116 K. N. Hoppe

TEREDO NAVALIS - THE CRYPTOGENIC SHIPWORM

Abstract

KAIN. HOPPE CRM Coastal Research & Management, Kiel, Germany Kai. H oppe@crm-online.de

The shipworm Teredo navalis is one of the most effective and harmful marine invaders world­wide. It is not c1ear, wh ether it invaded Europe from South East Asia or whether it originated in Europe and invaded the rest ofthe world from there. Recently it reappeared in the Western Baltic causing damages estimated at 25 - 50 Mio Euro along the Baltic coast of Germany alone. An immense reproduction rate and a high resistance to unfavourable environmental circumstances are the keys to its success. Even though mankind has tried to develop counter measures for thousands ofyears, still there is no easy solution to the shipworm problem in sight.

1 Introduction

The shipworms, wood boring bivalves of the family Teredinidae, belong to the oldest invaders. They naturally spread with wooden flotsam, but have also settled in all wooden boats and ships from the earliest dugouts to modem sailing yachts. With the trading vessels they travelled literally around the world. In fact, the most prominent shipworm, Teredo navalis, spread so early that we do not know where it originates. Even though seafaring nations have tried to protect themselves from shipworms for thousands of years, these animals still inflict immense damages on harbour construc­tions.

2 Biology

Teredo navalis is a bivalve with a wormlike, elongated body and a short, helmet-like shell at the anterior. It lives completely embedded in wood, with a tiny opening for two retractable siphons.

Teredo eats its way into wood digesting it with the help of endosymbiotic bacteria. T. navalis is unique in this regard, because it can survive on a wooden diet only, unlike other shipworms. However, it also filters and digests plankton from the seawater. Its symbionts are able to fix dissolved nitrogen from the water and incorporate it into es­sential amino acids. The shell is aboring organ with tiny teeth on the valves; it is used like a rasp. Up to 2,300 rasps are performed per hour, with a rotation of 3600 during 20-40 rasps (Mann & Gallagher 1985). Undigested sawdust is expelled through the exha­lant siphon, often accumulating around the siphons. The tunnel is always slightly greater in diameter than the lateral extension of the valves, so that the shipworm can retract to a certain extent. It usually tunnels along the fibres of the wood and avoids other shipworms. It stops or turns around at the end of the piece of wood it inhabits, or when it approaches another shipworm's tunnel.

The tunnel is lined with a calcareous excretion, and the opening can be closed with two paddle-like calcareous plates called "pallets", which are important in identifying ship­worm species. Sealed off the shipworm can survive anaerobically for about three weeks (Lane 1959), with the lining buffering the acid metabolites. Therefore it can withstand

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E. Leppäkoski et a!. (eds.), lnvasive Aquatic Species ojEurope, 116-119. © 2002 Kluwer Academic Publishers.

Teredo navalis - the cryptogenic shipworm 117

unfavourable conditions like exposure to air or freshwater. It even survives very cold winters, when the tunnelled wood is completely covered in ice. The larvae of T navalis can live and settle in salinities as low as 9 PSU, but the adults withstand even lower salinities. They grow to an average length of20-30 cm, the largest specimen recorded in the Baltic was 59 cm (Kristensen 1969; 1979).

Teredo navalis attacks wooden structures as a pediveliger larva with a diameter of ab out 1 mm. It detects wood chemically from a distance and actively swims the last centime­tres, before it attaches itself to the surface with abyssus thread. The soft-shelled larva penetrates the wood in an unknown manner, with maternal enzymes playing a role in softening the surface. The tiny borehole will remain the only connection to the outside for the shipworm. Teredo navalis is a protandric hermaphrodite, it starts to develop male gametes six weeks after metamorphosis (Nair & Sraswathy 1971). The transition from male to female is gradual (Sordyl et al. 1998); it can even self-fertilise. After re­production in the female phase it changes back to the male phase and starts another cycle. In warmer climates the shipworm is assumed to develop several cycles per year, however, in the Baltic it is unlikely to have more than one cycle. Fertilisation is internal, the developing larvae are kept for about 8 days until the straight-hinge veliger stage (Calloway & Turner 1988). They are implanted in the tissues of the gill, followed by a "placental reaction" (Lane 1959), whereby the larvae get nourishment from the maternal tissues.

The reproductive rate is immense, up to 2 million larvae are released per cycle under favourable conditions. They are dispersed by currents and are pelagic for two weeks, like other bivalve larvae. The high number of larvae and the resistance to various envi­ronmental factors are necessary to find and utilise the few pieces of tree that are natu­rally transported into the sea. After all, the shipworm genus is about 20 million years old (Turner 1966), and it holds an important niche in the marine environment, where the degradation of wood by bacteria and fungi is very slow. When humankind appeared at the shoreline with dugouts, boats and piers, the adaptations made the shipworm a might­ier enemy to seafaring than all pirates and warships.

