Ah yes, fishing. Lounging by the water waiting for a bite is a relaxing, and occasionally frustrating, pastime enjoyed by millions worldwide. Whether it’s for sport or sustenance, many have become enamored with the activity, dedicating time, money, and effort in pursuit of a “trophy catch.” Yet none match the dedication to this craft as one particular family. No, it’s not your cousins with the massive bass mounted above their fireplace. It is a family of clams.
This family, Unionidae, is composed of freshwater bivalves, meaning they possess a hinged shell that protects their slimy, gooey body from predators. They are part of the phylum Mollusca, which is composed of incredible animals such as snails, octopuses, cuddle fish, and slugs. Due to their shell’s elongated shapes, unionids are not technically clams but instead mussels. Unionids are distributed worldwide in freshwater lakes and rivers, with over 300 species in North America alone.
Unionids spend their adult life half-buried in sediment or slowly moving along the bottom via a muscular, tongue-like projection called a foot. Much like a straw, they use a siphon to suck in water and feed on debris and other yummy snacks floating through the water. They can live up to 10 years, and for the most part, unionids simply sit, eat, and be, well, as happy as a clam.
But much like your cousin’s favorite t-shirt says, unionids too would rather be fishing. After all, it’s essential to their reproduction and completing their lifecycle. If they cannot successfully catch a fish, their species will die. As a result, unionids have become quite good at their craft. Here’s a rundown on fishing like a clam:
Step 1: Be a female
Sorry guys, but for clams fishing is a strictly female sport. Unlike many mollusks, unionids are dioecious, meaning individuals possess either male or female parts (many mollusks, such as freshwater snails, possess both male and female organs). Female unionids produce eggs and hold them until they hatch into larval mussels, called glochidia (singular glochidium). These precious babies will soon become the bait.
Step 2: Select your bait
Once the eggs hatch into glochidia, they are put into a specialized flap attached to their mantle. The mantle, which also functions as the straw-like siphon, can vary widely in shape. Depending on the species, unionid mantle flaps can convincingly resemble an array of different creatures. Species of the genus Lampsilis have flaps disguised as a small fish, complete with fins, stripes, and eyespots. Other species mimic worms, aquatic insects, or shockingly realistic crayfish (the latter being the bait-of-choice for Villosa iris, or the rainbow mussel). Craftier unionids package their glochidia into a bait-shaped conglutinate and use a mucus “fishing line” to jig their bait.
Step 3: Attract a fish
This is when the game begins. Unionids face a unique challenge when fishing; they do not possess eyes, ears, or even a head. Despite this, unionids wiggle their lures with the skill of a seasoned fisherman, mimicking the behavior of a baitfish, crayfish, or worm. Often unionids are attempting to attract bass, saugers, drums, or other gamefish, although some unionids go after sculpins and smaller fish species. The target fish can vary between species, with some unionids being very particular about which fish they attract. Unionids continue to patiently sit and wriggle their lures until a curious fish takes a bite.
Step 4: Get a bite
Once a hungry fish takes a bite, the unionid’s mantle flap or conglutinate erupts into a cloud of glochidia. The unionid’s goal is to get as many glochidia into the gullible fish’s mouth as possible. Some unionids, such as the snuffbox mussel, will even close their shell around the fish’s face to ensure the fish gets a face-full of their larval young. This is an understandably unpleasant experience for the fish.
Step 5: Release the fish (and suck its blood?!?)
Once the glochidia are released, the fish is free to swim off. However, the fish is not without company. The released glochidia’s shells are sharp and resemble miniature bear-traps. Once in the fish’s mouth, the glochidia attach themselves to the fish’s gills and begin to drink the fish’s blood. This highly nutritious meal allows the glochidia to grow and develop, eventually falling off and settling to the bottom as a filter-feeding adult. From there, they continue the cycle, eventually becoming fishermen, or fisherclams, themselves.
Although glochidia suck the blood of fish, it is rarely a lethal affair. Unionids rely on fish to survive, so ensuring that there’s enough fish around for them to catch is crucial for the next unionid generation. For the most part, unionids in fact benefit fish species, although indirectly. Unionids modify the river bottom, or lake bottom, habitat by pulling out nutrients suspended in the water and making it accessible to water plants. The plants then provide food and shelter to baitfish and insects, which are then eaten by bigger fish.
Despite their stereotypically upbeat demeanor, unionids are suffering globally. Pollution, dams, invasive species, and warming water have all significantly impacted their populations. It is estimated that 45% of unionids are threatened with extinction and at least 25 North American species have already gone extinct. To help these shelled friends, you can support local water conservation groups, assist in data collection for unionid researchers, and share information with others about unionids. The more people that know about the issue, the more likely something will be done.
