Eurythenes atacamensis sp. nov. (Crustacea: Amphipoda) exhibits ontogenetic vertical stratification across abyssal and hadal depths in the Atacama Trench, eastern South Pacific Ocean

Abstract

Eurythenes S.I. Smith in Scudder, 1882 (Crustacea: Amphipoda) are prevalent scavengers of the benthopelagic community from bathyal to hadal depths. While a well-studied genus, molecular systematic studies have uncovered cryptic speciation and multiple undescribed lineages. Here, we apply an integrative taxonomic approach and describe the tenth species, Eurythenes atacamensis sp. nov., based on specimens from the 2018 Atacamex and RV Sonne SO261 Expeditions to the southern sector of the Peru-Chile Trench, the Atacama Trench (24–⁠21°S). Eurythenes atacamensis sp. nov. is a large species, max. observed length 83.2 mm, possesses diagnostic features, including a short gnathopod 1 palm and a chelate gnathopod 2 palm, and a distinct genetic lineage based on a 16S rRNA and COI phylogeny. This species is a dominant bait-attending fauna with an extensive bathymetric range, spanning from 4974 to 8081 m. The RV Sonne SO261 specimens were recovered along a 10-station transect from abyssal to hadal depths and further examined for demographic and bathymetric-related patterns. Ontogenetic vertical stratification was evident across the trench axis, with only juveniles present at abyssal depths (4974–6025 m). Total length-depth analysis revealed that the size of females was unrelated to depth, whereas juveniles followed a sigmoidal relationship with a step-up in size at depths >7200 m. Thus, these bathymetric trends suggest that juveniles and females employ differing ecological strategies in subduction trench environments. This study highlights that even dominant and ecologically important species are still being discovered within the abyssal and hadal environments. Continued systematic expeditions will lead to an improved understanding of the eco-evolutionary drivers of speciation in the world’s largest ecosystem.

Morphological and molecular evolution of hadal amphipod’s eggs provides insights into embryogenesis under high hydrostatic pressure

“…According to previous studies, larval and juvenile amphipods and fish species tend to occupy shallower waters than adult populations (Blankenship et al, 2006;Gerringer, 2019;Weston et al, 2021). Reduced hydrostatic pressure is beneficial to developing instars (Blankenship et al, 2006).…”

Barriers to gene flow in the deepest ocean ecosystems: Evidence from global population genomics of a cosmopolitan amphipod

“…For the few taxa with wide distributions, molecular taxonomy tends to challenge this notion by uncovering that a species is a complex of cryptic species, morphological indistinguishable but genetically distinct ( 21 , 33 ). One classic example is Eurythenes gryllus , a large amphipod historically presumed to live in all oceans between 550 and 8000 m but since 2013 has been known as a complex of at least 12 morphologically and genetically distinct species ( 16 , 32 ). Given that the hadal zone is defined as fragmented habitats, we expected B. schellenbergi to have high levels of overlooked diversity, such as E. gryllus , with multiple species inhabiting individual or neighboring features.…”

“…Specifically, ~95% of hadal amphipod species are only known to inhabit a single trench or a group of adjacent trenches (3,4). For example, the large amphipod Eurythenes atacamensis has only been found at the Peru-Chile Trench (16), while another large amphipod, Hirondellea gigas, is known to the deepest depths of the northwest Pacific Ocean trenches (17). However, many of these findings originated from the first hadal sampling campaigns, the RV Vityaz and Galathea expeditions of the 1950s and 1960s (3).…”

“…Genetic analyses of the limited number of specimens collected have primarily used single-gene mitochondrial markers, specifically DNA barcoding, to study fundamental taxonomy, uncover cryptic species, and assess biodiversity (14)(15)(16)19), with few studies directly assessing patterns of population connectivity among features. One of the only studies using genome-wide markers found enough gene flow among four Pacific Ocean trench populations of the amphipod Paralicella spp.…”

The Multi-Ocean Distribution of the Hadal Amphipod, Hirondellea dubiaDahl, 1959 (Crustacea, Amphipoda)

“…One H. gigas individual from 7,949 m in the Mariana Trench (Table 1) and two Hirondellea sp. individuals from 6,714 to 8,052 m in the Atacama Trench (Weston et al, 2021a) were included in the molecular taxonomy. A total of 24 sequences have been deposited on GenBank (Supplementary Material 2).…”