Species of the family Teredinidae are often referred to as "shipworms". They're not worms, but actually molluscan bivalves--clams. What gave them their nickname is their ability to eat wood--and any ships made of it! Wood is made of tough stuff, like the molecules cellulose and lignin, which most animals cannot digest and obtain nutrition from. But certain species, such as cows, termites, and shipworms thrive off of wood. How? They host bacteria in their bodies that do the digesting for them! This is a symbiotic relationship of mutualism much like what I shared in the "Dinoflagellates" post: both species benefit, as the shipworm can now eat wood while most other animals cannot, and the bacteria in its body gets free room and board.
Shipworm photo by Deplewsk, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0 via Wikimedia Commons. This shipworm was eating a mangrove plant; set by water bottle for scale.
Records of shipworms date as far back as Ancient Greece. In 350 B.C., Theophrastus, an early plant biologist, wrote about shipworms damaging the country's vessels. Christopher Columbus complained in his fourth and last voyage to the West Indies that his fleet was sinking, "riddled with holes as a honeycomb", simply from the damage that shipworms had caused. In 1503, his ships were finally wrecked by the shipworms where he remained stranded with his fleet on the coast of Jamaica for a disastrous year before rescue by Spanish ships. In 1780, the British navy won a major battle against the Spanish navy. What made the crucial difference? The English had ingeniously lined their ships with a copper sheath to keep away shipworms. The Spanish had not had the idea, and so their ships were sinking from shipworm damage in that very battle! Even today, if wooden pilings and docks are not regularly covered with deterrent chemicals, shipworms will ruin those too.
Shipworms are experts in their trade, carving wood with their shells before consuming it and able to survive for weeks without air or water while they burrow. They are also great team players that sense and avoid crossing the paths of other burrowing shipworms. Their siphon, a structure common among other clams, ejects waste, and their double shell can close up when they are disturbed or when the conditions of their environment are poor.
Shipworm-formed holes in driftwood, photo by Michael C. Rygel, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0 via Wikimedia Commons
Like many organisms that attach to floating debris, shipworms can travel very long distances--for example, in the Smithsonian magazine post shared below, one researcher found shipworms native to Japan instead attached to driftwood on western American beaches!
Not all shipworms eat wood. One species of shipworm, Lithoredo abatanica, bores through and digests limestone! Then there are the giant shipworms, which thankfully also do not eat wood. Instead, they host thioautotrophic bacteria--distinguished from photoautotrophs which include plants and algae, these bacteria get energy from hydrogen sulfide instead of sunlight. These kinds of bacteria are common in hydrothermal vents and some hot springs such as in Yellowstone, and hydrogen sulfide is the chemical in those locations that gives off a "rotten eggs" smell. These bacteria, like plants, are highly self-sustaining because of their autotrophy. By forming a symbiotic relationship with these bacteria, giant shipworms became highly isolated and sedentary (nonmoving) and were able to grow very large, hence their name.
The seeming ability of shipworms to float long distances and evolve from wood-eating to hydrogen sulfide absorbing has resurfaced an old idea by the scientist Daniel Distel called the wooden-steps hypothesis. In this hypothesis, it is suggested that the large mussels that live around deep-sea hydrothermal vents originated from smaller mussels that lived on sunken wood. Here's a basic outline of how it would work over a long period of time and many generations of shipworms:
Symbiotic bacteria that live in the shipworms' bodies digest wood and exhale hydrogen sulfide as a waste product (something created but useless or harmful to the organism).
Bacteria that use hydrogen sulfide (thioautotrophs) enter the gills of shipworms to harvest the hydrogen sulfide which the original bacterial symbionts (symbiotic organisms) exhaled.
The shipworms can obtain plentiful energy from the thioautotrophs, and change lifestyles to favor them rather than the wood-digesting bacteria. The wood-digesting symbionts become few to none in the shipworms' bodies.
When the shipworms change lifestyle, their body morphology (physical characteristics) changes too: their mouths are blocked off by an increased shell; their mouth-like region, digestive organs, and the muscles they used to bore into wood all shrivel. Their size also increases immensely, and their thioautotroph-hosting gills take over nearly the entire exterior of the shipworms' bodies.
Now, the newly-formed giant shipworms can thrive in the harsh environments where hydrogen sulfide is plentiful.
Many bivalve mollusks, organisms similar to the giant shipworm, can live equally well by digesting wood or near hydrothermal vents. Matching this fact with Distel's wooden-steps hypothesis allows us to visualize these bivalve molluscs as having mastered long-distance traveler both on the top and the bottom of the sea!
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