Amphipod (G. tigrinus) (Gammarus tigrinus)

Common Names: tiger scud, scud

Description:

Gammarus tigrinus (Gammarus tigrinus) is a tiny, shrimp-like freshwater crustacean ? an amphipod, or "scud" ? native to the Atlantic coast of North America [1][2]. It is not native to the Great Lakes: it was first found there in Saginaw Bay, Lake Huron, around 2001?2002, most likely carried in the ballast water of ships, and it has since spread through all five Great Lakes and into the upper Mississippi and Ohio rivers [1]. MISIN tracks it because it breeds explosively, tolerates poor water, and can crowd out and even prey on other amphipods that fish and wildlife depend on for food [1][2].

Identification:

This is a very small animal ? in Lake Huron males average about 10.5 mm (under ? inch) and females about 7.6 mm ? so it is seldom noticed without a close look [1]. Like other scuds, it has a shrimp-like body that is flattened from side to side, with a short snout and large, kidney-shaped dark eyes [2]. Mature males have a noticeably long, curly second antenna and tufts of bristles on their legs [1]. It lives in shallow water on silty sand, among stringy Cladophora algae, and in beds of underwater plants, and it can be found year-round, most easily when that shallow substrate is sampled [1]. The hard part is that it looks almost exactly like our native scuds; telling it apart for certain takes a microscope and an expert [1][2].

At-a-glance cues:

1. Tiny, shrimp-like crustacean (scud), only about ? to ? inch long, flattened side-to-side [1]
2. Short snout and large, kidney-shaped dark eyes; mature males have a long, curly second antenna [1][2]
3. Lives in shallow water on silty sand, in algae mats, and in underwater plant beds [1]
4. Cannot be reliably told from native scuds by eye ? confirming it needs a microscope and an expert [1][2]

Habitat:

This amphipod lives in shallow, sheltered water ? silty-sand bottoms, mats of stringy algae, brackish marshes, and beds of underwater plants ? and in the Great Lakes can be found down to about 65 feet (20 m) deep [1]. It is unusually tough: it tolerates a wide range of saltiness, from fresh water to seawater strength, handles a broad range of acidity, and does well in warm, nutrient-rich, and low-oxygen water that stresses many other species [1]. It dies off only in very warm water, around 90?93 ?F (32?34 ?C) [1]. It is native to the Atlantic coast of North America, from the Gulf of St. Lawrence to Florida [2]. In the Great Lakes, the upper Mississippi and Ohio rivers, and across much of Europe it is an introduced invader, not a native [1][2].

Reproduction:

Because it lives underwater, this scud spreads through water and the things that move between water bodies ? not across land [1]. It most likely first reached the Great Lakes in ships' ballast water, and it can also travel on boat hulls, in bilge water, live wells, and bait buckets, and through canals that connect waterways [1][2]. For anyone who boats or fishes, that means it can ride a trailered boat or a bucket of leftover bait from one lake to the next. Cleaning, draining, and drying boats and gear between waters is the best way to slow it down.

It breeds fast. A female carries roughly 30 eggs on average ? more in larger animals ? and in warm water the population can turn over many times in a single year, so numbers build very quickly once it arrives [1][2]. There is no practical "remove before it sets seed" step for a microscopic water animal; the realistic lever is keeping it from reaching new waters in the first place [1].

Impact and Damage:

This amphipod poses no danger to people, pets, or property ? it is a tiny water creature that does not bite, sting, or damage anything you own [1]. Its impact is ecological, at the base of the food web. It is an aggressive omnivore that eats algae, plants, detritus, and small animals, including the young of other amphipods and even its own [1]. In Great Lakes wetlands it has already become the second most abundant amphipod, after the native scud Gammarus pseudolimnaeus [1]. Its potential is clearest from Europe, where it was also introduced: there it has wiped out native amphipods in parts of the Rhine River and the Baltic Sea, often by out-competing and out-eating them, and in one Polish lagoon it became dominant while two native amphipods died out entirely [1][2]. Because amphipods are a key food for fish and waterfowl and a major grazer of algae, swapping native species for this one can ripple through the food web and, in some European waters, has led to more algae as the plant-eating natives disappeared [2].

Similar species:

Several small "scuds" share the same Great Lakes waters and look so alike that none can be told from this species by eye ? reliable identification needs a microscope and an expert.

Scud (Gammarus pseudolimnaeus) is a common native amphipod and the one this species most often turns up alongside in Great Lakes wetlands. The two are separated only by fine features of the antennae and legs under magnification [1].

Scud (Gammarus fasciatus) is another native amphipod of the same lakes and rivers, similar in size and shape, and likewise distinguished only by close microscopic study [1].

Amphipod (Echinogammarus ischnus) is a different non-native scud, from the Black and Caspian Sea region, that lives in the same Great Lakes habitats. Like this species it is a tiny invader best confirmed in a lab, not by sight [1].

Scud (Hyalella azteca) is a widespread native amphipod in the same waters. It tends to be smaller and is a different genus, but in the field it looks like the same tiny shrimp-like creature and needs lab examination to separate [1].

Monitoring and rapid response:

Signs to watch for:

1. Tiny, shrimp-like scuds in shallow water on silty sand, in algae mats, or in underwater plant beds [1]
2. Most likely to be noticed when scientists or volunteers sample shallow bottom habitat [1]
3. Remember: this species cannot be confirmed by sight ? a sample must be examined in a lab [1][2]

What you can do:

1. This is an underwater animal you cannot hand-remove like a weed ? the most useful action is stopping its spread between waters [1]
2. Clean, drain, and dry boats, trailers, waders, nets, buckets, and live wells before moving from one water body to another [1][2]
3. Never dump bait buckets, live wells, or aquarium water into a lake, river, or storm drain [1]
4. Lake associations and natural area crews can include this amphipod in routine bottom-life (benthic) monitoring, sampling shallow silty and algae-covered substrate, and prioritize preventing ballast and bilge transfer between waters [1][2]

When to call a professional:

1. Confirming the species and assessing a population require an aquatic biologist or lab ? collect a sample rather than relying on a field guess [1][2]
2. Lake- or river-wide infestations are handled by state natural resources agencies and aquatic invasive species programs [1]
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. U.S. Geological Survey, Nonindigenous Aquatic Species Database. Gammarus tigrinus (tiger scud) ? Species Profile. nas.er.usgs.gov
2. Smithsonian Environmental Research Center, NEMESIS database. Gammarus tigrinus. invasions.si.edu

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