Above the water, September seems like any other month in the shipyards of Charleston, Oregon, with yachts and wooden fishing vessels bobbing gently against a backdrop of emerald trees. But beneath the surface, especially beneath the boats and in the hulls, it’s a very special time of year when the wood-eating giant feathered shipworms throw sperm and eggs into open water in the hope that their genes will live in a new generation.
This is commonplace stuff for the spawning of the shipworm, which is not a worm but a highly adapted clam with a long, bare, and eerily smooth body that spends its entire adult life buried in wood. The only part of the shipworm that extends beyond the wood is two siphons that the animal uses to breathe and expel waste.
But on the most unfortunate boats, whose hulls are littered with mussel holes, shipworms go a step further by pulling up clumps of sperm with one of their siphons and introducing these drops into the siphons of other neighboring shipworms. This insemination can even take place at the same time, with one shipworm transforming its sperm into a second shipworm with one of its siphons, while its other siphon receives a drop of sperm from another shipworm neighbor.
“It’s really sophisticated behavior for a clam,” said Reuben Shipway, a research and teaching fellow at the University of Portsmouth, England.
This form of direct fertilization is known as pseudocopulation (copulation is an honor reserved for creatures with sex organs) and was first reported in shipworms in the 1960s. But no one was able to videotape it until Dr. Shipway 2017 recorded a madness of the pseudo-population through giant feathered shipworms with a GoPro, the results of which were reported in Biology Letters in December.
Documenting sex with shipworms with a GoPro is not part of Dr. Shipway, but something he felt was his duty as a scientist when the opportunity arose. At that time, Dr. Shipway postdoctoral fellow in the laboratory of marine biologist Dan Distel at Northeastern University, part of which is safely housed in a former WWII bunker. Dr. Distel, director of Northeastern University’s Ocean Genome Legacy Center and author of the paper, is studying shipworm symbionts, the bacteria in the animal’s gills that allow them to break down cellulose in wood.
Although shipworms can be found all over the world, Dr. Thistle some of its shipworms from the Pacific Northwest, where abundant piles of wood and the relatively warm temperature allow the giant feathered shipworm to grow very large very quickly. In Oregon, clams can grow to be as long as a cone in just nine months, according to Nancy Treneman, a research fellow at the University of Oregon and author of the paper. Ms. Treneman collected the shipworms filmed in the study by dropping pine planks from a dock in Charleston and collecting them nine months later.
Dr. Shipway had stopped at the bunker to routinely clean tanks – shipworms produce heaps of sawdust-like waste – when it discovered the tanks were full of white miasms and erupted from the shipworms in clouds. “It was like milk, literally hard to see what was going on,” said Dr. Shipway. Dr. Thistle added, “It’s really dramatic when they spawn like little white chimneys.”
When Dr. Shipway put the GoPro in the soup of the gametes, he noticed something unusual: the shipworms had devoured their siphons in huge knots. “Just a large snake pit of siphons,” he said. Each shipworm has a siphon that takes in water and a siphon that expels waste. In certain nodes, the excurrent siphons of various shipworms seemed to be wrestling with one another in competition, pulling the incoming siphons away from the groping siphons. The shipworms held up this competitive hand-to-hand combat for nearly three hours. “Tirelessly,” said Dr. Shipway.
“I was amazed to see siphons of the individuals of Bankia setacea” fighting “between them,” said Marcel Velásquez, marine biologist at the National Natural History Museum in France and at the University of Oriente Venezuela, who is not involved in research, said in an email. “This competitive behavior is very unusual in mussels.” Dr. Velasquez observed the pseudocopulation in three other shipworm species directly, but never in competition.
Certain shipworms, unfortunate enough to have buried themselves too far to touch siphons with another shipworm, dumped their eggs and sperm into the water on a proxy, while their more cosmopolitan neighbors pseudocopulated. “They did what they could,” said Dr. Shipway.
The researchers neither knew what gender the pseudocopulating shipworms were, nor did they try to find out. Although shipworm larvae all start out as males, adult shipworms can exhibit simultaneous, sequential, and rhythmically sequential hermaphroditism, which means that it is almost impossible to tell which sex a shipworm is while it is alive and in its timber structure.
“You can be anything anytime,” said Dr. Shipway. The only way to sex a shipworm is to dissect it, but even then, its sex is slippery. For example, if the shipworm you are dissecting has just gone through a pseudocopulation marathon session and has kicked out all of its sperm, it might appear female.
When Dr. Shipway analyzed his footage, there were certain scenes that he played over and over again. In one of them, which he called “The Wipe,” a shipworm botched its attempt to invade another shipworm, leaving its semen on the side of its potential mate’s siphon, which a third shipworm quickly and carefully wiped away. (Dr. Velásquez also rated The Wipe as the most memorable scene.) In another sequence – “The Smack” – a shipworm attempting to pseudocopulate with another shipworm is knocked out by a third shipworm.
In these cases, it is difficult to know whether their movement is intentional or just the accidental result of accidental siphon threshing or some other behavior altogether, even though the behavior of the shipworms looks coordinated. “Was it awkward? Am I turning it anthropomorphic? “Dr. Shipway wondered aloud about The Wipe.
For Ms. Treneman, the questions about perception of shipworms were the most fascinating aspect of this study: whether and how the animal can locate and identify the siphons and sexes of other shipworms. “How can you even notice what the competitor is doing?” Mrs. Treneman said. “You are in the forest.” The answer could include the shipworm’s sensory papillae, fleshy bumps on the animal’s excurrent siphons that straighten up during the pseudocopulation, said Dr. Thistle.
In the future, Dr. Shipway to secure funding for a more formal study of the pseudo-population with a discrete number of shipworms in the tank, to see how they interact, and perhaps to see if The Wipe is a brilliant strategy or a shaky coincidence. He believes that revealing the secrets of shipworm reproduction will help scientists understand how wood is recycled in the oceans, as mollusks play a crucial role in the global carbon cycle.
Back in Oregon, Ms. Treneman plans to collect more of her shipworm tablets this week and take them back to her lab to try to see the action for herself. She’s looking for The Wipe.
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