Diversity and spatial patterns of foraminiferal assemblages in the eastern Clarion–Clipperton zone (abyssal eastern equatorial Pacific)

Abstract

Foraminifera are a major component of the abyssal meiofauna in parts of the eastern Pacific Clarion-Clipperton Zone (CCZ) designated by the International Seabed Authority for polymetallic nodule exploration. We analysed the diversity and distribution of stained (‘live’) and unstained (dead) assemblages (0–1 cm layer, >150-μm sieve fraction) in megacorer samples from 11 sites (water depths 4051–4235 m) within three 30 × 30 km ‘strata’ in the United Kingdom 1 (UK1 Strata A and B; 5 and 3 samples, respectively) and Ocean Minerals Singapore (3 samples) exploration contract areas and separated by distances of up to 28 km within a stratum and 224 km between strata. Foraminiferal assemblage density, diversity and composition at the higher taxon/morphogroup level were largely consistent between samples. Stained assemblages were dominated (>86%) by single-chambered monothalamids, mainly spheres, tubes, komokiaceans and forms that are difficult to categorise morphologically. Hormosinaceans were the most common multichambered group (∼10%), while calcareous taxa (mainly rotaliids) represented only ∼3.5% of stained tests. Dead foraminifera were more evenly distributed between monothalamids (56%) and multichambered taxa (44%). Almost all test fragments were monothalamids, mainly tubes. Morphospecies were added regularly with each new sample and totalled 580 (stained + dead, complete + fragments), of which 159 occurred in all three strata, 222 were shared between UK1 Strata A and B, 209 between UK1A and the OMS Stratum, and 193 between UK1B and the OMS Stratum. Individual strata yielded 310–411 and individual samples 132–228 putative morphospecies. The majority (550) of the 580 species were represented by intact tests of which 462 included at least some that were stained. Most of the stained (∼80%) and stained + dead (∼75%) species were monothalamids, almost all of them undescribed. Many species were rare; 146 of the 550 species with complete tests (stained + dead) were singletons and 53 doubletons. Values of Morisita's index indicated that most individual species represented by ≥ 10 complete tests had aggregated distributions. In MDS plots, all 11 samples fell within the 95% confidence limit, consistent with a general uniformity in assemblage composition. However, there was also a weak grouping of UK1A samples, while a plot of Bray-Curtis similarity against distance between samples suggests that there may be some gradual change in assemblage composition related to distance. We conclude that the foraminiferal assemblages at our eastern CCZ sites are highly diverse, dominated by undescribed monothalamids, include many rare species, and are fairly similar across the study area, but with a patchy distribution at the level of individual morphospecies.