to produce organic carbons that feed the shipworms. The process is similar to the green plants' photosynthesis to convert the carbon dioxide in the air into simple carbon compounds during photosynthesis. Scientists found that K. 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. The scientists are planning to study the microbes found in the single gill of the K.polythalamia to find a new possible antimicrobial substance.
Genera within the family Teridinidae include:
- Bactronophorus Tapparone-Canefri, 1877
- Bankia Gray, 1842
- Dicyathifer Iredale, 1932
- Kuphus Guettard, 1770
- Lyrodus Binney, 1870
- Nausitoria Wright, 1884
- Neoteredo Bartsch, 1920
- Nototeredo Bartsch, 1923
- Psiloteredo Bartsch, 1922
- Spathoteredo Moll, 1928
- Teredo Linnaeus, 1758
- Teredora Bartsch, 1921
- Teredothyra Bartsch, 1921
- Uperotus Guettard, 1770
- Zachsia Bulatoff & Rjabtschikoff, 1933
Shipworms greatly damage wooden hulls and marine piling, and have been the subject of much study to find methods to avoid their attacks. 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 with HMS 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 the Netherlands the shipworm caused a crisis in the 18th century by attacking the timber that faced the sea dikes. After that the dikes had to be faced with stones. Teredo has recently caused several minor collapses along the Hudson River waterfront in Hoboken, New Jersey, due to damage to underwater pilings.
In the early 19th century, the behaviour and anatomy of the shipworm inspired the French engineer Marc Brunel. Based on his observations of how the shipworm's valves simultaneously enable it to tunnel through wood and protect it from being crushed by the swelling timber, Brunel designed an ingenious modular iron tunnelling framework—the very first tunnelling shield—which enabled workers to tunnel successfully through the highly unstable river bed beneath the Thames. The Thames Tunnel was the first successful large tunnel ever built under a navigable river.
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".
In the Norse Saga of Erik the Red, Bjarni Herjólfsson, said to be the first European to discover the Americas, had his ship drift into the Irish Ocean 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.
In Palawan and Aklan in the Philippines, the shipworm is called tamilok and is eaten as a delicacy there. It is prepared as kinilaw—that is, raw (cleaned) but marinated with vinegar or lime juice, chopped chili peppers and onions, a process very similar to ceviche. The taste of the flesh has been compared to a wide variety of foods, from milk to oysters. Similarly, the delicacy is harvested, sold, and eaten from those taken by local natives in the mangrove forests of West Papua, Indonesia and the central coastal peninsular regions of Thailand near Koh Phra Thong.
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