Effects of giant protozoans (class: Xenophyophorea) on deep-seamount benthos

Abstract

Biogenic sediment structures have been proposed to enhance diversity in deep-sea sediments. To evaluate this hypothesis we examined the influence of xenophyophores, giant sediment-agglutinating protozoans, on the structure of metazoan communities inhabiting sediments of deep (1000 to 3300 m) seamounts in the eastern Pacific Ocean. Xenophyophores provided habitat for 16 major metazoan taxa. Sediments immediately surrounding xenophyophores exhibited elevated faunal densities and species richness relative to control sediments collected 1 m from the tests. Amphipods were exclusively associated with the protozoan tests or sediments beneath them. Crustaceans, molluscs, and echinoderms exhibited enhanced infaunal densities in the presence of xenophyophores but polychaetes did not. Both horizontal and vertical distributions of infauna appear to be influenced by these protozoans. 234Th measurements suggest that xenophyophores and their associated fauna increase the particle flux of fine-grained material to the seabed and enhance subsurface mixing on a 100 d time scale. We propose that xenophyophores alter hydrodynamic conditions and provide deep-sea metazoans with substrate, food, and refuge. The resulting habitat heterogeneity may contribute to maintenance of high benthic diversity. The benthic fauna of the deep sea is noted for unusually high diversity relative to shallow-water fauna (Hessler & Sanders 1967, Sanders 1968). Among the many explanations offered for elevated deep-sea diversity (reviewed in Rex 1983), the habitat heterogeneity hypothesis (Jumars 1975, 1976) has received much recent attention (Thistle 1979, 1983, Jumars & Eckman 1983, Gooday 1984). Biologically produced sediment structures, such as mounds, tubes, tests, fecal casts, burrows, and tracks, may enhance diversity in one of 2 ways. Biogenic structures may persist for long periods under highly stable conditions in the deep sea and provide microhabitats within which species can specialize on different resources (Jumars 1975, Thistle 1979, Gooday 1984). In shallow environments, where the frequency of disturbance is orders of magnitude higher (Thistle 1981), such structures are likely to be obliterated rather quickly. Alter@ Inter-Research/Printed in F. R. Germany natively, local disturbances in the deep sea may allow species with similar resource requirements to coexist below carrying capacity (Grassle & Sanders 1973, Jumars & Eckman 1983). One important group of biogenic structures in the deep sea is comprised of the tests produced by giant, sarcodine protozoans of the class Xenophyophorea (Tendal 1972). Xenophyophores are common epifauna at depths greater than 500 m in regions of the Atlantic, Indian, and Pacific oceans exhibiting high surface productivity or topographic relief (Tendal 1972, Levin unpubl.). Xenophyophores agglutinate sediments to form large tests (0.5 to 25 cm diameter) which protrude above the seabed (Tendal 1972). In this paper we examine the influence of xenophyophores on the composition, diversity, and spatial distribution of sediment-dwelling assemblages on deep (1000 to 3300 m) seamounts in the eastern Pacific Ocean. We evaluate whether xenophyophores, which are abundant on seamounts (Levin 1984), provide the sort of habitat heterogeneity which could contribute to maintenance of high faunal diversity. Most of our knowledge of xenophyophores has been derived from analyses of bottom photographs (Lemche et al. 1976, Tendal & Lewis 1978, Tendal & Gooday 1981), or from haphazard collection of specimens (often damaged) in benthic sled hauls (Tendal 1972, Gooday 1983, 1984) or box cores (Tendal et al. 1982). Use of the submarine ALVIN has allowed observation and collection of intact xenophyophores. Materials and methods. Box cores, each containing 4 square subcores (7 X 7 X 15 cm), were collected from 7 sites on 4 seamounts in the eastern Pacific Ocean off Mexico (Table 1). One pair of cores was taken at each site. One core in each pair sampled a xenophyophore and underlying sediment, the other was taken less than 1 m away over sediment with no visible biogenic structures. Six xenophyophore tests were recovered M a r Ecol. Prog. S e r 29: 99-104, 1986 T a b l e 1. Dive s i tes a n d fauna l a b u n d a n c e s for a s tudy of xenophyophore effects on infauna. Paired b o x cores (196 cm2) w e r e t a k e n a t e a c h site, 1 o v e r a xenophyophore a n d 1 as a control