Evolution has been very creative when it comes to mimicry, but the most incredible in my opinion and therefore at number one in the scoundrel chart, are the freshwater pearly mussels (Unionida). These unassuming molluscs spend their adult lives embedded in the sediment and gravel of streams and rivers doing very little except filtering edible muck from the water and breeding. However, indolence aside, the mimicry exhibited by these mussels is surely one of the most bizarre examples of this phenomenon and one that evolved as a means of dispersal rather than as a way of catching prey or bamboozling predators.
In many respects, freshwater pearly mussels look like any other bivalve mollusc, but what sets some of them apart (notably Lampsilis spp.) is the unusual extension of their fleshy mantle that grows beyond the confines of the protective shell valves to wave around in the water. This fleshy protuberance can look astoundingly like a small fish and this is no coincidence because this fishy appendage is actually a lure to attract fish so they can be press-ganged into the mussel’s reproductive strategy. The lure is very convincing. Not only does it have markings that suggest eyes and skin patterning, but it is even moved by the mussel in a fish-like way. These details are more than enough to grab the attention of a real fish that mistakes the lure for a snack. The fish edges closer and makes a lunge for the fake prey nipping the membrane of a specialised brood gill the lure is concealing. This releases the mollusc’s larvae, nasty-looking miniature versions of the adult, known as glochidia. These larvae are parasitic and they get drawn under the fish’s gill plates where they latch onto the blood-rich tissues of the gills using a long adhesive hair and hooks on their tiny shell valves. Attached to their host, the larvae stimulate the development of a cyst which provides them with protection and nourishment for anywhere between 10 and 30 days.
After a couple of weeks spent feeding on the unfortunate fish’s blood and gill tissue, the larvae leave their cysts and sink to the bed of their aquatic home where they begin their sedentary way of life probably a long way from where their mother set her trap. Other freshwater pearly mussels dupe hungry fish into dispersing their young by releasing larvae in mucus encapsulated masses attached to the parent mollusc by a long, mucus tether. These ovisacs have more than just a passing resemblance to a juicy worm or insect larva (see below) and fish can’t resist taking a bite from them, which releases the glochidia within. Still more freshwater pearly mussels have evolved a more brutal way of securing a host for their larvae. When a hungry fish poke it’s head within the partly open shell valves the mussel clamps down, grasping the slippery customer with muscular force and small teeth on the outer edge of its shell valves. The edge of the mussel’s mantle swells to form a tight seal around the trapped fish and in sucking up water to try and ventilate its gills the fish is thoroughly infested with glochidia. After a short period of time the mussel relaxes its grip and the fish dashes off, none the worse for wear apart from its cargo of parasites.
The use of mimicry in these ploys allows a sedentary mollusc to attract a vehicle for the safe dispersal of its young and in my view it is the most incredible example of an animal pretending to be something else to give it an edge in the struggle for survival. For this reason the pocketbook mussels and their relatives top the scoundrel chart.
There’s more about these marvellous molluscs, including lots of photos and videos, on the website of Dr. Chris Barnhart, University of Missouri.