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Teredo navalis

By Maggie Ho

Geographic Range

The origins of Teredo navalis are unknown. Appropriately referred to as naval shipworms, T. navalis are molluscs that frequently dwell in the wood of ships. Due to the prominent use of ships in global trade and the consequent dispersal of the shipworms, the origins of T. navalis are uncertain. Masses of naval shipworms were first identified near the Netherlands in the North Sea. They are common to the Baltic Sea as well as the Atlantic and Pacific Oceans today. (Didziulis, 2007; Gollasch, et al., 2009)

Habitat

Naval shipworms are marine and estuarine organisms inhabiting various submerged wooden substrates including floating wood, ships, or wharfs. Part of their larval stage is spent free-swimming in water. While they can tolerate low saline levels (up to 5 ppt), they flourish at levels greater than 9 ppt. Their optimal temperature range is 15 to 25 degrees Celsius and, as a result, T. navalis can be found in temperate and tropical zones. (Didziulis, 2007; Tuente, et al., 2002)

Physical Description

While T. navalis looks like a brown worm on the outside, it is actually a bivalve. Its head is covered with two white, tri-lobed shells used to bore into wood. The shells are up to 2 cm long and have concentric ridges. Inside the shell is a hook-like process called a styloid apophysis. The foot is also at the anterior end. At the posterior end are two siphons: incurrent and excurrent. The former is used for respiration and feeding while the latter is where waste and sperm or larvae exit. Paddle-like pallets act as a lid to cover the siphons when not in use. Naval shipworms are about 20 cm in length but can range from 1.5 to 58 cm. They are 1 cm in diameter. Calcareous coverings are secreted from their mantles that coat the burrows they make. Male and female adults cannot be distinguished externally. (NIMPIS, 2011; Didziulis, 2007; Grave, 1928)

  • Sexual Dimorphism
  • sexes alike
  • Range length
    1.5 to 58 cm
    0.59 to 22.83 in
  • Average length
    20 cm
    7.87 in

Development

Teredo navalis takes about five weeks to develop from eggs to metamorphosing larvae. They spend half of this time in the mother’s gill chamber until they are released into the water as free-swimming larvae. As the larvae develop, they transition from being small and white to large and dark gray. Fertilized eggs develop into cilia-covered larvae, referred to as trochophores. Over time, cilia are seen covering only the velum in larvae, now called veligers. The velum serves as an organ participating in movement and feeding. A shell appears about the same time in development as the velum and becomes bivalved after formation. Older veligers are released into the water. During this free-swimming stage, the siphons, gills, and foot develop. Once shipworms attach onto a wooden substrate, metamorphosis is observed. (NIMPIS, 2011; Grave, 1928)

Sexes alternate in T. navalis. Young are hermaphrodites while adults are either male or female. Usually, organisms are male first and then become female later. A second male to female phase may occur but shipworms normally do not live long enough for completion of the second phase. (Coe, 1943)

Reproduction

While no direct information was found on the mating system of T. navalis, it can be inferred to be polygynandrous. Males release sperm into the water, which females pick up via the incurrent siphon. This occurs on multiple occasions, as females spawn 3 to 4 times per season. (NIMPIS, 2011)

Reproduction typically occurs in the summer months when temperatures reach 15 degrees Celsius. Females spawn 3 to 4 times per season, each time releasing 1 to 5 million larvae. Teredo navalis embryos spend the first 2 to 3 weeks in the mother’s gill chamber. They are then released into the water as free-swimming veligers. Released larvae are 88 by 75 microns with a depth of 55 to 57 microns. They reach sexual maturity 6 to 8 weeks after inhabiting wood. ("Teredo navalis", 2009; Culliney, 1975; Grave, 1928; "Teredo navalis", 2009; Culliney, 1975; Grave, 1928; NIMPIS, 2011)

As mentioned previously, shipworms alternate between sexes during their life. When larvae mature, half of their gonads become spermatocytes, the other half ovocytes. Usually, spermatocytes multiply faster and are released earlier. (Coe, 1943)

