Illinois Cave Amphipod

views updated

Illinois Cave Amphipod

Gammarus acherondytes

ListedSeptember 3, 1998
DescriptionLight gray-blue amphipod; first antenna is more than one-half the length of the body; primary flagellum has up to 40 segments.
HabitatDark zone of cave entrances.
FoodDead animal and plant matter or the thin bacterial film.
ReproductionClutch size of up to 21 eggs.
ThreatsDegradation of habitat through the contamination of groundwater.


Sexually mature Illinois cave amphipod, Gammarus acherondytes, males are up to 0.8 in (2.0 cm) long; sexually mature females are 0.5-0.6 in (1.3-1.5 cm) long. The amphipod's color is light gray-blue, and the eyes are reniform (kidney-shaped), small, and degenerate with the pigment drawn away from the facets in an irregular black mass. The first antenna is long and slender, more than one-half the length of the body. The primary flagellum has up to 40 segments and the secondary flagellum has up to six segments. The second antenna is about three-fourths as long as the first antenna. The flagellum of the second antenna has up to 18 segments and lacks sensory organs in either sex.


The clutch size is up to 21 eggs; egg-bearing females have been observed in summer and fall.

They are omnivorous scavengers, feeding on dead animal and plant matter or the thin bacterial film covering most submerged surfaces throughout their aquatic habitat.

This species is best differentiated from other amphipods in the field, especially from Gammarus fasciatus, which it resembles, by its color, small degenerate eyes, and a much longer first antenna. G. acherondytes is usually associated with the larger G. troglophilus but is much less common.


This amphipod is a troglobitic (cave-dependent) species that lives in the dark zone of cave entrances. G. acherondytes inhabits the dark zone of cave streams. As a group, amphipods require cold water and are intolerant of wide ranges in temperature. They are strongly sensitive to touch and react negatively to light. High levels of dissolved oxygen appear to be an environmental necessity.

From an ecological perspective, an amphipod belongs to a group of species called detritivores that consume dead and decaying organic matter, recycling their nutrients back into the environment. Nutrient recycling is a critically important function in all ecosystems, especially nutrient-poor cave ecosystems. Amphipods can also be considered to be indicator species; that is, species especially sensitive to physical and chemical changes in their habitat, which can tell us when there is something critically wrong in the environment in general.


The Illinois cave amphipod is endemic to the Illinois Sinkhole Plain of Monroe and St. Clair Counties and was historically known from six cave systems, which are all within a 10-mi (16.1-km) radius of Waterloo, Illinois. The main entrances to two of the caves, Illinois Caverns and Fogelpole Cave, are in public ownership and the other four are privately owned. The cave streams from which this species is historically known are each fed by a distinct water-shed or recharge area; and there are no known interconnections between them, or with other cave systems. Two of the six caves may become hydro-logically connected during extremely high rainfall over short periods of time. Thus, it is believed that there is virtually no opportunity for this species to become distributed to other cave systems via natural pathways.

There are few data or adequate survey techniques on which to base population, productivity, or trend estimates for this species. Sampling for cave fauna is difficult at best, and the challenges of surveying are compounded by the relatively small size of this species and the difficulty of researchers to distinguish it from other similar amphipods in the field. Thus, survey data are not sufficient to accurately record numbers of this small subterranean invertebrate; however, they do demonstrate a reduction in its range and the number of extant populations.

Since the initial collections in 1938 of unknown numbers from two caves, other collections were made. In 1965 at least 19 specimens were taken from the two caves sampled in 1938, plus a third cave. In 1972 unknown numbers were taken from two additional caves, and in 1974 six specimens were taken from one of the caves sampled in 1938. In 1986 there were two specimens taken from one of the caves sampled in 1938 and from a new, sixth cave. In 1992 a total of 20 specimens taken from one of the caves sampled in 1938, and in 1993 there were 11 specimens taken from the two caves sampled in 1938.

A final twentieth-century sampling effort, and the most extensive, was in 1995 in which the Illinois Natural History Survey (INHS) investigated 25 caves in the Illinois Sinkhole Plain and confirmed the presence of the species in only three of the original six cave systems, all in Monroe County. The species was not found in any additional caves. In 1995, 56 specimens were taken from Illinois Caverns, 19 specimens from Fogelpole Cave, and two specimens from a third, privately owned cave. The species appears to be extirpated from the two caves where no specimens were collected in 1965 or 1986. Its status in a sixth cave remained uncertain because the cave entrance was closed by the landowner, and no surveys of the cave have taken place since 1965.

