In Maryland, April is Archaeology Month and May is Preservation Month, so this is an appropriate time to consider these tiny creatures that pose a large threat to the preservation of submerged archaeological resources. While these marine woodborers have impacted commerce and safety since humanity took to the sea, changes in construction materials of ships and harbor infrastructure, as well as the use of effective but environmentally dubious chemical treatments, greatly reduced their negative effects. There is still a toll; damage to harbor infrastructure by shipworms was estimated at $1 billion USD globally in the early 21st-century (Cobb, 2002) but this still compares favorably to the $500-$900 billion (based on 2009 dollar values) in damage over just two years, 1919-1921, in San Francisco Bay alone (Rayes et al. 2015:488). However, climate change appears to be a factor in the spread and adaptation of these woodborers enabling them to tolerate both fresher and much colder waters and permitting them access to a veritable smorgasbord of historic vessels.

It should be noted that various fungi and bacteria also degrade and rot wood and are the subjects of ongoing studies regarding their effects and the extent to which climate change may be affecting them.  This discussion considers the three main categories of marine woodborers.

Pill Bugs (a.k.a. Roly-Poly, Rollie Pollie, Doodle Bug, Potato Bug, and more) are not insects but terrestrial crustaceans (Figure 1).  Although Armadillidium vulgare is one of the most common, they are so diverse that they are usually referenced by Family; Armadillidiidae.  Although they are crustaceans and breathe through gills they cannot live under water (Reconnect 2023) but live in wet environments like mangroves.  They can damage the latter extensively, which has a bearing on low-lying areas that are coming to rely on mangroves for protection against storms and sea level rise, as well as maritime infrastructure built in these environments.  Because of their larger size compared to the much smaller shipworm and gribble, they were recognized and studied earlier.  They do not eat wood but chew through it to create burrows for shelter and, historically, have had far less impact on ships and harbor structures than the other woodborers. 

It took longer to differentiate gribbles and shipworm because of their small size and the apparent similarity of the damage they caused, despite the gribble being a crustacean and the shipworm being a mollusk.  Both actually digest the wood as opposed to burrowing through it.  There are more than 50 species of marine isopod in the gribble Family Limnoriidae and many of these bore into plants and grasses as well as wood (Figure 2).  The gribble (Limnoria lignorum) as a threat to vessels was identified in 1799.  These are the smallest of the woodborers and leave tiny entry holes that belie the extent of the internal damage they can cause. On the wreck of the vessel believed to be James Cook’s Endeavour off Rhode Island, Reuban Shipway identified both gribbles eating the exterior of the vessel and shipworms devouring the interior of the hull (Kuta, 2022).  A further concern is that as the wood weakens and breaks, creatures that feed on the woodborers can cause additional damage by rooting for them.  There are not a lot of known predators as long as the piece of wood is intact, but when the wood disintegrates, they are rapidly eaten by fishes, crabs, and other predators. They are vulnerable to protozoan parasites, such as Minchinia teredinis, which can cause extensive mortality (Hillman et al. 1990)” (Smithsonian 2023).  Nelson (1925) also suggests the Warty Comb Jelly, or sea walnut (Mnemiopsis leidyi), a species of tentaculate ctenophore, that is known to be a significant predator of mollusk larvae.  Gribbles appear to be native to western Atlantic coastal waters, but have become established as an invasive species in European and western Asian regions.

The most infamous of these “termites of the sea” is the shipworm.  While there are a number of species, often named for the regions where they are found, the eponymous Teredo navalis can represent them all (Figure 3).  It does not look like the bivalve it is, because most of its body is external, taking the form of a worm, with the two shells being reduced to small plates at the head designed to auger through wood.  It has been found in fossil form dating from the Cretaceous period (145-66 MYA) but the earliest evidence of them impacting humanity comes from Egypt.  Hull planks from excavations show Teredo damage and efforts to address this through use of thicker planks on oceangoing watercraft versus river vessels, additional sacrificial wood at joints and seams, choices of denser, more finely grained woods like cedar (Cedrus libani) and Nile Acacia (Acacia nilotica), and the application of a coating of pine tar to the hulls (Rayes et al. 2015, Ksenija Borojevic et al. 2010, Polzer 2011, Ward and Zazzaro 2009). So it continued through time, with various coatings being applied; from Pliny the Elder’s reference to zopissa (bee’s wax and resin) to substances like tar, brimstone (sulphur), or arsenic. Then there were efforts to sheath the hulls from additional layers of sacrificial wood to lead sheathing from Greek and Roman times through similar endeavors by Spain and England in the 14th and 15th centuries.  The tacks holding the lead sheathing to the hull tended to corrode and the metal then fell off exposing the wood. Japanese boatbuilders even scorched the exterior of the hulls to deter the borers (Thunberg 1796).  Again, there was an effort to find Teredo-resistant wood species.  Two that offered promise were Cuban cedar (Cedrela odorata) and the Cabopa tree (Mitragyna stipulosa) from Cacheu; a region of what is now Guinea-Bissau, and since Spain built about a third of its Navy in Cuba in the 18th century this connection demonstrates the merit of studying placement of shipyards in proximity to where teredo-resistant timbers grew (Aderinto 2007, McNeill 2004). Copper sheathing had its first success when applied to the Royal Navy vessel Alarm in 1761 and became widespread thereafter whenever a builder could afford it.  The late 19th and 20th centuries saw a return to applied coatings, but of metallic anti-fouling paints of significant toxicity including mercury, and chrome copper arsenate, as well as somewhat less toxic turpentine and borax, although these are still undesirable (Paalvast and van der Velde2011) and most were outlawed by the late 20th-century.  The widespread use of ferro-concrete in harbor structures after 1900 also aided in reducing damage.  In the 20th-century, increased Teredo activity was experienced by urban and developed areas after steps were implemented to reduce the level of pollution in the waters.  New York City, after the Clean Water Act of 1972, saw improved water quality but also extensive woodborer damage between 1995-1997 such that a 21-meter(23-yard) section of a wharf dropped into the East River, and a 6-meter (6.6-yard) section fell from the Brooklyn pier (Rayes et al. 2015:488, Paalvast and van der Velde 2011:119). Similar situations have occurred in Maine in 2000, on the Rhine River and in the port of Rotterdam (Cobb 2002), and are currently occurring in Venice (Figure 4).  Ironically, pollution had been protecting these and, by extension, submerged heritage resources.

