Overview:

The fish hook water flea is a predatory cladoceran (tiny, aquatic crustaceans) native to Ponto-Aralo-Caspian Basin of Eastern Europe.¹ They have been introduced to other parts of Europe and the Great Lakes of North America accidentally via ballast water and dispersed via water currents, boat traffic, and fishing equipment.¹
C. pengoi reproduce both asexually and sexually. Asexual, or parthenogenic reproduction dominates during periods of rapid population growth.⁴ Sexual reproduction, or gametogenic, dominates during the last stages of population growth or may facilitate establishment in new habitats.⁴ Both females and males develop through three instars, which differ by the number of spines on the tail.²
Fish hook water fleas are generalist feeders of other cladocerans and zooplankton. They capture prey with their thoracic legs, crush it using their mandibles, and then suck out the prey's body contents.² The fish hook water flea is established in Lakes Erie, Ontario and Michigan.⁵
C. pengoi may be confused with the spiny water flea, Bythotrephes cederstroemii, but fish hook water flea has a loop in the caudal process (tail) and spiny water flea does not.¹
As of January 1, 2016, the possession, sale, or transport of this species in Alberta is unlawful under the Fisheries Act.

Habitat:

C. pengoi inhabits lakes, estuaries and offshore marine environments. It tolerates freshwater and brackish water up to 17% and water temperatures of 3 to 38 °C.¹ Highest populations are found at water temperatures 16-26 °C and salinities up to 10%. In Lake Ontario abundance increases with distance from shore.¹ In stratified waters it stays above the thermocline.²

Identification:

Fish hook water fleas are greyish white and almost transparent. The body is 1-2 mm long while the tail is 10 mm long. The tail has a loop-like curvature at the tip.¹ The most pronounced parts of the body are the head, 2nd antennae, thoracic legs, abdomen, tail and a brooding pouch in females.¹ The head is a large single eye and the black pigment covers less than half the diameter of the eye.¹ The second antennae are large appendages composed of two branches.² The first pair of thoracic legs are 3-4 times longer than the second pair. Abdomen length is about equal to the rest of the body, excluding the tail.² Spines are large - about 2-3 times the diameter of the tail.²
First-generation, parthenogenic females which hatch from resting eggs have a short, straight tail - not the characteristic looped tail.²

Ecology:

Between instars the fish hook water flea shed its exoskeleton to the base of the tail.² Both sexes, whether parthenogenic or gametogenic, go through three instars. Newborn, parthenogenic, first-instar females have one pair of barbs on the tail, compact oval embryos in the brood pouch without a pointed apex. Second instar females have two pairs of barbs and final instars have a large brood pouch with a pointed apex housing the embryos.² Third instar males have paired penes behind the last pair of thoracic legs and a toothed hook on the first pair of legs.² Subsequent generations of females only require two molts to reach maturity.²
Parthenogenically-produced young develop in the brood pouch which ruptures to release them. Later in the season parthenogenic females produce eggs which develop into males and gametogenic females, which mate.² Gametogenic reproduction produces resting eggs which are released when the brood pouch ruptures and overwinter in the sediment. Most gametogenic females carry two resting eggs.⁵ The following spring development resumes and the eggs hatch to re-develop the population.²
Sexual females reproduce only at instars 2 and 3, parthenogenic females produce 1-24 embryos and clutch size decreases gradually from first to final instar.² Depending on water temperature, there could be 5-7 generations over a summer.²

Economic Impacts:

C. pengoi fouls commercial fishing equipment by attaching to gears, lines, and clogs net and trawls. Costly impacts have occurred in some countries.¹ Infestations of fish hook water flea and fouling of sportfishing gear could have impacts on recreation and tourism.

Environmental Impacts:

C. pengoi may compete directly with other native aquatic organisms which rely on zooplankton for a food source.¹ It may reduce the density of native, smaller-sized cladocerans by predation. Depletion of zooplankton can result in higher concentrations of phytoplankton, contributing to eutrophication.¹

Sociological Impacts:

Some fishermen have reported allergic reactions after contact with C. pengoi while cleaning nets.² The transformation of native aquatic communities results in the intrinsic loss of natural capital and enjoyment of natural areas.

Prevention:

Learn how to identify fish hook water fleas and how to prevent spread. Being a pelagic (free-floating) species it is dispersed with water currents.¹ Report any sightings.
All aquatic recreational and fishing equipment should be inspected after use and cleaned of all mud and plant material. Do not release bait or bait water into a water body or transport from one water body to another.¹ All aquatic equipment should be rinsed with hot water (>40 °C) or high pressure spray, or let dry for at least 5 days before re-entering water. C. pengoi eggs can survive desiccation and freezing. Thoroughly drain the motor, bilge, transom, live wells, bait buckets, and fishing equipment (lines, nets, etc.).¹
The Canadian government's Ballast Water Program and The International Convention for the Control and Management of Ships' Ballast Water and Sediments are designed to prevent the introduction of non-native aquatic species.

Control:

No physical, chemical or biological control methods exist to date.¹

References:

¹Datasheet report for Cercopagis pengoi (fishhook waterflea). Invasive Species Compendium. www.cabi.org/isc/datasheetreport?dsid=89859 Accessed: March 30, 2017.
²Global Invasive Species Database (GISD) 2015. Species profile Cercopagis pengoi. www.iucngisd.org/gisd/species.php?sc=118 Accessed: March 30, 2017.
³Benson, A., E. Maynard, D. Raikow, J. Larson, T.H. Makled and A. Fusaro. 2017. Cercopagis pengoi. USGS Nonindigenous Aquatic Species Database, Gainesville, FL. nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=163 Revision Date: 6/4/2013. Accessed: March 30, 2017.
⁴Vadim E. Panov. Cercopagis pengoi. Delivering Alien Invasive Species Inventories for Europe. Date Last Modified: October 30th, 2006. www.europe-aliens.org/pdf/Cercopagis_pengoi.pdf Accessed: March 30, 2017.
⁵Spiny and Fishhook Waterfleas. Ontario’s Invading Species Awareness Program. www.invadingspecies.com/invaders/invertebrates/spiny-and-fishhook-waterflea/ Accessed: March 30, 2017.

Resources

• Fact Sheet - Ontarios Invading Species Awareness Program
• Fact Sheet - Ontarios Invading Species Awareness Program

Selected Images

Maps

Invasive Listing Sources

• Alberta Fisheries Act
• Idaho Invasive Species Act
• Maryland Aquatic Nuisance Species Plan
• New York Regulated and Prohibited Invasive Species - Prohibited
• Ontario’s Invading Species Awareness Program Tracked Species List
• Pennsylvania's Field Guide to Aquatic Invasive Species
• To be proposed for MISC
• Wisconsin's Invasive species rule – NR 40

References