10 Mar Mussel-porn: How mussels fool fish into rearing their young
If you’re ever inclined to doubt the wonders of biology, consider the reproductive policies of freshwater mussels, a subject I learned about Wednesday night when CBU instructor Kellie White spoke to the Cape Breton Naturalists’ Society about freshwater mussels, muskrats, and “what they have to say about each other.”
White has a special interest in the yellow lamp mussel, Lampsilis cariosa, one of 10 freshwater mussels found in Nova Scotia. Until 2014, the Canadian population of Lampsilis cariosa was thought to exist only in the Saint John and Sydney River systems. (It lives along the US East Coast as far south as Georgia.) Its recent discovery in two Cape Breton lakes leads White to suspect it exists in others, most likely including mainland locations. She is considering a citizen-science project this summer to search for more.
The muskrat angle is fairly straightforward. Muskrats love freshwater mussels, which they dig out of lake bottoms and bring ashore to eat—often choosing the same feeding area day after day. This produces “muskrat middens,” little piles of shells that guide naturalists to mussel beds without the bother of hip-waders and underwater searches.
As so often happens in life and literature, it was the mussels’ sex life, nicely captured on video, that captivated Wednesday night’s audience.
Briefly put, when ardour strikes, adult male mussels shower nearby females with sperm, which the females take into their bodies to fertilize their eggs. When those eggs hatch, still inside the adult females, the mothers must contrive a way to transfer them to fish that share the same watercourse, a feat they accomplish with magnificent deception.
The video below shows how the Missouri lampsilis, a cousin of our yellow lamp variety, hornswoggles the large-mouthed bass,* an anadromous species that preys on a much smaller fish called a darter. Lampsilis has developed an elaborate structure at the open end of its shell, which it can manipulate to imitate a darter—right down to its “eyes” and coloration (including a dark stripe running the length of its “body”), and the characteristic flicking of its “tail,” a motion that provokes bass to strike.
When lampsilis detects a nearby bass, it begins flicking its bogus darter tail. The moment the bass strikes, lampsilis sprays it with a cloud of larvae, each no bigger than a pinhead. These infiltrate the fish and clamp onto its gills. There they feed on bass blood for several weeks until, as tiny but fully formed mussels, they drop to the lake floor, where they will grow into adults and repeat the cycle.
The ruse makes for one of the best videos I’ve ever posted on Contrarian:
One other mystery remains. The yellow lamp mussel’s shell is hard and thick, more like a quahog’s than the mussels served at Governour’s Pub. Lamp mussels are hard to open. Yet the muskrat somehow manages this feat without scratching the shell or damaging its adductor muscle, the strong black hinge that connects the two halves of a shell. Figuring out how the muskrat manages this is another potential topic for citizen-science, perhaps for a young biologist with a GoPro.
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* Nova Scotia’s 10 freshwater mussel species parasitize at least 14 different fish into service as temporary hosts for their brooding larvae—two species of perch in the case of the yellow lamp mussel. Derek S. Davis’s 2007 report on the province’s freshwater mussels includes a table on page 6 listing all the unwitting partnerships, along with many other intriguing nuggets.