The origins of these marine woodborers are not clear since they were not studied until they became a threat.  Pill Bugs are believed to have originated in southern Europe and/or northern Africa (Higgins 2023).  Gribbles, as previously, noted, are native to the western Atlantic but Teredo are thought to have originated in the Pacific and Indian Oceans (WreckProtect 2023).  As they were in Egypt’s harbors so early, it may speak to their introduction to the Mediterranean via vessels brought overland from Egypt’s Red Sea/Indian Ocean trading and fishing expeditions and, like most invasives, they spread rapidly.  The introduction and widespread proliferation of shipworm outside of the Mediterranean correlates with the expansion of European maritime trade into the Indian Ocean and subsequently to the Caribbean and beyond.  Although they are considered warm water species, they adapted sufficiently rapidly to Atlantic waters to force the beaching of the vessels Capitana and Santiago during Columbus’s fourth voyage, in 1503.  In 1731, they caused extensive damage to the wooden seawalls holding back the ocean from the reclaimed lands of The Netherlands and kept the citizens living in fear of a disastrous flood for the two years it took to repair and reinforce the seawalls.  In an 1873 publication about the laying of communications cables, Sir James Anderson, complained about them boring through the core of the cable in shallow water, and devouring the hemp covering in a few months and inhabiting the interior gutta-percha covering at depths of 2.2 km (1.37 mi) and said that the only protection was burying the cables but noting that they could not be relied upon to stay buried (Anderson 1873).

Conventional wisdom has been that shipworm requires warm salty water to survive and this seemed to hold for the Mediterranean where ship remains only survived if they were buried or covered by cargo like amphorae.  Cold, more brackish, waters preserved vessels beautifully, as is evident in the myriad vessels in the Baltic Sea.  However, more recently it has become apparent that climate change is increasing salinity, as well as warming the waters, of the Baltic (WreckProtect) and, also, many European Rivers are experiencing a migration of salinity upstream (Paalvat and van der Velde 2011:120).  Species of Teredo are also adapting to tolerate much colder and fresher waters.   This has been reported by WreckProtect during a two-year project funded by the European Union between 2009-2011.  Also, a log was recovered by a research vessel from 250 m (273 yards) depth and only 1100 km (683 miles) from the North Pole and with a water temperature of -1.8°C (28.8°F) that was riddled with a living multi-generational colony of Teredo (Kintisch 2016).

The Chesapeake Bay covers an area of 4,479 square miles in both Maryland and Virginia, with five major rivers in Maryland and a further 111 square miles of State coastal waters.  Certainly, climate change is evident and being addressed through the State’s Climate Change Program and its sub-programs. However, Teredo and another woodborer present in the Bay, Bankia Gouldi L., are not considered in these.  Most of the studies of shipworm in Maryland date from the early 1950s (Maryland Tidewater News 1951, Schelema and Truitt 1956). The average depth of the Bay, outside of the 80-foot deep shipping channel, is about 45 feet, which means it is heating very rapidly; 1°C (1.8°F) over the last 30 years (NOAA 2023).  This is already being expressed in the proliferation of the flesh-eating bacteria Vibrio vulnificus (NOAA 2022).  At present, submerged historic resources in Baltimore Harbor may be benefitting from the double-edged sword of the extant pollutants, but the more than 5000 wrecks in Maryland waters are increasingly at risk.  Until more studies are undertaken, the best resources available for managing the maritime resources are the detailed manuals produced by WreckProtect which offer guidelines for protecting submerged wooden cultural sites (2011a) and guidelines for predicting shipworm damage (2011b).  Although the latter focuses on the Baltic, it is relatively recent and can be adapted.  While it will take time for the 100 WWI vessels in the Mallows Bay-Potomac River National Marine Sanctuary to be under direct threat from shipworm, it may be sooner than anticipated.  It would be a sad irony for ships that faced all the challenges of seafaring with storms, reefs, and battles, to be lost to a 15mm (0.5 inch) mollusk.