Amphipod (E. ischnus) (Chaetogammarus ischnus)

Synonyms: Gammarus ischnus, Echinogammarus ischnus

Common Names: scud, Ponto-Caspian amphipod

Description:

Amphipod (Chaetogammarus ischnus) is a tiny, shrimp-like freshwater crustacean ? an amphipod, often called a "scud" ? native to the Black and Caspian Sea region of eastern Europe and western Asia [1][2]. It reached the Great Lakes around 1994?1995, most likely in the ballast water of ocean ships, and has since spread through the lower lakes and connected rivers [1][2]. MISIN tracks it because it has largely replaced a native amphipod that fish and other wildlife rely on for food [2].

Identification:

This is a very small animal ? males are about ? to ? inch long (6?15 mm) and females a bit smaller (5?13 mm) ? so it is rarely noticed without looking closely [1]. Like other scuds, it has a shrimp-like body that is flattened from side to side, with no spines along its back [2]. It lives among rocks and cobble and especially loves the clumps of zebra and quagga mussel shells that now coat many lake bottoms, where it can be found year-round and is easiest to detect when sampling that mussel-encrusted substrate [2]. The catch is that it looks almost exactly like our native scuds. Telling it apart reliably takes a microscope and an expert, who look at fine features of its tail fan and antennae [1][2].

At-a-glance cues:

1. Tiny, shrimp-like crustacean (scud), under about ? inch (15 mm) long, flattened side-to-side [1]
2. Lives among rocks and especially in clumps of zebra and quagga mussel shells [2]
3. Body has no spines or teeth along the back; males have bushy, bristled second antennae [2]
4. Cannot be told from native scuds by eye ? confirming it needs a microscope and an expert [1][2]

Habitat:

This amphipod lives on the bottom of lakes and large rivers, favoring rocky and cobble shores [1][2]. It is most abundant in clumps of zebra and quagga mussel shells and in mats of stringy algae, which give it food and shelter from fish [2]. It tolerates a wide range of saltiness, from fresh water up to brackish water, which helped it travel and settle in new places [1][2]. It is native to the Black and Caspian Sea basins and the large rivers that feed them [1]. In North America it now lives through the lower Great Lakes and the St. Lawrence River, and has reached the upper Mississippi and lower Ohio rivers and several lakes in interior New York [1].

Reproduction:

Because it lives underwater, this amphipod spreads through water and the things that move between water bodies ? not across land [2]. It most likely first arrived in ships' ballast water, and it can hitch rides on boat hulls, in bilge water and live wells, on gear, and possibly tucked among zebra mussels, as well as moving on its own through canals [1][2]. For anyone who boats or fishes, that means it can travel between lakes on a trailered boat or in a bait bucket. Cleaning, draining, and drying boats and gear between waters is the single best way to keep it from spreading.

It breeds quickly. A female carries up to about 20 eggs in a brood pouch and the population can turn over two to three times in a single year, so numbers build fast once it arrives [2]. There is no practical "remove before it sets seed" window for a microscopic water animal ? the realistic lever is preventing its spread to new waters in the first place [1][2].

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 [2]. Its impact is ecological, at the base of the food web. Across many Great Lakes sites it has largely replaced the native amphipod scud (Gammarus fasciatus) through a mix of competition and direct predation, including feeding on native amphipods and their eggs [2]. It thrives in the dense beds of invasive zebra and quagga mussels, which shelter it from hungry fish and help it outnumber the native [2]. Amphipods are a key food for many fish and for waterfowl, so a shift in which scud dominates ripples upward through the food web [2]. Research suggests the newcomer and the native graze algae at roughly similar rates, so the main change is in which species is present rather than in the total amount of grazing [1].

Similar species:

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

Scud (Gammarus fasciatus) is the native amphipod this species is replacing, and the closest match ? similar size, shape, and rocky, mussel-bed habitat. The two are separated only by fine features of the tail fan and antennae under magnification [1][2].

Scud (Hyalella azteca) is a common 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 [2].

Scud (Gammarus pseudolimnaeus) is another native amphipod of lakes and streams that closely resembles this species in size and body form, and is distinguished only by close microscopic study [2].

Monitoring and rapid response:

Signs to watch for:

1. Tiny, shrimp-like scuds among rocks and zebra or quagga mussel clumps in lakes and large rivers [2]
2. Most likely to be noticed when scientists or volunteers sample mussel-encrusted or rocky bottoms [2]
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 [2]
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 [2]
4. Lake associations and natural area crews can include this amphipod in routine bottom-life (benthic) monitoring, sampling mussel-encrusted 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 [2]
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. Chaetogammarus ischnus (scud) ? Species Profile. nas.er.usgs.gov
2. Smithsonian Environmental Research Center, NEMESIS database. Chaetogammarus ischnus. invasions.si.edu

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