Background and Introduction
This predatory cladoceran zooplanktor, commonly known as the spiny waterflea (Bythotrephes longimanus)[formerly identified as Bythotrephes cederstroemi], is a crustacean (a relative of crayfish and shrimp). A native of the Ponto-Caspian region of Eastern Europe and Western Asia, Bythotrephes was first found in North America in December 1984 in Lake Huron. Spread through the Great Lakes was rapid, with the species being found in Lake Ontario in September 1985, Lake Erie in October 1985, Lake Michigan in September 1986, and Lake Superior in August 1987.This species is believed to be an international shipping ballast water introduction. Its rapid spread throughout the lakes most likely is the result of currents, inter- and intra-lake ballast transfers, and recreational boating on the lakes.
Biology and Impacts
Bythotrephes is planktivorous, consuming up to 20 prey zooplanktors per day. One major target species of Bythotrephes is Daphnia (another small water flea). Research has shown that a dramatic decrease in Daphnia abundance coincided with the introduction of Bythotrephes in Lake Michigan. Density of a native predatory zooplanktor, Leptodora, also dropped off coincident with the appearance of Bythotrephes, possibly because Bythotrephes was outcompeting it for Daphnia. It has been theorized that declines in the abundance of Daphnia and other Bythotrephes prey may alter the food web in the Great Lakes, reducing the number of young plankton-eating fish which survive their first year. Researchers have observed that chinook salmon, walleye, white bass, alewife, yellow and white perch, emerald and spottail shiner, and lake whitefish consume Bythotrephes. It is not known, however, how nutritional this water flea is for fish, given the amount of its mass made up by exoskeleton and the long tail spine. It is too soon to know the ultimate impact of Bythotrephes on Great Lakes ecosystems. If the water flea is found to be a preferred (and nutritious) food source for perch and other fishes, its impact on fish populations may be beneficial. If predation by Bythotrephes results in reduced populations of preferred prey, such as Daphnia, the water flea may result in negative consequences to native Great Lakes fish populations. Is has been theorized that the decline of alewife in Lakes Ontario, Erie, Huron, and Michigan may be related to the introduction of Bythotrephes.
Bythotrephes longimanus (also Bythotrephes cederstroemi), or the spiny water flea, is a planktonic crustacean less than 15 millimetres (0.6 in) long. It is native to fresh waters of Northern Europe and Asia, but has been accidentally introduced and widely distributed in the Great Lakes area of North America since the 1980s.[1][2] Bythotrephes is typified by a long abdominal spine with several barbs which protect it from predators.
Bythotrephes longimanus is a cladoceran crustacean (water flea) recognizable with its straight tail spine averaging about 70% of its length. Adult individuals have one to three pairs of barbs on the spine, while juveniles have only one pair. The females grow to become larger than the males. The animal has one large eye that is usually black or red. It also possesses a pair of swimming antennae and four pairs of legs, of which the first pair is used to catch prey. Mandibles are used for consumption of prey.
The spiny water flea is similar to another introduced Cladoceran of the same family, the fishhook water flea Cercopagis pengoi, which, however, has a more slender spine featuring a prominent loop-like hook at its end (see figure).[2] There is also variation in the shape of the more robust spine of Bythotrephes itself. Specimens arising from sexually produced eggs have a completely straight and relatively shorter spine. In parthenogenetically produced animals, the spine features a kink in the middle (see figure). Previously, the kinked-spined animals were thought to be a separate species – Bythotrephes cederstroemi.[2] After genetic analysis, it is now considered to be a form of B. longimanus, making Bythotrephes a monotypic genus, (one with only a single species).[3]
The spiny water flea preys on smaller planktonic organisms. Its diet consists mostly of zooplankton, including Daphnia and smaller crustaceans. Bythotrephes can consume 10–20 prey organisms a day. It may also eat other small organisms it comes across.
The spiny water flea is native to northern Europe and Asia, and some parts of central Europe. However, the water flea has spread in recent years to many areas throughout Europe, including some ports and inland lakes outside its natural range.[4]
In 1982, the crustacean was found in Lake Ontario, and soon spread to the other Great Lakes[4] and some inland lakes within the Midwest, including over 60 inland lakes in Ontario.[4] It is thought to have been introduced by untreated ballast water from international ships.[4] Concern has increased to the impact of this invasive species in the Great Lakes region and other areas it might have been accidentally introduced.[4] The spiny water flea, eggs and larvae may be caught up in fishing line, downriggers, fishing nets, and other fishing equipment,[4] which has caused the spread of the water flea to inland lakes and rivers.
Bythotrephes competes with several fish, including panfish and perch, for prey. It has been suggested that very small larval fish are not able to feed on the spiny water flea itself because of the barbs the flea possesses, but it is readily consumed by several fish species once the fish are larger. The spiny water flea's diet consists mostly of Daphnia zooplankton, leading to competition with small and baby fish, and also with native water flea species. Daphnia zooplankton populations have declined in recent years, though there is no conclusive evidence as to the cause.
The spiny water flea is causing serious concerns about the lakes of Canada, with the problem being that it feeds on zooplankton and can actually eliminate zooplankton species. As zooplankton is the backbone of aquatic food chains, this tiny crustacean presents a serious risk to the ecosystem. The eggs survive even after being dried out or eaten by fish.[5]
Invasion by the spiny water flea has also correlated with ecological changes in the Great Lakes. Since the species' introduction to the Great Lakes in the mid-1980s, there has been a decrease in species richness, as well as decreases in the late summer densities of several other Cladocera species, including Daphnia pulicaria and Daphnia retrocurva.[6]
Bythotrephes longimanus (also Bythotrephes cederstroemi), or the spiny water flea, is a planktonic crustacean less than 15 millimetres (0.6 in) long. It is native to fresh waters of Northern Europe and Asia, but has been accidentally introduced and widely distributed in the Great Lakes area of North America since the 1980s. Bythotrephes is typified by a long abdominal spine with several barbs which protect it from predators.
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