Amphipods belong to the Phylum Arthropoda, Class Crustacea, Subclass Malacostraca, and Order Amphipoda. The Malacostraca contains seventy percent of all known crustaceans. Over 10,000 species of amphipods are currently recognized. Twenty species of amphipods are known from Arkansas, with most being found in groundwater environments.
Traditionally, amphipods have been placed in four suborders: the Caprellidea, Gammaridea, Hyperiidea, and Ingolfiellidea. The Gammaridea, which contains the majority of species, includes all the freshwater and semi-terrestrial taxa. The Hyperiidea includes the pelagic amphipods, which are associated with other planktonic forms such as gelatinous zooplankton (medusae and ctenophores). Hyperiids are usually characterized by very large eyes, although some forms, like gammarideans, have normal-sized eyes. Hyperiid members are a polyphyletic group, and it is thought that several lineages are derived independently from various gammaridean ancestors. The Caprellidea are composed of over 300 species and are sometimes called the “skeleton shrimps.” All are highly modified for clinging to other organisms such as filamentous algae and hydroids. As a rule, caprellid amphipods have a very narrow and elongated body and long, narrow appendages. One specialized family of caprellid amphipods known as the Cyamidae (twenty-eight species) are obligate symbionts on cetaceans such as dolphins, porpoises, and whales. The Ingolfiellidea contains about thirty species that live primarily in subterranean waters; however, a few are marine and interstitial.
The classification of the Amphipoda is currently under revision to help better reflect their phylogeny, as the relationships within the suborder Gammaridea are the most complicated. A classification in 2013 was developed whereby a new large suborder Senticaudata was split off from the Gammaridea. That taxon, which also encompasses the previous Caprellidea, now comprises over half of the known species of amphipods.
With a diversity of over 10,000 species, most live in marine environments; however, amphipods can be found in most aquatic environments. Approximately 2,000 species occur in freshwater habitats, although, surprisingly, the order contains several semi-terrestrial species (e.g., Talitrus saltator).
By 2019, there were twenty species of amphipods known from Arkansas, with eighteen being found in a variety of groundwater environs. In the state, amphipods are distributed among four families: Allocrangonyctidae (one species of Allocrangonyx); fifteen species of Crangonyctidae (two species of Bacturus, five species of Crangonyx, seven species of Stygobromus, and a single species of Synurella); Gammaridae (three species of Gammarus); and Hyalellidae (one species, Hyalella azteca). Taxonomic identification of amphipods requires careful dissection of appendages and other external body parts. These are mounted either temporarily or permanently on a slide for careful examination with a binocular light microscope.
Morphologically, amphipods have no carapace, two pairs of prominent antennae, and compound sessile eyes, and are generally laterally compressed. The body plan of a typical amphipod contains thirteen segments, which can be divided into three regions: head, thorax, and abdomen. The head region is fused to the thorax and has two pairs of prominent antennae that are uniramous, with the first pair typically longer, and one pair of sessile compound eyes. The first pair of antennae has three basilar joints. Mouthparts on the head are generally concealed but are divided into an upper lip, a pair of mandibles, two pairs of maxillae, one pair of maxillipeds, and a lower lip. The thorax and abdomen are very distinct and possess different kinds of legs. The thorax contains eight pairs of uniramous appendages, the first of which are used as accessory mouthparts while the next four pairs are projected forward, and the last three pairs are directed posteriorly. The abdomen of an amphipod is further divided into two parts: the pleosome, which contains the swimming legs, and the urosome, which is made up of a telson and three pairs of uropods.
The amphipod respiratory system contains gills on the thoracic segments, and the circulatory system is open, with a heart that uses the protein hemocyanin to carry oxygen in the hemolymph to the body tissues. The antennae contain special glands that control the uptake and excretion of salts.
Amphipods are sometimes confused with isopods; however, isopods have only a single kind of thoracic leg and are dorsoventrally flattened. People who fish know amphipods by a number of names such as freshwater beach hoppers, landhoppers, scuds, shrimp, and sideswimmers. Amphipods exhibit a variety of feeding strategies ranging from scavenging to herbivory, carnivory, and even parasitism.
The family Talitridae (which includes semi-terrestrial and marine amphipods), also called the landhoppers, are semi-terrestrial and live in moist habitats such as leaf litter or supralitoral sandy beaches (beach hoppers). Landhoppers are common litter dwellers in New Zealand and Australia, the sub-Antarctic Islands, the Pacific Islands, Japan and Southeast Asia, Africa, India, and Central America. Landhoppers did not occur naturally in the Northern Hemisphere but have successfully invaded North America, the British Isles, Ireland, and other parts of Europe. A few species, such as the terrestrial Talitrus sylvaticus from the coastal region of eastern Australia, even live in greenhouses.
