Systematics and Biogeography of Antarctic Deep-Sea Gromiids

Abstract

Gromiids are large amoeboid protozoans with an organic test and filose pseudopodia, which protrude through a shell aperture that is surrounded by an oral capsule. The first shallow-water gromiid, Gromia oviformis, was described by Dujardin in 1835 and is found worldwide. It was only in 1994, however, that gromiids were found for the first time at bathyal depths in a nonfjord setting, when the first deep-sea gromiid, G. sphaerica, was recovered from below the oxygen minimum zone on the continental slope off Oman in the Arabian Sea. The present study is based on the largest sample set of undescribed deep-sea gromiids thus far. The core of this thesis is the description of twelve new morphospecies from bathyal and abyssal depths (1180 m – 4800 m) in the Weddell Sea, Antarctica, using a combination of morphological and molecular criteria. All species possess a distinct oral capsule and a layer of “honeycomb membranes”, which form the inner part of the organic test wall. Both these features are characteristic of gromiids. Their identification as gromiids is confirmed by analyses of partial small subunit ribosomal DNA (SSU rDNA) gene sequences for three of the twelve new species: Gromia marmorea sp. nov., Gromia melinus sp. nov., and Gromia winnetoui sp. nov. Phylogenetic analysis using the maximum likelihood method revealed that G. marmorea and G. melinus form two distinct clades reflecting the morphological differences amongst the Weddell Sea species, and between deep-water Southern Ocean Gromia and previously described shallow-water gromiid G. oviformis, as well as gromiids from the deep Arabian Sea. Gromia winnetoui forms a third clade which branches with one of the Arabian Sea species. Both these species are morphologically very distinct and therefore the molecular results are inconsistent with the morphological characteristics. Based solely on morphological criteria, another sixteen morphospecies from the Northeast Atlantic are described, including specimens from the Northwest African margin (1320 m), the Porcupine Seabight and the Rockall Trough (1028- 3000 m), the Skagerrak (664 m), and the fjords of Møre along the western coast of Norway (65- 667 m). Most deep-sea gromiids appear to range in size between 0.1 and 4.0 mm, except for morphospecies B2 from the Skagerrak as well as most of the Arabian Sea species, which display a wider size range. The different morphospecies exhibit a variety of shapes, including spherical, droplet, conical, grape, elongate oval, sausage, bowling pin, vermiform, and irregular. The results of this thesis support the hypothesis that gromiids occur at depths >200 m in all major oceans. They are predominantly found along continental margins between 1000 and 3000 m. This thesis presents the only two records of Gromia (G. melinus and Species 154) from depths >4000 m in the Weddell Sea. It is hypothesized that further sampling will increase the number of species and novel morphotypes. Gromiids appear to be found in highest abundances in areas with elevated fluxes of organic matter. They are likely to play an important role in carbon and nitrogen cycling, as well as increasing local biodiversity.

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