Whatever Sinks Your Boat: Pests as a Conservation Tool

Close-up of the apical end of a shipworm taken from the He'eia mangroves. Shipworms are not actually worms but bivalves (this picture shows one of the shells, the other is obscured by tissue).

Teredo worms (or shipworms), which are actually bivalves of the family Teredinidae, are legendary in their appetites for ship hulls, wood pilings, or any other wood found in the ocean. Like a clam or any other bivalve, they have two sharp shells on one end, but their long, soft body makes them look more like a worm. These shells make excellent tools for carving burrows in wood, which shipworms line with a calcareous shell. This can make them look like tube-dwelling worms at a glance. The calcareous shell protects the worm from the unstable environment of the wood-- much like humans build tunneling shields when tunneling in unstable substrates. The width of an individual's shell tunnel depends on species, but they can vary within species depending on the degree of crowding in a single piece of wood (Cragg et al. 2009). Members of this family are the primary cause of the characteristic round holes we see in driftwood. Though boat hulls are now usually made with metal or fiberglass, wood hulls used to be a frequent victim of shipworm infestations. The U.S. invests millions of dollars every year in protecting wooden structures from shipworm damage.

There are a few more sides to this voracious group of organisms, however:

1. They are delicious. In parts of Southeast Asia, they are found in abundance in mangrove forests, where humans harvest them for food.
2. They are extremely efficient at recycling decaying wood material and releasing carbon and nitrogen from the mangrove into the surrounding ecosystem. Like termites do on land, they eat wood pulp and digest the cellulose with the help of symbiotic bacteria. This is no trivial task. The tannins that normally protect mangrove from being eaten by herbivores do not deter wood-boring organisms like these, and even healthy mangrove can be damaged by fungi that take refuge in the calcareous tubes (Kohlmeyer 1969). 22-50% of the carbon produced by Rhizophora sp. is stored in woody parts and trunks (as opposed to leaf litter) (Robertson & Daniel 1989). In Rhizophora sp. forests in Australia, 50% of trunk mass decayed after 8 years, and by 4 years after deforestation, trunks were colonized by Teredinids.
3. They are in He'eia Fishpond. LAIP interns discovered high densities of boring bivalves during a POH workday when our task was to dig out mangrove stumps. The patch we dug in was cut down in 2007, and stumps contained live worms and calcareous tubes.

A shipworm and many calcareous tubes found in a mangrove removal area in He'eia Fishpond. The trunk on the left is hollowed out (the spongy interior has been mostly decomposed already), and the periphery bristles with the calcareous tubes of shipworms.

So can a pest be our best hope for returning this system to pre-invasion conditions? How long will it take them, and when they liberate carbon and nitrogen from the mangrove trunks, are there any organisms in the brackish, anoxic mud that can use it? Hawai'i lacks many of the other important species evolved to break down this tough material, but these worms are crucial nutrient cyclers for decomposing mangrove. If we don't have to count exclusively on bacteria to do the job, we may be looking at a faster recovery to pre-invasion conditions.

Cragg, S., Jumel, M., Al-Horani, F., & Hendy, I. (2009). The life history characteristics of the wood-boring bivalve Teredo bartschi are suited to the elevated salinity, oligotrophic circulation in the Gulf of Aqaba, Red Sea Journal of Experimental Marine Biology and Ecology, 375 (1-2), 99-105 DOI: 10.1016/j.jembe.2009.05.014

Kohlmeyer, J. (1969). Ecological notes on fungi in Mangrove forests Transactions of the British Mycological Society, 53 (2) DOI: 10.1016/S0007-1536(69)80058-6

ROBERTSON, A., & DANIEL, P. (1989). Decomposition and the annual flux of detritus from fallen timber in tropical mangrove forests Limnology and Oceanography, 34 (3), 640-646 DOI: 10.4319/lo.1989.34.3.0640