Crangonyx

Crangonyx sp., copyright Biodiversity Institute of Ontario.

Belongs within: Crangonyctoidea.

Holarctic subterranean amphipods, Batman!
Published 7 August 2007

Meet Crangonyx, the type genus of the amphipod family Crangonyctidae. This is an entirely Holarctic family (Holsinger 1986) that shows an interesting tendency towards a subterranean lifecycle. When Holsinger reviewed the family in 1986, 126 of the 154 known species where stygobionts (exclusively cave-dwelling). More species have been described since then, but the proportion of stygobionts is probably still roughly the same. Even those species that are not stygobionts are cold-stenothermal (only able to tolerate cold water temperatures) and photonegative (keep away from light). Many are stygophilic (not exclusively cave-dwelling, but often found in cave habitats). Crangonyx, with 47 species, is one of only three genera of crangonyctids with epigean (surface-dwelling) species (the others are Synurella and the monotypic Lyurella hyrcana, which may be a species of Synurella—Holsinger 1986). Epigean species of Crangonyx have small but distinct eyes and pigmentation between light brown and pale green in colour, while stygobiont species may retain vestigial eyes and traces of pigmentation (in contrast to exclusively stygobiont crangonyctid genera, which are invariably eyeless and colourless). The greater number of known Crangonyx species come from North America, with only a few from Eurasia—however, I would be inclined to suspect (admittedly without real evidence) that a certain degree of researcher bias may be at fault here.* Perhaps the most widely distributed species is Crangonyx pseudogracilis. Originally native to North America, this species has been introduced to Europe.

*The vast majority of species of organisms on this planet remain undescribed—there are probably about 1.5 million described species, and while it is difficult to estimate how many species remain to be discovered, estimates of 20 million are not impossible. As a result, there are many higher taxa for which the current known species distribution does not accurately reflect reality. Major factors influencing this discrepancy will be collection and study bias—for instance, if most of the researchers on a given taxon have been North American, then there might be expected to be an inflated view of North American diversity as opposed to elsewhere. In the case of Crangonyx, 24 of the known species were erected in a recent review of the North American fauna (Zhang & Holsinger 2003) and one can’t help wondering what would be uncovered if, say, the Siberian fauna received the same treatment.

Crangonyx pseudogracilis, copyright Malcolm Storey.

One species of Crangonyx, C. islandicus Svavarsson & Kristjánsson, 2006 was recently described as part of the Icelandic subterranean fauna. This has interesting implications for the biogeographical history of Crangonyctidae—due to the family’s exclusively freshwater, stringent habitat requirements, it is believed to have originated prior to the tectonic separation of Europe and North America and spread by vicariance as the continents divided. The Crangonyctidae are certainly very old—Palaeogammarus from Baltic amber is essentially indistinguishable from modern crangonyctids. Iceland is far younger than the opening of the Atlantic. However, Kristjánsson & Svavarsson (2007) indicate that Crangonyx, as well as the ancestors of the endemic Icelandic Crymostygiidae, may have spread along the Greenland-Iceland ridge as the geological hotspot that is responsible for the formation of Iceland drifted east, if at least a part of the ridge was above sea-level and held groundwater during that time (note that it does not have to have always been the same part—if extra land was raised above sea-level in the east at about the same rate as land eroded below sea-level in the west, that’d do).

Systematics of Crangonyx
<==Crangonyx Bate 1859H86
    |--*C. subterraneus Bate 1859B77
    |--C. antennatus Packard 1881H86
    |--C. arsenjeviH86
    |--C. chlebnikovi Borutsky 1928H86
    |--C. dearolfi Shoemaker 1941H86
    |--C. floridanusH86
    |--C. forbesiH86
    |--C. grandimanus Bousfield 1963H86
    |--C. hobbsi Shoemaker 1941H86
    |--C. packardi Smith 1888H86
    |--C. paxi Schellenberg 1935H86
    `--C. pseudogracilisH86

*Type species of generic name indicated

References

[B77] Bousfield, E. L. 1977. A new look at the systematics of gammaroidean amphipods of the world. Crustaceana Suppl. 4: 282–316.

[H86] Holsinger, J. R. 1986. Holarctic crangonyctid amphipods. In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 535–549. E. J. Brill/Dr W. Backhuys: Leiden.

Kristjánsson, B. K., & J. Svavarsson. 2007. Subglacial refugia in Iceland enabled groundwater amphipods to survive glaciations. American Naturalist 170: 292–296.

Svavarsson, J., & B. K. Kristjánsson. 2006. Crangonyx islandicus sp. nov., a subterranean feshwater amphipod (Crustacea, Amphipoda, Cranonyctidae) from springs in lava fields in Iceland. Zootaxa 1365: 1–17.

Zhang, J., & J. R. Holsinger. 2003. Systematics of the freshwater amphipod genus Crangonyx (Crangonyctidae) in North America. Virginia Museum of Natural History, Memoir 6: 1–274.

Leave a comment

Your email address will not be published. Required fields are marked *