Yesterday, after much grousing, I introduced you to last week's belated taxon of the week. So as not to get caught out like that again, today I'm introducing this week's title-holder - the amphipod genus Crangonyx. The image at left comes from here.
Crangonyx is the type genus of the family Crangonyctidae. Crangonyctidae 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, shown above in a photo from Bioimages (copyright Malcolm Storey, 2004). 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 (Harrison et al. have a brief review here). 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.
One species of Crangonyx, C. islandicusSvavarsson & 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 (Kevin Z has already commented on this here) 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).
Holsinger, J. R. 1986. Holarctic crangonyctid amphipods. In Stygofauna Mundi: A faunistic, distributional, and ecological synthesis of the world fauna inhabiting subterranean waters (including the marine interstitial) (L. Botosaneanu, ed.) pp. 535-549. E. J. Brill/Dr. W. Backhuys: Leiden.