Amphipods range in size from 1 mm to 25 cm (0.04 to 9.8 in.). While they are typically small, there are large members such as the deep-sea, benthic species that reach 25 cm. Interestingly, researchers have discovered that the size of amphipods depends on the availability of dissolved oxygen in their aqueous environment. For example, amphipods found at an altitude of 3,800 m (12,500 ft.) in Lake Titicaca, in Bolivia and Peru, can only grow up to 22 mm (0.87 in.), whereas amphipods residing at 455 m (1,500 ft.) in Lake Baikal, southern Siberia, may reach lengths of 90 mm (3.5 in.).
Ecologically, amphipods exhibit a wide difference in tolerance to salinity in aquatic environments. They are found in most all aquatic situations, ranging from fresh water to water with two times the salinity of sea water. In North America, more than fifty percent of all amphipod species occur exclusively in subterranean waters, and this applies to several species in Arkansas. Ten of the twenty documented species of amphipods that occur in the state are stygobites (organisms that live in limestone areas, caves, lakes, and rivers), and two others are stygophiles (organisms that may live part of their lives in caves but do not complete their life cycle in them) that are closely associated with cave waters. These species are typically troglomorphic (i.e., eyeless, unpigmented) and obligatory to subterranean groundwaters. Approximately 900 species of stygobiontic (spring and artesian well) amphipods have been described, including Niphargus (Europe) and Stygobromus (North America), each with over 100 species described to date. Twenty percent of the total amphipod diversity inhabit freshwater or other non-marine waters. Rich endemic amphipod faunas have been found in the ancient Lake Baikal and in the world’s largest inland body of water, the Caspian Sea basin.
Amphipods function as important mesograzers of any aquatic ecosystem. Many gammaridean species are epibenthic, whereas all members of the Hyperiidea are planktonic and marine and associated with gelatinous animals during some part of their life cycle. In addition, amphipods have a symbiotic relationship with such marine organisms as colonial radiolarians, ctenophores, medusae, and siphonophores. Amphipods play an essential role in controlling brown algae growth in benthic ecosystems. Interestingly, amphipods display a strong preference for brown algae in benthic ecosystems. However, due to removal of these mesograzers by predaceous fishes, brown algae are able to dominate these communities over green and red algae.
Reproductively, the sexes are separate, and a mature female amphipod possesses a structure called the marsupium, or brood pouch, that holds her eggs until the young are ready to hatch. There is an age-related fecundity, as older females produce more eggs in each brood than younger ones. Mortality is relatively high and has been measured at twenty-five to fifty percent of the eggs. There is no larval stage—rather, eggs hatch, juveniles emerge from the brood pouch, and sexual maturity is typically attained after about six molts.
Most amphipods are detritivores or scavengers with some being grazers of algae, or predators of crustaceans and small insects. Prey items are grasped with the front two pairs of legs, which are armed with large claws. However, the incidence of cannibalism and intraguild predation is relatively high in some species, although adults may decrease this behavior when directed at juveniles when they are most likely to come in contact with their own offspring.
When compared to isopods and copepods (other crustaceans), relatively few amphipods are parasitic on other animals. Two suborders, Hyperiidea (254 species) and Caprellidea (300 species), contain specieswith a parasitoid or parasitic lifestyle. The former includes a group of marine pelagic parasitoids that associate with gelatinous zooplankters such as ctenophores, medusae, siphonophores, and thaliaceans.
Most of what is known about North American amphipods was provided by John R. Holsinger (1934‒2018). He was a pioneer in the field of amphipod taxonomy and produced numerous published works on the subject as well as having several species named in his honor.
Three species of Arkansas amphipods are endemic, and each are known only from a single site in the state, including the Magazine Mountain amphipod (Stygobromus elatus) known only from Mount Magazine (Logan County); Rich Mountain amphipod (S. montanus) known only from Rich Mountain (Polk County); and Crangonyx aka from a stream in Pope County, two streams in Van Buren County, and a seep in Saline County. The range of the latter species is known from only four collections from central Arkansas and covers a linear distance of about 80 km (50 mi.) from north to south and spans parts of three different physiographic regions. In terms of conservation, seven species, Allocrangonyx hubrichti, Bacturus pseudomucronatus, C. aka, C. forbesi, S. elatus, S. montanus, and S. onondagaensis, are of special concern due to their rarity of known localities in the state.
