Mussels That Lure Fish.

I recently participated in an OLLI¹ class, Aquatic Biodiversity in the Southeast, presented by Duncan Elkins, a Professor in UGA’s Warnell School and the River Basin Center. The purpose of the class was to introduce us to the diversity of the fish, fresh water mussels, crayfish and salamanders in the southeast. Professor Elkins did a wonderful job and had outstanding slides for his presentation.

Fresh water Mussels

I was least familiar with the fresh water mussels and found their life cycles fascinating. North America has more species of freshwater mussels than the rest of the world. But the North American mussel fauna is highly endangered. A greater proportion of NA mussels are endangered than any other well known group of animals. The southeastern United States is a biodiversity hotspot for fresh water mussels, in spite of the fact that many have become extinct.

Mussel natural history

Mussels spend their lives buried in the sediments at the bottom of lakes, rivers, ponds and streams. They feed by filtering microorganisms and dissolved organic matter from the water. Like plants, they live where they grow up, and face the same problem that plants have: how to disperse their offspring. To understand freshwater mussel reproduction and dispersal you need to know a little about their anatomy.

Mussel anatomy, shells & mantle

A mussel or clam has two shells that hinged together along one side. The living part of the animal is in between the left- and right-hand shells. Everything inside is enveloped by a sheet of tissue called the mantle. The mantle actually secretes the shells and is firmly attached to the inner side of both shells. If a foreign object, like a grain of sand, is placed between the mantle and shell, a pearl will be produced as the mantle secretes shiny shell material around the object. As the mussel grows the mantle adds new material to the shells, making them thicker and larger.

Mussel anatomy, inside the mantle

Between the two mantle sides you find the rest of the animal: the body, containing internal organs, a muscular “foot” that can extend outward, through an opening in the mantle, and a pair of gills on each side of the foot-body. The innermost of each gill pair is specialized to be a nursery chamber. It holds the developing embryos in the female mussel.

Mussel anatomy, water flow & gills

 The mantle has two more openings to allow water to circulate through the mantle chamber and over the gills. Water is circulated by ciliated cells on the interior surface of the mantle. By their synchronized beating the water enters the first mantle opening (called the incurrent siphon), flows across the gills, and exits the mantle chamber through the second mantle opening (the excurrent siphon)

Mussel anatomy, filter feeding

Mussels are filter feeders. The water coming into the mantle cavity carries microorganisms (algae, protozoans, bacteria) and dissolved organic matter. These are trapped in mucus secreted by the gills and the mucus is carried toward the mouth, where it is swallowed and digested. Feces are eliminated at the other end of the digestive tract where they are carried away by the flow of water out of the excurrent siphon.

Mussel reproduction

The sexes are mostly separate in mussels. In the spring males broadcast sperm through the excurrent siphon. Females in the neighborhood pull the sperm in through their incurrent siphon. The sperm make their way into the nursery gill that contains the mature eggs. Fertilization takes place inside the gill chamber and the fertilized eggs grow into tiny clam-shaped larvae. But these are not just baby mussels. They are parasites called glochidia, and each one will need to find a fish to complete their development. In order to become a free-living mussel the glochidia must spend several weeks feeding on the tissue of a fish.

Here is where the story gets weird. When a gravid mussel is approached by a fish the mussel expels a batch of glochidia from her nursery gills. Those glochidia either grab hold of the fish’s fins or they are accidentally swept into the fish’s mouth where they attach to the gills. Some species of glochidia have tooth-like structures; they look like tiny vampire clams.

Those glochidia that were able to attach become encapsulated and feed on fish flesh for a week or more. They grow into tiny mussels that leave their feeding capsule and drop to the sediments where they dig in and start filter-feeding.

Attracting host fish with a lure

Bizarre as this life cycle is, the way some mussels attract suitable fish is even more bizarre. In some mussels the edges of the mantle are developed into flaps that resemble small fish. The flaps even have dark spots at one end that resemble the eye of a minnow. And, making the illusion more remarkable, they wiggle the mantle edge so it looks like an injured minnow. A fish that is attracted to the motion strikes at the “minnow” and is greeted with a mouthful of glochidia.

You can see all these features in this outstanding video.

The evolutionary biologist Stephen Jay Gould devoted an essay in one of his early books² to the question of how such a complex anatomical feature and correlated behavior could have evolved. Gould suggested that the early predecessors lacked the elaboration of the mantle edge, but still attracted fish because the mantle openings (the incurrent and excurrent siphons) open and close in an alternating, coordinated rhythm. (This can be seen in the video linked above.) First the incurrent siphon opens and then, as it is closing, the excurrent siphon begins to open. This staggered opening and closing creates the illusion of a moving darker area, that, Gould proposed, could attract a fish. You see a similar optical illusion when your computer shows you a series apparently moving dots when it is doing something that takes a little time. The dots don’t really move, they are just turned on and off in sequence to give the appearance of movement. Then change in the fringes of the mantle that enhanced the attraction to fish would be selected for. And, finally, the decorative touches of spots would further enhance the lure. (Note that this is really a plausibility argument, not a proof. It’s an hypothesis, not a theory.)

Footnotes:

1 OLLI is the acronym for Osher Livelong Learning Institute at UGA. Its mission is: “to meet the intellectual, social, and cultural needs of individuals aged 50+ in Athens through lifelong learning.” For more information visit OLLI’s website.

2 Stephen Jay Gould, 1977. The Problem of Perfection. Natural History 86(1):32-35. reprinted as: The Problem of Perfection, or How Can a Clam Mount a Fish on its Rear End, in Ever Since Darwin, 1977, New York, W. W. Norton, pp. 103-110.