CLASSIFICATION AND IDENTIFICATION OF CRANGONYCTID AMPHIPODS
Crangonyctidae is the largest family of freshwater amphipods in North America and one of the largest in the world. Although the family occurs primarily in North America, it is also recorded from Europe and Asia. Six extant genera are currently recognized. Bactrurus, Stygobromus, and Stygonyx are composed exclusively of stygobionts, whereas Crangonyx and Synurella contain both epigean and hypogean species and also several stygophiles. Lyurella, which is closely allied with Synurella, is represented by a single epigean species. Palaeogammarus, an extinct genus known only from Baltic amber fossils, is also included in the family. Approximately 80 percent of the crangonyctid species inhabit subterranean waters, and most of the remaining species live in small surface streams, ponds, bogs, seeps, wetlands, etc. that may have connections to underlying groundwater aquifers. Please see table for a breakdown of the number of species per genus.
Stygobromus gracilipes (Holsinger) (ca. 10 mm long) from cave in Virginia, USA (photograph by Lynda Richardson)
Crangonyctid amphipods are easily distinguished morphologically from other common groups of freshwater amphipods in North America such as Gammarus (family Gammaridae) and Hyalella (family Hyalellidae).
Gammarus differs from the crangonyctid genera in having few peg-like spines on the palmar margins of gnathopods 1 and 2, dorsal spines on the last three body segments (urosomites), a well developed third uropod with relatively long, setose rami, and a deeply split telson. Hyalella differs from the crangonyctids in having antenna 2 significantly longer than antenna 1, a greatly reduced propodus of gnathopod 2 of the female, and a very small, subtriangular-shaped telson without an apical notch.
The crangonyctids can be distinguished from both Gammarus and Hyalella by the presence of a double row of robust, bifid spines on the palmar margins of gnathopods 1 and 2 (although in a few species these spines are lacking or greatly reduced), and by the absence of dorsal spines or spine-like projections on the pleonites and urosomites. They can be further distinguished from Gammarus by the reduced third uropod, which bears a relatively short and poorly setose outer ramus (sometimes vestigial or even lacking) and a tiny, scale-like inner ramus that is often absent; and a proportionately shorter telson, which is often notched but never split to the base. Finally, crangonyctids differ from Hyalella by having two pairs of relatively large gnathopod propodus, which although sexually dimorphic in some species, are never drastically reduced in size in the female; and a "conspicuous" telson, which is typically subquadrate or subrectangular in shape and often has a notched apical margin. Please see Key to crangonyctids for assistance with identification of the genera.
Line drawing of Stygobromus quatsinensis Holsinger & Shaw (female, ca. 6 mm long) from cave on Vancouver Island, British Columbia, Canada
Line drawing of Crangonyx forbesi (Hubricht and Mackin) (17.0 mm female) from a cave in Pike Co., Missouri. This species is a widely distributed stygophile in east-central North America, where it inhabits many cave streams and surface springs. It is especially common in the caves of Missouri.
Line drawing of Crangonyx hobbsi Shoemaker (9.0 mm female) from Homosassa Spring, Citrus Co., Florida. This stygobiont species is the most troglomorphic member of the genus known to date and along with Crangonyx grandimanus, it is endemic to subterranean groundwater habitats in several karst areas of Florida.