Macrostylis metallicola spec. nov.—an isopod with geographically clustered genetic variability from a polymetallic-nodule area in the Clarion-Clipperton Fracture Zone

Abstract

Background The Clarion-Clipperton Fracture Zone (CCFZ) in the Northeast Central Pacific Ocean is a region of heightened scientific and public interest because of its wealth in manganese nodules. Due to a poor ecological understanding at the abyssal seafloor and limited knowledge of the organisms inhabiting this area, huge efforts in alpha taxonomy are required. To predict and manage potential hazards associated with future mining, taxonomy is an essential first step to grasp fundamental ecosystem traits, such as biogeographic patterns, connectivity, and the potential for post-impact recolonization. Amongst samples from the Global Sea Mineral Resources NV exploration area (EA) in the CCFZ an undescribed species of the isopod crustacean family Macrostylidae was discovered. Previously, it has been reported from two other nearby regions, the Institut Français de Recherche pour l’Exploitation de la Mer and BGR EAs. There it was one of the more widely distributed and abundant species of the benthic macrofauna and exhibited geographically structured populations. It nevertheless remained taxonomically undescribed so far. Methods The new species is described by means of integrative taxonomy. Morphologically, macro photography, confocal microscopy, scanning electron microscopy and light microscopy were used to describe the species and to get first insights on its phylogenetic origin. Additionally, mitochondrial DNA markers were used to test the morphological allocation of the two dimorphic sexes and juvenile stages, to analyze geographic patterns of genetic differentiation, and to study intra-and inter-species relationships, also in light of previously published population genetics on this species. Results The new species, Macrostylis metallicola spec. nov., is a typical representative of Macrostylidae as recognizable from the fossosoma, prognathous cephalothorax, and styliform uropods. It can be morphologically distinguished from congeners by a combination of character states which include the autapomorphic shape of the first pleopod of the copulatory male. A sexual dimorphism, as expressed by a peculiar sequence of article length-width ratios of the male antennula, indicates a relationship with M. marionae Kniesz, Brandt & Riehl (2018) and M. longipes Hansen (1916) amongst other species sharing this dimorphism. Mitochondrial genetic markers point in a similar direction. M. metallicola appears to be amongst the more common and widely distributed components of the benthic macrofauna in this region which may suggest a resilience of this species to future mining activities because of its apparent potential for recolonization of impacted sites from adjacent areas of particular environmental interest. The genetic data, however, show geographic clustering of its genetic variability, pointing towards a limited potential for dispersal. Local extinction of populations could potentially not be compensated quickly and would mean a loss of genetic diversity of this species.

Adult life strategy affects distribution patterns in abyssal isopods – implications for conservation in Pacific nodule areas

“…6). If a real identification as M. metallicola can be provided for clade 5, this would follow the wide distribution of this species across the CCZ according to Riehl and De Smet (2020). These authors stated that they found molecular hints of M. metallicola being a complex of more (cryptic) species, and thus the morphologically uniform appearance leads to underestimating biodiversity, or possibly the population is in the early stages of a radiation.…”

“…It is notable that the percentage of species new to science was quite high, reaching 94.1 % in our dataset. Of the 10 described species, five were either described in the course of the JPIO sampling campaign or other CCZ and DISCOL collections (Malyutina and Wägele, 2001;Malyutina, 2011;Malyutina et al, 2020;Riehl and De Smet, 2020). The remaining species, all belonging to the Munnopsidae, appear to have wide (pan-oceanic) distributions.…”

“…However, molecular studies have shown that morphologically similar, but genetically distinct (cryptic), species are common among deep-sea lineages, fundamentally changing our understandings of deep-sea species distributions (e.g., Vrijenhoek et al, 1994;Pfenninger and Schwenk, 2007;Raupach et al, 2007;Havermans et al, 2013;Brix et al, 2014Brix et al, , 2015Jennings et al, 2018Jennings et al, , 2019. Conversely, for some species there is morphological and genetic support for wide geographic distributions even across major topographic barriers (Brix et al, 2011;Menzel et al, 2011;Riehl and Kaiser, 2012;Janssen et al, 2015;Easton and Thistle, 2016;Bober et al, 2018;Brix et al, 2018, Christodoulou et al, 2020. However, biological data on dispersal distances of deep-sea species are still fragmentary due to the low sampling effort compared to the sheer area of deep-sea floor and the scant knowledge of species' taxonomy.…”

“…However, some species, like the Macrostylidae and some Haploniscidae, are limited in their distribution, having been found in just one of the contractor areas. Therefore, their local loss may thus become a global loss, but even for widely distributed species, extinction of connecting populations may result in loss of genetic diversity and recovery potential (Riehl and De Smet, 2020). Janssen et al (2019) stated that in the case of polychaetes with long-and short-distance dispersal capabilities, large populations are continuously distributed over large geographic scales.…”

“…Finally, the Macrostylidae, due to their infaunal tubicolous mode of life, are expected to have the least dispersal potential and thus the smallest distributional ranges. However, their sexual dimorphism may allow males of some lineages to be more mobile on the suprabenthos compared to the females (Harrison, 1989;Hessler and Strömberg, 1989;Riehl and Kaiser, 2012;Bober et al, 2018).…”