  • Breeding interval
    Shipworms spawn 3 to 4 times each season
  • Breeding season
    Breeding season is usually in the summer
  • Range number of offspring
    1,000,000 to 5,000,000
  • Range gestation period
    2 to 3 weeks
  • Range time to independence
    2 to 3 weeks
  • Range age at sexual or reproductive maturity (female)
    6 to 8 weeks
  • Range age at sexual or reproductive maturity (male)
    6 to 8 weeks

Females carry offspring in gill chambers during early development. When larvae reach the advanced veliger stage, they are released into the water. There is no evidence that mothers assist veligers in finding wooden substrates to inhabit. (Grave, 1928)

  • Parental Investment
  • female parental care
  • pre-hatching/birth
    • protecting
      • female
  • pre-weaning/fledging
    • protecting
      • female

Lifespan/Longevity

The lifespan of shipworms is 1 to 3 years. (NIMPIS, 2011)

  • Average lifespan
    Status: wild
    1-3 years

Behavior

Released veligers are free-swimming. Upon attachment to a wooden substrate, veligers undergo metamorphosis to become adult shipworms. They burrow into the wood and stay there for the duration of their lives. Burrowing occurs either upward or downward; there does not seem to be a preference.

Neighboring burrows never come into contact with each other. Naval shipworms are able to somehow sense when they are close to another's burrow and respond by digging in a different direction or ceasing to grow. Similar behavior is seen when T. navalis reach the end of the wooden substrate and are able to turn around to delve in parallel to the original burrow. (Grave, 1928)

Communication and Perception

There is limited information on how T. navalis communicates.

Food Habits

Naval shipworms primarily feed on wood. They are able to do so because of enzymes produced by the nitrogen-fixing bacteria within them. Teredo navalis use their shell to cut into the wood. The pieces are then transported into the mouth via cilia. Organisms from the water may also be taken up for food via the inhalant siphon. Free-swimming veligers feed on plankton. (NIMPIS, 2011; Grave, 1928)

  • Other Foods
  • microbes

Predation

The calcerous covering they secrete not only act as a lubricant but also deters predators or poisons in the water. In response to these conditions, thicker calcareous material is secreted at the anterior end in addition to the sides. Predators include bacteria and parasitic protozoa like Architophrya. Native Australians and snails also eat naval shipworms. (Grave, 1928)

  • Known Predators
    • Bacteria
    • Parasitic Protozoa
    • Architophrya
    • Marine snails
    • Humans, Homo sapiens

Ecosystem Roles

Naval shipworms break down submerged wooden substrates. The holes they create in the wood can be used by crustaceans such as Idotea. Teredo navalis share a symbiotic relationship with the nitrogen-fixing bacteria within them that help the shipworms digest wood. Some protozoa are known to parasitize this species. (NIMPIS, 2011)

Economic Importance for Humans: Positive

There are not any mentioned effects of T. navalis that are positive to humans. They do serve as food for Australian natives. (NIMPIS, 2011)

  • Positive Impacts
  • food

Economic Importance for Humans: Negative

Naval shipworms have many negative effects due to their wood boring activity. They have been noted to cause damage in ships and dikes. Weakening of dike gates, combined with a heavy storm, resulted in flooding of the Netherlands in 1731. Teredo navalis also eat away at piers and wharfs. In San Francisco Bay, they can cause 200 million dollars worth of damage yearly. (NIMPIS, 2011; USDA, 2006)

Conservation Status

The conservation status of T. navalis has not been evaluated.

Other Comments

There are few ways to effectively prevent destruction of wood by Teredo navalis. Other non-wooden materials have been used to build objects like ships or wharfs. Biocides are only temporary answers and are hazardous to humans. Use of geotextiles to protect antique ships serves as a physical barrier to naval shipworms. (NIMPIS, 2011)

Contributors

Maggie Ho (author), University of Michigan-Ann Arbor, Phil Myers (editor), University of Michigan-Ann Arbor, Renee Mulcrone (editor), Special Projects.

