The Great Lakes contain small, freshwater invertebrates commonly referred to as shrimp, though they are not the large decapod crustaceans typically associated with seafood. These organisms belong to the order Mysida and are often described as “shrimp-like.” The lakes host both a long-standing native species and a more recently arrived invasive species. Both play significant roles within the complex aquatic environment of the Great Lakes ecosystem.
The Native Crustacean: Opossum Shrimp
The native mysid of the Great Lakes is the Opossum Shrimp, a clear crustacean that can grow up to 25 millimeters, making it the largest zooplankton in the lakes. Its translucent body features long antennae, prominent stalked eyes, and a deeply forked tail.
The common name comes from a specialized structure on the female’s underside called a marsupium, or brood pouch. Females carry their developing young within this pouch for extended periods, a reproductive strategy shared with marsupials. This species is adapted to the deep, cold, and dark conditions of the Great Lakes’ profundal zone.
The Opossum Shrimp exhibits diel vertical migration (DVM), a daily journey between the lake bottom and the water column. During daylight, they remain near the lake floor to avoid visual predators that rely on light to hunt. Under the cover of darkness, they migrate upward to feed on suspended matter and smaller organisms.
This migration pattern links the deep, less-productive benthic zone with the more active upper waters. This native species is a “glacial relict,” meaning it is a survivor from the last ice age, thriving in the deep, oxygen-rich waters of the Great Lakes.
The Invasive Arrival: Bloody Red Shrimp
The invasive mysid known as the Bloody Red Shrimp arrived from the Ponto-Caspian region of Europe and Asia. First reported in the Great Lakes basin in 2006, it was likely introduced through the discharge of ballast water from commercial vessels.
The Bloody Red Shrimp is smaller than its native counterpart, typically measuring less than 1/2 inch long. It is distinguished by its reddish-orange hue and the presence of two prominent spines on its flat-ended tail. The reddish pigment is due to chromatophores, which can expand and contract to change the color’s intensity.
Unlike the native Opossum Shrimp, this invasive species prefers warmer, shallower, nearshore habitats, often congregating around structures like docks, piers, and breakwalls. They form dense, reddish swarms in the shadows of these structures during the day to avoid direct sunlight.
The species has a shorter lifespan but a much faster reproductive cycle than the native mysid, reproducing multiple times a year. This high reproductive rate and its preference for nearshore areas have allowed it to spread rapidly across the lower Great Lakes since its initial discovery.
Function in the Ecosystem: Food Web Dynamics
Both the native Opossum Shrimp and the invasive Bloody Red Shrimp serve as a fundamental trophic link, connecting the base of the food web to higher-level consumers. As omnivores, they consume a variety of food sources, including organic detritus, algae, and zooplankton, transferring energy up the food chain.
The Opossum Shrimp is an important food source for fish species, such as lake trout, lake whitefish, and deepwater salmon. These fish rely on the large, energy-rich mysids, particularly in the deep, cold areas of the lakes. Their nightly migration makes them available prey for fish that ascend to feed in the water column.
The introduction of the invasive species has created new dynamics in the nearshore environment. By consuming large amounts of native zooplankton and competing with the larvae of native fish species for food, the Bloody Red Shrimp can alter the composition of the nearshore food web. Its presence adds a new layer of competition and energy flow in the shallower waters, affecting the growth and survival of young fish.