3 Teredo navalis in Europe

Teredo navalis is a cryptogenic animal, it is nor entirely clear where it originated. The species was first recorded in 1731 (Sellius 1733), when it destroyed wooden dyke gates in the Netherlands, causing a terrible flood ("Holland in peril"). According to the Wad­densea web page (www.waddensea.org) the Dutch believed it was introduced from Asia and sent as a punishment from God.

However, it is very likely that it was this shipworm that attacked the Spanish Armada, while it was waiting in French and Portuguese harbours for the invasion of England in 1588. It has been proposed to originate in the North Atlantic area (Schütz 1961), its resistance to low temperature supports this theory. Nowadays T navalis is one of the most successful invaders reported from Indonesia, Japan, Australia, Brazil, the Atlantic and Pacific US and Canada. In Europe it is found along the Atlantic and North Sea coast. There were sporadic mass invasions into the Baltic Sea, which lasted for two or three years in the 1930s and 1950s (Schütz 1961). The easternmost border of shipworm

118 K. N. Hoppe

settlement in Germany wasthe Island of Rügen, where a salinity of 9 PSU stopped the establishment of Teredo larvae. After three years the animal died out, presumably be­cause no reproduction was possible in low salinity.

However, the latest invasion in the Baltic is taking place since 1993, and the population appears to have established itself for good. W ooden pilings used in harbours, sea bridges and groynes are mainly attacked, with even large pieces being destroyed in two years. Up to 40,000 larvae m-2 were recorded in one month of exposure (pers. obs.). It is now proven, that T navalis can reproduce in the Baltic at least near Rostock (Sordyl et al. 1998). It was suggested that shipworm larvae conditioned to live in lower salinities, for example in estuaries, were transported in ballast water tanks from the North Sea area to Eastern Germany and established a foothold there (H. Rosenthai, pers. comm.). From Eastern Germany they spread to the West German Baltic, where they were recorded 3 years later. However, there is a conflicting theory of a small population that was always present and triggered by an unknown combination of environrnental factors to mass reproduction and subsequent damage. This is supported by the fact that a survey in 1989 demonstrated the presence of low numbers of shipworm at several places along the Schleswig-Holstein Baltic coast (Schweimanns 1993).

4 Fighting the shipworm

Any wooden ship or structure in the sea has to be protected from marine borers. The earliest strategies ineluded pulling the dugout up on the beach, taking the ship upstream into fresh water for extended periods or charring the bottom of the ship (Nair & Sras­wathy 1971). Chemical defences like paints or copper or lead plating were used by early Chinese, Egyptian and Roman seafarers. Unfortunately, there are no reports whether Viking ships were attacked by Teredo navalis, as this could indicate the origin of the species.

European me die val seafaring nations used tar, which was quite effective as long as it could be reapplied at least every year. When the merchantmen travelled for a long time in tropical waters, where shipworm attack usually is more severe, more ships were sunk by shipworms than by pirates. Another product called creosote, used in apressure im­pregnation process, is probably the most effective shipworm deterrent. However, it is also highly toxic and carcinogenic, so its use is banned in many countries. The modem alternatives are heavy metal salts, containing copper, chromium and arsenic (CCA) or borax (CKB). These salts are often used in high retentions (up to 40 kg m-3) and even though they are supposed to bond firmly with the wood fibres, they are still controver­sially discussed by environmental biologists (Weis & Weis 1996).

Another modem alternative is to wrap wooden pilings with plastics. Polyethylene or polyvinyl wrappers are very widespread in US harbours, but not in Europe. Canadian logging companies used to detonate dynamite charges elose to floating timbers in estu­aries to kill the shipworms with the shock waves (Quayle 1992). This method proved to be quite effective, but is not very popular among German harbour masters (pers. obs.). Protection tests with electric currents or the release of certain poisonous chemical into harbour waters have not been successful (Nair & Sraswathy 1971).

Teredo navalis - the cryptogenic shipworm 119

Some tropical timbers offer a good natural protection, but with the over-exploitation of rain forests by logging companies, the use of these timbers has become restricted in Germany and other countries. Local oak trees also offer some protection, especially when they are used with the bark still intact (that is where most of the deterring tannic acids are), but there are obviously different qualities to oak piling. Some last for 30 years, others only for two (pers. obs.).

The detection of shipworms is very difficult. Only the 1-2 mm large hole is visible, with the siphons only sometimes extracted. The best detection method is by scuba diving and probing for tunnels with a knife. Also the rasping can be detected with a stethoscope. X­rays are possible, but very complicated especially under water. This method is very helpful to record the advance of the shipworm tunnelling in smaller pieces of wood. In conclusion, it is quite amazing that after thousands of years of fighting the shipworm, man has still to come up with a solution for detection, prevention and protection for wo oden installations along our coasts.