The dead do tell some tales! Amphipod taxonomy revealed from sequencing of extinct population
Freshwater organisms occurring in arid systems have long been highlighted for not only the unique positions they occupy, but also their high rates of endemicity These trends have been well documented in the Gammarus pecos complex of the Chihuahuan Desert of New Mexico and Texas.
The complex had long been comprised of at least four species (Gammarus pecos, G. hyalelloides, G. desperatus, and G. seideli) all of which occupy single springs or spring systems. However, owing to the great diversity that can be present in both arid environments and within members of the Amphipoda further, cryptic, diversity has been observed even in a group such as this! Previous investigators such as Adams et al (2018) had identified putative cryptic species within the complex. Particularly within species such as G. desperatus and G. hyalelloides which possess “wide-ranging” distributions, occupying spring systems instead of single sites.
Gammarus seideli
Caroline Spring, Terrell County, Texas.
Scale bar = 1 mm.
As prophesized by the Linnean Shortfall of Lomolino et al (2004), phylogenetic analyses have overtaken taxonomy, and these putative species had yet to be described. This had been due, in part, to confused taxonomy; especially for species such as Gammarus desperatus.
Gammarus desperatus was originally described from a spring (North Spring) in a country club in Roswell New Mexico. Owing to its specific epithet, its situation was perilous, due primarily to groundwater withdrawal in the region, and subsequently, the species was extirpated from its type locality. However, thankfully, additional populations were discovered from springs in the nearby Bitter Lake National Wildlife Refuge. Analyses performed by Adams et al. (2018) and others suggested that populations on the refuge likely represented two species. (I) a “main group” occupying several springs on the core of the refuge and (II) a population on the Rio Hondo, a tributary of the Pecos River. Given the fact that Rio Hondo animals were thought to be endemic to that site, it was assumed that these represented a new species and all other populations were G. desperatus. However, without genetic comparison to the type population, this could not be confirmed.
Very little material was ever collected from the type locality, however, amazingly, we were able to obtain some and extract/sequence their DNA! Phylogenetic analyses suggested that the type population was conspecific with the Rio Hondo population, overturning the previously held assumption and circumscribing G. desperatus to again only occur at a single site! Populations from the refuge were then described as Gammarus acerbatus. Of note is the staggeringly small distances that separate these species. Species that have been isolated for several million years! Rio Hondo is only separated from the nearest record of G. acerbatus by an overland distance of ~ 5 km.
A similar scenario was observed in Texas within the species Gammarus hyalelloides. Here populations occupying springs removed from the species type locality were observed to be phylogenetically distinct and were described here as G. balmorhea in reference to the chosen type locality (Balmorhea State Park) and nearby city/lake. Results generated by this study don’t clarify the taxonomy of the complex but also highlight the processes of speciation nat local/regional spatial scales.
This paper, published in the Zoological Journal of the Linnean Society closes the book on several years of taxonomic/phylogenetic analysis of Gammarus spp. in the Chihuahuan Desert, and can be viewed on the Zoological Journal of the Linnean Society website (https://doi.org/10.1093/zoolinnean/zlae108) or requested through Researchgate.
Literature Cited
Adams, N.E., Inoue, K., Seidel, R.A., Lang, B.K., & Berg, D.J. (2018). Isolation drives increased diversification rates in freshwater amphipods. Molecular Phylogenetics and Evolution, 127: 746-757.
Cannizzaro, A. G., & Berg, D. J. (2024) Molecular phylogenetic analyses reveal a radiation of freshwater Gammarus (Amphipoda: Gammaridae) in the northern Chihuahuan Desert. Zoological Journal of the Linnean Society 202: zlae108.
Lomolino, M. V. (2004). Conservation biogeography. In: Lomolino L, Heaney LR, eds. Frontiers of biogeography: new directions in the geography of nature. Sunderland: Sinauer, 293–296.
Gammarus spp. examined as a part of this study. (A) G. desperatus North Spring (B-C) G. desperatus, Rio Hondo male & female, (D-E) G. acerbatus male & female (F+G) G. balmorhea male & female, San Solomon Spring. Scale bar = 1 mm.
