Gammarus acherondytes

Illinois Cave Amphipod

FWS Focus

Overview

Characteristics
Overview

The Illinois cave amphipod is a troglobitic, meaning cave dependent, species that inhabits the dark zone of cave streams. They are endemic to the Illinois Sinkhole Plain, which covers an area that is approximately 89 square miles (230 square kilometers) in southwestern Illinois. Little is known of the biology and specific habitat requirements of this species, although it has been collected in mainstream gravel riffles, smaller tributary streams, rimstone pools and from streams that overly bedrock. Habitat loss and degradation of groundwater quality, resulting from urbanization and land use practices, as well as an influx of human and animal waste, are the principal threats to the species.

Scientific Name

Gammarus acherondytes
Common Name
Illinois cave amphipod
FWS Category
Crustaceans
Kingdom

Location in Taxonomic Tree

Identification Numbers

TSN:

Characteristics

Characteristic category

Physical Characteristics

Characteristics
Size & Shape

MeasurementsLength for sexually mature males: 0.8 in (20 mm)Length for sexually mature females: 0.5 to 0.63 in (12 to 16 mm)

Weight

 

Sound

 

Color & Pattern

Illinois cave amphipoda are usually light gray-blue, and their eyes are a small irregular black mass.

Characteristic category

Similar Species

Characteristics
Similar Species

Given the relatively small size of this species, it can be difficult to distinguish it from other amphipods in the field. 

Characteristic category

Behavior

Characteristics
Behavior

Given that the Illinois cave amphipod lives in the dark zone of cave streams, they are sensitive to touch and react negatively to light.

Characteristic category

Food

Characteristics
Food

Amphipods are typically thought of as scavengers, as noted by R.W. Pennak in 1953, that shred coarse organic debris. J.J. Lewis observed the Illinois cave amphipod grazing on substrate by slowly walking the bottom in 2001, at which Lewis presumed that the individuals were harvesting a mixture of inorganic substrate material, along with the microbiota that were present.

Characteristic category

Life Cycle

Characteristics
Lifespan

The life span of the species is unknown; however, M.P. Vernasky and others documented the species living between 14 to 16 months in 2007.

Reproduction

Population peaks from late winter to late spring and from late summer to fall suggest two reproductive events per year. Up to 21 eggs have been reported in an Illinois cave amphipod, as documented by J.R. Holsinger in 1972.

Lifecycle

In 2007, M.P. Venarsky and others collected ovigerous, or egg-bearing, females year-round and found that their numbers peaked during two distinct periods of recruitment - one from later winter to late spring, February to May, and the second from late summer to fall, August to October. An incubation time of 90 to 120 days was estimated for young that were first sampled in spring. This sampling coincided with lower water temperatures and took 30 to 60 days for young first sampled in summer, when water temperatures were warmer. Researchers found that Illinois cave amphipods reached the immature stage in seven to eight months, and estimated the time to reach maturity between 14 to 16 months. Although they found the amphipod to live to 14 to 16 months, life expectancy could not be calculated. 

Characteristic category

Habitat

Characteristics
Habitat

The species generally inhabits shallow water that is less than 15.7 inches (40 centimeters), that are characterized by having gravel or cobble present, as documented by J.J. Lewis in 2001. Like other amphipods, this species needs cold water and does not tolerate a wide range in water temperatures. Limiting factors may include increased nutrient load, sedimentation, hydrologic changes and changes in water quality.

Cave or Karst

Geography

Characteristics
Import/Export

 

Range

This species is endemic to the Illinois Sinkhole Plain, which covers an area that is approximately 89 square miles (230 square kilometers) in Monroe and St. Clair counties in southwestern Illinois. Historically, the Illinois cave amphipod was known from six cave systems. However, since the time of listing, there have been several additional caves and groundwater basins that have been added to the species range. Currently the species is considered present in seven caves, extirpated from one and has an unknown status in eight.

Launch Interactive Map

Timeline

Explore the information available for this taxon's timeline. You can select an event on the timeline to view more information, or cycle through the content available in the carousel below.

20 Items