Taxonomic, ecological and morphological diversity of Ponto-Caspian gammaridean amphipods: a review

Abstract

Thanks to its dynamic geological history the Ponto-Caspian region harbors a unique and unusually adaptable fauna, notorious for its invasive species. Gammarid amphipods attained considerable diversity, becoming the world’s second most speciose ancient-lake amphipod radiation. Nonetheless, apart from a few invasive species, this group remains poorly studied. Herein, we review and quantify the taxonomic, morphological and ecological diversity, as well as phylogenetic context of Ponto-Caspian gammarids within the adaptive radiation framework. Published molecular phylogenies indicate that this radiation has a monophyletic mid-Miocene Paratethyan origin, and is nested within the morphologically-conserved Atlanto-Mediterranean genus Echinogammarus. We find extensive disparity in body shape, size, ornamentation and appendage length, along a broad ecological gradient from mountain springs to depths exceeding 500 m, on virtually all substrate types (including symbiosis). We propose four putative ecomorphs that appear convergent with distantly related oceanic and Baikal Lake taxa. Thus, the identified patterns support the adaptive radiation model, although extensive further research is needed. A checklist and provisional key to all known endemic species are provided to facilitate taxonomic research. Ponto-Caspian gammarids could be a potentially powerful model for studying adaptive radiations and invasive species evolution.

Global distribution and diversity of alien Ponto-Caspian amphipods

“…The boundary between the native and non-native areas of a species’ distribution is often not straightforward to define (Courchamp et al 2020; Pereyra 2020). We acknowledge that a single definition applicable to all APCA species is difficult given the high taxonomic and ecological diversity of the group (Copilaș-Ciocianu and Sidorov 2021). Many species have well-documented introductions (Grigorovich et al 2002, 2003; Arbačiauskas et al 2011) and spatio-temporal dispersal patterns (Bij de Vaate et al 2002) outside the native area, while in some cases it is assumed that they dispersed since the Late Pleistocene (Cristescu et al 2004).…”

“…Only four non-alien species that belong to the Ponto-Caspian amphipod group (ca. 4%) are found exclusively outside the Ponto-Caspian realm, all of these being found in the nearby Balkan Peninsula or the Caucasus (Copilaș-Ciocianu and Sidorov 2021). Therefore, our definition is conservative as it does not include lower sections of rivers that stretch further than the deltaic regions (Gogaladze et al 2020).…”

“…As a taxonomic framework we used a recent review of Ponto-Caspian amphipod diversity (Copilaș-Ciocianu and Sidorov 2021). Two species were found to be taxonomically ambiguous and should be treated with caution.…”

“…Following Grigorovich et al (2002) we distinguished five types of species introduction modes: deliberate, accidental, facilitated by hydrotechnical constructions (canals and reservoirs), shipping, and natural dispersal. Ecomorph and taxonomic assignment followed Copilaș-Ciocianu and Sidorov (2021). The corophiid genus Chelicorophium was not included in the ecomorphological classification as ecomorphs have been established only for gammaroidean Ponto-Caspian amphipods (Copilaș-Ciocianu and Sidorov 2021).…”

“…This is mainly because Ponto-Caspian amphipods have inhabited and diversified in this area (i.e. Paratethys Sea) since the late Miocene, as evidenced by fossils as well as time-calibrated phylogenies (Derzhavin 1927;Hou et al 2014;Hou and Sket 2016;Copilaş-Ciocianu et al 2020;Copilaș-Ciocianu and Sidorov 2021). Only four non-alien species that belong to the Ponto-Caspian amphipod group (ca.…”