As a result of the extensive searches conducted by INHS, it is possible, but unlikely, that there are populations in other caves in the Illinois Sinkhole Plain. The INHS made an intensive effort to survey all the small side rivulets and drip pools in the 25 caves it sampled and believes that the collection results reasonably reflect the relative abundance of the species in cave streams of the Sinkhole Plain.


The degradation of habitat through the contamination of groundwater is believed to be the primary threat to the Illinois cave amphipod. Karst terrain, where this amphipod is found, is a geologic land formation typified by sinkholes and fissures that provide direct and rapid conduits for water and water-borne material from the surface to the ground-water, thereby avoiding the filtering and cleansing mechanisms normally provided by overlying soils. Water movement from the land surface to the water table in karst terrain often is nearly instantaneous, and flood pulses following a rainstorm may cause levels of contaminants to become up to 10,000 times higher than before the event.

There are several sources of groundwater contamination affecting the amphipod's habitat. The application of agricultural chemicals, evidence of which has been found in spring and well water samples in Monroe County, is one source. Also, bacterial contamination from human and animal wastes, which finds its way to subsurface water via septic systems, the direct discharge of sewage waste into sinkholes, or from livestock feedlots, is problematic. Likewise the application of residential pesticides and fertilizers can cause contamination, as can accidental or intentional dumping of a toxic substance into a sinkhole.

This primary threat is believed to be caused by a reduction in the dissolved oxygen content of underground cave streams which, at times, may fall below life-sustaining levels. To a certain extent, this is a natural phenomenon that occurs during a rainstorm event. Storm water runoff is typically low in dissolved oxygen; when the runoff enters the groundwater, it depresses the ambient dissolved oxygen level in the cave stream. Under natural conditions, cave stream fauna can survive these short term, probably rare, depressions that may reach lethal levels.

However, human activities on the land surface have resulted in changes to this natural condition that make lethal levels of depressed ambient dissolved oxygen more common. With agricultural, residential, and municipal development, storm water now runs off the land more rapidly, reducing the time in which it reaches underground streams. Because of this more rapid runoff, the ambient dissolved oxygen in the cave stream will be depressed to a greater degree and can reach lethal levels faster. Furthermore, pesticides typically bind to soil particles; with the loss of vegetated buffers around sinkholes and fissures, more soil particles erode from the land surface and enter the groundwater carrying more pesticides with them. In addition, nitrogen-based fertilizers and organic wastes increase the demand for dissolved oxygen to accomplish biochemical breakdown. These factors exacerbate the natural depression of dissolved oxygen levels. Agricultural chemicals may either be lethal in themselves at certain concentrations, have chronic effects such as inhibiting reproduction, or can leave the amphipod in a weakened condition and less able to cope with short term depressions of dissolved oxygen.

Conservation and Recovery

Recovery can be achieved by protecting the quality of its habitat and by restoring stable and viable populations to the caves from which it has been extirpated. The U.S. Fish and Wildlife Service, the Illinois Speleological Society, and the Illinois Department of Natural Resources, are investigating the significance of cave visitation as a threat to the species.

The Illinois cave amphipod is listed as an endangered species under the Illinois Endangered Species Protection Act. As such, it is protected from direct taking (i.e., injury or mortality) regardless of whether it occurs on public or private land. However, state law does not protect species from indirect harm, such as habitat alteration. As long as the actions of private landowners are otherwise in compliance with the law, actions that destroy or degrade habitat for this species are allowed under Illinois law.


U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1 Federal Drive
BHW Federal Building
Fort Snelling, Minnesota 55111
Telephone: (612) 713-5360

Rock Island Field Office
U.S. Fish and Wildlife Service
4469 48th Avenue Ct.
Rock Island, Illinois 61201-9213
Telephone: (309) 793-5800
Fax: (309) 793-5804


U. S. Fish and Wildlife Service. 3 September 1998. "Endangered and Threatened Wildlife and Plants; Final Rule To List the Illinois Cave Amphipod as Endangered." Federal Register 63 (171): 46900-46910.

About this article

Illinois Cave Amphipod

Updated About content Print Article


Illinois Cave Amphipod