Water flea (D. lumholtzi) (Daphnia lumholtzi)

Description:

Water flea (Daphnia lumholtzi) is a tiny crustacean in the water-flea family (Daphniidae) that drifts in open water as plankton [2]. It is native to warm lakes and rivers of Africa, Asia, and Australia, reached the United States in the 1980s, and has since spread to reservoirs across the southern and midwestern states [1][3]. MISIN tracks it because its unusually long spines make it hard for young fish to eat, so it can crowd out the native water fleas that baby fish depend on for food [1][4].

Identification:

Water flea is tiny ? only about 1/10 inch (2 to 3 mm) long ? so you will not spot it without a magnifier or a plankton net, and it drifts in open water rather than living along the shore [1]. Under magnification it is unmistakable among water fleas: it has a tall, pointed "helmet" on its head and a straight tail spine that is at least as long as its whole body, with a row of about ten small spines along the lower edge of its clear, shell-like body [1]. Native water fleas have neither the tall helmet nor the long tail spine [3].

Populations build in the heat of late summer, when the water is warmest and native water fleas are scarce ? that is when a water sample is most likely to turn one up [1][3].

At-a-glance cues:

1. Tiny (about 1/10 inch), clear, and drifting in open water [1]
2. A tall, pointed helmet on the head [1]
3. A straight tail spine at least as long as the body [1]
4. About ten small spines along the lower edge of the shell-like body [1]

Habitat:

Water flea is native to tropical and subtropical Africa, Asia, and eastern Australia, where it lives in warm lakes and rivers [1][3]. In North America it has spread to reservoirs, lakes, river oxbows, and slow rivers across the southern and midwestern United States, and is now recorded in more than 20 states and provinces [3].

It lives in open water as plankton [1]. Its edge over native water fleas is heat: it thrives in warm water above about 77?F (25?C) and tolerates up to about 90?F (32?C) ? warmer than most native water fleas can handle ? and it survives cold winters as tough resting eggs [1][3]. It even tolerates slightly salty water [3].

Reproduction:

Like other water fleas, it multiplies fast. In good conditions the population is nearly all females that clone themselves, so numbers can explode over a warm summer [1]. When conditions turn harsh, it switches to making resting eggs sealed in a tough case (an ephippium) that survives drying, freezing, and years of dormancy until conditions improve [1].

Those resting-egg cases are how it moves between waters. They are covered with hooked spines and hairs that snag on boats, trailers, fishing gear, and even birds, and they ride along in any water left in a livewell, bilge, or bait bucket [1][3]. It has also spread through fish stocking and shipments of fish [1][3]. For this species, control is about stopping that ride ? not seasonal removal.

Impact and Damage:

Despite the name, a water flea is not a flea. It does not bite or harm people, pets, or swimmers [1].

Its impact falls on fish and native plankton. Water flea grows rigid head and tail spines when it senses chemicals from fish in the water ? a built-in defense that makes it too spiny and awkward for larval and juvenile fish to swallow, because their small mouths cannot handle the spines [1][4]. Young fish visually reject it and eat the softer native water fleas instead [3]. The concern is that where water flea replaces those native water fleas, it removes food that baby fish can actually eat, which may hurt fish that hatch and grow in late summer [1][3].

It also fills a seasonal gap: its numbers peak in the late-summer warm water when native water fleas are naturally scarce, and its arrival has been linked to drops in native plankton such as Diaphanosoma and to the suppression of native water fleas in fall [1][3]. A peer-reviewed study found that when fish are present, this spiny invader can outcompete a native water flea that would otherwise dominate ? its ability to grow spines on demand is a key reason it invades so successfully [4]. Competition with native species is complex and not always harmful, but the loss of edible plankton for young fish is the headline concern [3].

Similar species:

Water flea is most likely to be confused with other small planktonic crustaceans in a net sample, but its tall head helmet and long, straight tail spine set it apart from all of them [1][3].

Spiny waterflea (Bythotrephes longimanus) is another invasive water flea, but it is a larger, predatory species with a single very long tail spine bearing barbs, and it has no tall head helmet ? it is a Great Lakes invader rather than a warm-reservoir drifter [3].

Fishhook waterflea (Cercopagis pengoi) is also invasive and predatory, named for the sharp hook and loop at the end of its long tail spine ? unlike the straight, helmet-topped body of this water flea [3].

Native water fleas (Daphnia species) share the small, clear, drifting body but have at most a low, rounded head and a short tail spine ? never the tall pointed helmet or the body-length tail spine of this invader [1][3].

Monitoring and rapid response:

Signs to watch for:

1. You are unlikely to see water flea directly ? it is microscopic plankton. Suspect it in warm reservoirs and lakes in late summer, where monitoring programs find it in water samples [1][3]
2. Its resting eggs and hitchhiking adults travel in any water or mud left on boats, trailers, nets, and gear [1]

What you can do:

1. Clean, Drain, Dry ? the core habit for every boater and angler. Before leaving the launch, inspect and remove all plants, mud, and debris from your boat, trailer, motor, and gear, and leave them at the site [1]
2. Drain every bit of water on land ? livewell, bilge, motor, and bait bucket ? so you never carry water from one lake to another [1]
3. Dry everything completely for at least five days, or rinse with hot water (at least 104?F / 40?C) or a high-pressure sprayer, before entering another waterbody [1]
4. Never dump bait buckets, livewell water, or aquarium water into a lake, river, or storm drain [1]
5. For lake associations and stewards: post Clean-Drain-Dry signage at launches, support boat-inspection and monitoring programs, and prioritize keeping it out of uninvaded nearby and upstream waters [1][3]

When to call a professional:

1. Water flea cannot be hand-removed from a lake once it is established ? there is no practical way for one person to eradicate a plankton species. If you think you have found it, report it and let agency biologists confirm and monitor [3]
2. If you manage fish stocking or water transfers, coordinate with your state aquatic-invasive-species program before moving fish or water
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. IUCN SSC Invasive Species Specialist Group, Global Invasive Species Database. Daphnia lumholtzi. iucngisd.org
2. Global Biodiversity Information Facility. Daphnia lumholtzi Sars, 1885. gbif.org
3. Smithsonian Environmental Research Center, NEMESIS. Daphnia lumholtzi. invasions.si.edu
4. Engel K, Tollrian R (2009). "Inducible defences as key adaptations for the successful invasion of Daphnia lumholtzi in North America?" Proceedings of the Royal Society B 276(1663):1865?1873. semanticscholar.org

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