For those interested in collecting and studying amphipods, various kinds of nets should be set in an aquatic site to reach the bottom of the water body, as amphipods generally live on the substrate or within vegetation. Artificial samplers such as mesh baskets filled with a combination of stones, gravel, and detritus, can be weighted or partially buried into a stream bed.
For additional information:
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———. “Insect Endemism in the Interior Highlands of North America.” Florida Entomologist 73 (1990): 539–569.
Barnard, J. L., and C. M. Barnard. Freshwater Amphipoda of the World Parts I & II. Mt. Vernon, VA: Hayfield Associates, 1983.
Bousfield, E. L. “Fresh-Water Amphipod Crustaceans of Glaciated North America.” The Canadian Field-Naturalist 72 (1958): 55–113.
———. “An Updated Commentary on Phyletic Classification of the Amphipod Crustacea and its Implication to the North American Fauna.” Amphipacifica 3 (2001): 49‒119.
Cather, Mary R., and George L. Harp. “The Aquatic Macroinvertebrate Fauna of an Ozark and a Deltaic Stream.” Proceedings of the Arkansas Academy of Science 29 (1975): 30‒35. Online at https://scholarworks.uark.edu/jaas/vol29/iss1/11/ (accessed December 27, 2019).
Creaser, E. P. “A New Genus and Species of Blind Amphipod with Notes on Parallel Evolution in Certain Amphipod Genera.” Occasional Papers of the University of Michigan Museum of Zoology 282 (1934): 1‒5.
Culver, David C., T. C. Kane, and Daniel W. Fong. Adaptation and Natural Selection in Caves. Cambridge, MA: Harvard University Press, 1995.
Dunivan, James D., C. Renn Tumlison, and V. Rick McDaniel. “Cave Fauna of Arkansas Further Records.” Proceedings of the Arkansas Academy of Science 36 (1982): 87‒88. Online at https://scholarworks.uark.edu/jaas/vol36/iss1/28/ (accessed December 27, 2019).
Graening, G. O., Michael E. Slay, and John R. Holsinger. “Annotated Checklist of the Amphipoda of Arkansas with Emphasis Upon Groundwater Habitats.” Journal of the Arkansas Academy of Science 59 (2005): 80‒87. Online at https://scholarworks.uark.edu/jaas/vol59/iss1/12/ (accessed December 27, 2019).
Graening, G. O., Michael E. Slay, and Karen K. Tinkle. “Bioinventory and Bioassessment of Caves of the Sylamore Ranger District, Ozark National Forest, Arkansas.” Journal of the Arkansas Academy of Science 57 (2004): 44–58. Online at https://scholarworks.uark.edu/jaas/vol57/iss1/8/ (accessed December 27, 2019).
Holsinger, John R. “Biodiversity of Subterranean Amphipod Crustaceans: Global Patterns and Zoogeographic Implications.” Journal of Natural History 27 (1993): 821‒835.
———. Biota of Freshwater Ecosystems, Identification Manual No. 5: The Freshwater Amphipod Crustaceans (Gammaridae) of North America. Cincinnati (OH): Environmental Protection Agency, 1972.
———. The Freshwater Amphipod Crustaceans (Gammaridae) of North America. Cincinnati (OH): United States Environmental Protection Agency Office of Research and Development, Aquatic Biology Section, 1976.
———. “Systematics, Speciation, and Distribution of the Subterranean Amphipod genus Stygonectes (Gammaridae).” United States National Museum Bulletin 250 (1967): 1‒176.
———. “Troglobites: The Evolution of Cave-Dwelling Organisms.” American Scientist 76 (1988): 146‒153.
Holsinger, John R., T. R. Sawicki, and G. O. Graening. “Bacturus speleopolis, a New Species of Subterranean Amphipod Crustacean (Crangonyctidae) from Caves in Northern Arkansas.” Proceedings of the Biological Society of Washington 119 (2006): 15‒24.
Hubricht, Leslie. “Studies in the Nearctic Freshwater Amphipoda, III: Notes on the Freshwater Amphipoda of Eastern United States, with Descriptions of Ten New Species.” American Midland Naturalist 29 (1943): 683‒712.
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Youngsteadt, Norman, and Jean Youngsteadt. “A Survey of Some Invertebrates from Northern Arkansas.” Arkansas Cave Studies Publication Number 1, 1978.
Henry W. Robison
Chris T. McAllister
Eastern Oklahoma State College
Last Updated: 12/27/2019