Glossary

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

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Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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Neotropical

living in the southern part of the New World. In other words, Central and South America.

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Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

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Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

biodegradation

helps break down and decompose dead plants and/or animals

brackish water

areas with salty water, usually in coastal marshes and estuaries.

coastal

the nearshore aquatic habitats near a coast, or shoreline.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

filter-feeding

a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.

food

A substance that provides both nutrients and energy to a living thing.

herbivore

An animal that eats mainly plants or parts of plants.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

introduced

referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

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pelagic

An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).

phytoplankton

photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)

planktivore

an animal that mainly eats plankton

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

protandrous

condition of hermaphroditic animals (and plants) in which the male organs and their products appear before the female organs and their products

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

2009. "Teredo navalis" (On-line). Accessed June 01, 2011 at http://www.itis.gov/servlet/SingleRpt/SingleRpt.

Bartsch, P. 1923. The status of Teredo beachi and Teredo navalis. Science, 57 (1485): 692.

Blum, H. 1928. On the physiology of the pallet mechanism of the shipworm, Teredo navalis. Physiological Zoology, 1(3): 416-418.

Carlton, J. 1992. Introduced marine and estuarine mollusks of North America: an end of the 20th century perspective. Journal of Shellfish Research, 11:2: 489-505.

Coe, W. 1943. Development of the primary gonads and differentiation of sexuality in Teredo navalis and other pelecypod mollusks. Biological Bulletin, 84: 178-186.

Culliney, J. 1975. Comparative larval development of the shipworms Bankia gouldi and Teredo navalis. Marine Biology, 29: 245-251.

Didziulis, V. 2007. "NOBANIS-invasive alien species fact sheet, Teredo navalis" (On-line pdf). NOBANIS-European network on invasive alien species. Accessed June 01, 2011 at http://www.nobanis.org/files/factsheets/Teredo_navalis.pdf.

Gollasch, S., D. Haydar, D. Minchin, W. Wolff, K. Reise. 2009. Introduced aquatic species of the North Sea coasts and adjacent brackish waters. Pp. 507-528 in G Rilov, J Crooks, eds. Biological Invasions in Marine Ecosystems, Vol. 204. Germany: Springer.

Grave, B. 1928. Natural history of shipworm, Teredo navalis, at Woods Hole, Massachusetts. Biological Bulletin, 55 (4): 260-282.

Grave, B., J. Smith. 1936. Sex inversion in Teredo navalis and its relation to sex ratios. Biological Bulletin, 70 (2): 332-343.

Grave, B. 1942. The sexual cycle of the shipworm, Teredo navalis. Biological Bulletin, 82 (3): 438-445.

Mann, R., S. Gallager. 1985. Growth, morphometry and biochemical composition of the wood boring molluscs Teredo navalis, Bankia gouldi, and Nototeredo knoxi (Bivalvia: Teredinidae). Journal of Experimental Marine Biology and Ecology, 85: 229-251.

Mann, R., S. Gallager. 1985. Physiological and biochemical energetics of larvae of Teredo navalis and Bankia gouldi (Bivalvia: Teredinidae). Journal of Experimental Marine Biology and Ecology, 85: 211-228.

NIMPIS, 2011. "Teredo navalis, general information" (On-line). National Introduced Marine Pest Information System. Accessed June 01, 2011 at http://adl.brs.gov.au/marinepests/index.cfm?fa=main.spDetailsDB&sp=6000016293.

Scheltema, R., R. Truitt. 1956. The shipworm Teredo navalis in Maryland coastal waters. Ecology, 37 (4): 841-843.

Tuente, U., D. Piepenburg, M. Spindler. 2002. Occurence and settlement of the common shipworm Teredo navalis (Bivalvia: Teredinidae) in Bremerhaven harbours, northern Germany. Helgoland Marine Research, 56: 87-94.

USDA, 2006. "Submodule 7: The Price We Pay -- Economic Impacts" (On-line). National Invasive Species Information Center. Accessed June 06, 2011 at http://alic.arid.arizona.edu/invasive/sub7/p2.shtml.

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