The Down and Dirty on a Most Unusual Bivalve

Close-up image of small blue-ish, purple-ish tubes underwater

Xylophaga wood sample

This is the first in a series of posts by Associate Curator of Invertebrate Zoology Janet R. Voight as she heads out on an expedition to Norway. There, she and colleagues will look for a wood-boring clam, Xylophaga dorsalis, to study its, well, poo. Stay tuned for upcoming posts to find out what they discover.

One of the first things they teach you in Invertebrate Zoology (the study of animals that do not have a vertebral column or backbone) is that all animals have to overcome two identical problems: to get oxygen into their bodies and to get their waste products, especially their feces (poop, waste matter, manure, etc.), as far away from them as possible.  

xylophaga.jpg

A white clam with a tube protruding from the shell, photographed on a back background. Arrows point to different elements.
Xylophaga dorsalis. The arrows indicate the direction of water (and fecal) flow from the inhaling siphon (i) inside the shells where the gills are, and out, following the arrow by the exhaling siphon. 

However, I know one animal, a bivalve mollusk, that violates this basic animal need: it saves its poop, keeping it next to itself throughout its life. This animal, Xylophaga dorsalis, makes its living by boring into and eating wood. It starts its life as a tiny swimming larva that finds a piece of sunken wood and settles on it. Then, it undergoes a transformation: its tiny shell grows teeth that it will use to scrape water-soaked wood. By scraping the wood, it digs itself a hole where it will spend the rest of its life. The tiny clam eats the wood shavings and is able to digest them because it has bacteria that live in its gills. (As far as we know, no animal can actually digest wood without help from a microbe.)  

Eating leads to feces. To understand how clams release their feces, you have to understand how they breathe. Although sophisticated mammals like us breathe in and out through the same set of pipes, bivalves have two sets that we call paired siphons. One is devoted to bringing in oxygen-rich water, the other to exhaling low-oxygen water. When a bivalve has to go, it releases feces into the stream of water leaving the body—which usually carries them away.  

Xylophaga dorsalis has this basic anatomy, but their siphon for exhaling is so short that it ends inside the hole the clam lives in. Feces pile up in the hole and become packed against the animal’s siphons. The older feces get packed down to form a “chimney” of clam poop, and the siphon extends through a hole in the middle. Our question is: Why? Other, related wood-boring and -eating bivalves have normal, equal-length siphons and behave as the books say they should; that is, they deposit their feces away from their body, maintaining the minimum recommended level of hygiene.

To answer these questions, I am traveling with colleagues Reuben Shipway from Northeastern University and Ken Halanych of Auburn University to Norway. There, X. dorsalis lives in relatively shallow water. This species occurs between two and 300 meters deep, although it’s been reported as deep as 2500 meters (over 8200 feet!). Although typically found in deep water, being in the far north means we should be able to collect these animals in the wealth of sunken wood from deep fjords near the marine station. If and when we find the bivalves, they, in their wood homes, will go into aquaria in the lab and we will see what we can learn about their feces.

We hypothesize that the feces culture bacteria that either help digest the wood or provide the bivalve with nutrients that the animal absorbs through the flesh of its siphon. Or, maybe the feces moderate the environment somehow, to help the animal thrive.

Read on in Part 2: Arctic Ice and Wood: What’s the Connection?

Funding for this project was provided by the Robert A. Pritzker Center for Meteoritics and Polar Studies established by a grant from the Tawani Foundation.